Ignacio, all:
Your observations are consistent with mine. My point of reference is a
new Pwr SDR 1.18.0 with a F5Ka, compared to Hammarlund and
Hallicrafters non-coherent AM detection (Hammarund wins, by the way).
The F5Ka coherent AM detection is quite impressive in terms of its
resistance to fading. The non-coherent detection and AGC action is
pretty good, but maybe a little problematic. I have also observed
that listening to AM in SSB mode, independent of bandwidth, as along
as the comparison bandwidths are same, produces more pleasing audio
quality. I have not taken the time to fiddle with the AGC threshold or
time constants yet. But I will.
In the grand scheme of things, I am not sure how important it is to
emulate the performance and audio characteristics of the old, high
quality AM receivers since AM transmission, even on the commercial,
and propaganda side of things, is slowly dying. Sure, I would like to
say that my new bleeding edge digital radio can perform like the best
analog radios of yore, but I still have the old radios too (hi).
Thanks for your insight.
Bob KF6BC
On Jun 21, 2009, at 11:52 AM, EB4APL wrote:
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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