May 2012 correspondence with Greg "Pete" Peterson of "the Digital Group"
edited by Herb Johnson. Last updated Sept 17 2012, edited by Herb Johnson

Digital Group cassettes


See Web document "dg_peterson.html" on this site for more information. This edited document is copyright (c) 2012 Herb Johnson. 

Greg's Digital Group cassette tapes

....I also stumbled across my collection of cassette tapes from that era while
cleaning out the garage. I have had to re-build 2 tapes so far, but I am
recovering the data on them [as digitized audio files] and will save it
eventually on a CD rom.

The cassettes are generally Suding / DG format. I also have some
Tarbell format cassettes. The tapes contain the DG os, Floppy OS,
 various basics, etc. etc. 

[As of late May 2012] I am maybe half way through the cassettes
 at this point. I have roughly 2
dozen or so. So maybe another long night of copying tapes or so to go.
It is pretty boring listening to the two tone audio streams. You can tell the
DG and Tarbell tapes apart, they sound different.

DG cassette standards

> I'm unclear on what DG owners used for tape cassette hardware. There's
> some suggestion they used S-100 Tarbell floppy controllers, or the
> Tarbell format. Tarbell was an analog solution, with portable cassette
> recorder/players. Then there was the Phi-Deck, that's digitally
> controlled, and I have an impression that DG hardware read data almost
> directly from the Phi-Deck drive. I simply don't recall if Phi-Decks had
> read/write electronics or the format.

The Suding format is a narrow shift RTTY format as I remember.
2125 and 2975 Hz. Robert Suding came to computers from ham radio,
so this standard would be his logical choice.

Most all of the cassette standards were 300 Baud. Suding did not
use a UART, but instead just bit-banged the data stream in from
a parallel port, shifting it into a register bit by bit, then storing it
away in memory. Very simple, very inexpensive.

Tarbell used a serial to parallel chip. His frequencies were different,
but I believe he ran around 300 baud as well. The Tarbell VCO was
an 8T20 Signetics part. Good luck finding that one today!

The DG standard-issue audio tape player was a Superscope C-104 (Might be
a D-104, there is some abrasion damage on the label on the bottom). This
unit was chosen primarily because it sported a variable speed playback.
This occasionally was needed to match up tone frequencies and baud rate.
Other than that, it was a pretty standard player. Yes, I do still have mine.
Not sure it works, the years have not been kind to it. I am sure with a little
grease, oil and a few new belts I could press it back into service.

You had asked about the [binary] format of the Suding tapes. I have the
source code for the read routines in the original DG documents.
It is unfortunately hand assembled and in Octal. So it is a bit hard
to follow. MSB appears to be first, as ROL R is used for byte assembly.
Bit rate in the documentation is stated as 1100 BPS. Hard to say how
many stop bits are used. I, am really a hex guy, not an octal guy. I can
translate, but I don't think in octal. I immediately recognize 'C3'to as a
jump in hex. I have to translate octal to hex to provide the same insight.

It is interesting, the listings I have were produced before DG had an
assembler. They are all hand coded, without labels and all the other
nice things assemblers bring. But that was the way it was back then.
Amazing to think how far we have come. I was hand assembling off of
code sheets back then. I would commonly skip a dozen or so memory
locations at the end of each page to provide room for any future
patches I might need to my code. Another world, another time.


> [I asked about how one might directly read the tapes as intended, using
> something built today. ]

If and/or when I get around to trying to recover the data, I will first try to
get the DG system running. Failing at that, I would re-build the Suding
circuit from the schematics, the parts are all pretty run of the mill stuff.
I would probably use an 8251 Intel UART that I fed a solid 300 baud clock
to and not try the bit-banging.

archiving DG cassettes as audio

The cassettes are generally Suding / DG format. I also have some
Tarbell format cassettes. The tapes contain the DG os, Floppy OS,
 various basics, etc. etc. The tapes
so far have come back far better than the original labeling on the cassettes.
I have had to strain to read some of the markings. The signals are strong though.

I would envision someone taking my CD and copying that to a cassette for
subsequent feeding to a DG system (if one exists). Anyway, the code is
preserved in a CD format as tapes eventually (longer than 35 years evidently)
loose their signal.

digitizing cassettes

You asked about my process for converting cassettes to CD's.
It turns out I am something of an audio buff as well. Figures,
doesn't it, computers and audio, all technology stuff.

I built a stereo system up using mostly classic McIntosh hardware.
I guess the easiest way to describe it is: it is a surround sound
system, implemented in both solid state and vacuum tube technology. Not
just mixing them, but two separate systems running in parallel.

I use the Lexicon CP-3 audio processor. Many regard this as the
pinnacle of audio processors, it is much more geared to audio than
to home theater, which ultimately replaced it, sometime in the
80's or so. Anyway, you can still find them on Ebay and in classic
audio stores, though they are getting more expensive than when I
bought mine.

The basic question was, if you drive 2 CP-3's in parallel with the
same audio stream, will they make the same audio decisions in
creating the surround sound. Not an easy question, 'cause at the
heart of each CP-3 is a DSP chip running at a very high data rate.
It turns out they do, so one CP-3 feeds [my] solid-state [transistor]
amps, and one feeds Tube amps.

Using this system, I can select how much of the solid state brilliance
I want, and how much of the very mellow, warm tube sound. My
speakers are all classic Acoustic Research, JBL, or Klipsch. All
have been rebuilt with modern drivers and crossovers. Driver
technology has improved substantially since the 70's, but speaker
enclosure technology has not. So it makes sense to rebuild [speakers
when they wear out].

Anyway, as part of this system, I have a pair of Nakamichi cassette
tape recorders. I put the DG tape in the Nakamichi, that feeds through
a McIntosh tube preamp. I take the signal out of the Preamp to a
small mixing console. The mixing console feeds an Edirol
UA-1000 audio sampler, a 1 inch rack mount unit, intended for a studio audio
rack. [It's of] musician quality, way beyond Soundblaster [audio] 
computer boards. The Edirol outputs a USB stream to a small
lunchbox size computer running Windows XP. Audio is sampled and
captured with a computer program by Adobe called "Audition".
I like this program as it allows me to edit the captured waveform
if necessary, so I can put in standard lead in and lead out silences
etc. I also use the standard CD sampling rate.....
so the CD is compatible with all commercial players.

[As of late May 2012] I am maybe half way through the cassettes
 at this point. I have roughly 2
dozen or so. So maybe another long night of copying tapes or so to go.
It is pretty boring listening to the two tone audio streams. You can tell the
DG and Tarbell tapes apart, they sound different.

The last step is to burn the audio images to CD. I usually use Nero
for that. I have every reason to believe the CD I produce will
accurately reflect the cassette data stream audio.

I also like classic vinyl records, and have accumulated quite a few.
I frequently capture the vinyl audio image and transfer that to
CD as well. To some extent, I can even remove pops and cracks
from vinyl using Audition.

So I am no stranger to this process of moving audio onto CD.

Greg Peterson, July 2012

Resulting digitized cassette files
====================================

Pete completed the work and sent me a DVD of the files.
These are available from me on DVD, for $5 to be postal-mailed within the USA.
They are too big to make available on my Web site. Contact me for details.

A list of the files is in this text document:

dg_peterson_dvd.txt

According to Windows XP, the .WAV files have these features:

bit rate 1411kbps
sample size 16 bits
2 channels
audio sample rate 44kHz
audio format PCM

Greg added these comments about the DVD he made, in mid-Sept 2012:

> I did not screen the tapes for content (obviously, since I have no
> working system yet), and some of the tapes were given to me by other
> people. So what is on them is what is on them, content un-screened and
> un-verified. Some of the labels were hard to read, but the files are
> labeled as best I could read them. My eyes were better 35 years ago too.
> 
> I did cut out dead space between software segments. I often made
> short recordings at increments of 100 on the foot counter, so why
> listen to 75 feet of dead air? Anyone recovering the data can arrange
> it as they choose on their medium of choice.


Herb Johnson, Sept 19 2012