I received an urgent phone call yesterday from a man who had digitized several reels of 2″ 24-track analog recordings that he wished to re-mix.

The tapes were originally recorded in about 1978-1979 and he said that he needed them to have Dolby C noise-reduction processing applied to the files.

I did a bit of research, as that did not sound correct from an historic point of view.

Here is an approximate chronology of the major noise-reduction systems and their dates of introduction:

DOLBY
A – 1967 (pro)
B – 1971 (consumer)
C – 1983 (consumer/prosumer)
SR-1986 (pro)
S – 1990 (consumer/prosumer)

dbx
I (pro) & II (consumer) – 1971

Telefunken (later ANT)
C4 – 1977

He later wrote me back saying the engineer was pretty sure it was Dolby A.

When I applied Dolby A, Dolby B, Dolby C, dbx I, and Telcom C4, only the dbx I sounded close to correct.

Fortunately, dbx I is less critical than the Dolby noise reduction systems for accurate level setting, since there are no test tones digitized along with the audio.

This work requires playing the digital files out through the D-A converter and then re-recording them via the A-D converter.

The Lunar Orbiter tape digitization folks have just posted a commentary that bears reading by all archivists who are holding tapes. You may link to it here. The main site is www.moonviews.com

NASA, in their press conference yesterday held at The Newseum, admitted that the original 14-track 1-inch instrumentation (IRIG) tapes that contained the slow-scan video direct from the moon were most likely recycled and reused for later missions. Apparently, over 350,000 reels of instrumentation tape were recycled by NASA over time. No one apparently thought to preserve the 45-odd reels of the original moon walk.

The loss of the original IRIG tapes of the moonwalk is truly sad because this data could be re-converted to standard television formats using far superior methods than were available in 1969. There may be 2-inch helical Ampex VR-660 video tapes still extant of the slow-scan data,  but those have not surfaced. It appears that all surviving copies of the moonwalk videos are ones that had gone through optical standards converters. An optical standards converter is one that has a monitor displaying the image in real time in the transmitted standard and a television camera taking a picture of that monitor using the desired standard. Even the Australian Broadcasting Corp. tapes would have gone through this type of device, although they would be in PAL rather than the U.S.’s NTSC versions.

Lowry Digital is doing a great job of restoring what they have, but the Polaroid screen shot that survives of the slow-scan monitor is alluring of what could have been preserved. More information is available on the Parkes website and from NASA.

Vigilant migration of data as new storage techniques become available is the only way to assure long-term preservation. Even if the IRIG tapes are found, we are almost at the point where the tapes would be un-decipherable. I think one of my machines could play them (I say think as I’ve never tested it to full 500 kHz bandwidth), but I don’t have the specialized video decoder. NASA apparently preserved some equipment should the tapes ever show up.

This also raises another spectre. We MUST be selective as to what we keep in our archives because if we keep everything we won’t be able to afford it–or find it. This is one of the key jobs that archivists do. However, blindly following retention practices, as was done by NASA for the IRIG Apollo 11 tapes, needs to be tempered by historians as well. Certain small subsets of data (moonwalk slow scan video) are much more important than others (astronauts’ blood pressure and other biometrics throughout the entire flight).

All organizations who keep archives need to address this. In a generation (or less) if we save everything, it will become an overwhelming burden and the high points will be lost if they are not properly indexed.

Please note that connecting any microphone other than the intended one to any of the adapters shown in the link may severely damage the microphone. In general, while the vast majority of dynamic microphones and some ribbon microphones can work with phantom powering, it is a good idea not to send power to microphones that don’t need it.

In the 1960s, transistorized microphones from AKG, Neumann, Schoeps, and Sennheiser became available. There are several niches of early microphone powering that continued on for many years. Perhaps the easiest way to look at it is backwards.

I have adopted the terminology “hot” and “low” to refer to the audio signals in a balanced line. Hot is defined as the voltage going positive in respect to the low lead with a positive pressure at the microphone diaphragm. I’m using “hot” in this context rather than “hi” because so many people have heard of “pin two hot” or “pin three hot” that I wanted to be consistent with that nomenclature.

PHANTOM POWER

Today, 48 V phantom powering is almost universal. In phantom powering the positive voltage is fed through a pair 6k81 ohm resistors, one to each modulation lead. The matching of these resistors is often done to 0.1% to maintain common mode rejection. The negative power runs on the mic shield. XLR: Pin 1-shield; Pin 2-audio hot, +48 V; Pin 3-audio low, +48 V. Tuchel: Pin 1-audio hot, +48 V; Pin 2-shield; Pin 3-audio low, +48 V. This was standardized in the 1960s in DIN Standard 45596.

A caveat about phantom powering voltages. There are a wide range of microphones that will work with phantom voltages from 9-52 V, but many that are rated at 48 V will not work well outside of the +/- 4 V tolerance in the standard. AKG, Audio Technica, and Schoeps, for example, make many 9-52 V powered microphones, while DPA, Neumann, and Sennheiser mics generally need 48 V. Some devices (e.g. the first version of the MicroTrak digital recorder) had an odd 30 V phantom that probably worked with a number of mics, but might have degraded their performance. M-Audio took this to heart and the MicroTrak II has real 48 V phantom power. There once was a 24 V phantom power option in the standards, but apparently it was never adopted in practice and it has since disappeared.

Prior to standardization, in 1964, Schoeps produced the CMT-20 microphone which used negative 8.5 V phantom power. The CMT-200, according to Schoeps drawing SB316, dated 1964-10-14, used the same -8.5 V phantom. Later this was broadened to negative 8-12V phantom followed by the switch to positive phantom at some later point. With vintage microphones, at least from Schoeps, be very careful as they might be negative phantom.

For more details about phantom power, please see Rick Chinn’s page here. This page on Wikipedia has some further history.

NOTE: You can use a polarity-reversal cable with phantom power, but not with any of the other schema.

CAUTION: Phantom Power can damage some battery-operated microphones like the Audio Technica AT822 stereo mic. It can also damage ribbon microphones which are not floating (i.e. those that have their centre tap grounded).

T or AB POWER

Moving backwards in time, the next most widely used microphone powering scheme is called “AB power” “T-power” or “Tonader Power” and is standardized in the 1960s in DIN standard 45595. In this design, both the power and the audio are on the same wires. The hot audio and the positive power is on one conductor while the low audio and the negative power is on the other conductor. The shield is just a shield in this scheme. This was widely used by Sennheiser and Schoeps in the film industry. Neumann made a FET-70 series that used this powering scheme and many of the mics in that series are the same as the much more widely known FET-80 series (as in U-87 and KM-84, for example). Rick Chinn has more information here.

If you have T-powered microphones, you can power them off phantom power WITH THE APPROPRIATE ADAPTER.  These adapters can be purchased or made. Rick Chinn has a transformer design and a transformerless design. I developed a similar transformerless design,  but used 680R resistors instead of the 4k7 resistors that Rick used and I placed a 180 ohm resistor between the zener and the filter capacitor to reduce the noise of the zener even more.

T-Power, as introduced by Schoeps with the CT100 series in 1965 was wired to XLR connectors with the hot/+ connected to pin 3, following the Ampex standard. This polarity practice is documented on the same Schoeps drawing referenced above. Sennheiser, to the best of my knowledge, always connected the hot/+ to pin 2, which became the international standard. Many Nagra recorders came with pin 3 hot, but I believe they could be ordered either way. Sennheiser introduced this powering scheme in late 1963 based on catalog research by Lonn Henrichsen.

It got to the point where there was the term “red dot” microphones which had been rewired for pin 3 hot/+. If in doubt, this is the one legitimate use of a polarity reversal cable with T-power. Sennheiser adopted T-Power with the MKH-105/405/805 microphones in the mid 1960s. They later provided “P48″ versions of these microphones and at that point also designated the T-powered mics with a “T”. So there could be an MKH-416T and an MKH-416P48 which differed only with the powering. This appears to have been introduced with the XX6 series of microphones.

CAUTION: T-Powering WILL damage ribbon and may damage dynamic microphones.

EARLY ANOMALIES

In addition to the Schoeps NEGATIVE phantom mentioned above, Sennheiser introduced their first line of RF condenser microphones with an unbalanced, negative power connection. The MKH-104/404/804 have odd wiring. Pin 1 of the Tuchel connector is audio output. Pin 2 is ground (audio low and power +). Pin 3 is -8 V power. This series of microphones was introduced in 1963 and discontinued between the 1968 and 1969 catalogs.

Later, Sennheiser introduced the extended-low-frequency special-purpose microphones, the MKH-110, with the same powering scheme, except Pin 3 is +8 V power!

It is trivial to power an MKH-x04 microphone from a 9 V battery, and after having noise problems with an inexpensive off-the-shelf DC-DC converter, I ended up with an alternate powering scheme, described here.

I’ve decided that this will be an application to keep in the Tuchel connector domain, so this oddball powering doesn’t get into any other microphones and possibly fry them.

CAUTION: These powering schema can damage many types of microphones.

CONCLUSIONS

While this may sound complex, in practice certain combinations of mics/recorders are used together and it’s fairly trivial to keep track of what’s what.

For example, for almost four decades, I’ve had AKG C451 microphones in my kit, and I’ve adapted almost everything to power them. Other mics that take 9-52 V phantom obviously will work there as well. I’ve had several (currently three) Sennheiser MKH-416T short shotgun mics for about a decade (I got them used). I made a P48-T12 adapter for each of them, and I keep them with the mic. When I grab the mic, I grab the adapter. Sometimes it goes right on the mic, other times it goes at the end of the first mic cable coming from the mic. Placing this at the mixer increases the risk of damaging other microphones.

When I first got the MKH-416T mics, I made a stereo powering box that had a toggle switch that could select T12 or P12 so I could use one box with a pair of short shotguns or C451s. This had unbalanced outputs (all resistor-capacitor networks, not transformers) that connected to the portable DAT Walkman recorder and used 8 AA batteries for long running time.

I recently got an AKG C460/CK63 and that’s still 9-52V.

Some of the newer mics in my collection (Neumann TLM-103, KMS-105; DPA 4006 TL) are P48 only and, in fact, most of the new equipment I have has true 48 V powering. This includes a Sound Devices 722 portable recorder, a MOTU 828MK II multichannel FireWire audio interface for my laptop, a Mackie 1402VLZ mixer, and a Shure FP410 mixer. The church I do sound for has a Mackie 1604VLZ that I previously owned, so P48 is very common in my world. People who have used Nagras in the field report adapting everything to one scheme. In one case it was the “red dot” mentioned above.

All in all, don’t be afraid of some of the oddball powering schema, just work through what is needed. Since all of these schema are low-powered, the likelihood of any significant damage to a microphone is probably low, but still, don’t take chances with expensive, excellent performing antiques.

It is my understanding that European broadcasters at least influenced the various powering schemes by requiring compliance to their specific powering standard across several manufacturers. When I started working at ABC-TV in New York City in the early 1970s, there was a system of remotely powered microphone preamplifiers built into extended length female Cannon UA cable connectors. So this is yet another odd scheme, although it was an accessory to the microphones rather than in the microphone itself. This preamplifier was used to boost the signal level at the microphone in an attempt to overcome hum and noise on the long lines.

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Here are the ones with the picture of the chamber orchestra
http://www.richardhess.net/restoration_notes/111.jpg (red 7″)
http://www.richardhess.net/restoration_notes/120.jpg (orangy-brown 7″)
http://www.richardhess.net/restoration_notes/200.jpg (blue 7″)
http://www.richardhess.net/restoration_notes/311.jpg (grey-green 5″)

The moiré pattern you may see is the screening of the printing beating with your monitor.

These are earlier boxes for two if the above
http://www.richardhess.net/restoration_notes/111(A)_early.jpg
http://www.richardhess.net/restoration_notes/120(A)_early.jpg

And then it all became standardized in the 1970-era box
http://www.richardhess.net/restoration_notes/200_1970.jpg

When I get a chance, I’ll scan the box between the musicians one and the 1970s one.

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.

The article also heralded this as the saviour of tape, and talked about the “old” two-track format running at 7.5 in/s — the cartridges ran at 3.75 in/s (and on some models 1.88 in/s was also available). It goes on to say later that 7.5 in/s quarter track tapes are still a high-fidelity medium. The article referred to cartridges and quarter-track reels as the “one-two punch” against stereo records which seemed to take over from the two-track pre-recorded tapes. The open-reel tape at 7.5 in/s would be the “only choice for the quality-conscious stereophile” since the cartridges were only available in 3.75 in/s versions.

There was no mention of the later name “Sound Tape” in the article, but that appears to be the semi-official if not official name of this format. Thanks to Bill Schuh for that piece of information. Bill also provided a link to The Tape Place which specializes in out-of-print commercial tape releases. I have not used The Tape Place, so this is just being passed on, not a personal recommendation.

The Sound Tape cartridges used the standard 1/4-track interleaved format which prevailed for a decade as the consumer open-reel format.

Details about these formats can be found here and here in the Formats and Resources subset of this website.