Restoration Tips & Notes Working with audio media (mostly tape) restoration


Cassette Equalization: The 4 dB ambiguity at 16 kHz

Filed under: cassettes,Tape Aging — Richard L. Hess @ 19:54

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 εm 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.

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