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Cassette equalization redo

Filed under: cassettes,history,Tape Aging — 2010-10-08 by Richard L. Hess — Last Edit 2010-10-08 by Richard L. Hess

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.

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