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

[Donald Tillman]

Now now now, hold on a second.  Magnetic tape doesn't alias.

Yes, a high frequency bias signal is added during recording so that combination spends more time in the linear region of the magnetic curve and less time in the nonlinear zero-crossing region.  The bias frequency is audible if you slow the tape down sufficiently during playback, or if you were happened to be recording while the tape was rewinding.  'Happens if you work with real-to-real machines.

And yes, when played back, the tape nonlinearities will create some intermodulation products between the higher audio frequencies and the bias oscillator.  But that's not aliasing.

Aliasing is the specific case of the sampling process unable to distinguish frequency components on either side of the Nyquist frequency.  Hence the name: one frequency is an alias of the other.

And since they're indistinguishable, aliasing components occur at the same volume level as the intended audio components, so that's really bad.

That mechanism doesn't happen in magnetic tape.  In tape there is some difference frequency content, but it's at a much lower level since it's a side effect of the nonlinearities.

Refs:
    High Frequency Bias Requirement for Magnetic Tape Recording, 3M
    http://www.aes.org/aeshc/docs/3mtape/soundtalk/soundtalkv1n2.pdf

    Biasing in Magnetic Tape Recording, Ampex
    http://thehistoryofrecording.com/Papers/Jay_McKnight/Biasing_in_Magnetic_Tape_Recording.pdf

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com


> On Jan 28, 2016, at 7:46 AM, nvawter <nvawter at media.mit.edu> wrote:
> 
> holy cow, that's awesome :)
> 
> It's begging to be exploited!  I wonder what it would be like to make lo-fi bias signals?  Low-frequency?  non-sinusoidal?  wow and flutter in the bias signal?
> 
> On 2016-01-28 10:38, Sarah Thompson wrote:
>> 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.