How did Commodore call their tape format, if there ever was a specific name for it?

OK, let me clarify. I would be interested in the technical term of the encoding as well as any specific name, should it have ever gotten one. Why? I am interested in it. The only terms regarding tape formats I have found are KCS and CUTS, but neither have been used by Commodore.

AFAIK the tape format used by Commodore was identical starting from the PET onwards, comprising all of the 8 bit product line.

I would be interested to learn the correct technical term for the way the data was saved to tape as well.

  • 7
    "It must have had a specific name, right?" If you've only got one format, "the Commodore tape format" seems like an adequate name.
    – dave
    Commented Sep 11, 2023 at 21:27
  • 3
    Welcome to SE/Retrocomputing! Please take the tour to learn how this site works, and read "How to Ask". Then come back and clarify by editing your question: Do you really want to know the name of the format? If so, why? Or do you want to know the technical term of its encoding? Like FSK, Manchester, ...? Commented Sep 12, 2023 at 6:08
  • 2
    I’m voting to close this question because I can’t see it’s point OP does not seem to have done any research at all, OP did not look at questions guidelines, We are currently under a big flood of dubious questions, I’d like to keep the high quality of answers AND questions
    – Olivier
    Commented Sep 12, 2023 at 6:20
  • 1
    I find the question clear enough as it is. The edit in queue would improve it, but I am not sure this is what the asker meant. Commented Sep 12, 2023 at 8:42
  • 4
    The OP asks two questions - what was it called, and what was its name? Haddocks' Eyes. :-)
    – dave
    Commented Sep 12, 2023 at 14:27

1 Answer 1


How did Commodore call their tape format?

Hmm, what's wrong with Tape Format?

It must have had a specific name, right?

Does it? If you insist, maybe try CTF or 'Commodore (PET) Tape Format'?

But serious, why should Commodore call it anything if the there is neither a need to distinguish it from others supported formats - as there are none (*1) - nor intend to sell it as dedicated product?

The only terms regarding tape formats I have found are KCS and CUTS

KCS and CUTS are two identifiers for the very same format(*2).

  • CUTS is what Processor Technology named their format
  • Provisional Audio Cassette Data Interchange Standard ((P)ACDIC) is what Byte Magazin, the organizer of the Audio Cassette Standards Symposium called the result of that meeting (*3)
  • KCS is what other (participating and non-participating) magazines (*4) called it - usually to avoid naming Byte being the driver for standardization.

I would be interested to learn the correct technical term for the way the data was saved to tape as well.

Well, seems less about format - which would be the way data is structured - but encoding. How to call this encoding may depend on your view point:

  • Technical the encoding must be described as an FSK variant using sequences of three frequencies to encode a byte using a single full wave each.

  • For all practical purpose it might be better viewed as PWM encoded as that's the way the cassette software handles it (*6,*7).

This comes not at least due the way the Datasette hardware accepts and returns the data signal (*5). When seen as PWM, those three frequencies manifest themself as symmetric (50% duty) pules (*8). For a PAL C64 On/Off time is each

  • 182.7 µs (2737 Hz) for Short
  • 265.7 µs (1882 Hz) for Medium
  • 348.8 µs (1434 Hz) for Long (or Mark)

For an NTSC C64 it comes to

  • 176 µs (2840 Hz) for Short
  • 256 µs (1953 Hz) for Medium
  • 336 µs (1488 Hz) for Long

The difference is due different CPU speed as the signal is software generated. Those ~4% doesn't really matter as the reading routine is made to adapt to accept a way wider speed margin to accommodate different cassette speeds.

Several seconds of 'S' are used for synchronisation - that's the high pitch everyone who has ever put such a cassette into an audio player. Beside that, always two of those pulses form a symbol for data encoding:

  • L+M -> Byte Mark.
  • S+M -> Zero-Bit
  • M+S -> One-Bit
  • L+S -> End-of-Data Mark

Only those for are valid symbols, all others (*9) should result in an error.

Further down a Byte is formed from

  • Byte Mark,
  • 8 Data Bits and
  • 1 Bit of Odd Parity

This concludes encoding as used to write the further format.

All of that can be in even greater detail be found at the C64 Wiki's age about Datasette Endcoding.

*1 - That is unless one includes the KIM format - a rather unlikely case.

*2 - As a rather quick search on RC.SE might reveal.

*3 - Which essentially just adopted CUTS as that standard.

*4 - And Byte Magazin in later years as well. Georg Jellinek's Normative Kraft des Faktischen at work :))

*5 - Nicely described in the Encoding section of the Wiki page for the Commodore Datasette.

*6 - This goes not only for the genuine Commodore variant but all Fast-Loader variants as well.

*7 - Also the reason why the .TAP format for storing of cassette content is build as a sequence of duration markers.

*8 - The basics are as well Nicely described in Tommy's answer to a question asking How did the C64 interface tape drives?. Which also provides a section about the (block) format.

*9 - These are S+L and M+L - the others would be repetition of the same signal, thus being indistinguishable of a continuous one, making it non-symbols. Although continuous S is used as synchronisation pattern.

  • 5
    I recall that the Unix file system was called "the Unix file system".
    – dave
    Commented Sep 12, 2023 at 0:17
  • 2
    > Jack Sparrow: You know, for all that pirates are clever clogs, we are an unimaginative lot when it comes to naming things. > Gibbs: [nods] Aye. > Jack Sparrow: I once sailed with a geezer lost both his arms and part of his eye. > Gibbs: What did you call him? > Jack Sparrow: [pauses] Larry.
    – Olivier
    Commented Sep 12, 2023 at 6:27
  • I don't think the term FSK is really applicable, since that would imply that data could contain runs of multiple pulses having the same length, but I think that, except within the leader tone, any pair of pulses having the same length must be followed by one which is longer or shorter.
    – supercat
    Commented Sep 15, 2023 at 17:24
  • FSK does not imply anything about how many pulses are allowed in sequence or not.
    – Raffzahn
    Commented Sep 15, 2023 at 17:34
  • @Raffzahn: Generally the notion of "frequency" implies the notion of a periodic waveform. Given that the a tape has some pulses that occur at fixed times regardless of the data encoded therein, and some that occur earlier or later within that framework, I'd view that as being much more like phase-shift modulation.
    – supercat
    Commented Oct 16, 2023 at 14:41

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