| Geoff Emerick de Fake wrote on Sun, 29 November 2009 05:36 |
| Schallfeldnebel wrote on Fri, 20 November 2009 17:49 |
In the analog days, some engineers were making analog tape
copies in reverse mode. They said it sounded better that way, because the slew rate of the whole system was better.
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This is not correct. The only reasons I know of for doing reverse copies were:
a) in the (false) hope of avoiding print-through pre-echo
b) to save time when numerous copies were to be done (saves rewinding time)
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It also saves fragile tape from overuse. Simultaneous ingest of two sides of a half-track mono or 1/4-track stereo reel can be realized this way.
| Quote: |
What happens in a tape recorder, besides the NAB or IEC/CCIR equalisation, the recordhead needs also EQ to be able to get a flat response into the higher regions. The phase shift from the NAB and IEC corrections is canceled during playback. But the phase shift caused by the record amplifiers' eq, not... | Quote: |
This is absolutely untrue: the phase-shift is, as in any minimum-phase process, the derivative of the frequency response. If the frequency response is flat, the phase response is too.
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This is absolutely untrue. Looks at the step response graph of the Revel loudspeaker. The frequency response is near perfectly flat (is ironically too flat?), but the phase of the groups is not flat as evidenced by delayed flatulences of woofers wrt tweeter.
| Geoff Emerick de Fake wrote on Sun, 29 November 2009 05:36 |
| Schallfeldnebel wrote on Fri, 20 November 2009 17:49 |
By reversing the tape direction of the playback tape, the record amplifiers apply the same equalization for the copy, but in reversed direction. (music played backwards and recorded backwards on the copy) When later the copy is played back in the right direction again, this process has equaled out all phase shift from the record amplifiers. A reversed copy results in a phase linear analog recording, and therefore it sounds better.
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Anyone with a half-decent technical knowledge knows this is pure invention. There is no relation with the electronic phase-shift introduced by the record EQ and the travel direction of the tape.
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Anyone with half-decent technical knowledge knows your name is pure inwention! (; Seriously, you are misinformed, or else AES Fellow, Robert Orban, and analog tape machine guru, Cary Cornett, are! Here's what Orban, writes to Ampex user list:
{begin quotations}
"There's a fairly well-known trick in DSP to make phase-linear
non-realtime IIR filters -- run the signal once through the filter, reverse the result, and run it through the filter a second time. It can be shown that the magnitude response (in dB) is twice that of a single filter (an intuitively obvious result) but any group delay distortion is completely eliminated (perhaps not so intuitive).
The "copy tapes backwards" technique works on the same principle."
[Bob Orban]
{end quotations}
And here is what Cary Cornett wrote at same group list:
{begin quotations}
"The equalizer networks used in recording and playback are used to correct for frequency response changes that happen between the heads and the tape. The EQ networks introduce phase shifts that change with frequency. The heads/tape do not have these variable phase shifts. As a result, the record/play process "smears" the phase of the audio in a way that alters the waveform.
Copying a tape backwards causes the shifts between the original recording and the copy to sort of self-cancel. Sneaky trick here: if you are making a copy of a "reversed" copy, you need to run that one in the FORWARD direction, otherwise you are adding more distortion, just in the other direction. If you go more generations, the 1st, 3rd, 5th, etc. should be run in reverse and the 2nd, 4th, etc. should be run in the forward direction.
"There is an old AES paper somewhere that explains this much better than I just did. I bet Jay knows which paper.
"To make this whole thing more complicated, some professional tape machines include a phase rotation network [to] compensate for this problem. In some 3M recorders, this compensation was put in the playback circuit. In some Otari recorders, the compensation was put in the record circuit.
In the Ampex ATR-100, the record equalizers are designed so that they don't create the problem in the first place. A tape originally recorded on these latter machines does not need the "reversal" for correction. If you are playing a master on a machine that "corrects" in playback, again you don't want to do the reversing bit. The nasty thing about all this is you need to know what kind of machine made the original tape to know whether it needs reversal.
"Cary B. Cornett
Cornett Technical Services"
{end quotations}
Laars
