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Re: digital recording mystery
Now ya' got me thinkin'!
Interesting problem...you've convinced me that the minidisc is
the major culprit.
So I ran a couple of experiments. (I used my Kyma system
for all of this, operating it at 44.1 kHz sampling rate, and a Sharp MD
MS-702. Your milage may vary.)
Experiment 1:
I generated a 20 kHz sine wave of 4 seconds duration. I
recorded it to minidisc. I played it back and re-recorded it. Items
of note: 1) The playback signal was actually longer than 4 seconds. 2) The
signal played back as a 20 kHz sine wave.
I'd expect the signal to be slightly longer due to filtering
so (1) doesn't surprise me too much. (2) is due to the adaptive nature of
ATRAC. I understand you can record a pure sine wave and play it back
unchanged (well pretty much unchanged anyway considering the ADC and DAC).
And this experiment confirmed that. But consider that there is not the
storage capacity on the minidisc for a full bandwidth signal. I.e., on a
74 minute minidisc, I can't record 74 minutes of a 20 kHz sine wave.
There's simple not enough room as Nyquist states.
Experiment 2:
Same procedure repeated with white noise instead of a sine
wave. 1) I still see the playback signal as longer than 4 seconds, but
only slightly. 2) The playback signal has been limited to 18 kHz.
There's good evidence of two filters, the first at approximately 17 kHz and the
second at at 18 kHz.
Four seconds is not very long. One
of these days, I should repeat the experiment with real music of four minutes or
more.
