Docs: ZX81 audio (EAR/MIC) i/o jacks

Physical

The ZX81's audio i/o ports "MIC" and "EAR" are designed for old cassette tape recorders, which have an auto-gain amplifier at the MIC input (from ZX81 "MIC" output), and provide a high (phones) level signal on the (EAR)PHONES output (to ZX81 "EAR" input) suitable for those old little (mostly white with white cable) mono ear plugs.

Mechanically, they're 3.5 mm mono TRS jacks.

The ZX81's "MIC" output has a very low volume. It's too low to be recorded and played back to the ZX81 ("EAR") as-is.

The ZX81's "EAR" input expects a high volume signal, meaning at least (near) full range of a line-level signal.

A high volume signal from the "MIC" output can be achieved by electrical amplification (cassette tape recorder auto-gain, headphones amplifier, mixer desk, etc.), modification to the ZX81 (higher output volume), or in software by applying "normalization" or "gain" to the recorded audio data.

When full-volume source audio is played back to the "EAR" input at line-level, it might still be too low for the ZX81, requiring electrical amplification. 

Signal logic

The audio signal, as created and read by the ZX81, is a sequence of bits, each represented by either a short or a long sequence of high signals, and a pause after each bit:

A '0' bit is represented by four pulses, a '1' bit by nine pulses. 

Each pulse consists of 150 microseconds of high signal, followed by 150 microseconds of low signal (silence).

Each bit is followed by a pause of 1300 microseconds.