Hi Guys,

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

25 dBµV.

 

This tells us that when you do a scan, only stations with >25 dBµV RSSI are eligible to be stored.

Bill
RADIO-TIMETRAVELLER
https://radio-timetraveller.blogspot.com

Update: the PL-330 does not defeat Soft Mute by tuning +/- 1kHz on the SW bands. Checked 9690 Spanish and it did not work. I know my PL-310 (before the 'et') also did this. One does need a reletively strong SW station. For example 9690 was 42:19 on frequency and when offset.The indication Soft Mute is defeated is the double zero on the right of the strength display. I get xx:00 on AM-BCB, not on SW

Regards
Paul S. in CT FN31nl

Thank ypu Paul.  Now, if only this pandemic situation would vanish.

Richard

On Oct 11, 2020, at 17:35, Paul Blundell <tanger32au@...> wrote:

Great to hear that things are improving for you on the bands.

Paul,_._,_

TP DX this morning was the best here in quite some time (LSR at 1233 GMT).
567 JOIK fair at 1226, audible to 1235.
594 JOAK good at 1219.
693 JOAB fair at 1222 until 1237.
747 JOIB fair at 1201 until 1241; overall the best signal of the session.
774 JOUB poor 1159, signal ploughed under by semi-local KSPI on 780.
828 JOBB fair at 1208; poor at 1239.
972 HLCA audible from 1155 until 1238 often peaking at a good level.
1287 JOHR poor (fully readable) at 1230.
1566 HLAZ poor at 1241; barely audible at 1245.
Skywave SSB with 8-inch FSL.

Richard Allen,
near Perry OK USA.