Cobra SoundTracker Compandor
Certain Cobra CB radios of the late 1990s through the early 2000s including the Cobra 75 WX ST had a compandor circuit known as “SoundTracker” to reduce noise. The IC used for the SoundTracker included the TA31101AF, which is a generic audio compandor IC used in various radio communication devices. Compandoring compresses the dynamic range of an audio signal before transmission and expands it back to its original range upon reception, typically with a frequency vs. amplitude response curve tuned for the communication mode of the radio (say FM cordless phone vs. AM CB radio). Compandoring helps improve apparent audio SNR, but it can introduce artifacts, especially when only one side uses companding. Field experience indicates that SoundTracker can improve intelligibility for weak to moderate signals, including with non-SoundTracker radios, while adding a “huffy” character to recovered audio in some cases. Reported improvement can be up to about 20 dB when both sides enable SoundTracker, and up to about 10 dB when only one side enables it; artifacts are usually more noticeable in one-sided use. The digital noise reduction (DNR) circuit in the modern Cobra 75 All Road is generally superior to SoundTracker and avoids those specific compandor artifacts.
For the 2000s-era Cobra 75 WX ST, transmit modulation is high-level (yielding better sound quality and punchiness vs. low-level modulation of less expensive radios) and the radio includes an automatic noise limiter (ANL). Internal adjustments in the Cobra 75 WX ST allow optimizing transmit carrier power, modulation limiting, and other parameters. After two decades, the cord covering may crack and fall off, revealing the wires and antenna coax inside. In contrast to the modern Cobra 75 All Road, the 75 WX ST has all the RF hardware in the microphone, with the small gray box being an interface to the antenna, power, and speaker jack. The 75 WX ST is a good spare radio, but if purchasing a radio, consider instead a modern radio with DNR instead of SoundTracker, as the DNR is superior to the compandor.