I responded some time ago to this subject, saying that I didn't see any
problem with my S-1000; since the topic continues open I though it
twice and made some test before giving more opinions.
Listening carefully to strong (and not so strong) AM voice
transmissions, I must agree that the situation is not as good as I had
stated before, my apologies for that. I think there are two causes:
AGC and demodulation.
1.- AGC. It is clear that this radio uses something similar to the
"voice (or RF) derived AGC" concept, with all the improvements explained
by Phil Harman in his document about the SDR-1000 AGC, and the software
reorganization incorporated in SVN 2797. This implementation works
extremely well for SSB, CW and digital modes.
But when listening to AM broadcasts this solution is not so good, since
the current AGC raises the gain in speech pauses and you hear it (studio
noises, in fact) and the effect can be very annoying. When dealing with
AM you has at your disposal a constant reference of the transmitted
signal, the carrier, which can be used as a "pilot" for the AGC instead
of the audio (or IF signal). This is the old method of "carrier derived
AGC" used since almost the beginning of broadcasting. Really the reason
for its universal use was its simple implementation once the variable-mu
tubes were introduced, but still has advantages when listening AM voice
broadcasts, because the AGC still controls the gain in the speech pauses
without recurring to very long time constants which are not desirable
for SWL DX listening.
So my suggestion is to include a "Carrier derived AGC" for AM and SAM,
which could include some of the refinements to deal with impulse noise
as already described. This AGC mode could be selected from the setup.
2.- Demodulation. Both the AM and SAM detectors recovers a non
symmetrical signal. I just don't have an idea of the reason, but the
test is easy: Tune an AM broadcast station transmitting speech, and
look the audio on the scope or panascope display. Switch between AM an
USB and see the result (select bandwidths of 6 kHz and 3 kHz to listen
in the same conditions and set the scope time to 5000 us). If you
listen carefully the audio quality improves when switching to USB/LSB.
If you tune to an empty frequency you will see unsymmetrical noise also.
This effect is not related to the AGC, even fixing the gain level the
asymmetry remains.
There are bug reports about this: 2240 and 2282. Also 2276 may be related.
73 de Ignacio, EB4APL
Frank Mayer wrote:
Audio limiting on AM and SAM receive very evident in the new release, just like it was in the previous release 1.18.0 with the SDR-1000. This issue started with the SVN test branch, SVN 2797 when the changes were made to the AGC.
Frank, WA3JBT
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