Audio Cassette formats

From: Allison J Parent <allisonp_at_world.std.com>
Date: Thu Nov 26 18:57:25 1998

< Many game tapes for e.g. ZX Spectrum, Commodore 64 used turbo loaders. N
< what the highest data rate was, though at least some C64 programs loaded
< 3000 baud.

300 cps, maybe. Sill the bandwidth had to be under 7-9khs as the group
delays in portable cassettes are terrible as you near their upper 3db
point. Still that is only twice the Tarbel data rate.

< 3db point may not be completely relevant; at least for the old ZX Spectr
< had to hook up the tape player with volume set at or near the maximum.

It is relevant as that is the result of the head gap and media
limitations. Most portables a tone at 15khz was usually at least 20db
down if not worse. The HF falloff for casette is poor and portables
of the era terrible!! Most the amplifier chain was easily good for more
than that! A fair number the bias osc for recording was only 35khz so
anything over 17khz would alias till the cows came home but the head
never saw it nor the media as it couldn't do it.

The bulk of audio casettes were on the 30 cps range using 300baud systems
or the later ones used 150-190cps using phase encoding like tarbel, trs80
or cosmac(lots of others too). Most all were most sensitive to:

        *Dropouts. short periods of far lower level (or none!).
         *Speed variations due to transport not being able to hold
         or not being on speed.
        *Group delays (phase shifts) from trying to put HF (above 5-6khz)
         through the electronics, heads and media.

Compared to CDrom the bandwidth, speed and noise are TERRIBLE and barely
approximate 1/4 the cdrom capability.


Allison
Received on Thu Nov 26 1998 - 18:57:25 GMT