Re: Cassette Capacity

["Michael J. Mahon" <mjmahon@aol.com>]

Mike Westerfield wrote:
> I was just moving gigabytes of data around on terabyte sized drives,
> and was remembering the good old days, when you could more or less
> grasp how much stuff was on a disk. That led me to wonder how much
> stuff was on the first storage device I ever used on an Apple II. Does
> anyone recall about how much information could be stored on a cassette
> tape on an Apple II? I realize it varied by tape size, so the
> appropriate unit is probably bytes per minute.

Apple cassette data rates are 1kbits/sec for 1s and 2kbits/sec for 0s.
If we assume that 1s and 0s are equally probable (though "typical"
uncompressed data tends to have more 0s than 1s), then the average
data rate is 1500bits/sec.

So one side of a C60 (the most common and most durable cassette) holds
30 minutes of recording, or 1800 seconds, or 2.7 megabits, or 337,500
bytes--if it contains only data.

But since each data "record" consists of ten seconds of "header"
tone, the fraction of a recording that actually consists of data
depends strongly on the length of data recorded in each record and
the blank space ("gap") that the user puts between records or groups
of records.

If we assume that a typical data record contains 10,000 bytes, it
takes 10 seconds for header and 53.3 seconds for data.  Then if a
typical user leaves about 5 seconds of gap between records to
make record cueing easy, that would cause each record to take 68.3
seconds of tape.

With these assumptions, a full side of a C60 holds 260,000 bytes,
and both sides hold 520,000 bytes.

Of course, what is usually recorded on cassettes is BASIC programs,
and they consist of <user gap> <length record> <program record>,
where each record still has a 10-second header and the short length
record contains a negligible amount of data.

So a BASIC program of 10,000 bytes takes 5 seconds for user gap,
plus 20 seconds of header for two data records, plus 53.3 seconds
for the program, or 78.3 seconds of tape.

Under these assumptions, a C60 side holds almost 230,000 bytes, and
a whole cassette holds about 460,000 bytes.

As you can see, Apple cassettes are relatively capacious, particularly
in terms of cost per byte!

Of course, if short programs were recorded, then the effective
capacity would be much lower, since most of the tape would consist
of gaps and headers, with relatively little data.

For example, if we repeat the above calculations assuming a BASIC
program length of 2,000 bytes, then each side of a C60 would hold
only about 100,000 bytes.

BTW, cassettes longer than C60s are made of thinner tape, which makes
them more delicate and less reliable, and also increases the rate of
"print-through" between adjacent layers of tape wound on the spindle
for long periods.  As a result, longer tapes are not recommended for
long-term data storage.

I've actually had good results with both C60s and C90s, and have found
both to be reliably readable after 28 years of storage.  ;-)

-michael

NadaNet and AppleCrate II: parallel computing for Apple II computers!
Home page: http://home.comcast.net/~mjmahon

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