Best Recording Practices
When a signal is encountered, there are generally three ways to preserve it: demodulated audio recording, full-rate I/Q (RFRadio Frequency) recording, and intermediate frequency I/Q (IF) recording.
IF I/Q Recording
I/Q recording allows playback of raw spectrum data, which allows the listener to listen to the signal as if they were receiving it on their own SDR. In essence, it downshifts the tuned frequency to 0 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz)., and encodes amplitude and phase as sine (In-phase) and cosine (Quadrature) in two respective audio channels. I/Q files are not demodulated, and require demodulation in order to be directly useful to humans and most decoders. Because typical I/Q recordings produce very large files at the full sample rate of the radio, it is almost always necessary to record the intermediate frequency (IF), which is a narrow I/Q stream containing the section of RFRadio Frequency spectrum that is demodulated by the SDR software. Typical sample rates for the SDR input are over 2000 kHzKiloHertz (kHz) 10^3 Hz, while the IF sample rate is usually 16 kHzKiloHertz (kHz) 10^3 Hz to 48 kHzKiloHertz (kHz) 10^3 Hz; the reduction in file size is proportional.
Guidelines for High-Quality Recording
When recording the intermediate frequency, it is important to ensure that the entire signal is being recorded, and that sufficient bit depth is present to capture the full dynamic range of the signal in question. Increasing the bit depth increases both file size and dynamic range; an 8-bit IF recording is half the size of a 16-bit IF recording, but has half the dynamic range (56dB vs. 112dB). The dynamic range of the IF I/Q recording should exceed the dynamic range of the SDR to prevent loss of signal SNR. For 8-bit SDRs such as RTL-SDR and HackRF, recording should be done in 16-bit depth; higher bit-depth SDRs such as USRP, AirSpy, LimeSDR, and ADALM-PLUTO should use 24-bit depth; and higher-depth SDRs such as the 22-bit AirSpy HFHigh Frequency (3-30 MHz)+ in full oversampling may see benefit to 32-bit depth recording.
When sample rate is selectable, the lowest samplerate that captures the entire signal bandwidth should be used. If the bandwidth is too wide, unnecessary spectrum is captured, producing an unnecessarily large file. If the bandwidth is too narrow, signal features such as sidebands can be lost, and a constant-amplitude signal may not appear as such: FMFrequency Modulation demodulation will clip, FSKFrequency-Shift Keying carriers can be lost, and QAMQuadrature Amplitude Modulation will lose constellation points.
Due to the nature of SDR operation, the center (Local Oscillator / LO) frequency is filtered to remove DC offsets; without this filtering, a large spike appears in the center of the spectrogram. Since removing this applies a notch filter, spectrum information at the notch filter's frequency is lost; a perfectly centered AMAmplitude Modulation carrier will disappear entirely, as the SDR software can't differentiate the carrier from the SDR's DC offset (and in an I/Q stream, the carrier would appear as a DC offset in that scenario). Because of this necessary filtering, it is advisable to keep the LO frequency outside of the IF filter passband.
SDR# does not include a straightforward method of recording the IF; the stock Recording plugin only offers recording for raw input (full-rate I/Q) and demodulated audio, and an IF recording option is not immediately presented. To record the IF, set the demodulation mode to RAW, set the bandwidth as high as possible (32000), and use the Recording plugin to record demodulated audio. The RAW demodulation mode causes the demodulator to be bypassed, and I/Q data from the IF is passed directly to audio output. Recording this audio output creates an I/Q file of the IF.
Unfortunately, due to the design of SDR#, the IF is exposed after bandpass filtering; reducing the demodulator's bandwidth reduces the amount of captured spectrum with no effect on file size, causing only a loss of information. For this reason, IF I/Q files should be recorded at the highest possible demodulator bandwidth setting. All files recorded by SDR# are in 16-bit depth; sample rate varies on demodulation mode when using independent IF recording plugins, and is fixed to 32 kHzKiloHertz (kHz) 10^3 Hz when using the stock recorder.
HDSDR can simultaneously record full-rate I/Q input, unprocessed IF I/Q, and demodulated audio. However, HDSDR does not record IF I/Q files by default. To enable IF I/Q recording and disable full-rate RFRadio Frequency I/Q recording, right-click the Record button, and ensure only the IF recording mode option is checked.