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sergeauckland

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I'm sorry, but this is a gross oversimplification. The only way I can explain how bias worked, in a few words, is this: magnetic recording is, inherently, highly non-linear. By using an AC bias signal (typically >8× the maximum audio frequency of interest) whose magnitude was significantly higher than the wanted audio, as the tape entered the magnetizing zone of the head it would be pulled and pushed through several non-linear cycles of the bias waveform which gradually reduced to zero plus the required audio as the tape left the vicinity of the head.

In the absence of the bias signal, the residual audio would be entirely at the mercy of squat S-shaped hysteresis loop but the fact that the magnetizing force on the tape builds to a peak and the opposite peak and the opposite peak and the opposite peak and then declines gradually at a rate several times higher than the maximum audio frequency leaves a residual that is a relatively linear approximation to the audio waveform you want to record.

Ages ago, you may remember Hugh Ford, of sainted memory, who was an expert in the forensic evaluation of tape recordings. I think he was one of the people who analysed the Nixon tapes after Watergate. Anyway, he published one or more articles in Studio Sound on forensic analysis, and bias continuity was one way of checking whether a tape had been edited. Bias leaves a signature on the tape, and any discontinuity in this indicates that the recording was stopped and restarted, and/or erased and recorded over, so bias does leave traces on the tape, but well down into noise levels.

S.
 

Pluto

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That was why Sel-Sync was developed on multitrack machines. (Selsync was the Ampex term for it, I think Studer made up their own) This used the record head for playback into the monitoring system, so, for example, a singer could add their contribution without the timing error between replay and record
But there still exists the erase head timing problem, especially when dropping-out! If you are listening to the sync head (Studer's term) and you punch out at exactly the right instant, the couple of inches between the erase head and sync head have already gone! :eek:

In practice, ISTR that the answer to this was that, at the punch of the button the erase head was de-energised and the record head still driven until the gap-period between the two heads has passed. This got you a technically clean drop-out with the possibility of some unwanted residual material.
 
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sergeauckland

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There goes your argument then :facepalm::D
No, because recording audio on a video recorder is not the same as recording digital audio as video on a video recorder. When video recorders were used as CD mastering machines, the audio was encoded to look like a legal video signal, for either PAL or NTSC.

S.
 

sergeauckland

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But there still exists the erase head timing problem, especially when dropping-out! If you are listening to the sync head (Studer's term) and you punch out at exactly the right instant, the couple of inches between the erase head and sync head have already gone! :eek:

Yes indeed, I was thinking more overdubs rather than punch-ins.

S.
 

voodooless

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No, because recording audio on a video recorder is not the same as recording digital audio as video on a video recorder. When video recorders were used as CD mastering machines, the audio was encoded to look like a legal video signal, for either PAL or NTSC.

S.

Sure, but I cannot imagine this being one of the primary reasons for going for 44.1?
 

Pluto

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Bias leaves a signature on the tape
I seem to remember that this residual signature was typically at a frequency well below what you might expect (i.e. the actual bias frequency), suggesting that it was (probably) an intermodulation product of some kind.

Recall anything in this respect?
 

Pluto

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I cannot imagine this being one of the primary reasons for going for 44.1
Sorry, this is getting all topsy-turvey, but I believe that the prime reason for the choice of 44.1kHz was to get something that was not 48kHz but fully compatible with both the PAL and NTSC* video technologies of the time. The fact that the chosen sampling rate meant that the Nyquist frequency was perilously close to the audible range (espcially in the days of analogue-only filters) was an added bonus.

*afterthought edit although PCM-1630 U-Matic tapes sent to the CD factory were ever and always NTSC, if I remember rightly
 
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Blumlein 88

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I will measure it anyway after warranty expiry, for my own curiosity, and I did not make any claim, I am only curious in the context of #153 if it might be... I hope being curious is kindly allowed. If everything always was as it should, many things (experimenting, prototyping) would be unecessary. Veryifying people's crazy ideas is the business of science, sometimes (not in this case because I can do it myself).

But to be clear: nobody needs to do anything, it's not a matter of life or death.
Peace...
Why do this after the warranty expires? Seems like it is a good idea to do it before the warranty expires.
 

sergeauckland

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I seem to remember that this residual signature was typically at a frequency well below what you might expect (i.e. the actual bias frequency), suggesting that it was (probably) an intermodulation product of some kind.

Recall anything in this respect?
I vaguely remember that it was something to do with noise modulation but it was a very long time ago. Hugh taught me an awful lot, as I young engineer, I was fortunate to have visited him a few times at his lab in Sunbury.

S.
 

sergeauckland

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Sorry, this is getting all topsy-turvey, but I believe that the prime reason for the choice of 44.1kHz was to get something that was not 48kHz but fully compatible with both the PAL and NTSC* video technologies of the time. The fact that the chosen sampling rate meant that the Nyquist frequency was perilously close to the audible range (espcially in the days of analogue-only filters) was an added bonus.

*afterthought edit although PCM-1630 U-Matic tapes sent to the CD factory were ever and always NTSC, if I remember rightly
I suspect, but don't know for sure, that the use of NTSC U-Matics was de-facto rather than de-jure, and the original specs assumed either would be used. As Japan also used the NTSC system, it's quite likely that they produced an NTSC encoder first, then never got round to doing a PAL version, and anyway, CD production facilities would have had their NTSC machines by then, so the need for a separate PAL version went away.

S.
 

Guermantes

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Ages ago, you may remember Hugh Ford, of sainted memory, who was an expert in the forensic evaluation of tape recordings. I think he was one of the people who analysed the Nixon tapes after Watergate. Anyway, he published one or more articles in Studio Sound on forensic analysis, and bias continuity was one way of checking whether a tape had been edited. Bias leaves a signature on the tape, and any discontinuity in this indicates that the recording was stopped and restarted, and/or erased and recorded over, so bias does leave traces on the tape, but well down into noise levels.

S.
The erase and record heads also leave magnetic signatures on the tape, especially at start and stop events, that can be revealed by coating the tape in ferrous particles. Sometimes it is even possible to match a tape recording to a particular machine. Forensic authenticity analysis of magnetic tape sometimes requires this and it can be quite destructive unfortunately.
 

Pluto

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Forensic authenticity analysis of magnetic tape sometimes requires this and it can be quite destructive unfortunately
Were I a defence lawyer I could make a meal of that.

“How fortunate for the prosecution, members of the jury, that the process purporting to provide evidence of my client's guilt has conveniently resulted in the destruction of that evidence, rather limiting any prospects for further examination”
 

Spkrdctr

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Tweeter response is the never mentioned elephant in the room for all these formats. Every argument about the additional music content above 20kHz conveniently ignores the tweeter response, and the unfortunate reality that very few tweeters have any useful response above 20kHz, many are already diving down, and many have quite nasty resonances above 20kHz.

The winner! I like it when someone points out the elephant in the room that everyone ignores. I could talk about probably 5 other elephants but it would shock the ASR faithful. I don't want to be run out of town! Even our scientifically minded crew here has standards that are too high for 95% of the human race. The entire audio community is trying to find elusive pennies while passing by all the $20 bills laying around. Oh well, that's audio!
 

respice finem

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A hobby is a diagnosis. Apart from these elephants, there's also an invisible T-Rex, the adult hearing that ends somewhere between 8 (yes eight) and 18 kHz at sane levels, depending on age and hearing condition.
 

respice finem

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Why do this after the warranty expires? Seems like it is a good idea to do it before the warranty expires.
Not for me, as it is "just for fun" and not to actually improve anything. If it was a 100€ part, maybe I would think differently ;)
 

Guermantes

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Were I a defence lawyer I could make a meal of that.

“How fortunate for the prosecution, members of the jury, that the process purporting to provide evidence of my client's guilt has conveniently resulted in the destruction of that evidence, rather limiting any prospects for further examination”
Yes, of course and I would expect a good lawyer to make that point. In most cases, however, it is the defence that are alleging the tampering.

The tape is not destroyed, it can still be examined afterwards. It's playback that is compromised because of handling and ferrous residue so a high quality copy is made first. Damaging evidence is anathema to me and all care is taken, but fragile tapes such as C90 cassettes don't stand much physical manipulation.

I'm also going to look at "baking" some unplayable cassette tapes in a desiccator soon. If there is interest, I'll do a post on the process.
 
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amirm

amirm

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On response above 20 kHz, I am testing this Barefoot speaker and while I didn't go up that high, the company did:

index.php


They claim flat to 40 kHz!
 

Francis Vaughan

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As Japan also used the NTSC system, it's quite likely that they produced an NTSC encoder first, then never got round to doing a PAL version, and anyway, CD production facilities would have had their NTSC machines by then, so the need for a separate PAL version went away.
Quite likely. The early Sony machines were pretty much wedded to the U-Matic system, and actually used slightly modified recorders - ones that disabled the chroma signal, so they were B/W recorders anyway. So there were arguably neither PAL or NTSC. There was never any idea that the recorders were multiple use. The entire recording chain occupied a good slab of a rack. So what frame timing was in use on the tape would have been essentially invisible.
At the start I can't imagine that the politics of the situation would have left Philips happy with an NTSC only solution. They would have been arguing for PAL compatibility, if only to protect their European sensibilities. So even as NTSC became the default, the reasons for 44.1 remain.

This brochure for the Sony PCM-1610 (the second iteration of their digital recording adaptor) makes for interesting reading.
 

Somafunk

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I am testing this Barefoot speaker and while I didn't go up that high, the company did:

Keen to see the review of the barefoot,
 
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