Then I have analyzed the code in detail and came to conclusion that a low pass filter will be fine, as the GET-BIT routine only cares wheter the bit coming is 1 or 0 and instead of counting transitions (like MSX for instance).
Another issue that might be relevant is that if a TTL chip is used as a input buffer it should have a pull down on the (ear) input pin as the ZX81 expects the silence (no audio-in signal) to be at LOW level.
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|----- "0" bit ----|
/\/\/\/\____________ as coming from ear
________
\___________ as should be seen by GET-BIT
1200us 1300us
[]s