Computer Data Cassette Formats


This document was first produced in 2014, with updated content to Oct 25 2019, (c) 2019 Herb Johnson

In May 18 2018, I gave a talk at the VCF-East vintage computing convention on audio cassettes for data storage; based on the content of this Web page. It went well. - Herb

Summary

[cassette]

Audio tape cassettes of the 1960's (developed by Phillips) were used with the first microcomputers of the mid-1970's, to store and recall programs. Several digital standards and circuits and programs were developed in the period. Personal portable cassette recorders were inexpensive, reliable, available; the circuits needed on the microcomputers were simple. Into the 1980's personal computers in mass-production continued to use cassettes, because of their cost advantage and simple digital hardware over floppy diskettes and drives. Decades later, vintage computer owners and museums of computing technology, are recovering the binary (and audio) content from these decade-old tapes. Many of these programs are games, and there is particular interest today by "gamers" in vintage computing. - Herb

Background

When microcomputers became available and affordable in the mid-1970's, they did not include "mass storage", as we think of it in the 21st century. That came later, when floppy diskette drives and hard drives were priced at ONLY a few THOUSAND dollars apiece. Programs, therefore, were stored on paper tape, as punched holes in groups of eight. Even that technology - paper tape readers, punches, Teletypes - were expensive devices, in commercial use for data or text communications and on minicomputers and industrial controllers.

One solution was to use audio tones as recorded on audio cassette tapes, to represent binary data. Phillips developed this standard. Cassette recorders and tapes were available, inexpensive and popular. The cassettes held 1/8-inch width tapes and were playable for tens of minutes per side. Music of the era was sold on cassettes for automobiles and home and played on "personal" portable players. The players and tapes were a convenient and portable alternative to vinyl records, or older reel-to-reel 1/4-inch audio tape. Most of those devices could also record - people copied tapes, recorded music from broadcast and vinyl records, voice dictation, and so on. And, some data terminals (keyboards and CRT displays which used serial links to operate with computers) and minicomputers of the era included digital data cassettes devices.

[cassette]

Here's a photo of a Panasonic brand cassette recorder, in a mid-1970s' COSMAC prototype computer "FRED 2". from the Sarnoff Collection of RCA artifacts. Use of audio cassette technology for personal computers, was described in 1967 by Claude Kagan of Western Electric(and updated in 1973 by Kagan and Larry Schear), in a journal article The Home Reckoner": "The Philips cassette was chosen as the local storage medium because of its size, attractive package, tape protection, and its capability to record medium-speed frequency-shift-keying tones with inexpensive equipment, improvable with a Dolby noise reduction system." This is not likely the earliest mention of audio cassettes for data use.

[cassette]

There were many schemes for encoding binary data as tones on tape. Essentially, one or a few cycles of audio could encode a bit of data, either by cycle-count or by frequency (period). Here's an oscilloscope image of a COSMAC scheme of two or five cycles to represent 1's or 0's. Some terminals and computers used specialized tape-decks under computer control; some recording methods were entirely digital and capable of more storage.

In any event, designers and hobbyists of the "personal computing" era came up with various ways to convert logic levels into either bursts of pulses or brief tones for data and program storage. Interfaces to audio recorders were simple to primitive, but largely worked. Several standards were developed, and it was not uncommon for a microcomputer manufacturer to have their own standards. There was a software publishing industry; it was obliged to either produce multiple formats, or only supply programs for specific microcomputer brands. But brands as large as Apple's Apple II computer, and Radio Shack's TRS-80, supported cassettes.

When floppy disk drives became more common, these audio cassette data schemes fell out of use. However, for the very low-cost, mass-produced microcomputers of the 1980's - such as the sub-$100 Timex/Sinclair ZX80's - cassettes continued to dominate. Several software companies provided games on cassette. Few remember, the first "IBM PC" of 1981 had a cassette interface.

Recovering data from digital audio cassettes

Today, when restoring and reviving microcomputers from the past, that may mean recovering programs from audio cassettes. Some of them are now 35 years old. There's numerous problems in reading magnetic media that old, as follows. First, the mylar base material can become brittle or the iron oxide coatings can flake off, lose lubricants. Also, wound tape can become demagnetized. Tapes stretch, which changes the timing of the bits. Also, the original hardware to read these may not be available. Or the technical understanding of the format and frequencies may not be readily available.

This is not terribly different from issues of recovering data from "ancient" floppy diskettes, some of which were recorded in formats now just as obscure. I have several pages on floppy diskette and drive technology; That work is one reason I'm interested in audio cassette data.

"Restorers" often ask if there are "standard" ways to recover or archive digital audio cassette tapes. As of 2014, there's no "consensus" about how to preserve programs, nor any consensus "standards" for either saving as audio, or saving as binary/digital files. The reason for a lack of consensus is simple: there never was a single original standard to encode digital files into audio signals. Each manufacturer developed their own methods, and their own "file systems". Such standards as were suggested for encoding 1's and 0's, didn't necessarily cover how to represent filenames or directories (if any). Translation: you cannot "play" tapes from one computer brand, on another brand's computer. There was a kind of "universal" standard - Kansas City - but it was popular for a limited time.

So what can you do to "archive"? I believe you can determine the "brand" of computer, and look for Web sites where individuals have established archives on behalf of that brand, or model. Then consider what some archiving group has done, and do that, and add to their archive. It's my observation, that each archive for each computer brand, "converts" their tapes in a different way. My experience is, the people who operate these archives, aren't interested in standards, or archiving beyond their "brand"; so don't expect to use one method upon another "brand" of computer's data. But archiving data, is either a matter of digitizing the audio; or recognizing the binary values in the data with hardware or software, and saving the "binary". I'll discuss both below.

Many "archives" are made by people with no access to running vintage hardware. So they simply save the audio digitally, in some WAV or similar format. Those formats already sample at rates (44 kHz) and resolutions ("16 bits") far beyond what's likely necessary to recover the binary data. After all, these were recorded on inexpensive 1970's cassette recorders, with limited fidelity and full (by today's standards) of wow and flutter. Data rates in the era were on the order of kilobits asecond or less. But frankly, I don't know of many efforts to *verify* that some WAV file, actually *works" on vintage hardware. Again - check specific Web sites per brand.

Some archives have access to some vintage hardware, and they produce "binary" files that are the digital equivalent to whatever was on the tapes. Or, they do what's probably "best" - read the tapes with vintage hardware, and save the files as binary programs files. That is, the actual binary code which would appear in memory and "executed".

It's popular in the 21st century to run "emulators". HOw do they deal with cassette storage? Various vintage computer emulators, have their own "standards" on loading programs. Of course, they are emulators, and expect a file of binary information, not "real" audio (as far as I know....I could be wrong).

Recovering the binary files, preserves the "content", but it does not preserve the "media" as an audio file. Preserving some audio files, allows someone in the future to actually operate vintage hardware (cassette recorder, cassette interface, vintage computer) in the original manner. But preserving the "data" allows people with emulators (a common circumstance) to "run" the programs.

As I've mentioned, decades-old cassettes may be damaged. Reading them may damage them. There's an art to doing that, look for details on the Web. So making a digital-audio "backup" is probably a good first choice. If you have any access to any vintage hardware, you can create test tapes from the "backups" to verify your results before risking your original tapes.

My Web pages about audio cassette digital storage

COSMAC 1802

Netronics ELF II, recovery of cassette data, manuals and documents. On loan from Vintage Computing Federation museum at Infoage in New Jersey.

RCA COSMAC 1802 - QUEST Super ELF, ELF II Cassette tape support

RCA COSMAC 1802 - COSMAC VIP Cassette tape; also CUTS - KC standards

RCA COSMAC 1802 - COSMAC "FRED" and coin-op? tape recovery

[cassette]

RCA prototype COSMAC or 1802 computers of the early 1970's used audio cassettes as data storage. Data was recovered from archived RCA cassettes in late 2017, here's some background about those cassettes and those COSMAC video-arcade and other COSMAC prototype systems. Here's some early 2018 activities to recover FRED 2 systems artifacts and data cassettes. Those efforts continued with the Hagely Library's and the Sarnoff Collection's archives of early COSMAC tapes. By Fall 2018, binaries were recovered from those tapes and run on emulators. The image shown of several cassettes, are from the Sarnoff Collection's archives of COSMAC recorded cassettes, as discussed on the linked Web pages.

other systems

In the late 1950's, RCA announced a 1/4-inch stereo audio cartridge and player. While a number of companies produced compatible blank and prerecored cartridges, the format was not terribly popular, and RCA stopped promoting the product by the mid-1960's. There's a number of possibly reasons for its failure as an audio format. Of course it's ultimately a "decision of the market" and not some particular cause that makes consumer items popular or not. I include this product among my cassette data entries, because Scientific Data Systems used the format on a minicomputer for data files. This Web link shows you my efforts in 2019, to recover SDS computer data from an RCA-format tape cartridge. - Herb

SD Sales 1977 audio cassette digital interface kit

The MCM/800 computer was an 8008 based computer with a digital cassette deck. I assisted a Canadian museum in determining the details of a cassette tape deck, so they could have it restored to recover programs.

Digital Group and Greg Peterson's efforts to recover cassette data

Don Tarbell developed a popular S-100 audio cassette board.

Dwight Elvey recovered data from Polymorphic brand S-100 computer data cassettes. an image of one of my Polymorphic "Poly" cassette tapes.

includes a description of using a MITS Altair 8800 and IMSAI 8080 computers with cassette tapes

includes a description of moving from cassette to an early floppy disk system in 1977. The writer STILL has that floppy and IMSAI system in 2012!

Here's an industrial/hobby microcontroller based on the 1802, produced in 1982. A cassette interface option is described.

Web links and comments about audio cassette data

Marcel van Tongeren, producer of the EMMA 02 emulator of COSMAC 1802 computers, worked on early RCA cassette-data recovery in 2018. Here's his review of those tape formats and how to recover them from recorded WAV files. See my COSMAC notes above for more work by other COSMAC enthusiasts.

Many S-100 computer companies of the 1970's produced either cassette interface boards, or included cassette interfaces on their I/O cards. There's too many brands to list here. You can check my list of S-100 companies on my S-100 home page. Each name links to a pages by brand, or to a page where I list the documents I have. A 2018 Google search of my Web site retrotechnology.com finds almost 50 references to "cassette". HEre's some more information from users of cassettes-as-data. - Herb Johnson

Thanks to William Sudbrink for this reference: "I was thinking about doing a VCF exhibit, displaying the "secret messages" in Tomita's 1978 "_Bermuda_Triangle_" album, using a Tarbell (note that the album notes call it TARBEL (one L) format) S-100 interface card and a real vinyl record." Here's a Web page link to a description of the album and something about the encoded messages.

Tony Brogan told me in May 2018, about Apple II's early cassette history, and a Web site which archives those old cassette programs. here is the link for the French group BrutalDeluxe I mentioned. This is a direct link to their apple II cassette page. They also give instructions on how to acquire a program from a cassette, for preservation with a modern computer. Up to over 650 apple II cassettes have been digitized." Tony added, " Many of the cassette tapes I have seen, supported 3 or 4 different [computer] systems on a single tape. Common groupings were Apple II, CBM [Commodore], Atari."

Tony followed up, with how the Apple II can load programs or disks, as files via the cassette port. "Someone with an Apple II but has no disk drives (or just no disks) can download the audio file(s) to a cell phone or MP3 player and load them onto the machine to play/use. Sort of a reverse audio cassette where the floppy is made into an audio file to be loaded through the cassette port on an Apple II. There's a Web site that has over 1500 [Apple II] disks (system software, utilities, games, CP/M software etc) that has been made into audio files." The program tool in use is called "c2t".

Jeffrey Brace is a VCF officer and works at the Vintage Computing Federation's museum at Infoage in New Jersey USA. They operate vintage microcomputers every week. "We use a mini iPod to load programs into the Apple 1 Mimeo [replica]. We could use it for the Apple II as well. I know that Corey has used it to load programs into the [Processor Technology] SOL." Jeff also recalls "[some] tape programs and games on the [Commodore] C64, would have a loader game that one could play while the program or game loaded from tape. Something small and simple. Others would load songs or graphics to keep people entertained while the tape took a long time to load."

In May 2018, Rich Cini who wrote the Altair32 emulator wrote: "This is an interesting topic because I did some work on this for the Altair32 Emulator. About 10 years ago, I was trying to add the ability to directly read WAV files (22050Hz, 8-bit) from within the emulator. Bob Grieb helped me with the Fast Fourier Transform code needed to make it work and taught me a bit about DSP and filter design. The add-on basically became a WAV-to-digital converter based on the Altair ACR recording format. I don't think it ever made it into the main code tree because of the inability to locate WAV images at the time." Rich found the code and sent me the documentation header to it. Read it for further description and check Rich's site for more information.

In Oct 2019, I stumbled over This github project by, er, "datajerk", to create audio files from binary program files. While it's part of Apple II efforts, for some reason the developer came up with COSMAC VIP support! So this got my attention.

alternatives to audio cassette tapes for data

Fidelipac and related

Kyle Owen who has restored some PDP-8's, noted in May 2018 on the VCFed email discussion list: "This may be too beyond audio cassettes, but I figured I'd share. I never really was interested in this portion of my first PDP-8 haul, but I ended up with a Tennecomp Fidelipac drive. For those unfamiliar, these [magnetic tape cartridges for audio] were commonly used for commercial breaks in radio stations and for the stations' jingles. Similar [in size] to 8-track cartridges, but the drives [tapes?] didn't have a built-in pinch roller. Sent the scans of my Tennecomp Minidek manuals along to Al at bitsavers.org."

"I have yet to see if the unit is functional, and need to figure out where the interface card is (if it came with the interface, even), as I would like to digitize the tapes. Fortunately, I've got a friend with some radio station cart machines, so I could possibly record them to audio files and reverse engineer them from there. So yes, even a PDP-8/M, ca. 1974, and possibly prior, used an audio tape system for data storage!." images on this Google drive.

However, Matt Patoray noted in reply: "Since that is a 4 track drive/cart that will NOT be compatible with the broadcast Fidelipac drives, as those are 2 track running at 7.5 IPS. 4 track would be a 4 track "Muntz" stereo cartridge running at 3 3/4 IPS, which is what William Lear based the "Lear stereo 8". For the Fidelipac and the Muntz machines the cartridge is exactly the same with the pinch roller swinging up into the cart for playback, which is an advantage in this case as the pinch roller that normally turns to goo, can be replaced in the machine saving the tape carts from destruction." [Links to Wikipeda by Herb.]

Michael Thompson on the same VCFed list, noted that Kyle's drive is also found on other PDP-8s: "There is one of those on the PDP-8/I at RCS/RI and on the Classic PDP-8 at the Computermuseum der Fakultšt Informatik."

other

PDP-8's also used Phillips-type cassettes, but recorded in a digital format and driven by logic-controlled drives. Thus multiple file and a file directory could be put on those tapes. Some of these digital cassettes had a square notch on the "top" of the cartridge. DEC used a digital cassette product. I know of a non-DEC product product used on PDP-8's to operate commercial sewing machines. The Phi-Deck brand digital cassette product was available on some microcomputers of the mid-1970's including the Digital Group. DG also used audio-format tapes and some content is available via the linked Web page. Texas Instruments used similar tape technology on some TI terminals. So did the MCM/800 8008 computer which used MFE model 250 digital cassette drives. - Herb

Here's a Web site about DC100 small data cassette tapes and drives which were used in the HP 9800 series of desktop calculator/computers.


Herb Johnson
New Jersey, USA
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Copyright © 2019 Herb Johnson except for content as per sources or authors described herein.