Using a ZOOM H4 digital recorder with the ZX81

The ZX81 has very simple, apparently quirky audio in- and outputs, for loading and saving files. But in fact these are designed to work with cassette tape recorders built about 40 years ago, which are slightly different from modern audio devices.

You can read a bit about the ZX81's audio i/o characteristics if you like - they appear to be a bit unusual on first sight.

Will a modern ZOOM H4 digital recorder be able to handle the task?

The program on the screen was saved to and loaded from Zoom H4.

The good news first: It works! :-D

One or two extra steps are required to handle the main issue with the ZX81's tape ports: low volume. The ZX81's MIC/EAR tape ports are designed for old cassette tape recorders, which handle audio signal slightly different from modern digital equipment. Zoom H4 digital recorder provides enough audio processing functions to make it work.

Saving

Connect ZX81 "MIC" to Zoom Input 1 or 2, and switch that input to high sensitivity. In Zoom's "Input menu", set "Mono mix" to "On" (so you don't have to worry about left/right channels), all other settings to "Off", and turn "Level" all the way up (0 db). 

Saving/recording from the ZX81 to the Zoom H4 will still have only ~50% amplitude - select "WAV" recording format, so you can "normalize" (via "Menu"->"File"->"Normalize") to full volume. (Zoom H4 can't normalize mp3.)

Loading

For loading/playback use "phones" output jack, and near-full volume (~90% or so).

Are there any bad news? Not really. A little understanding of what's going on is required, as are the necessary steps to make things happen successfully, which is really manageable for everyone.

The perfect solution would either require an old cassette tape recorder with built-in recording amplification, if you like the original 1981 workflow, or some sort of hardware amplification to the signals, so no more post-processing is required on the digital recorder. This could be an external amplifier, or a modification to the ZX81's mainboard.

But unless you're a hardcore coder who does constant saving and loading, the Zoom H4 is quite a usable storage device for ZX81 tape port i/o, even without extra circuitry.