Field Guide · term

Also known as: cs16, sc16, complex int16, ci16

cs16 (“complex signed 16-bit”, also written sc16) is a raw IQ format: an interleaved stream of little-endian 16-bit signed integers, I₀ Q₀ I₁ Q₁ …, with no header, no metadata, and nothing else in the file.1 One complex sample costs 4 bytes — half the size of the float32 cfile — and because 12–16-bit ADCs produce integer samples natively, cs16 is often a bit-exact record of what the hardware delivered rather than a converted copy.

I₀Q₀I₁Q₁I₂Q₂ … byte 0 — no header each box = one little-endian int16 (2 bytes); one complex sample = 4 bytes
File offset zero is already sample data: everything you need to know about the capture — rate, centre frequency, scaling — has to travel alongside the file.

Conventions

  • Scaling. Full scale is ±32767, which maps to 0 dBFS; consumers typically divide by 32768 to get ±1.0 floats. A capture’s peak level in dBFS is meaningful and portable — one reason integer captures are convenient evidence when overload or gain staging is in question.
  • Endianness and order. Little-endian, I before Q, is the near-universal convention (matching SoapySDR’s CS16 stream format and USRP sc16 wire format), but nothing in the file enforces it — a byte-swapped or IQ-swapped read produces plausible-looking garbage, so trust but verify with a spectrum sanity check.
  • No metadata. Sample rate and centre frequency must be carried in a sidecar file, the filename, or a note. SigMF exists precisely to formalise this; a bare .cs16/.raw file depends on discipline.

Against its neighbours: a cfile (float32) spends twice the bytes to preserve processing headroom beyond the ADC’s range — worthwhile for processed signals, moot for raw captures of a 12-bit front end. 8-bit formats (rtl_sdr’s unsigned u8) halve the size again at the cost of dynamic range. A 2-channel 16-bit WAV is cs16 plus a 44-byte header that documents the sample rate — a big usability win the raw format trades away for simplicity.

Relevance to SDR

cs16 is GopherTrunk’s workhorse capture format: gophertrunk replay -in capture.raw -format cs16 -sample-rate … replays one, the repository’s regression fixtures (testdata/*.cs16) and operators’ field captures use it, and the pre-combine diversity_capture tap writes one headerless cs16 file per receiver branch plus a JSON sidecar carrying the metadata the format itself omits. The half-size-of-float economy is the reason: a 30-second two-branch capture at 250 kS/s is ~120 MB in float32 and ~60 MB in cs16, with nothing lost — the samples were 16-bit integers on the wire anyway. The habits that make headerless captures useful later are the same ones this format demands: record the sample rate, centre frequency, gain, and device alongside every file, at capture time, every time.

Sources

  1. SoapySDR wiki — Pothosware, on the CS16 complex-int16 stream format and its role as a native SDR sample type. ↩

See also