Interesting this subject comes up on soft mute. I've been working on a blog post about soft mute and just what it is and its technical parameters.
Here's the data on soft mute:
What is soft mute?
Soft-mute is a further lowering of the audio level of the received signal when it drops below a prescribed signal-to-noise ratio. It was implemented in consumer grade DSP radios to provide a more "comfortable listening experience" for the casual listener and not the DXer. The idea is to relieve the listener from all that nasty low level "static" and "interference", or as Silicon Labs states: "....to attenuate the audio outputs and minimize audible noise in compromised signal conditions."
Soft mute attenuation is available in the Si473x digitally-tuned series of chips as well as the Si483x analog-tuned series of chips. The soft mute feature is triggered by the SNR (signal-to-noise) metric. The SNR value is directly readable by the chip's software when you tune to a station. The SNR threshold for activating soft mute is programmable, as are soft mute attenuation levels, attack/release rates and attenuation slope.
The Tecsun PL-380, PL-310, PL-330, and other radios all may set different soft mute values than the chip's default values shown below. Settings for soft mute are initialized during the power up sequence.
The 4 soft mute parameters: Rate, Slope, Max Attenuation, Threshold.
Rate (default): 278 dB/second (range 1-255, actual figure 278 = setting * 4.35)
Determines how quickly soft mute is applied/released when soft mute is allowed (enabled).
Slope (default): 2 dB (range 1-5 dB per dB of SNR)
The attenuation slope for soft mute application - in dB of attenuation per dB SNR below the soft mute SNR threshold. Translated: how much audio attenuation to applied as the SNR and signal quality decreases. A setting of 2 will lower the audio by 2 dB for each 1 dB reduction of SNR below the starting threshold at which soft mute kicks in. An example: soft mute starts to kick in when the SNR decreases to 10 dB. At 10 dB, there is 0 dB of soft mute. When the SNR decreases to 9 dB, soft mute reduces the audio level by 2 dB. When the SNR decreases to 8 dB, soft mute reduces the audio level by another 2 dB (4 dB total). By the time the SNR hits 2 dB, the soft mute has reduced the audio level to a max of 16 dB. It will go no lower as the max soft mute has been applied. Note that every 6 dB of audio reduction is a halving of the audio voltage level. 12 dB of reduction is then 1/4 of the original audio voltage level. 16 dB (max soft mute) is a reduction of 84.2% (0.158).
Max Attenuation (default): 16 dB (range 0-63 dB, max attenuation of soft mute)
If set to 0, soft mute is disabled entirely.
Threshold (default): 10 dB (range 0-63 dB, SNR at which soft mute starts to engage). Silabs states, "for a tuned frequency".
Note that the Threshold setting is applicable only "for a tuned frequency". I take this to mean that soft mute is dis-engaged totally when not tuned to an exact 9 or 10 KHz channel, which is apparently why the 1 KHz off-tuning hack works.
What you're hearing when a signal's SNR lowers below the threshold and the soft mute kicks in is the Slope factor in action. The Slope factor is lowering the audio volume accordingly.
Two other interesting parameters effecting tuning and seeking.
AM Seek/Tune SNR Threshold.
SNR Threshold which determines if a valid channel has been found during Seek/Tune.
Specified in units of dB in 1 dB steps (0–63). Default threshold is 5 dB.
This tells us that when you do a scan, only stations with >5 dB SNR are elegible to be stored.
AM Seek/Tune Received Signal Strength Threshold (RSSI).
RSSI Threshold which determines if a valid channel has been found during
Seek/Tune. Specified in units of dBµV in 1 dBµV steps (0–63). Default threshold is