View Full Version : OT Old time computer question...

The Artful Bodger
05-27-2011, 01:47 AM
Not really, really old like Hollerith days, but rather the dawn of the PC age, TRS80s et al.

I have a couple of 'computers' from that era, very small Z80 jobs. Now the question is regarding the audio tape data storage.

The idea is to write the data on a regular PC to a CD and play the 'data' back as audio for the antique to read, using a portable CD player.

I know the data was written with a bit cell method but I dont know the exact format of the data.

Anyone played with this?

Pete F
05-27-2011, 01:57 AM
Wow, I remember using those audio cassettes... Even as a 12 year old, it seemed like kind of a hack, but that may be because I was jealous of my friend who had an Apple II with floppy disk.

Anyway, this guy seems to have some useful stuff: http://www.trs-80.com/wordpress/disk-and-tape-conversion-utilities/


05-27-2011, 03:36 AM
well many moon/eons ago I did a scratch build of a Zilog Z80 based computer which ran at 4 mzh and had 64 k of ram ( 4116 chips )

anyway most of the audio tape interfaces were of the cuts type ,they used two tones high and low to record the data high = 1 low = 0
therefore the data was sound hence the use of the audio tape.

unless you know the frequency you will be in trouble.

my build included a 8 inch floppy drive which I designed and built the interface and wrote the machine code to run it


05-27-2011, 05:29 AM
I built a S100 system at one point, there were more than one system iirc the one I used was based on what the trs80 used. the encoding was based on NRZI. http://en.wikipedia.org/wiki/NRZI#Non-Return-to-Zero_Inverted_.28NRZI.29

I used a SDLC chip, and iirc recorded the NRZI stream directly to the recorder. some where I have a notebook . . .


Weston Bye
05-27-2011, 05:52 AM
I have a couple of 'computers' from that era, very small Z80 jobs.....

Would they be the Sinclair ZX-80?

I still have mine from that era.

The Artful Bodger
05-27-2011, 07:00 AM
Thanks Pete, that is a very good site and I will be going back there..

Stuart, the tape modulation was quite simple I do know that as I have the circuit which was just two gate outputs and a resistor voltage divider.

Fred, thanks, that what I needed to know, NRZI.

Weston, VZ200 and VZ300. I have a cassette tape (which I hope is still good) with an assemblerand a bit of technical information. These little machines used the same tape format and I think much of the same firmware as the TRS80. Neither machine has a disk drive so it is all cassette tape.

I am considering how I can do something useful with these but I well recall how patient one had to be to get a program to load from the audio tape, hopefully that would be glitch free if the sounds are digitally created and played on a CD player.

05-27-2011, 09:19 AM
I think that this may be exactly what you need.


05-27-2011, 10:49 AM
I had a TRS-80 Color Computer back then, and it read tapes pretty reliably. The dedicated cassette deck for TRS-80's had its frequency response tweaked to emphasize the upper midrange, as I recall. The bass was pretty well filtered out. It was a pretty standard portable cassette player except for that, and the omission of the microphone. When I first got the computer I tried using a regular cassette player and it had poor results, but the dedicated one was quite reliable. So I'm guessing that you'll want to limit the bandwidth of whatever source you substitute.

05-27-2011, 11:19 AM
Not sure about the creating new data on the PC part.....

As to the storage part.... I don't see why one couldn't route the output from the old PC's tape output into a soundcard and create a wav file containing your data. Once you created the file the PC could playback your new wav file into your old PC. Think of the PC system with a soundcard as an expensive cassette data recorder. With some sound editing sofware one could likely cleanup the data to make the transfer more reliable.


05-27-2011, 11:21 AM
Ahh, old computers!

Over the years I've had (in no particular order);

Olivetti P652
Acorn Atom (two + floppy drive)
BBC B (about 6 + Music 500 + 6502, Z80 & 68000 co-processors)
BBC Master
Jupiter Ace

Back in the ZX81 days a friend managed to transfer programmes to another friend several houses away using a pair of FM trainceiver modules! He also had a 2K ram chip giving him 5 times the programme space!

Weston Bye
05-27-2011, 11:43 AM
I did some piggybacking of chips in my ZX-80, doubling the onboard RAM.

In search of a genuinely useful application, I piggybacked a zero insertion force socket onto the system ROM chip and added a few logic gates to program 1702 EPROMS. Wow, took me a while to remember the 1702 - only 256 bytes!

Still have the little bugger.

Optics Curmudgeon
05-27-2011, 12:06 PM
Lookie here: http://www.threedee.com/jcm/audio/

05-27-2011, 12:20 PM
I have one or more of each of these.


05-27-2011, 12:20 PM
Yeah, I've got a box full of Sinclair ZX-81 (and T1000) systems and parts, including a Wildonix eprom programmer and a bunch of ram packs. I still have the wirewrap expansion board I made too, and a bin full of eproms. No idea why I still have them. Sentimental I guess. Someday I'll browse Ebay, become totally disillusioned ... and then probably just bin them. Well, maybe I'll keep them until my kid gets old enough to complain about how slow his computer is... then I'll show him what I had at his age... he should get a good laugh out of that. 2K of RAM? 2K? My phone has 8 million times more!

You do know that you can emulate these systems on a modern computer, including downloading binaries - doing completely away with the audio cassettes? My little Nokia N810 Linux pocket computer even has an emulator for the Sinclairs, and the TRS-80, and ...

Lew Hartswick
05-27-2011, 12:43 PM
If there is anyone here that collects this sort of stuff I have half a
file drawer full of such.
A sort of list if of interest:
Timex 1000 , Commodore Plus 4 , TI 99/4A
All the manuals for the above.
Other booklets etc.
Mostek Z80 tech manual
ZX81 Basic programming, Assembly instructions, Software catalog.
SYNC special issue ( The magazine for Sinclair users )
Syntax ZX80 Vol 3 Nr 2 Feb 1982 (a pub of the Harvard Group )
Plus a bunch of misc schematics and interface info.

05-27-2011, 02:01 PM
I have one or more of each of these.


I have an old Kaypro 1 at my parents place:


06-03-2011, 06:12 AM
I Built a Sol-20 in my early twenty's, try goggling 'Kansas City tape storage'. Kansas City was standard for cassette tape storage. They had a 300 & 1200 baud standard.

06-03-2011, 07:14 AM
Check out a film/documentary called "micro men" it's very entertaining , well worth watching.

Micro Men

http://www.bbc.co.uk/iplayer/images/episode/b00n5b92_303_170.jpg (http://www.bbc.co.uk/iplayer/images/episode/b00n5b92_640_360.jpg) Affectionately comic drama about the British home computer boom of the early 1980s.
Legendary inventor Clive Sinclair battles it out with ex-employee Chris Curry, founder of Acorn Computers, for dominance in the fledgling market.
The rivalry comes to a head when the BBC announce their Computer Literacy Project, with the stated aim of putting a micro in every school in Britain. When Acorn wins the contract, Sinclair is furious, and determines to outsell the BBC Micro with his ZX Spectrum computer.
Home computing arrives in Britain in a big way, but is the country big enough for both men?

all the best.markj

06-03-2011, 01:18 PM
I actually wrote some software for tape storage ages ago. There were different formats in use. The aforementioned kansas city standard was one. CUTS was another. These seemed to be used on the more serious S-100 bus standard based computers of the day and not so much on the proprietary mass market consumer computers (commodore, sinclair, apple, IBM, etc.).

They are very simple to encode. Not to hard to decode with very simple algorithms that were typically used but those were susceptible to noise and other degradation of the tape. A CD would be an improvement in terms of longevity. Machines which had their own tape drives used saturation encoding (just saturating the magnetic field on the tape) which was better than using an audio cassette recorder.

You may be able to use the original software via an emulator. For example, xtrs will apparently read and write TRS-80 cassettes.

Documentation on the various formats can probably be found on the net as there are active devotees of vintage computers. But you will need to search based on the specific computer. Also, you can look at disssembly of the ROM firmware for exact details.

Here is a description of the ZX-80 format:
Basically the data itself is one cycle of 1024 or 2048 hz per bit.

Once you know the format, writing the software is really easy. They were generally simple pulse frequency or pulse width modulation square waves or huffman coding. And this is key, normally the modulated frequencies were exactly aligned with the data, produced by bit-banging. I.E you wrote out a specific number of cycles of the square wave for each bit. If you tried to use a modulated analog oscillator chip, it would tend to fail miserably (except in some cases on ones which used many cycles of a waveform and were originally implemented using analog modem type hardware).

I would suggest connecting the vintage machine to the sound card on your PC and skipping the CD for most purposes. Where RS-232 ports are available, skip the cassette mode and use that to load programs.

06-03-2011, 02:03 PM
I have one or more of each of these.


I have or had all but the one in the middle right.

No idea how many programs I wrote or modified for the C=64 or 128, or even the amigas.
When I first started playing with computers, the pet was still commanding fairly high prices on the used market, but was considered 'old' if not obsolete.

Interestingly, I gave an estimate to a small shop last month, and noticed that their book keeping was being done on an old c=128-d running cpm