Good question. It is just a volume change in the audio
processing. In the audio world, this would be called
'downward expansion'. (Well, not quite. Expansion would
just be based on the signal volume, whereas the soft-mute
is based on calculated SNR.)
This suggests one way to defeat the effect: just off-tune
from the carrier by 1 kHz, which engages the soft-mute.
Then turn your volume knob up by 2-clicks (PL-380) or
4-clicks (PL-300). As long as you can live with the offset
filtering, this should get rid of the pumping effect. Since
you are already dealing with low-SNR signals, this technique
should not further degrade reception.
--- In email@example.com, Richard Berler > wrote:
> Hi Scott
> When you say that the volume is lowered by 6dB (or 12 dB on the 310), does that mean that the signal has been attenuated or that the volume alone is impacted? If volume only, would one recover the equivalent audio by adjusting the volume?
> --- On Sun, 1/31/10, sdwillingham ...> wrote:
> From: sdwillingham ...>
> Subject: [ultralightdx] Re: Measuring PL-380 Soft-Mute
> To: firstname.lastname@example.org
> Date: Sunday, January 31, 2010, 1:00 AM
> Further info: Via my USB-equipped PL-300wt, I can read out that
> its soft-mute is configured with a threshold at SNR = 6 dB and
> the soft-mute slope is 2 dB/dB. This means that for each dB
> that the SNR reads below 6, the volume is reduced by 2 dB. At
> SNR = 0 dB, the maximum volume reduction is 12 dB.
> I suspect that the PL-380 simply changes the soft-mute slope
> parameter to 1 dB/dB, so it reduces the gain more gently and to
> a maximum of 6 dB. I'll have to confirm that when I get around
> to modifying my PL-380 with a USB interface.
> --- In ultralightdx@ yahoogroups. com, "sdwillingham" wrote:
> > For comparison, my stock PL-300wt measures about 12 dB soft-mute.
> > -Scott-
> > --- In ultralightdx@ yahoogroups. com, "sdwillingham" wrote:
> > >
> > >
> > > Hi All,
> > >
> > > It has been debated occasionally whether the PL-380 has
> > > soft-mute. The short answer is: mine does (PL-380 2009.9 ver 1)
> > > and it's fairly easy to measure. The procedure is to tune a
> > > strong, steady station, measure its average audio level, then
> > > off-tune by 1kHz, and finally remeasure its level.
> > >
> > > To measure the average level, use a free computer recording
> > > program. I think many of you have Audacity and, while it's not
> > > my favorite, it works fine for this purpose. Here's what I did:
> > >
> > > 1) Install Audacity and launch the program.
> > > 2) Attach an audio cable from your PL-380 to the computer's
> > > mic or line input.
> > > 3) Turn the radio on, tune in a strong station with good
> > > SNR, and set the radio's volume control to 7 or 8. Also,
> > > set the IF filter to 6 kHz.
> > > 4) Back in Audacity, locate the input meter (rightmost), and
> > > click on meter to start monitoring the input. (Stretch the
> > > meter toolbar out to see the levels better.)
> > > 5) Next, you'll have to locate your system's recording mixer
> > > and adjust the recording level. I can't tell you the
> > > specifics, since this depends on your sound card. If you
> > > are using a 'mic' input, try to turn off its 'boost'
> > > setting. Adjust the mixer level until the average signal
> > > shows about -15 dB or so on the Audacity meter.
> > > 6) Now monitor the signal's running peak (a dark-red
> > > vertical line on the meter). Determine a where this peak
> > > sits for a particular talker's voice or song. For
> > > example, I'm seeing about -12 dB on my meter now.
> > > 7) Tune the radio off-carrier by 1kHz. Even for strong
> > > signals, this causes the radio to indicate and SNR of 0 dB
> > > which kicks-in the soft-mute function. Measure the peak
> > > level again. I see about -18 dB now. So my conclusion
> > > is that my radio has about 6 dB maximum soft-mute.
> > >
> > > The conclusion is, PL-380 2009.9 ver 1 radios have about 6 dB
> > > soft-mute. This probably also means that the soft-mute
> > > threshold is 6 dB, but that is not certain. We can also
> > > conclude that Tecsun is setting this deliberately, since it does
> > > not match with the Si4734 defaults (neither firmware 2.0 or
> > > 4.0). Perhaps one of you with a 'new' model can test if the
> > > settings have been changed.
> > >
> > > -Scott-
> > >