Outboard compandoring for radio comms
Compandoring for two-way radio discussion:
Inexpensive integrated circuits like the On Semiconductor SA572 can be used for compandored audio. Compandored audio increases the perceived SNR by greatly reducing audio gain for signals below a threshold, and greatly increasing gain for signals above that threshold. It could be of interest to amateur radio operators to have such a circuit for noisy HF SSB / AM / FM. CB radios before digital NRC features used compandoring e.g. Cobra SoundTracker or Realistic “Audio Clarity Enhancer”. Narrowband FM two-way radios (2 kHz maximum deviation) often use compandoring to improve the audio quality.
Note that compandoring has a downside – when communicating with non-compandored stations, the audio may sound unnatural. Even communicating with another compandored station may sound odd if the settings are not matched well. Compandoring has fallen a bit out of favor due to DSP-based noise reduction (NRC) generally doing a better job of improving apparent SNR with fewer audio artifacts vs. compandoring.
A compandor IC such as used in commercial two-way radio can improve the apparent audio SNR of analog voice communications, whether AM / FM / SSB. The circuit thresholds and compresses transmitted audio, in a better fashion than the simpler clipping circuits sometimes used in ham radio. The circuit also has the capacity to “expand” the receive audio when the transmitting station is transmitting “compressed” audio.
The apparent SNR increase can be on the order of 20 dB if the transmit and receiver compandoring settings are matched. Note that this apparent increase is caused by increasing the average power of the transmission; peak power is still limited by the radio.
The compandor IC SA572NG is available from Mouser for $2. It needs a few common resistors and capacitors external to the IC as well.