Audio Cassette formats

From: Mark <mark_k_at_iname.com>
Date: Thu Nov 26 17:56:29 1998

On Wed, 25 Nov 1998 Sam Ismail wrote:
>In my experience, sound files do not compress well at all with PKZIP. I
>don't know about WAV files in particular, but I'm lucky if I can squeeze
>10% off the ADPCM files that I work with.

Are you talking about digitised computer tapes or music? I have some digitised
(VOC format) files of ZX Spectrum game tapes. For example, Altered Beast. Not
sure at what rate this was sampled, but:
    uncompressed file size = 12,725,061 bytes
    compressed file size = 504,546 bytes

Which is a pretty good compression ratio. That is using the lharc archiver; I
imagine zip would give similar or better compression.

There are lossless compression systems designed for audio, but I bet they
weren't designed with computer tapes in mind.



On Wed, 25 Nov 1998 Allison J Parent wrote:
>FYI, protable casette recorders 3db point is 11-14KHz for the better ones
>and they typical ones used 9.5-12.5KHz is more realistic.
>
>Sampling above 20Khz is wasting bandwidth and recording time.
>
>Even the fastest audio formats (sudding and TARBEL187bytes/sec) had
>bandwidths below 5KHz.

Many game tapes for e.g. ZX Spectrum, Commodore 64 used turbo loaders. Not sure
what the highest data rate was, though at least some C64 programs loaded at
3000 baud.

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

For systems which (notionally) record a bilevel signal -- this may include
almost all of them -- it may be that sampling at a very high rate and then
doing some kind of post-processing of the sample to reduce it to 1-bit
resolution would be beneficial; the result could be closer to the original
signal output from the computer when the master tape was created.



On Wed, 25 Nov 1998 Marvin wrote:
>Someone mentioned that there are copy protection schemes for cassette tape,
>and I was curious what these might be and how they might interfere with
>recording the tape onto my HD.

Sure. Tape copy-protection was used by almost all commerical (game) software.
This involved some devious tricks, as well as turbo-loading routines. It was
designed to prevent people from copying games by reading into the computer and
saving out again. It shouldn't have any bearing on sampling the tapes at all.

Some techniques used were auto-run loaders, fast loaders and custom formats in
general, greater-than-RAM-size blocks of data.


-- Mark
Received on Thu Nov 26 1998 - 17:56:29 GMT