altair 680 tapes
dkelvey at hotmail.com
Wed Jan 3 09:59:27 CST 2007
>From: Grant Stockly <grant at stockly.com>
>I have never been able to get a computer recorded file to load into the
>Altair. This may be caused from a lack of audio amplitude? I was
>surprised that the KCS.EXE utility only found 3 errors in the cassette tape
>wav file but found 11,000 errors with the remodulated WAV file. There was
>a slight DC offset and a background hum (not 60hz)
There is usually some compensation circuits that are intended to
correct for the phase shift of frequencies caused by the tape
recorder. Without feeding it through a tape recorder, you are
not adding this additional phase error.
Most of these early machines did simple zero crossing type
detectors. This means that the phase of the harmonics of
the signal going to the tape are important, since they effect
the timing of the zero cross. For the most part, harmonic
phase is not that important to our ears since we are most
sensitive to spectral content and less so to phase.
To a zero cross detector, it is more important. This
phase shift is caused by the fact that the tape head only
redords by the trailing edge of the head gap while the
playback is from the entire width of the head gap.
This causes both a high frequency rolloff and a phase
shift. From an audio perspective, they use pre-emphasis
and de-emphasis circuits to compensate for the gain.
The problem is that some phase error is always left.
It is the high frequency content that creates the sharp
edge at the front of a square wave. Move the high
frequency part around and it doesn't look too square anymore.
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