Intro msg: Teraks, emulators, reviving cassette data

From: John Foust <>
Date: Tue Nov 18 15:51:45 1997

I'd like to introduce myself to the list.

I maintain the somewhat imaginary "Jefferson Computer Museum" at
<>, which has info about my several
Terak systems. The Terak was a PDP-11/03-based graphics workstation
circa 1978-85. My "Terak Museum" web page is the proud recipient of
the "Geek Site of the Day" award for October 16, 1996.

I also have historical info about the UCSD P-System, including an
emulator, source code and very early Pascal compilers. In the months
to come, I will add info about other systems I have, including a
Zilog MCZ Z-80 development system, CBM PET and NEC systems.

I've also been given permission by Claus Giloi to distribute the
C source to his Windows-based Altair and IMSAI emulators. If you haven't
seen this, it's a nifty graphical recreation, letting you click on
the toggle switches to drive the emulator and watch the LEDs blink.
In the years to come, I'd like to enhance it to include virtual
peripherals, or with inexpensive recreations like an opto paper
tape reader.

Recently, I've been researching the possibility of reviving old
audio cassette tape storage of computer data. With today's PC
audio digitizers and a little software, it should be possible to
decode and synthesize tapes in formats such as Tarbell tapes for
S-100 systems, 88-ACR, Commodore PET, VIC and C-64, Bell 103
recordings, etc. A software approach would have several
advantages: you don't need the original hardware, and it has
a better chance of restoring out-of-spec data.

A little digging revealed the "soundmodem", a driver for Linux and DOS
that is a software-based FSK modem that can handle 300 to 9600 baud
in real-time using a SoundBlaster as the digitizer / DSP. It is
used by ham radio operators.

To experiment, I digitized an old wobbly CBM PET tape and did a bit of
post-processing in contemporary sound software and it normalized the
volume quite well. I suspect with commercial audio software, one
could even invoke filters to remove print-through.

At 22 or 44 kHz mono 8-bit samples, there's certainly enough
headroom to distinguish these relatively slow-speed signals. I'm
sure with some formats, just watching the zero-crossing timing would
work. I wonder if this technique could be used to rescue old N-track
reel tapes that have become unreadable over the years, by intercepting,
digitizing, and post-processing the tape-head signal.

What would help the most is to get specifications for the old
standards. I don't have any documentation, although I'm digging through
the basement to find my old Kilobauds. I sent an e-mail
to Don Tarbell, who is apparently still on the Internet, but
no response so far. It would also help to see more samples!
If you have any old cassettes, please consider digitizing them
and sending them to me.

In another area of the JCM, I've begun to collect ancient ASCII art
from the 60s and 70s: Einstein, Spock, Snoopy, etc. I've written a
program that converts teletype-style overstrike art into Adobe Illustrator
documents, which are easy to re-size and print on today's laser printers.

I have dozens of pictures from DECUS tapes, but I'm always looking for
more, and I'd like to record personal anecdotes about the creation
of these old artworks. I'd also like to get an actual print sample
of the entire printable font from an ASR-33 teletype, in order to
scan and convert it into an authentic Postscript bitmap font.

- John
Received on Tue Nov 18 1997 - 15:51:45 GMT

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