One of the features of the Atari machines was that the left audio channel on the cassette was connected through to the television speaker. This was used to produce things like "Conversational French" which would play phrases through the speaker (or headphones) while watching the right channel for small bursts of 1's that indicated it should pause and run the related quiz.

I always thought this was unique to the Atari until around 1984/5 when I saw a TI-99 at the local mall doing the same thing. Looking around today, I can't find any references to this ability. Does anyone have any books from the era mentioning this and how to use the system to create such tapes?

1 Answer 1


I always thought this was unique to the Atari until around 1984/5 when I saw a TI-99 at the local mall doing the same thing

Well, similar but different (*1), as the Atari mixes the AUDIO-IN signal from the SIO port, which carries the left channel, with the sound output of the POKEY, while the TI inputs the (mono) cassette output into the TMS 9919, wh then mixes it with its sound channels for a common output signal.

The TMS 9919's AUDIO-IN is connected in parallel to

  • The Peripheral Connector, where for example the Speech Synthesizer inserts his output
  • The Cassette input, providing everything the cassette plays (mono)
  • A pin of the 9901 port device

Well, the last one isn't really a separate 'input', but meant to enable/disable transport of the cassette signal, so regular loading sounds could be suppressed. This is done by simply pulling the cassette signal hard to ground. Since the amplifier output is held up with a 330 Ohm pullup, this the port can produce a rectangular signal, much like the Apple II does. Essentially adding a fifth sound channel to the 9919 :))

Within the 9919 the external Audio is mixed straight with the generators and equalized. The feature was BTW officially removed from the later SN76489 and the pin is marked N.C.

*1 - The TI99/4's cassette interface is kinda interesting. It is setup to serve two cassette recorders in combination:

  • Two cassette devices using standard recorders
  • All Interface lines bundled on a DE9 connector
  • Connected via a breakout cable with 5 audio jacks
  • Two red for the microphone input of each recorder (same signal on both)
  • Two black for motor control of each recorder (different signals)
  • One white for ear output of recorder #1
  • All organized into two whips, marked 1 and 2

In addition a 'MUTE' signal (which is related to Audio In) is present but not wired.

With this

  • Recorder #1 can be used for reading data
  • Either of both recorders can be remote controlled for writing data

Quite useful for data management. There's also a cable for only a single recorder, for default operations.

  • So the input from the cassette is mono? Voice and data are recorded to the same channel (or both channels)? Commented Nov 14, 2021 at 15:46
  • @MauryMarkowitz IIRC, yes. let me check.
    – Raffzahn
    Commented Nov 14, 2021 at 15:47
  • @MauryMarkowitz Sorry, took me a bit, as the circuit is kinda unusual, as it operates two recorders, can start/stop each separate, but read only from recorder #1, whose signal is then transferred (as well) to the 9919 input. So yes, voice and data comes from the same channel of recorder #1. It's strictly mono, channel depends how the mic/ear plug is wired within the recorder. So as I said, similar but different :))
    – Raffzahn
    Commented Nov 14, 2021 at 16:18
  • Interesting. This being the case, they must have used some other system to indicate starts and stops. On the Atari, the sound was on the left, so you recorded a block of 1's to the right and when you saw them come out of the POKEY you dropped MOTOR. Commented Nov 15, 2021 at 15:31
  • @MauryMarkowitz There's always a way of embedded data. Like a beep when an interactive sequence starts, or a section to be listened too ends. Or simply at the start of a data packet. Of course with muting the sound output as soon as recognized. And of course timed reaction. And yes, the combination is kinda odd, but understandable if one sees not only language courses, but tape based data processing. Here reading from one device and writing to another without tape change is a must. In Many ways TI engineers had still a professional, mini based PoV.
    – Raffzahn
    Commented Nov 15, 2021 at 15:41

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