Restoration Tips & Notes » cassettes

IASA TC04 Guidelines on the Production and Preservation of Digital Audio Objects available online.

Richard L. Hess — Fri, 08 Oct 2010 22:26:25 +0000

The International Association of Sound and Audiovisual Archives (IASA) has released their landmark Guidelines on the Production and Preservation of Digital Audio Objects as a free web (HTML) edition, available here.

I provided some information for the listing of tape equalizations, and I find the compiled table (here) most useful.

Thanks to Kevin Bradley and the IASA team for their work in making this available. If you want a PDF copy, join IASA and it’s available.

Cassette equalization redo

Richard L. Hess — Fri, 08 Oct 2010 22:25:34 +0000

There has been much discussion on some web fora about the differences between different brands’ cassette equalization standards.

As I stated here in 2006, there is a 4 dB ambiguity at 16 kHz.

Many things conspire to make this 4 dB ambiguity essentially meaningless in a generally low-fi medium. The only reason I’m mentioning this now is that I’ve been bombarded with email from more than one participant in this discussion and apparently there may be some editorial judgment attached to what is posted.

Jay McKnight has graciously permitted my posting of his comments to me:

The problem, I think, is that people now-a-days are used to measuring digital equipment with digital measuring equipment, and think that precision measurements are always possible. As you well know, THIS JUST AIN’T SO WITH AN ANALOG MAGNETIC TAPE RECORDER! We try hard, and often come close, but there are just a lot of complicating factors, and most engineers are not aware of them.

For instance, there has NEVER been a measurement standard for tape flux vs frequency (“frequency response”) in ANY format. I have published on it (link here), but when we approached the IEC committee way back when, they said “we don’t write measurement standards like that”, which I think really meant “don’t confuse me with facts, my mind is already made up”.

There is also a problem revealed in the excessive spacing loss document, here . We suspect that this is the cause of the discrepancy in the wavelength-response of the Philips cassette calibration tapes, but they would never admit that. Note that the German Open Reel calibration tapes even at the higher speeds 15 in/s (380 mm/s) also show this problem.

Note also that the AES Standard for measuring the medium-wavelength fluxivity ( AES Standard AES7-2000 (r2005): AES standard for the preservation and restoration of audio recording — Method of measuring recorded fluxivity of magnetic sound records at medium wavelengths (Revision of AES7-1982)) does not exist as an IEC standard, and we think that the amplitude of the medium-wavelength fluxivity on the German Open Reel calibration tapes at the higher speeds are about 10 % in error (link here).

When we approached BASF with these problems on their Calibration Tapes circa 1978, they said something to the effect “Your measurements are probably right, but we’ve been doing it this way for years, and we’re not going to change it now.”

So between the technical problems and the political problems with the IEC Committee (which, to a great extent WAS Philips and BASF), plus the fact that this is OBSOLETE technology, I think that trying to solve the problem with a 4 dB error at a 3 um wavelength on a cassette tape is futile. Take it for what it is. If it sounds bad, fix it as best you can.

To this, I might add that a colleague (and former member of the Ampex Standard Tape Lab) who would prefer not to be mentioned by name (and I can understand why after this week’s barrage of emails) has noted in at least some high-end cassettes back in the 1980s and 1990s that, if put away in storage for a year, they would lose substantial amounts of high-end. Some might have lost close to 10 dB at 10 kHz.

This high-frequency loss due to aging has never been studied, but it is one potential explanation for the very poor Dolby tracking with older tapes.

The same colleague also noted that in his measurement of cassette calibration tapes all of them were hot at the high end. The ones prior to the Prague Compromise were hotter than the ones after, but all were hotter than what the standard states.

A hot calibration tape will cause the repro EQ to be turned down. Adjusting record EQ to match playback EQ will mean that the tapes recorded on a machine calibrated with a hot calibration tape will be hot. Tapes recorded on machines that meet the standard will play back sounding dull on machines calibrated with the hot calibration tapes.

Please note, according to IASA TC04 IEC Type I tape reached its final equalization curve in 1974, and that was 3180/120 µs, and the change was in the low end from 1590 to 3180 µs. IEC Type II and IV tape reached their final equalization in 1970, and that was 3180/70 µs.

Five historic standards now linked from the history page

Richard L. Hess — Thu, 21 Aug 2008 23:14:30 +0000

About two years ago, I asked the EBU to make available a copy of their historic document, Review of existing systems for the synchronisation between film cameras and audio tape-recorders and they complied, making it available on their website.

I asked the National Association of Broadcasters about their Cartridge, Cassette, and Reel tape standards as well as their Disc standard and they gave me permission to post these standards at my website.

These five standards plus some other articles of historic interest are available here in the history portion of this website. I hope that you find these of use in unraveling some of the challenges that old media present.

Mono and stereo cassettes

Richard L. Hess — Thu, 22 May 2008 18:29:26 +0000

The format page for 0.15 inch wide tape has a drawing (click for large version) that clearly shows that mono cassettes have one wide track and stereo cassettes split this track in half and add a small guard band. Most mono cassette recorders follow this format. It turns out that the mono Marantz PMD201 uses a two-channel head and records dual mono. Most other mono recorders seem to follow the standard.

While this is a theoretical problem, few if any good mono recorders are available for reproducing these tapes anyway, so most of us in the domain transfer field use good quality stereo machines for all cassette transfer work.

This is an interesting anomaly that should not effect operations. If you had a full-width mono head, there would be at most about a 4 dB improvement in signal-to-noise ratio vs. using one track of a stereo head (which usually results in better sound). Since the premium cassette decks, such as the Nakamichi Dragons that I use, provide very low wow and flutter, flat frequency response, a means to adjust azimuth of the play head, and gentle tape handling, all of these benefits outweigh the 4 dB or less reduction in signal-to-noise performance. If the tape hiss is a real problem (as opposed to noise recorded in from the recorder’s electronics), some of it can be effectively removed using a noise-reduction plug-in. Generally a 4 dB noise reduction in a good plug-in is transparent.

We do recommend archiving both tracks in a professional archival scenario.

Slow speed 4-channel cassette digitization

Richard L. Hess — Sun, 24 Feb 2008 00:38:37 +0000

There has been some discussion recently about the 4-channel cassette recorders that were used for court reporting and other logging- or court-reporter-type applications. It seems that the players only have one output and can select any combination of one or more playback channels into that one output.

This monitoring topology is actually identical to two 1-inch 40-channel reel-to-reel logging machines I have where one can listen to any combination of one through forty tracks on a single output.

The solution for the reel machine is that I have about half figured out how to create 40 different outputs–and then I have to figure how to digitize 40 channels simultaneously. All can be overcome, but the cost to do it generally terminates the inquiry.

Fortunately, there is a solution for the 4-track cassette machines: use the higher-quality 4-track machines designed for music recording. I have a Tascam 234 (as well as a 238 8-track unit). Yes, I know these operate at 3-3/4 in/s while the logging recorders are running at 15/16 in/s (normal cassettes are in the middle at 1-7/8 in/s).

What I do is record the four tracks playing at 3-3/4 in/s into the computer at 88,200 samples/second (s/s). In samplitude, after the recording is made, I make a new virtual project that has a project sample rate of 44,100 s/s. I load the tracks into that. I then adjust all four tracks to -50% speed in the object properties panel. I use resampling for highest quality. This provides a 1/4-speed playback of the original files while maintaining a 44,100 s/s output file.

The digital data, of course, is actually at some point being processed at 22,050 s/s, placing the Nyquist frequency at 11,025 kHz, for an effective bandwidth of perhaps 10 kHz.

But, that isn’t a problem as only a very few Nakamichi cassette recorders ever made better than 10 kHz at 15/16 in/s — this wasn’t even officially in the Philips standard.

So, there you have a way to migrate these recordings into 44,100 ks/s WAV files while doing the bulk of the work in 4x real time.

You may add equalization and other filtering to improve the usually poor sound after the output is at the correct speed.

I actually had to put the recordings back on this infernal format after repair of the defect (very poor recording speed due to a broken machine), so I reversed the process with the Tascam 234 without adding any equalization and the client was apparently happy (I received payment and no feedback).
As an alternate to the speed/pitch adjustment in the virtual project, one could bounce the 88,200 s/s track played at 44,100 s/s to a second 88,200 s/s track and then repeat the process of loading that as a 44,100 s/s file and it will be in time. I prefer the single-pass approach that I can do in Samplitude.

One of the things that affects my procedure is that my audio interface (RME Multiface) does not work below 44,100 s/s.

Good 4-track recorders like the 234 have not been made for a while. Find them while you can.

UPDATE 2011-11-12: I sold my broken Tascam 234 a few months ago as I have not received requests for this format in years and I have obtained two DataTape CMS-1000 professional voice logging systems. The first one I tried worked and they are modular. I didn’t even try the second one. These run from 15/32 to 7.5 in/s and have four parallel outputs. Also recommended and perhaps better at longevity as I believe the Tascams run on belts which failed.

Tape recorder bias frequencies

Richard L. Hess — Sat, 02 Feb 2008 20:44:02 +0000

The discussion of what bias frequencies were used over time keeps recurring. Special thanks to Jay McKnight of Magnetic Reference Lab, Tom Fine, and Brian Roth for input to this list. I posted this to the ARSC list, but wanted to include it here as well. This knowlege is useful for those who wish to archive the bias along with the audio for future application of time-base-error correction tools such as the Plangent Processes.

Bias frequences started low, apparently with 60 kHz for early consumer recorders, but Ampex started with 100 kHz. Other later machines used different bias and erase frequencies. We can see with a few exceptions, the top bias frequencies were commonly limited to 250 kHz for audio, with the Sony APR series and the Ampex ATR series in the 400 kHz region. For cassettes, a practical maximum appears to be about 150 kHz. Much higher frequencies (up to at least 8 MHz) were used in instrumentation recorders. An enumeration of several machines follows.

Wire and Reel Tape Audio Recorders

In the early days, apparently wire recorders used bias as low as 30-40 kc, but Jay McKnight recalled in the pre-Ampex days, 60 kHz was common.

Ampex

The Ampex 200A used a bias frequency of 60 kHz [Jay McKnight 2012-03-25 post to Studer List] He indicated that this was probably the lowest for any professional recorder.

The Ampex Standard was 100 kc up to the MR-70.

With the MR-70, Ampex switched to 150 kHz bias frequency (and adopted the Hz) [Larry Miller, ex Ampex]

Ampex AG-440 (A) stayed with 150 kHz [manual]

Ampex ATR-100 144 kHz erase, 432 kHz bias (1:3) [manual]

MCI-Sony

MCI JH-24 Multitrack 210 kHz bias, 105 kHz erase [manual via Brian Roth]

Sony APR-5000, APR-24 100 kHz erase, 400 kHz bias (1:4) [manual]

Otari

Otari MTR-10/12 II Bias 250 kHz (erase not spec’d) [manual]
Otari MTR-90 (original) 246 kHz bias, 123 kHz erase [manual via Brian Roth]

Studer

Studer A80VU (All versions) 80 kHz erase, 240 kHz bias (1:3) [manual]
Studer A80 RC 150 kHz [manual]
Studer A810, A807, A820 2CH, A820 MCH, A827 153.6 kHz [manual]
Studer B67 150 kHz [manual]
ReVox A77 120 kHz [manual]
ReVox B77 150 kHz [manual]
ReVox PR99 150 kHz [manual]

Tascam

Tascam 32/44-OB — 150kHz [manual via Tom Fine]

Technics

Technics 1500/1506/1520 — 120kHz [manual via Tom Fine]

Cassette Audio Recorders

Here is a quick sampling of published bias frequencies for two top-of-the line cassette recorders, a better-than-average portable, and an early compact portable.

Nakamichi Dragon (Along with the Nakamichi CR-7A, perhaps the finest machines ever made for overall audio quality) 105kHz [Service manual dated 1985 (scan) 1990 (copy)]
Nakamichi MR-1 — 105kHz [manual via Tom Fine]

Studer A710 (a high-end cassette recorder, without the auto-azimuth that makes the Dragon superior) 150kHz [no date, scan on Studer ftp site]

Sony TC-D5M (a workhorse, good quality stereo portable) 85 kHz [Svc Manual dated 1980]
Sony TC-55 (an early compact — jacket pocket — mono portable) 41kHz (as low as I’ve ever seen) [Svc Manual dated 1972]

Reel Instrumentation Recorders

Honeywell 101 Medium Band: 4 MHz, Wide Band: 8 MHz [1977 manual] (note that the direct recording bandwidth on Medium Band was 600 kHz and 2 MHz on Wide Band at 120 in/s)

Racal Store / DS (Dual Standard) series 1.2 MHz [1982 manual] (note that the direct recording bandwidth was 300 kHz at at 60 in/s, the same wavelength limit as the Medium Band version of the Honeywell 101)

Aligning a tape recorder

Richard L. Hess — Sat, 02 Feb 2008 19:18:03 +0000

It seems some people new to tape are confused over how to align a tape recorder. This is the abbreviated version.

If you want to record on a tape recorder (and I do not recommend doing that these days as you’re just generating more tapes that will need to be transferred later) the first thing to do is get the playback correct.

CLEAN the machine.
If you haven’t done it in the last year or after a move (depending on the machine), demagnetize the heads and guides (using a strong demagnetizer like the Han-D-Mag).
Get a NEW (or trusted) calibration tape from MRL
The MRL tapes are supplied tails out. Rewind carefully and slowly onto a large-hub reel.
The first tone is a lineup tone, set for 0 on the VU meters of all channels.
If you are compulsive, the first time you do this, check the VU meter calibrations using an external AC voltmeter with wide frequency response. Most professional decks have very flat VU meters, so once you confirm that, you can just use the VU meters for the alignment.
There is a second lineup tone at different levels. If it is one of the -10 dB levels, take the machine out of playback cal and increase the level so that the meter again reads 0 VU.
On the 8 kHz azimuth section align the playback head azimuth (with an oscilloscope or a scope-application in the DAW) for minimum phase shift. Also check in mono sum.
Adjust the EQ trims (Trans-treble on the Studer A810/flashing treble light) for 0 VU.
On the 16 kHz tone, readjust the azimuth for minimum phase difference and maximum amplitude as above. Check in mono sum as well. It will never be perfectly stable.
Low frequency adjustment cannot be accurately accomplished off a test tape due to fringing unless the test tape and the play head track width is matched. However, one can often get close a test tape, but don’t necessarily tune for flat. It’s best to leave this alone if you can. The right channel of quarter-track machines will show more bass than the left as the fringing effect is coming in from both sides. Read the material on the MRL website.
Finally, recalibrate the playback level setting on the last tone. Leave the tape in a played wind on the reel it came on.

This completes the playback adjustment. Now you are ready for record adjustment.

Place a piece of blank tape on the machine (NOT your calibration tape from MRL)!
Record a 700 Hz tone at 0 VU on the meter when monitorin input and adjust the record level calibration for 0 VU when monitoring the output. Do this for all tracks.
Increase the frequency to 10 kHz (and drop it 10 dB at slower speeds, making up the gain in the uncal portion of the playback gain controls).
Decrease the bias level slightly so that you can find the peak. Then increase the bias past the peak until the 10 kHz level off the tape drops by the amount specified for that particular tape. It’s often somewhere around 3 dB. There are other, more precise ways to do this, but this should get you close.
Do a sweep of the high frequencies and adjust the HF record equalization for response closest to the response you got from the test tape. DO NOT try and improve the response from the test tape while adjusting record EQ as that will give you non-standard tapes.
Do a sweep of the low frequencies and then you can better adjust the PLAYBACK LF equalization.
Go back to 700 Hz and adjust for 0 on the VU meters when reading input.
Adjust record level control for 0 on the VU meters when reading output.

That should do it. I generally do a quick check flipping between input and output monitoring and you should hear no difference.

A word about levels. In the old days, I used to record at 185 nWb/m with Dolby A. With more modern tapes, 250 nWb/m will provide adequate headroom in most cases and may reduce the need for noise reduction processing. However, some have complained that 250 nWb/m is too low as it sounds too “digital” (i.e. “clean”). If you want to use tape as an effect, increase the record level to taste.

I really love recording with my Sound Devices 722 or somewhat less so with my MOTU 828 MKII, though there is nothing wrong with the MOTU that an RME FireFace 800 wouldn’t fix! Of course, now MOTU has the new 828 MKIII and it seems they have improved some of the things I complained about, but … twice burned (8Pre, also) … Anyway, I think that quality digital recording will capture sounds closer to the original than analog magnetic tape. This has been true in most tests run since the early days of digital recording and why most of the classical engineers who are looking for accuracy and not colouration were early adopters of digital. If you wish to record on analog that’s wonderful, but consider that analog tape is being used as much as an effect or sound-colourant as it is a storage medium. Also, remember that your legacy of tapes will be much more costly to preserve and migrate than digital files, although they may withstand neglect better.

Doug Pomeroy commented:

After aligning the deck for playback, per Richard\’s list, there is a simple way to set bias on any machine, recording on any tape. Use a 1000 Hz test signal and set bias current for maximum recording sensitivity (VU meter reading). Then for 15 and 30 ips recording, increase the bias until the
output level drops 0.2 dB. For 3.75 and 7.5 ips recording, decrease the bias until sensitivity drops 0.1 to 0.2 dB. One slight problem with this is being able, accurately, to read such small values on a conventional VU meter! (This method comes from Jay McKnight, of MRL Labs.)

Another method, also requiring a tone generator, is to record a low frequency, such as 30 Hz, at a very low level – at least 20 dB below normal operating level – and crank up the playback level enough to hear the output clearly, then adjust bias for the minimum amount of distortion (modulation noise, actually). This method allows one to easily adjust the bias by ear, listening for the cleanest reproduction of the low tone. The point of minimum modulation noise will very closely match bias settings arrived at by more elaborate means.

OF COURSE, after setting bias one must always go back and look at the high frequency response (10 kHz) and readjust the recording eq for flattest response.

Yes, bias setting is somewhat complicated, but it is good to remember it is always a matter of compromise, between the least distortion on the one hand, and the flattest overall high frequency response on the other.

Thanks for the comment, Doug. These are also good ways to set the bias. It is always a tradeoff. I no longer remember all the details, but biasing some old Magnetophonband Typ L from circa 1943 was a real challenge on a modern recorder as it is a homogeneous tape which means the \”magnetic coating\” is much thicker than on any coated tape, so the thickness loss is greater, and its basic sensitivity was far less than even something like 3M/Scotch 111.

Magnetic Tape Splicing

Richard L. Hess — Mon, 07 Jan 2008 21:36:18 +0000

I received an email asking me to discuss tape splicing. Most of my work is now repairing old splices so I try and butt them together as best I can in an Edi-Tall block and use the blue Quantegy splicing tape (which will become harder to find with Quantegy exiting the business). I will not be evaluating a replacement for several years as I bought a large supply a few years ago.

I use paper leader tape as I’ve seen too many tapes damaged by the oxide adhering/laminating to plastic leader. I also don’t know where to get any more of this as I’ve laid in a large supply of this as well. Sorry.

Edi-Tall blocks are also out of production, sadly. They show up on eBay from time to time.

The splicing angle for new cuts is not all that important, but the angles were used to minimize disturbance and to provide a short crossfade at the splice (I used to hand-draw dovetail splices that were about an inch long to make the crossfade work in stereo).

The “MTA” or minimum track angle blocks for multi-track tapes were based on the speed of the tape and how much of a scatter was acceptable between the tracks as the splice rolled through.

Perhaps one of the hardest Edi-Tall blocks to find is the 0.150-inch one for cassette splicing and its use is really important for that application. There are blocks that look like Edi-Talls, but are not. Edi-Tall became a part of the Xedit corporation which has morphed into making ServoReelers for hanging microphones.

These blocks aren’t EdiTalls, but they are worth the price and are way better than nothing:
http://www.tapecenter.com/tapecare.html

While on the subject of tape splicing, here are few good Web links:
http://www.aes.org/aeshc/docs/3mtape/soundtalk/soundtalkbull26.pdf

http://www.ischool.utexas.edu/~smegison/Susan/Management%20e-portfolio%20iSchool%20projects/392L%20Intro%20to%20Audio%20Preservation/Tape%20repair%20and%20splicing%20paper%20and%20manual.doc

On January 16, 2008 Seb commented:

I use a â€˜portable\’ Studer-Revox Editing Cutter for my 1/4â€³ tapes that works perfectly !
You can find some similar editing block on players like B67â€¦

I use an EMTEC 500 x 5,8 x 30 mm Splicing Tape Dispenser as tape splices. They are plastic-made, so the seem good for long-term resistance but, as I haven\’t had the the opportunity to test it on a very long period yet, I don\’t know how the glue which is used to manufacture them is evoluting.

I use mainly EMTEC, AGFA and BASF leader tape. They are matt and stretchable (not breakable) when strongly pulled, so I guess they should be Polyester or Mylar-madeâ€¦

For the moment, I am completely satisfied by these productsâ€¦
But any comments would be welcome !

Cheers,

Sebastien Vanhove,
Memnon Archiving Services,
Brussels, Belgium

Cross-point screws take two: JIS vs. PoziDriv

Richard L. Hess — Mon, 26 Nov 2007 16:13:42 +0000

I received the following in an email from a person only identified as Ross. I thank him. He sent me the following in reference to this post. concerning Philips and PoziDriv screws as used on Nakamichi Dragons and other Japanese equipment. I, too, have a set of Hozen drivers which I obtained from www.escience.ca

“You may also be encountering JIS (Japanese Industrial Standard, IIRC) crosspoint (not sure if “Phillips” is correct here, although people do say it) screws. These are a bit different than a standard Phillips, but don’t necessarily have telltale markings. One dot between two slots and near the center is the potential marking that I’m aware of. This is part of why I think you may be seeing JIS, not PoziDriv:
http://realbig.com/miata/1999-09/2918.html

“I just encountered, AFAIK, my first PoziDriv’s, on a Maxtor HD.

“You might encounter these on Japanese-made RC vehicles (helicopters), cameras, print/copier machines, bike parts, (old, at least) Honda motorcycles, and probably a lot more things than people realize.

“PB Baumann (SwissTools) claims their Phillips tools are designed for DIN/ISO and JIS, but I don’t know if that’s truly possible. Hozan provides screwdrivers. Vessel (another Japanese company) makes some. Wiha, I heard, doesn’t know what JIS crosspoint is. Moody (RI company, specialize in “precision” drivers) makes some. Ames offers some. Jensen (part of Stanley?), I’ve heard does.

“Products:
http://www.heliproz.com/jisdrivers.html
http://www.escience.ca/jensen/RENDER/1/26/235/3483.html
http://www.centralhobbies.com/Tools/jis.html
http://www.ikaswebshop.com/jisphilips.html
http://amessupply.com/products1.cfm?aid=1&cid=D&sid=DE&fid=1404070

“Discussions:
(there is useful information in here, but might have to do some sifting)
http://www.mail-archive.com/pdml@pdml.net/msg211895.html
http://www.practicalmachinist.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=1;t=021387;p=0
http://www.runryder.com/helicopter/t136466p1/
http://support.conurus.com/viewtopic.php?t=10
http://www.instructables.com/id/Enhance-the-functionality-of-many-screwdrivers./

“I hope that this helps.”

Noise reduction plug-ins

Richard L. Hess — Mon, 24 Jul 2006 12:37:52 +0000

The question seems to regularly arise on mailing lists and chat rooms about Dolby and dbx plug-ins. I don’t think it will happen and I added that comment and some hopefully helpful operational hints to my noise-reduction page, here.

Cassette Equalization: The 4 dB ambiguity at 16 kHz

Richard L. Hess — Thu, 18 May 2006 00:54:54 +0000

There have been rumours that Nakamichi used a different cassette standard than the other manufacturers. This is not really the case. Everyone thought they were using the same 3180/120 or 3180/70 microsecond equalization as specified in IEC Pub 60094-1, 1981. There is further discussion from 2010 here.

As I understand the history, both Nakamichi and STL in the late 1970s discovered that when they made calibration tapes based on the published time constants in the standards, their response showed that the then-common BASF alignment tapes were approximately 4 dB high (hot) at 16 kHz.

It is assumed that BASF, who made the calibration test tapes made an error in calibrating their reproduce heads’ response in one of two areas:

(1) The gap loss was miscalulated, but that would require a misinterpretation of the gap length from approximately 450 nm to 1.4 µm.

(2) The excess spacing loss of Alfenol heads (where the head’s active pole piece appears to be below the finished surface of the head) only needs to add 200 nm to the spacing to increase the loss by 4 dB. See McKnight: “Flux and Flux-frequency Measurements and Standardization in Magnetic Recording”, JSMPTE, Vol. 78, pp. 457–472 (1969-06), available here, and also McKnight: “Excess Spacing Loss with Alfenol-Core and Ferrite-Core Magnetic Tape Reproducing Heads”, J AES Vol 42 Nr 3, pp. 141–146 (1994 Feb).

This was brought to the attention of the IEC, and ultimately Annex A to IEC 94-2 was added, stating:

“Over the last few years it has become evident that due to differences in the primary measurement of tape flux and the change in performance of replay heads based on new technology, calibration tapes from different sources have led to divergent results when played on any one particular player.

“Steps have been taken to unify the methods of measurement and these steps have resulted in a unified calibration tape which conforms to the established recording characteristics as specified in IEC Publication 94-1.

“Calibration tapes made in accordance with these unified methods of measurement shall be identified by the words ‘IEC (Prague) 1981′ and can be obtained from [addresses omitted] A-Bex Laboratories, AGFA-Gevaert, B.A.S.F. AG, Guiyang 4th Radio Factory, Sony Corporation, Teac Corporation” but including neither Nakamichi nor STL.

The document also sets tolerances on the flux levels of +/- 0.5 dB up to and including 6.3 kHz; +/- 1 dB from 6.3 kHz to 14 kHz; and no mention of any tolerance above that frequency. Actually, the Calibration Tape tones above 12.5 kHz (14, 16, 18, and 20 kHz) are all marked as “optional”.

It is my understanding that these “Prague” test tapes embody a compromise between the original BASF test tapes which were made in error and the Nakamichi and STL test tapes that were made using what they considered good accuracy, taking into account all effects that changed response from the ideal. The reports indicate that both the Nakamichi and STL test tapes agreed.

While the discussion of motivation with 20/20 hindsight is probably counter-productive, the challenge remains that there is at least a 4 dB variation at 16 kHz in how the alignment tapes were recorded. Obviously with 16 kHz marked as “optional” as well as with no specified tolerance, there was no true standardization above 12.5 kHz in cassettes.

One speculation that is worth noting, however, is that the original alignment standard could have been arranged so that the specific playback head gap loss for the 1.4 Îµm playback gap length would be compensated for on the record side so the additional circuitry would not have to be added to the playback electronics. Where this fell down was when Nakamichi and others started making heads with shorter gap lengths to attempt to improve the sound quality on cassettes. However, it is interesting to note that the 1.4 µm Philips playback head in the Studer A80QC appears to be properly resonated with a 100 pF capacitor which would compensate for its gap loss out to 20 kHz, assuming the following amplifier’s input impedance is adequately high. This design, however, post-dates the Prague compromise, so we’re not sure precisely what this is telling us.

While many of us eschewed cassettes as a serious high-fidelity medium, we were wooed by the cost, quality, and portability (for the time) that the cassette machine offered. Many people recorded important things in the intervening years on high-end (and not so high-end) cassette machines. In order to provide the best possible reproduction of high quality cassette tapes, this ambiguity needs to be understood.

While a difference of 4 dB at 16 kHz is not the end of the world (especially for us older folks who are a bit challenged at 16 kHz), getting it “right” or as good as possible is still a worthwhile goal to pursue. Please note, however, that the 2010 discussion of this topic brings up further important sources of uncertainty when attempting proper recovery of these tapes.

Further, understanding these variations and their timelines would create an important “Rosetta Stone” to leave next to the cassettes that haven’t been transferred. It appears that the IEC documents that we have found to date do not provide all the information we would like to carve into our cassette “Rosetta Stone” to aid future generations in understanding the playback issues.

If one reads the IEC standard, one is led to believe that proper response will be achieved by simply following the 3180/120 or 3180/70 curves. I believe at this time, that is not the case and that another term has been introduced that modifies that curve, and that term is only documented by the protocols involved in making the test tapes and not actually in the standard.

Historical research is still in progress, and we hope to add more information, especially concerning the original error in the cassette measurement head calibration— gap length, “dead layer”/spacing loss, or something else.

Thanks to Frank Lennert (STL, retired), John Leslie (Ampex, retired), Jay McKnight (MRL), David Sarser (NBC Orchestra, various cassette duplicators, retired), and Ross Snyder (Ampex, retired) for input on this.

Project Notes: Advanced oxide delamination of a cassette

Richard L. Hess — Sat, 01 Apr 2006 02:58:06 +0000

A client phoned me and said a cassette he was playing started to shed in his machine and he stopped and took it out. He sent it to me and as I pulled a little bit of clear leader out of the middle of the tape, this is what I found:

Notice how the complete strips of oxide exist on their own, independent of the clear “leader” to which they previously were attached.

I immediately called the client and informed him that this portion of the tape would not be salvageable. He requested I attempt to salvage as much as possible as it contained many family voices from Europe.

I opened the cassette and it looked intact, except the tape was flaking. The flaking had only just started as he was playing, so we went and pulled off the smaller takeup hub (at the bottom in this picture) and found solid tape.

We know nothing of the storage history of this tape, other than it was sent from Europe about 2004-2005 (this is written in March 2006). The dark spot on the side 2 label has me wondering what befell this tape. Nothing looked overheated or charred inside. The specks on the paper and on the window of the side 2 shell half are pieces of flaking oxide.

We proceeded to unspool the supply side, in some instances, the backing and oxide would come off as two interleaved strands, totally unattached to each other. Of course there is no practical way to re-adhear the oxide to the backing. We kept pulling and pulling and finally found solid oxide. We spliced the two halves together.

Note the pile of tape and oxide strands behind the two spools, and the intermittant nature of the firmly attached binder right to the right of the splice. Also note the small chips of oxide all over the place.

Finally, we re-loaded the tape into a new shell as the old one was too contaiminated with shards of oxide and it was easier than cleaning the old one.

We were able to play this and interspersed with music the client didn’t want, there was a total of ten minutes of family voices that we were able to recover and then clean up in the computer. The tape had not been well recorded, either. One section required a 30 dB boost to normalize the level. Algorithmix Noise Free Pro did an admirable job of reducing the noise. Note in the above photo, the white splicing tape to the left side of the window.

Note, in the above photo, the colour of the pancakes on the hubs appears to shift. The oxide coating is probably dual layer (see notes below) and the brown side attaches to the base film while the black side faces the head. The colour shift in the photo is related to this and the angle of the camera. When handling the pancakes we didn’t see any noticeable damage as one correspondent suggested from heat.

I wonder how many other tapes will behave like this in the years to come. This was the worst I have seen to date. Others have reported similar conditions with some French Kodak tape from the 1960s, and some acetate tapes from the 1950s or 1960s.

Ben Torre emailed me some comments that help put this into perspective:

BASF FeCr and CrO2 tapes were among the best of the day, and the SK/SM housings were marvelous in the days when cassette machines had crappy mechanisms that never allowed for proper tape pack. (other than Nakamichi, of course) [You can see the SM feed arms in the second photo of the opened original cassette.] I’m guessing the one shown is early-mid 1980s vintage based on the label. These were good enough for Mobile Fidelity to use when they were selling 1:1 dupes done on JVC decks. (they used the CrO2 tapes.) They were not cheap. I have one they did of Pink Floyd’s Dark Side Of The Moon, and it sounds impressive for a cassette.

The FeCr tapes were dual-layer, so I wouldn’t be surprised if the two layers of oxide had different rates of expansion and contraction. Put them through enough heat-cool cycles, and you might see this. Only way to tell for sure is to do an accelerated aging test.

[As to the cause,] my money is on heat-cool cycles over a 20 year period. Shoe-box in the attic, or more likely a tape left near a heat source over the years. Not hot enough to melt anything, but hot enough to provide lots of dry heat for a long time. Might even have been from intense sun exposure in a window. Look at that tell-tale discoloration on the top of the label.

Don’t bet on [this being a one-off].

One other I can recall in the mid-70s. Memorex had a ferric cassette tape… I forget the designation… that after repeated plays started exhibiting dropouts. Sure enough, one look at the tape showed that the oxide was breaking off in chips from the mylar base….

I’ve also received several other emails on this post that confirm the dual-layer nature of this type of tape and also reminding me that it is a “Type 3″ tape. Type 3 tapes didn’t stay long in the marketplace and were considered quite fragile at the time.

Comments:

Mikkel Breiler Posted Apr 5, 2006 8:55 PM

This cassette I bet it not newer than 1982. I believe that the FerroChrom name was derived from AGFA patents which BASF utilized after their take over of AGFA in very early 80s to make the best tape available at the time – debate we can if Chrome will stand out as the overall winner, as Chrome tape held up better due to it being a one layer tape and the fact that consumers reluctance to adopt the new TYPE III formula (probably becuase it was competing with TYPE IV) we never saw too much development at the time to get a better tape from the manufacturers who wanted to push the media. But I will vote for Chrome.
The SM mechanism was popular because it worked, most decks back in the day had awful take-up capabilities and the Security Mechanism seemed like a straight forward solution to the problem.
So when AGFA was incorporated into BASF they took a tape type that they hadn\’t developed themselves and added their – if I recall only type of mechanism that worked towards an even spooling – SM invention. And I believe were it not for all the diseases the TYPE III tape-infant had to endure the type might have survived longer – but personally either TYPE IV or TYPE III would have eventually won most ground.
I have some one or two AGFA FerroChrom cassettes and I believe two BASF FerroChrom cassettes. And also a Sony 7â€²\’ reel of FrCr-7-550-BL tape, and I have them only for reference because I happen to have decks which allow for selecting such. Neither tapes in my collection display the problem seen here.
And I am surprised that the BASF tape had problems like the one pictured above. But as the patents were developed otuside of their lab this poor tape from BASF may be the results of poor management at lower levels where AGFA engineers and other knowledgable people did not receive enough credit for their pioneering efforts and were not allowed to continue to make even better versions of the TYPE III tape. Or hindered in doing so.

Now for the observed discolouring. I have seen this one several BASF cassettes of the early 80s, with the SM invention. And I must conclude that it is simply poor glue for the labels. This is not something only found with BASF tapes of bygone times, but something several brands have ocasionally exhibited. Though in my collection TDK, Maxell and Sony have never had the issue. I have more pre-recorded cassettes with this problem (and the glue drying out so the labels is loose/missing) than I have BASF or AGFA tapes with poor glue. But it may happen.
Storage is the key, and apparantly the brave little tape you got never stood a chance.

It does strike me as odd when BASF – knowing that the brand covers a lot of factories that produce this or that chemical and – would have enough ties with other companies to come up with better glue for labels than what we see was apparently the case. maybe this is just another anomaly we can chalk up to â€˜management success\’.

But not every company can draw a winner when several years on, one engineers idea that looked very effective, cost saving, and or which the marketing department found would be useful as a buzzword, became a wrong decision. Hindsight is always 20-20.

So the FerroChrom tape had been out for a few years in its heyday, but when this baby hit the street either BASF was asleep at the factory or just did not bother too much with a slow seller which showed no improvement in the market, and poor storage just got the better of it.

Sincerely
Mikkel Breiler

Tony Amato Posted Mar 23, 2006 7:04 PM

These FeCr dual layer tapes were one of three brand formulations available between 1980 and 1986. Sony, Scotch and BASF were the 3 major players for Type III formulations. Dual layer configuration ~ top coat (layer) of chromimum dioxide over a base coast (layer) of ferric oxide. Great combination when you think of it ~ yet prone to drop outs when to subjected to climatic changes that can occur in an automobile or just general improper storage. Many considered it a low cost alternative to the then expensive Metal Type IV formulations that were emerging.

I still have a cache of Sony FeCr tapes recorded in the early 80\’s ~ recordings of the Mobile Fidelity Beatles Collection vinyl. They have mostly survived due to careful storage, but some are now suffering from signal loss and a couple have begun to shed in similar fashion as the one depicted above.

Anyone having critically archived voice or music on tapes of this age, let alone tapes of this formulation, should seriously consider digital transfer before Oxidation and Age catch up and steal the recordingsâ€¦

Tony

More from Tony on Apr 17, 2006 11:51 AM

With regard to the top of label, centered discoloration ~ It strikes me most obviously as natural oil/soil remnants from handling by a greasy/dirty set of fingers (more than likely build up over time) and then, subsequent aging, oxidation and environmental attack to an already weakened label surface. I\’ve seen many a tape label look this way by way of friends who have massive tape collections and not a single sense of how to handle such a delicate productâ€¦

B77 Experience Posted Jun 4, 2006 10:29 AM

I recently bought a lot of still sealed BASF CSII from about the same time (a slightly newer model, but still a SM edition, with the large window).

The tape is perfect (typical BASF Chrome, no dropouts, good high frequencies, not too tolerant to hot levels), but the untouched labels exhibit dark spots all over (although they are distributed in a uniform area).
Strange enough, Side 2 is worse than Side 1 on all of the 4 tapes I bought. More glue, perhaps?
So, either a problem with glue or ink.

Demagnetizing recorders and heads

Richard L. Hess — Fri, 24 Mar 2006 05:03:35 +0000

Demagnetizing tape heads and recorder parts is a ritual of magnetic recording. If any part that touches the tape is magnetized beyond a certain level it will begin degrading the tapes played on it.

While early machines may have had an issue with magnetization, most late-model machines rarely become magnetized. The source for real information on this is Jay McKnight’s Magnetic Reference Lab Web site.

Demagnetizing a tape recorder discusses the issues and the need to demagnetize.

Field Strength for Partial Erasure of Magnetic Tape discusses the minimum strength that constitutes a risk to a magnetic record. The challenge in analyzing a tape recorder’s magentization is that it is very difficult to measure a small part since the area of magnetization is generally far smaller than most sensors, thereby providing a faulty (i.e. too-low) reading.

I use the Annis Han-D-Mag, available here. While I bought the kit, the Pocket Magnetometer is not that useful due to the relatively large size of the sensor area. I often demagnetize head assemblies on a bulk tape eraser.

Project Notes: Crosstalk on a cassette

Richard L. Hess — Fri, 24 Mar 2006 03:20:43 +0000

I received a cassette from a client and he complained that the previous recording was audible as well as the new recording.

There are several ways this can happen:

The erase head can be dirty—this usually leads to high frequencies being erased and lower frequencies still audible
The erase head can be misaligned—this often provides a partial erasure, but careful use of track selection can find a section of track with less crosstalk.
A similar problem occurred on quarter-track reels with misaligned record heads where recordings from the opposite direction would invade the tracks for the forward directions. Again, a specially adjustable narrow head usually solves this.
A completely non-functioning erase system—this is what we suspect happened with the current project. There were no track dissimilarities nor any other way we could find, including looking at the tape with the 8-track cassette recorder to separate the underlying, unwanted recording from the wanted one.
A totally unrelated mechanism that may sound the same is if the microphone or tape recorder picks up a broadcast or other radiated signal and records that along with the desired signal.

We opted not to proceed with any noise gating as it would not improve the overall audio quality for listening and may actually impede transcrption.

While not a success, we were able to confirm to the client that there was no way that could preserve the fidelity of the desired sound and remove the undesired sound. The desired interview was completely intelligible and could be transcribed. It was just distracting to listen to.

Noise Reduction Basics

Richard L. Hess — Sun, 19 Mar 2006 02:56:33 +0000

\”Noise Reduction\” is a potentially confusing topic, partially because it has come to be used to mean two different things.

Today, it means removing noise from a recording by means of a single-ended post-production device or plug-in, such as Noise Free Pro by Algorithmix that I use (among other tools).
Historically, the term was used for a double-ended process where the dynamic range of the program material was reduced in an unobtrusive way, transported by the noisy channel (be it tape or broadcast) and then expanded in a complementary fashion at the end/output of the noisy channel. These devices were generally referred to as companders (a contraction of compressor/expander).

When reviewing old tapes, they may be marked in various ways to indicate their noise reduction compression, or processing. As of this writing, there are no software plug-ins that accurately mimic the action of the hardware compander acting as an expander. We use actual companders from the original systems when restoring tapes made with these systems.

We currently handle nine different formats from four manufacturers and are always on the lookout for more. Most manufacturers sold a variety of systems tailored to the needs of different kinds of transmission/recording channels

See our special page under formats (click here) to see some of the information we’ve gathered about noise reduction techniques used over the years. Check back as we uncover additional formats and information.

How to archive recordings — a quick guide to resources on this site

Richard L. Hess — Wed, 15 Mar 2006 19:40:52 +0000

You’ve been asked to digitize recordings in your collection and don’t have any idea where to start. There are several resources on this site which might be of use.

What I use is shown on my facility page. That\’s one of the main reasons it is there. If I’m using it, it’s because I like it or it solves a problem for me. If I’m not using it, either I don’t have an opinion about it, won’t spring for it, or don’t like it.

It seems the good stand-alone CD recorders (like the Sony CDR-W33) are going out of production. Your best bet is to use a computer. Please consider a good USB/1394 audio interface and a good audio editing program.

Pro Tools is a good program, but I think Samplitude (which I use) offers more bang for the buck and is very, very clean sounding. Many musicians that I know use the Steinberg lineup of software. Adobe Audition (the former Cool Edit) is also widely used.

Browsing all the topics in this section, reviewing the Formats & Resources section, and looking at my facility page should give you a good idea of where to start and why.

I’d be happy to answer any questions submitted via email. Some may be answered here, so please let me know if you wish to remain anonymous or quoted by name/email/website.

Analog "Warming up" of sound & the use of obsolete formats in contemporary recordings

Richard L. Hess — Wed, 15 Mar 2006 04:27:45 +0000

I received a phone call today from someone who wanted my opinion on a Tascam 238 8-track cassette recorder for recording his music.

This was like the person who wanted to know about the DCC recorder for the same purpose yesterday.

People keep hearing that “analog sounds great” or that this or that format “sounds great” and they want to buy in.

I am telling people that if they want to use analog to “warm up” their tracks, it should be a high-quality reel-to-reel machine—or a plug-in. I also tell people when they ask me what to use that perhaps they should look at what I’m using on my facility page. That’s one of the main reasons it is there.

Sure an 8-track cassette (how DO you fit EIGHT tracks into a 0.150-inch tape?) is analog, and it might add warmth to a recording, and noise, and probably mis-tracking dbx if you don’t want the noise, and dropouts, and low headroom.

I have a 238 and it solves problems, the biggest of which is people who used to use this format who want to recover and digitize what’s already on the format.

I also tell people DO NOT use obsolete formats (reels are an exception). Please don’t create more obsolete material that needs to be transferred down the road. This goes double or triple for obsolete digital formats. I would not suggest anyone buying into any of the dedicated digital formats today. That includes ADAT, DTRS, DAT, MD, DCC, or DASH.

There is a trend to look at great older recordings and then try and copy it by getting similar equipment. Well, there are lots of things that went into the older recordings, and, in some instances, the great recordings were made despite the limitations of the equipment. Some old equipment is great, but the marketplace has moved on and the best use the old equipment can be put to is rescuing old recordings made in the old formats — obviously I think that as that is what I’m spending my time doing.

If you have a garage band or are a singer-songwriter, please consider a good USB/1394 audio interface and a good audio editing program. The knee-jerk reaction is “oh Pro Tools.” Pro Tools is a good program, but I think Samplitude (which I use) offers more bang for the buck and is very, very clean sounding. Many musicians that I know use the Steinberg lineup of software.

Is it really a Phillips screw or is it a Pozidriv screw?

Richard L. Hess — Tue, 07 Mar 2006 03:56:47 +0000

Many pieces of equipment with cross-headed screws actually have Pozidriv screws rather than Phillips screws in them. This is especially true of Japanese equipment. [EDIT 2007-11-26] Or are these yet different JIS screws? See the updated post about this here.

I bought a set of Hozan [JIS] drivers, but now that I’ve learned that PB makes them [maybe] (see tools article) I’ll buy any additional ones from them. Pozidrive screws have “tick” marks between the slots–or should.

Here is an interesting explanation of the different screw heads in the context of cabinet/furniture making.

After some testing with both Phillips and Pozidrive drivers, it seems that some/many of the inexpensive screws that come packaged with home hardware-type items are non-descript and perhaps don’t meet either standard!

Loading C-0 cassettes

Richard L. Hess — Tue, 07 Mar 2006 03:05:06 +0000

One way of loading C-0 cassettes is to unscrew the shell and drop the old tape into the new shell.

I have found that reloading using a modified cassette machine is much faster and easier. The following images should explain the process:

Splice one end of the original tape into the C-0:

Start winding tape into C-0 in modified recorder:

Another view of the winding machine:

Transfer of tape in process:

Finish by splicing the other end of the tape to the C-0 leader:

Remember, the oxide faces out and the splices are on the inside (backing) of the tape.

In response to some questions regarding where to obtain C-0 cassettes, Chris Goosman responded on Mar 12, 2006 at 10:55 AM:

Full disclosure: I used to work for the owner of World Class Tapes, and my wife is a satisfied customer of theirs.

That said, WCT always has C-0\’s of various flavours around since they do cassette duplication (and more of it that you\’d think these days) They can sell you them in small quantities.

Call (734)662-0669 and talk to Sue, you can email her at suegillis@worldclasstapes.com. They are located in Michigan, and you can find them on the web at www.worldclasstapes.com.