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.