Re: Tape input to speaker

[mjmahon@aol.com (Michael J. Mahon)]

Simon Williams asked:

>Is there any way to route the audio from the tape input to the speaker
>on a IIe.

It can be done in software with a short machine-lauguage program.

>What does the A2 'hear' when it reads from tape? Is it simply SOUND/NO 
>SOUND or can it actually differentiate tones?

The cassette input is simply a 1-bit A/D converter--that is, a comparator
on the AC-coupled input.  By reading the cassette input I/O location, you
can tell whether the input is positive or negative--that's it.

By watching the input change and keeping track of time, you can
measure the frequency of the input signal (if it is simple), or just find
the zero crossings if it is complex.

In its intended use, the cassette waveforms were approximations (!)
of squarewaves with frequencies of x and x/2, with x = 2kHz, so it
is often said that the mean bit rate of the cassette interface is 1500
bps.

If you write a short machine language program to sample the state
of the cassette input (high bit only) and, when a change is sensed,
toggle the speaker, then you will reproduce exactly what the Apple
"hears" at the cassette input.  It will, of course, sound like "infinitely
clipped" audio, or "full fuzz".  For all the information destruction, it
is surprisingly intelligible (at least for some voices), but hardly
pleasant.  ;-)

A number of programs were written to "record" infinitely clipped
audio through the cassette port, and then play it back through the
Apple speaker.  I myself wrote a program to read back a region of
storage as a hex dump, so I could proofread typed-in hex programs
without an assistant.  It was serviceable, but was certainly not
"hi-fi"!  After about 10 minutes of listening, it would start to sound
"normal" and my head would hurt!  ;-)

Radio amateurs and other radiotelephone addicts are relatively
accustomed to this highly clipped speech, and have cultivated
a higher tolerance for the extreme distortion.

I was motivated to find a way to produce high quality digitized
audio from an Apple II, but that's a story contained in the
electronic Call-A.P.P.L.E. and on my website.  ;-)

-michael

Check out 8-bit Apple sound that will amaze you on my
Home page:  http://members.aol.com/MJMahon/