rtzx

rtzx is a command-line utility for interacting with ZX Spectrum .tzx and Amstrad CPC .cdt tape data files. It supports inspecting .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. Rtzx is written in Rust.

Given the relative difficulty of transferring 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.

Usage

tldr;

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

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

inspect

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.

convert

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 option, and timings can be adjusted with --playback-duration-percent / -d as per the play command.

play

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.

You can also skip backwards and forwards through the blocks using the left and right arrow keys. This is usefl for skipping pas long opening pauses, or to skip forwards to a particular block in a multi-program file.

Sample rate

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 usually 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

Playback duration

Default playback timings can sometimes be too fast for real hardware to cope with. This appears to be particularly the case with CDTs for newer software that have been optimised for emulator loading. the --playback-duration-percent / -d option can be used to slow down (or speed up) the playback by increasing / decreading the length of all timing pulses:

# Increase duration by 10%
rtzx play -d=+10 path/to/my-cdt-file.cdt
# Decrease duration by 5%
rtzx play -d=-5 path/to/my-cdt-file.cdt

Note that this option does not affect pauses: these are defined in milliseconds and always play out as specified.

Platforms

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.

At present rtzx supports two platforms: the ZX Spectrum and the Amstrad CPC.

The platform is determined automatically from the file name (.tzx => ZX Spectrum, .cdt => Amstrad CPC), but can be overruled with the --platform / -p option, although this no longer has any affect on playback or vonversion as of rtzx version 0.3.0:

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

For playback, there's no difference between platforms. Earlier versions of rtzx erroneously adjusted playback timings, resulting in slower playback for CDT files. CDT files now play back / convert using the default timings as per TZX playback, however fine-grained playback timing adjustments can now instead be made with the --playback-duration-percent / -d option (see above). -d=+15 will produce a playback speed close to the default speed in versions 0.1.0 and 0.2.0.

Supported block types

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:

ID	Block Type	Parsed?	Implemented?
0x10	Standard Speed Data Block	Yes	Yes
0x11	Turbo Speed Data Block	Yes	Yes
0x12	Pure Tone	Yes	Yes
0x13	Pulse Sequence	Yes	Yes
0x14	Pure Data Block	Yes	Yes
0x15	Direct Recording	Yes	Yes
0x16	C64 ROM Type Data Block	Yes (as unsupported)	No
0x17	C64 Turbo Tape Data Block	Yes (as unsupported)	No
0x18	CSW Recording	Yes (as unsupported)	No
0x19	Generalized Data Block	Yes (as unsupported)	No
0x20	Pause Or Stop Tape Command	Yes	Yes
0x21	Group Start	Yes	Yes
0x22	Group End	Yes	Yes
0x23	Jump To Block	Yes	No
0x24	Loop Start	Yes	No
0x25	Loop End	Yes	No
0x26	Call Sequence	Yes	No
0x27	Return From Sequence	Yes	No
0x28	Select Block	Yes	No
0x2a	Stop Tape If 48K	Yes	Yes (as zero length pause)
0x2b	Set Signal Level	Yes	Yes
0x30	Text Description	Yes	Yes
0x31	Message Block	Yes	Yes
0x32	Archive Info	Yes	Yes
0x33	Hardware Type	Yes	Information print only
0x34	Emulation Info	Yes	Information print only
0x35	Custom Info Block	Yes	Limited information print only
0x40	Snapshot Block	Yes	Snapshot type print only
0x49	Instructions Block	Yes	Printed only when length matches 'nstr' magic, payload ignored otherwise
0x5a	Glue Block	Yes	Yes

Playback compatability

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!

Building

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

Contact and Contributing

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 phil.stewart@lichp.co.uk .

Phil Stewart, October 2025