That is, indeed, a cassette player in the foreground. It’s been modded in a couple ways, the key one here being that the speed of the playback can be manipulated electronically. Specifically, the sort of control voltage that works between synthesizer modules can be applied externally to the speed of the cassette. In this case, a slow waveform is increasing and reducing (back and forth, pendulum-like) the pace of the cassette playback, lending it that slurry, warbling quality. (Note the long, pink cable that plugs into cassette player.)
The recorded sequence itself is a Buddha Machine as sampled, sequenced, and played by Teenage Engineering Pocket Operator (PO-33 K.O.). I recorded that PO-33 K.O. sequence onto the tape, and then rewound the tape and played it back as controlled by the control voltage (CV) output of the synthesizer (seen in the background). This is the first patch I’ve tried out with the CV cassette player, which I received on Friday in the mail and have been eager to give a test run.
The slow wave form, an LFO (low-frequency oscillation), is from a Batumi (by Xaoc), its highs and lows compacted by the SPO (by WMD / SSF). SPO stands for Scaler / Polarizer / Offset Generator. The cassette player mod is by the awesome Chester Winowiecki, of Whitehall, Michigan. (The other mod is it can take an audio line in. The standard device only used its own microphone.) I shared some photos of the tape cassette player a few days ago: “Cassette Bent.”