rtzx is a command-line utility for interacting with ZX Spectrum .tzx and Amstrad CPC .cdt tape data file, written in Rust. It supports insepcting .tzx/.cdt files, converting to wav, and real time playback with a nice user interface for loading tape programs on a real Spectrum or CPC computer.

Given the relative difficulty of transfering data to floppy disk in this day and age, tape playback is one of the easiest ways of loading programs on actual hardware, requiring only a suitable audio cable to connect your PC's audio output to the tape input of the 8-bit computer.

tldr;

rtzx play path/to/my-tzx-or-cdt-file.cdt

Running rtzx, rtzx help, or rtzx --help will display help information.

To inspect a file, use the inspect command:

rtzx inspect path/to/my-tzx-file.tzx

Inspecting a file causes it to be fully parsed, but does not prepare waveforms, so no timing information is shown other than the lengths of pauses and other durations defined directly by blocks.

To convert a file to wav, you also need to specify an output file:

rtzx convert -o my-cdt-as-wav.wav path/to/my-cdt-file.cdt

Outputted wav files are single channel using a 44.1k sample rate by default. An alternative sample rate can be specified using the --sample-rate / -s command as per the play command.

The play command allows you to play back a file directly to audio output. This is the recommend way

rtzx play path/to/my-cdt-file.cdt

This plays back the specified file and displays a user interface showing the real-time progress of the playback.

The user interface allows you to pause and unpause playback using the space key. This is useful when a program stops the tape part way through and resumes later on (e.g. a game loading a subsequent set of levels after playing through the first levels): you can press space during a defined pause after you hear the tape relay click. Another use for this feature is when a tzx/cdt file defines a pause at the end of a data block that's too short: you can manually pause and unpause to compensate.

The sound is output with a sample rate of 44.1k by default, as recommended by the spec: most TZX / CDT files will have been generated 22.05k / 44.1k recordings, so timings work best with this sample rate. You can specify alternative sample rates with the --sample-rate / -s option:

rtzx play -s 48000 path/to/my-cdt-file.cdt

The TZX file format was created for digitising tapes made for the ZX Spectrum, and as other platforms used sufficiently similar tape loading schemes, the file format is also used for these other platforms.

The main difference between platforms is the playback timings: all timings defined in the TZX format use the ZX Spectrum clock speed of 3.5MHz, so when playing back for another platform that uses a different clock speed the timings must be used accordingly.

At present rtzx supports two platforms: the ZX Spectrum and the Amstrad CPC. When playing back or converting to wav in Amstrad CPC mode, timings are adjusted for the CPC's 4MHz clock (i.e. multiplied by 4/3.5).

The platform is determined automatically from the file name (.tzx => ZX Spectrum, .cdt => Amstrad CPC), but can be overruled with the --platform / -p option:

rtzx play -p amstrad-cpc path/to/tzx-file-with-cpc-timings.tzx

The TZX file format defines a wide variety of block types to support different tape loading schemes and to ease use of tape files in emulators. rtzx parses all known block types and will parse unknown types starting with 4-byte length fields as per 1.10. This should ensure all valid TZX / CDT files can be parsed, and rtzx successfully parses test2.cdt from Kevin Thacker's CDT/TZX test suite.

The subset of block types that are fully implemented is enough to play back many files, but some files using more complex block types won't work.

The following table lists the current status of block type support in rtzx:

So far all of my testing has been done with a real Amstrad CPC, where I have successfully loaded a number of games from CDTs using Turbo Speed Data Blocks. I've had success with games using Direct Recording blocks and more esoteric loaders modeled with Pure Tone, Pulse Sequences, and Pure Data Blocks. I've not yet encountered any CDT files using CSW Recording blocks, or the more complicated logic of loops, call sequences, etc.

While I've tested rtzx with a some TZX files and verified that files parse and Standard Speed Data Blocks generate output, it is completely untested with a real ZX Spectrum, as I don't have access to one of those :-( . Any feedback from ZX Spectrum users would be most welcome!

rtzx is written Rust, so to compile it from source you'll need to install Rust.

To build you can then run cargo build from a checkout of the repo and you'll then get an executable in the target/debug folder.

You can also build and run directly with cargo, using the -- argument to make cargo pass the subsequent args to rtzx:

cargo run -- play path/to/my--cdt-file.cdt

rtzx is very new and will have bugs, so please do open an issue if you encounter any problems with it! For playback issues I'm particularly interested in and TZX / CDT files that are known to work when using other software.

If you'd like to contribute code please feel free to create an issue for a bug or feature, fork and open a pull request.

If you use rtzx with a real ZX Spectrum I'd love to hear from you, as I don't have any way of testing with a real speccy at present!

You can also reach me by email on [email protected] .

Phil Stewart, October 2025