oh no, another crazy audio idea from Holland

[pd0psb]

Hi all,

There have been some reports (f.i by Rob Sherwood) of AGC's in modern DSP receivers being a bit "restless" (being over-responsive or "dancing on the noisefloor")  The K3 AGC is most certainly on the right track but has had some work in the past.

Could the following be the case:

Besides prevent clipping in RF/IF stages, in the end of the day our AGC is mainly there to serve our ears. Since the AGC detector stage is triggered by "white" bandnoise, is it not relatively(!) more sensitive to sounds near the high flank of our passband? In other words being triggered relatively more by static and pulse noises, which undermines it's control over the lower voice-content (the vocal-cord part of the spectrum).

Up till now I've only seen approaches in the time and gain domain (delaying the attack,modifying the attack/decay timeslopes and onset threshhold) but not in the frequency domain, adjusting some frequencies to have more grip on the agc and some less.

In audioland it's quite common to "side-chain" a dynamics processor (compressor/limiter) with a filtered version of the program material you wan't to treat. If you offer the AGC detector an input with a HPF inline it will become less sensitive to bass content. If you offer it an input with a LPF inline it will become less sensitive to high tones. In other words you get "frequency-dependent dynamics processing".

If you would use some sort of sloping (pink? ;-) LPF in front of the AGC detector in a DSP receiver, could you prevent it from being overly sensitive to white noise content? It might do a better job on tracking voice and be less responsive to pulse and static type noises near the high slope of the passband.

Would this (combined with Rob Sherwoods "delayed attack" transient approach) be an option to really tame the AGC in a near perfect DSP receiver?  Excuse me if i'm kicking in open doors...

73'
Paul
PD0PSB

[Leigh L. Klotz Jr WA5ZNU]

I thought of this recently as well, in another context.  With PSK and
other digimodes, the wide AGC doesn't do the right thing.
Especially with modes which aren't amplitude-sensitive, an AEC
(automatic equalization control) would be better than an
across-the-board AGC.
I think it should be possible to do a crude AEC by monitoring the power
spectrum and sending RS232 commands to control the RXEQ buttons.
An on-device version would be better, though, as it would be be able to
use smaller bin sizes.
Of course, this wouldn't help digimode reception when the AGC action is
strong enough to cause hardware AGC, but when the dynamic range is such
that it's within the range of the K3 DSP, but the generated signal
dynamic range exceeds that of the PC sound card, it could be useful.

You can decide yourself if this would be a good idea for your
computer+rig by performing the following test:
1. go to 14.070 on a busy day
2. turn off the speakers and turn off AGC
3. ride the RF gain control and see if you can find one spot that shows
all weak signals without driving your sound card into distortion from
the strong ones.

It'd be nice if Lyle KK7P or Wayne N6KR would comment on whether they
think this is valuable.

Legih/WA5ZNU

> Hi all,
>
> There have been some reports (f.i by Rob Sherwood) of AGC's in modern DSP
> receivers being a bit "restless" (being over-responsive or "dancing on the
> noisefloor")  The K3 AGC is most certainly on the right track but has had
> some work in the past.
>
> Could the following be the case:
>
> Besides prevent clipping in RF/IF stages, in the end of the day our AGC is
> mainly there to serve our ears. Since the AGC detector stage is triggered by
> "white" bandnoise, is it not relatively(!) more sensitive to sounds near the
> high flank of our passband? In other words being triggered relatively more
> by static and pulse noises, which undermines it's control over the lower
> voice-content (the vocal-cord part of the spectrum).
>
> Up till now I've only seen approaches in the time and gain domain (delaying
> the attack,modifying the attack/decay timeslopes and onset threshhold) but
> not in the frequency domain, adjusting some frequencies to have more grip on
> the agc and some less.
>
> In audioland it's quite common to "side-chain" a dynamics processor
> (compressor/limiter) with a filtered version of the program material you
> wan't to treat. If you offer the AGC detector an input with a HPF inline it
> will become less sensitive to bass content. If you offer it an input with a
> LPF inline it will become less sensitive to high tones. In other words you
> get "frequency-dependent dynamics processing".
>
> If you would use some sort of sloping (pink? ;-) LPF in front of the AGC
> detector in a DSP receiver, could you prevent it from being overly sensitive
> to white noise content? It might do a better job on tracking voice and be
> less responsive to pulse and static type noises near the high slope of the
> passband.
>
> Would this (combined with Rob Sherwoods "delayed attack" transient approach)
> be an option to really tame the AGC in a near perfect DSP receiver?  Excuse
> me if i'm kicking in open doors...
>
> 73'
> Paul
> PD0PSB
>  

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[pd0psb]

Hi Leigh,

Doesn't the K3's roofing do the job in PSK?
I would think that,with the smallest roofing filter, the AGC would track very narrowband signals perfectly..

My thought is that especially in wider modes,where you have a mix of wanted/non-wanted signals, to let the AGC focus on the wanted signal and prevent it from overreacting to unwanted signals by "pre-EQ ing" the the AGC detector.

I'm aware though that dynamics processing in audio is an artform as such.
Frequency-dependent (or "multiband") dynamics processing multiplies all variables so is an even higher art :-)

But it is capable of very well controlled results with a minimum of artifacts!

73'
Paul
PD0PSB

[Geoffrey Mackenzie-Kennedy]

Hi Paul,

How well your idea performs in practice would, I suggest, depend on the
strength of the wanted and unwanted signals of course, but also on the *in
passband* and skirt region IMD behaviour of the roofer(s), and all 'stages'
that follow vs.applied AGC. Any IMD 'weakness' in this area would result in
a compromise.

73,
Geoff
GM4ESD


Paul, PD0PSB wrote on Tuesday, April 28, 2009, at 8:16 AM

> My thought is that especially in wider modes,where you have a mix of
> wanted/non-wanted signals, to let the AGC focus on the wanted signal and
> prevent it from overreacting to unwanted signals by "pre-EQ ing" the the
> AGC
> detector.


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[Brendan Minish]

On Tue, 2009-04-28 at 00:16 -0700, pd0psb wrote:

>
> I aware though that dynamics processing in audio is an artform as such.
> Frequency-dependent (or "multiband") dynamics processing multiplies all
> variables so is an even higher art :-)
>
> But it is capable of very well controlled results with a minimum of
> artifacts!

In the pro Audio world compression is a different game to the AGC in a
receiver.
In music we are controlling dynamic range over a few dB only, even over
this limited range it's arguable that this results in 'minimum
artefacts' but then most of my work in pro audio was spent working with
Acoustic Music with an inherent large dynamic range, even small ( a few
dB on transients etc) can alter the overall colour of a recording in
unacceptable ways, This is one of the reasons that I used to go to a
very good mastering engineer in a separate mastering studio when we
needed to muck about with the overall Dynamic range of a good acoustic
recording .

In Rock/pop and in broadcast radio the effects of multi-band compression
are easy to hear and often not very pleasant at all to listen to
although they have long been part of the overall 'sound'  


The K3 has an excellent AGC, if you wish to weight it towards the lower
(af) frequency then try AGC SFT (Soft) in the new firmware and set some
slope. AGH HLD is also a great feature, RACAL used an AGC system like
this on the RA-3720 and it's wonderful to see the same AGC feature in
the K3.
Experiment with the release speed and threshold too.
transient reject is already in there and has been for a long time (AGC
PLS = NOR) the K3 ran rings around my Icom 7800 in this regard, the only
thing I miss about the 7800 is all the money I lost in deprecation
whilst I owned it.


Multi (audio) band AGC belongs in outboard gear if this is something you
really feel will add to your enjoyment of the radio

Multi-band compression (IF DONE RIGHT..) may further improve the average
power in SSB whilst maintaining intelligibility, However most of the
Multiband processed audio I hear on the HF bands is too wide (lots of LF
energy complete with the in-band IMD it can cause) or over-done, enough
fidelity is lost that it actually degrades the ability to copy the
station on a noisy band.

73
Brendan EI6IZ
--
Don‘t complain. Nobody will understand. Or care. And certainly don‘t try
to fix the situation yourself. It‘s dangerous. Leave it to a highly
untrained, unqualified, expendable professional.

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[pd0psb]

Hi Brendan,

Thanks for you reply!

If you think, my idea is to bring an audiostudio into an HF tranceiver, that's not at all what I would like.
I quite dislike all the bass heavy overprocessed signals on the band.
My thoughts are only "what ifs" for best SSB reception..

Great SSB receivers from the past I think stood out for:
-excellent RF capabilties (K3 status : done cum laude)
-the most pleasant/natural audio curve to listen to (K3 status: ...well...)
-the most "unnoticable" AGC action (K3 status: almost all the way there)

I just wonder if  slightly desensing the AGC for frequencies in the 1-3KHz audio range in voice mode may help to add that last bit of "rest".

73'
Paul
PD0PSB


Multi (audio) band AGC belongs in outboard gear if this is something you
really feel will add to your enjoyment of the radio

Multi-band compression (IF DONE RIGHT..) may further improve the average
power in SSB whilst maintaining intelligibility, However most of the
Multiband processed audio I hear on the HF bands is too wide (lots of LF
energy complete with the in-band IMD it can cause) or over-done, enough
fidelity is lost that it actually degrades the ability to copy the
station on a noisy band.

[Barry N1EU]

pd0psb wrote

Great SSB receivers from the past I think stood out for:
-the most "unnoticable" AGC action (K3 status: almost all the way there)

If you really want great sounding ssb, turn off the AGC, turn on the AF Limiter and throttle RF/AF gain.

73,
Barry N1EU

[n7ws]

I'm sorry, but IMHO, if you have to do this, you don't have a well-designed receiver.

As far as I'm concerned, these super-duper DSP radios should be smarter than I am and they should know when to switch the preamp in or out, when to switch the attenuator in or out and how to proportion the AGC between hardware and DSP for best IMD performance and so that SNR is maintained.

Arguably the best sounding SSB (and for a DSP radio, AM) receiver around in the one in the TS 870.  I *never* fiddle with the RF gain on mine, the AGC works dandy and I can listen to it for hours (phones on) without fatigue.

Wes Stewart, N7WS


> If you really want great sounding ssb, turn off the AGC,
> turn on the AF
> Limiter and throttle RF/AF gain.
>
> 73,
> Barry N1EU


     
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[pd0psb]

True Barry!

That's the most natural sound, but not very practical in a 80dB difference per signal contest situation ;-)
You'll get a beep in your ear that won't be auto-notchable HI.

73'
Paul
PD0PSB




If you really want great sounding ssb, turn off the AGC, turn on the AF Limiter and throttle RF/AF gain.

73,
Barry N1EU

[Barry N1EU]

Wes Stewart wrote

I'm sorry, but IMHO, if you have to do this, you don't have a well-designed receiver.

Nah, the K3 AGC is quite good - speech will ALWAYS sound more natural without AGC and its inherent dynamics-limiting action.

73,
Barry N1EU