Introduction to I/Q formats

SDR data is exchanged and saved in different formats. There are formats for raw I/Q sample data and formats with demodulated pulse data.

I/Q sample data formats

I/Q stands for "In-phase / Quadrature", the raw data format used by SDR receivers and transmitters. A sample consists of an I and Q value, each commonly of 8, 12, or 16-bit. This is called "interleaved" in audio or video data.

The data can be processed similar to a two-channel audio signal, although at a much higher sample rate.

Tips

Common sample rates with RTL-SDR receivers are 250 kHz and 1024 kHz, also 1 MHz (1000 kHz).

The nature of an I/Q sample allows to use a bandwidth equal to the sample rate (with a purely real signal the Nyquist-Shannon sampling theorem would only allow half the bandwidth).

Generally the data formats are header- (and thus metadata-)less, the used center frequency and sample rate must be transferred separately or encoded in the filename.

Formats differ in sample-(bit-)width and (bit-)number format, used bit-widths are 4, 8, 12, 16, 32, and 64, bit-formats are unsigned integer, signed integer, and float:

  • .cu4: Complex (I/Q), Unsigned integer, 4-bit per value (8 bit per sample)
  • .cs4: Signed integer
  • .cu8 (.data .complex16u): 8-bit per value (16 bit per sample)
  • .cs8 (.complex16s)
  • .cu12: 12-bit per value (24 bit per sample)
  • .cs12
  • .cu16: 16-bit per value (32 bit per sample)
  • .cs16
  • .cu32: 32-bit per value (64 bit per sample)
  • .cs32
  • .cu64: 64-bit per value (128 bit per sample)
  • .cs64
  • .cf32 (.cfile .complex): Float, 32-bit per value (64 bit per sample)
  • .cf64: Double Float, 64-bit per value (128 bit per sample)

Also used but rarely supported are audio files containing I/Q data:

  • .wav
  • .bwf: Broadcast Wave Format

The "native" format for RTL-SDR receivers is .cu8, for other receivers likely .cs16. Most receivers only sample with 12-bit per channel, using .cs12 will be more compact although not as widely supported.

The rtl_433 program supports most of these formats and allows to read, write, or convert them, e.g.:

  • rtl_433 -w FILE.cu8: write received data to sample file
  • rtl_433 -w FILE.cu8 FILE.cs16: convert sample file

Pulse data formats

Demodulated data can be stored in a readable text-format with file extension .ook, also .fsk or .psk. A header contains meta data about the demodulation (2-ASK / ook, 2-FSK / fsk, ...) and the extension is informational only. Each regular line in the file format contains a number for pulse duration and a number for gap duration. For FSK or PSK demodulation these are mark and space duration.

There is also the .vcd format which can carry the same information and might be useful with traditional signal data software. It can optionally also encode more than two states, e.g. (4-FSK), this isn't used however.

A very compact format is rfraw:, usually just one line of code. This format encodes quantized pulse/gap durations with a maximum of eight different durations.

There are also formats for demodulated but "raw" amplitude or frequency, e.g. .am.s16, .fm.s16 similar to the above formats but with only one "channel".

The SigRok .sr format is a Zip and combines multiple files for easy viewing with SigRok Pulseview.

Tips

Install SigRok Pulseview and write a SigRok file. The overwrite option (uppercase -W) will automatically open Pulseview.

The rtl_433 program can create all these formats from live data or sample files, e.g.:

  • rtl_433 -w FILE.ook: write received data to ook file
  • rtl_433 -w FILE.ook FILE.cu8: convert sample file to ook file

File name meta data

In addition to the file extension meta data about the center frequency and sample rate are encoded in the filename.

  • 433.92M : A decimal number suffixed with M denotes the center frequency
  • 1000k : A decimal number suffixed with k denotes the sample rate

Each part of the filename must be separated by an underscore. Even with low frequencies or high sample rates the suffix is fixed,

Warning

433920k is not a valid frequency specification and 1M is not a valid sample rate specification in filenames.

File viewers

All raw I/Q sample data formats and most demodulated pulse data formats can be visualized with the triq I/Q Spectrogram and Pulsedata viewer.

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Last Updated:
Contributors: Christian W. Zuckschwerdt, dforsi