Re: Recording digital data to analog tape... revisited

From: Lewin A.R.W. Edwards (larwe_at_larwe.com)
Date: 10/03/04 

Date: Sun, 03 Oct 2004 03:31:18 GMT

Okay... followup. I got lots of interesting comments and suggestions
here, thanks.

This problem turns out to be one of those utterly "DUH!" sorts of
things. I started by connecting the tape recorder in question up to a
ZX81. I saved a program and attempted to load it. No dice. The on-screen
datablocks didn't look right either (if you've used a ZX81, you'll know
what I mean - but in brief, the ZX81 outputs the audio data to the
raster while loading/saving).

Tried it with another tape recorder. No problems there. WTF?

To cut a long story short, this particular recorder I'm using, although
it is mono, uses stereo connectors for both mic and ear. Somehow (and
I'm not clear on why this is happening) when I insert my mono plug, it
doesn't get a reliable ground connection. If I use clipleads to connect
to the appropriate points directly on the recorder's PCB, the problem
goes away.

So, my workaround is to use the tip and barrel lines on a stereo plug
for both ear and mic, and with this arrangement I get the following:
<http://www.larwe.com/camctrl2.jpg> (lower trace). Observe that it is
pretty darn clean, 1V p-p centered around 0V, and somewhere between a
sine and a triangle. The decoder can read this signal happily with a
much lower BER than the spiky signal. No other changes were made in the
circuit.

By the way, the top trace in these pictures I've linked is 3kHz digital
out directly out of the micro's pin. I showed it mainly (a) because ch1
happens to be connected to that line, and the STK500 board can't be
switched off while connected to a PC, but also (b) to indicate what sort
of signal I was trying to lay down. It just so happened in the first
picture that the edges lined up very closely. There's a small phase
difference in the picture linked above. This doesn't mean anything at
all, since the signals are not directly related.

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