[sdiy] "Digital vs analog waveforms" [was: Ways for innovation]

[Needham, Alan]

Genuine question, self-taught.

I thought the higher bias frequency was a requirement of the smaller head gap of better quality machines, the two factors going hand-in-hand to allow a higher upper end, thus "sounding better" (assuming a comparable tape speed).
I can't see how this bias can be seen as equivalent to 'sampling' as it is more like a constant 'nudge' to avoid signals dropping into the hysteresis dead band, their combined peak being one side or the other of it, the head gap is a sort of low-pass filter preventing the bias frequency being detected during playback. It can only see an average of the magnetic field within the gap at any point in time so it naturally loses higher frequencies, hence the requirement for pre-emphasis during recording.
I thought the minimum bias frequency was determined by tape speed and head gap (pole-swap speed to exceed gap dwell by a factor of more than two) and its amplitude by the tape hysteresis, exceed both of these minima slightly, more than that and lose power as heat  and headroom, respectively.
Can you explain where this aliasing occurs please?


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Sarah Thompson
Sent: 28 January 2016 15:39
To: Tom Wiltshire
Cc: synth-diy List
Subject: Re: [sdiy] "Digital vs analog waveforms" [was: Ways for innovation]

It seems to me that nobody ever really acknowledges that analog tape actually also samples the recorded signal in time, though doesn't quantize in level. The bias signal, usually a sine wave at 50 - 100KHz or so, is added to preemphasized version of the audio as it is fed to the record head. This overcomes the large amount of hysteresis in the magnetic materials in the tape by rapidly flipping between linear(ish) regions. This overcomes the coercivity of the iron oxide, but has the side effect that the audio is only really recorded during the peaks of the bias signal. Though not identical to digital sampling, this does still cause very similar aliasing issues, so the signal path still needs antialiasing filters, even though they are typically not thought of as such.

This is also why higher bias frequencies often sound better, though were harder to achieve due to the relatively high voltages needed.

Sarah

Sent from my iPad


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