- Mar 16, 2016
- Suffolk UK
My first job after University was partly designing electronics for high-speed tape duplication. As part of that, I did a fair amount of work on record and replay amplifiers, and can confirm that replay amplification is very benign, similar in many respects to the more familiar RIAA replay amplifiers for LP playback. By far the greatest amount of noise and distortion comes from the record process, mostly the tape itself rather than the electronics.It is well known that in magnetic tape recording the distortion (and compression and saturation) are caused by the combination of recording head and tape, and to a lesser extent by the recording amplifier if of incompetent design (which happened a lot).
Proving by measurement that the replay side of things does not introduce distortion is a bit tough, as it would require me to disconnect the heads and inject signal straight into the replay amplifier at levels comparable to what a head produces, which is very feeble. This is very hard to do without picking up noise and hum, or sending the replay amp into oscillation. Moreover, the stimulus would have to be equalised prior to injection, at least when multiple frequencies are used.
So: no. I do not have measurements for that.
But I do have other measurements for tens of decks and hundreds of tapes. I also happen to have a lot of alignment tapes, the genuine stuff from ABEX, TEAC, and BASF, costing a small fortune. Such tapes were typically made for one or two specific purposes (i.e. playback level, replay azimuth, speed + wow&flutter, replay head height, ...) but a tape for replay distortion simply did/does not exist. No specific attention was spent on distortion during the recording of alignment tapes: distortion only had to be adequate so as not to endanger the tape's primary function. In fact, some of these tapes were not even produced on a recorder, but on machinery imprinting specific magnetic patterns.
When I play a cassette recorded on deck A on deck B, I see the distortion spectrum of A. When I play a cassette recorded on deck B on deck A, I see the spectrum of B.
Something like a Sony Metal ES on a Nak CR-4 has less than 0.3% third harmonic at 400Hz at 218 nWb/m (DIN), and happily takes peaks 10dB over that for 3%. Total A-weighted dynamic range is 68dB without Dolby! That is today. With tape and deck 30+ years old.
As an aside: all measurements should be referenced to a specific magnetic flux. '0VU' is meaningless without stating the flux level the deck was calibrated for.
I also entirely agree that terms like 0VU is meaningless without reference to the flux level. Noise and distortion are trade-offs, and I remember some specs that played this, using different flux levels for noise (a high flux level) and distortion (substantially lower). Frequency response also is very much flux related, on domestic machines it was generally measured at -20dB relative to whatever 0VU was, the better Pro machines used -10dB. I don't know of any machine that could support a frequency response at 0VU, unless that 0VU was deliberately done at low flux, relying on Dolby Noise Reduction to keep noise half-sensible. The only possible exception was the Ampex ATR100, but I left Ampex just as that machine was being introduced, so didn't spend much time with one.