3 Aug
2026
3 Aug
'26
9:59 a.m.
At Sat, 1 Aug 2026 20:42:23 -0400, David Gesswein <djg(a)pdp8online.com> wrote:
The reader is a bunch of instrument amps that amplify
to levels usable by
Saleae logic 16 pro which has analog digitization mode and way too many
times repurposed code to decode it. Doesn't look like I have put it online
probably because the code is embarrassing but ok with giving it away if
someone has use for it.
Here's an alternative to consider: Years ago I wrote a program to decode
waveforms digitized from cruddy old mag tapes in various formats,
including 7-track NRZI, 9-track NRZI, PE, and GCR, and 6-track Whirlwind.
https://github.com/LenShustek/readtape
We've read hundreds of tapes, mostly digitized using a Saleae analyzer.
I never wrote a DECtape decoder, but it would be doable.
3 Aug
3 Aug
10:41 a.m.
New subject: New TU56 Tape Images Available for simH use
On Aug 3, 2026, at 1:59 PM, Len Shustek via cctalk
<cctalk(a)classiccmp.org> wrote:
At Sat, 1 Aug 2026 20:42:23 -0400, David Gesswein <djg(a)pdp8online.com> wrote:
Clearly, yes. One problem is that you'd need a suitable tape head. All the rest is
just software.
Did you use one of those high track count MR heads that some others have used, so you can
deal with funny track counts? I know of some 1/2 inch tape with 10 tracks (Electrologica
X1).
paul
The reader is a bunch of instrument amps that
amplify to levels usable by
Saleae logic 16 pro which has analog digitization mode and way too many
times repurposed code to decode it. Doesn't look like I have put it online
probably because the code is embarrassing but ok with giving it away if
someone has use for it.
Here's an alternative to consider: Years ago I wrote a program to decode
waveforms digitized from cruddy old mag tapes in various formats,
including 7-track NRZI, 9-track NRZI, PE, and GCR, and 6-track Whirlwind.
https://github.com/LenShustek/readtape
We've read hundreds of tapes, mostly digitized using a Saleae analyzer.
I never wrote a DECtape decoder, but it would be doable.
11:35 a.m.
New subject: New TU56 Tape Images Available for simH use
At 11:41 AM 8/3/2026, Paul Koning wrote:
Clearly, yes. One problem is that you'd need a suitable tape
head. All the rest is just software.
It sounded like David has already done that, and has digitized the analog
track data. That's what my "just software" program expects as input.
On Aug 3,
2026, at 1:59PM, Len Shustek via cctalk
<cctalk(a)classiccmp.org> wrote:
At Sat, 1 Aug 2026 20:42:23 -0400, David Gesswein
<djg(a)pdp8online.com>
wrote:
The
reader is a bunch of instrument amps that amplify to levels usable by
Saleae logic 16 pro which has analog digitization mode and way too many
times repurposed code to decode it. Doesn't look like I have put it online
probably because the code is embarrassing but ok with giving it away if
someone has use for it.
Here's an alternative to consider: Years ago I wrote a program to decode
waveforms digitized from cruddy old mag tapes in various formats,
including 7-track NRZI, 9-track NRZI, PE, and GCR, and 6-track Whirlwind.
https://github.com/LenShustek/readtape
We've read hundreds of tapes, mostly digitized using a Saleae analyzer.
I never wrote a DECtape decoder, but it would be doable. Did you use one of those high track count MR heads that
some others
have used, so you can deal with funny track counts? I know of some
1/2 inch tape with 10 tracks (Electrologica X1).
No, we used vintage 7- and 9-track heads. The oddballs were the 6-track 1/2"
Whirlwind tapes, but it turns out that 6 of the 9 tracks read on a 9-track
head are close enough to the Whirlwind tracks to get strong signals without
any track-to-track crossover.
4 Aug
4 Aug
9:51 a.m.
New subject: New TU56 Tape Images Available for simH use
On Mon, Aug 03, 2026 at 10:59:12AM -0700, Len Shustek wrote:
At Sat, 1 Aug 2026 20:42:23 -0400, David Gesswein
<djg(a)pdp8online.com> wrote:
I looked at your code at the start. DECtapes aren't anything like 9 track.
They are closest to wirlwind so thought about trying to modify that. Ended up
adding the analog to digital decoding to my existing digital decode code vs
learning a new code base and modifying it. Was harder getting good bits from
the analog data than I thought so may not have been the best choice. The code
works pretty well now.
The reader is a bunch of instrument amps that
amplify to levels usable by
Saleae logic 16 pro which has analog digitization mode and way too many
times repurposed code to decode it. Doesn't look like I have put it online
probably because the code is embarrassing but ok with giving it away if
someone has use for it.
Here's an alternative to consider: Years ago I wrote a program to decode
waveforms digitized from cruddy old mag tapes in various formats,
including 7-track NRZI, 9-track NRZI, PE, and GCR, and 6-track Whirlwind.
https://github.com/LenShustek/readtape
We've read hundreds of tapes, mostly digitized using a Saleae analyzer.
I never wrote a DECtape decoder, but it would be doable.
9:58 a.m.
New subject: New TU56 Tape Images Available for simH use
On Aug 4, 2026, at 1:51 PM, David Gesswein via cctalk
<cctalk(a)classiccmp.org> wrote:
On Mon, Aug 03, 2026 at 10:59:12AM -0700, Len Shustek wrote:
Dealing with analog stuff can be a mind-bender for us digitally oriented folks. Reading a
good book about DSP can be a big help. Lots of good things can be done with filtering,
and FIR filters are very powerful and flexible. One book I've used a lot and would
recommend as a starting point is by Marvin Frerking, Digital Signal Processing in
Communications Systems. As the title indicates, its oriented to readers who do radio
receivers and transmitters, but all the common basics are there that would apply to any
place you would consider DSP.
paul
At Sat, 1 Aug 2026 20:42:23 -0400, David Gesswein
<djg(a)pdp8online.com> wrote:
I looked at your code at the start. DECtapes aren't anything like 9 track.
They are closest to wirlwind so thought about trying to modify that. Ended up
adding the analog to digital decoding to my existing digital decode code vs
learning a new code base and modifying it. Was harder getting good bits from
the analog data than I thought so may not have been the best choice. The code
works pretty well now. The reader is a bunch of instrument amps that
amplify to levels usable by
Saleae logic 16 pro which has analog digitization mode and way too many
times repurposed code to decode it. Doesn't look like I have put it online
probably because the code is embarrassing but ok with giving it away if
someone has use for it.
Here's an alternative to consider: Years ago I wrote a program to decode
waveforms digitized from cruddy old mag tapes in various formats,
including 7-track NRZI, 9-track NRZI, PE, and GCR, and 6-track Whirlwind.
https://github.com/LenShustek/readtape
We've read hundreds of tapes, mostly digitized using a Saleae analyzer.
I never wrote a DECtape decoder, but it would be doable.
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days inactive
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4 comments
3 participants
participants (3)
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David Gesswein -
Len Shustek -
Paul Koning