architecture

name	architecture
description	Architecture of airspy-fmradion — signal flow from SDR source to audio output, key type aliases, the Source abstraction, and the FM decoder pipeline. Use when reading or modifying the DSP chain, sources, decoders, buffers, or audio output.

Architecture

Signal flow

SDR hardware / file
    ↓
Source subclass (AirspySource / AirspyHFSource / RtlSdrSource / FileSource)
    ↓  [separate thread, pushes IQSampleVector]
DataBuffer<IQSample>  ← thread-safe queue with condition variable
    ↓  [main thread pulls]
FourthConverterIQ     ← Fs/4 downconversion for zero-IF devices (Airspy HF+, RTL-SDR)
    ↓
IfResampler           ← r8brain-based rational resampler to decoder's target rate
    ↓
FmDecoder / AmDecoder / NbfmDecoder
    ↓  [SampleVector at 48 kHz]
AudioOutput (SndfileOutput or PortAudioOutput)

Key type aliases (include/SoftFM.h)

Alias	Underlying type	Use
IQSample	std::complex<float>	Raw IF samples
IQSampleVector	std::vector<IQSample>	Block of IF samples
IQSampleDecoded	float	Phase-discriminated baseband sample
Sample	double	Audio sample
SampleVector	std::vector<double>	Audio block
IQSampleCoeff	std::vector<float>	FIR filter coefficients for IQ path

Enums also live in SoftFM.h: FilterType, DevType, ModType, OutputMode, PilotState.

Source abstraction

All SDR frontends implement the abstract Source interface (include/Source.h):

• configure(string) — parse key=value config string and open device
• start(DataBuffer<IQSample>*, atomic_bool*) — begin streaming in a background thread
• stop() / is_low_if() / get_sample_rate() / get_frequency()

Low-IF sources (Airspy R2/Mini output real low-IF, not zero-IF) skip the FourthConverterIQ step — controlled by is_low_if().

FM decoder pipeline (sfmbase/FmDecode.cpp)

Inside FmDecoder::process():

1. Optional IQ bandpass filter (LowPassFilterFirIQ)
2. IF AGC (IfSimpleAgc)
3. Optional multipath filter (MultipathFilter — normalized LMS, enabled with -E)
4. Phase discriminator → baseband
5. Audio resampler (r8brain via AudioResampler)
6. Pilot PLL for stereo detection (PilotPhaseLock, also generates PPS events)
7. Stereo L−R demodulation, de-emphasis, DC blocking