AVLC Automatic Volume Limit Control

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  • Updated 4 years ago
  • (Edited)
Auto Volume Limit Control, AVLC to allow setting the max audio you want so if listening to a weak station a strong station does not come through excessively loud.

I thought about a Auto Volume Control, AVC but these tend to boost the weak stations and the noise when no one is talking. This might take the form of an upward compression function with a slider for how much it limits the loud signals.

This differs from the AGC-T in that some strong stations have weak audio and some weak stations have strong audio.  
   
I believe this function would be really great for listening to and running a net.
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ai6re

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Posted 5 years ago

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Steve N4LQ

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This idea is desirable  because the inherit AGC of the 6000 series doesn't function until you turn up the "AGC-T" control which Flexers are trained not to do. With AGC-T set low, it's like using an older receiver without any AGC whereby you constantly ride the RF Gain. So AI6RE's idea would limit "blasting" and save eardrums. 
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M0GVZ

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I set AGC-T to 100% then back it down to just where the background level starts to drop. Seems to give a nice balance of background noise without having the "AGC-off" effect on strong signals.
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IW7DMH, Enzo

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Hello,

what about this feature in the next 1.6.x release?
Contesters and DXers would appreciate it while listening to weak stations and setting AGC to OFF.

73' Enzo
iw7dmh
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Ken - NM9P

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I think that when properly adjusted, AGC-T should take care of this very nicely on CW and SSB signals.  But many are still having problems because the haven't really caught on to the proper adjustment technique.

On FM and AM, however, I can see that this would be very handy.  I have noticed a lot of difference between modulation levels on AM and FM signals.  AGC keeps the "RF" gain equal, but if you have a S9+20 AM signal with low modulation then it is very difficult to understand the station without turning up the AF volume drastically, only to be blasted by noise or the next signal when the station unkeys.  Some form of audio level AVLC could be handy for these situations.

Ken - NM9P
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Lee, Elmer

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I would like this as long as it works with AGC off and I would like a control to set this at some max db limit in audio so I can adjust for a given set of headphones.  Sometimes local noise/static is best handled with AGC off on bands like 160 but there is always a danger of someone coming up on freq at s-10 to  blast your ears off so  some kind of limiter would be great.  Not sure I would make it "automatic" just some max value which would limit the top of the dynamic range

73  W9OY
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IW7DMH, Enzo

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I agree with you 100%.
Automating this feature probably could vanish any "Agc-off" benefit.
I think also it is more important for 6300 owners than 6500/6700 ones. Anyway others great contester machines, like the K3, have this handy feature.

73' iw7dmh
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Ken - NM9P

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Lee, I agree that as you describe this would indeed be a handy "ear saving" feature, especially when running AGC-OFF as I also do occasionally on 160. 

Ken - NM9P
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KC2QMA_John

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I agree Audio limiter separate from AGC-T would be great. Since the FLEX SDR has so much dynamic range and the fact that the “AGC-T” does not work the same as a traditional “AGC” control, having an “AVC” or audio limiter/compressor would great to take advantage of this awesome dynamic range and enjoy a better listening experience


73/KC2QMA

John
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Eric - KE5DTO, Official Rep

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Correct me if I'm wrong, but isn't this what the combination of AGC-T and the Volume controls offer?  The AGC-T control allows the user to control the maximum amount of gain to apply to the signals.  Then the volume control lets the user set the signal to a comfortable listening setting.

If this is not what you are after, I am confused on how this new control would interact with the existing AGC-T and volume controls.
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IW7DMH, Enzo

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Hello Eric,

my experience using 6300 says that it is ok 95% of the times.
But when I am in a pileup, or more reasonably running in a contest, often I can see in the panadapter traces of weak stations near the traces of stronger stations.
I can't hear them unless I try to turn off AGC and close the filter at about 60-80 Hz.
Setting the gain for the weak station sometime causes booming signals just from the near station. In this scenario a volume limiter feature could help a lot.
I try to show it in the following picture. The red square show the trace of a weak station I can't hear using the standard controls. The weak signal appears only when the stronger one isn't transmitting. To get its audio as well I try with AGC-OFF and often I am lucky.

73' Enzo
iw7dmh

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Eric - KE5DTO, Official Rep

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Given the scenario described, I think AGC is actually the answer.  By turning AGC off, you are indicating that you want to control the gain manually and you set the gain higher.  Then a louder signal comes on and the signal goes to full scale likely distorting.  AGC was designed to address this issue.  

Rather than turning AGC off, why not set it to Fast and turn the AGC-T value higher to allow appropriate gain for the low signals?  This would prevent a stronger signals from being significantly louder than the the weaker ones.
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Lee, Elmer

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The reasn you turn off the agc is because under certain conditions no agc is better than fast agc. This is empirical.

73
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Eric - KE5DTO, Official Rep

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What do you mean by "better?"  I would presume you mean that the gain is not rapidly changing and thus making the weak signal harder to hear.  

But isn't this precisely what the proposed feature would do?  It would have to adjust the gain to keep a strong signal from exceeding the limit thus "hiding" the weak signal behind the stronger one once again -- in essence, this is just an AGC system which we already have.

Rather than add another piece to the system, I would propose that we figure out what it is about the AGC system that causes you to prefer turning it off.  
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Ken - NM9P

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I think that Lee may be describing more of a "limiter" than a gain control. On some bands like 160 or 80 when noise levels are high, or on VHF bands when signals are extremely weak, it is sometimes helpful when copying into the noise to turn AGC off to prevent any signal from taking over the gain, allowing the human ear to do its magic and detect signals into the noise floor.  This is hindered when even moderate signals take over the gain control.  (even when set to FAST.)

I have experienced this on both 160 and 6 meters, with ACG OFF allowing me to copy signals that somehow are hidden in the pumping of an active AGC, even after finely adjusting the Threshold.  It is just "different." 

The problem comes when a moderate, or worse a strong signal close by blasts the ears.  AGC would keep him at an even level, but even with FAST mode, it tends to obliterate the weak one.  Having a limiter would limit how loud the strong signal gets (perhaps by simple soft clipping) but should not lower the total gain of the system, thus reducing the weak signal to nothing.  Yes, the strong one might be distorted, but it is not the desired signal, which is then copyable underneath the noise of the loud one. 

There is a slight difference in concept between Automatic Gain Control and absolute limiting (which could just be seen as a fail-safe threshold limiter) without controlling the total gain of the system. 

Does this paint a better picture?  (Lee, John and others, I hope I did your concept justice.)

Ken - NM9P
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Eric - KE5DTO, Official Rep

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How do you "limit" without changing the gain?  I think the answer is: you can't.  Thus, the suggested feature is an algorithm that would detect large signals and prevent them from overloading the audio subsystem.  This is what AGC already does.

But part 2 is that it is supposed to limit the strong signals while preserving the weak ones.  This is where I am confused.  How do you limit the gain of one signal and amplify another?

Instead, I would recommend that you leave the AGC system engaged and narrow the filter to reject the offending signal if possible.  If the signal is right on top of the signal of interest, I don't see how another gain system is going to help you here.  If you keep the gain high enough to hear the weak signal, the strong signal is far too loud.  If you lower the gain to limit the strong signal, the weak signal can't be heard.
(Edited)
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ai6re

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My original intent was for a way to absolutely limit the maximum audio to a preset adjustable level. I normally don't try to hear what a weak station is saying while being covered by a louder signal but, I also don't like to be surprised by a sudden signal that is way to loud for my speakers and headphones. This would also address the people that press their Key to tune their Legal Limit amplifier at 600hz above the carrier frequency and send a piercing signal through my house at 5am. Fortunatly our flex 6K radios Tune button generates a signal on the tuned frequency and most people never hear it.

There are certainly ways to limit the audio with the AGC-t and the volume control but not without leaving yourself open to an unanticipated level signal. This has become and interesting discussion.
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Eric - KE5DTO, Official Rep

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This is exactly what the AGC system is built to handle.  It anticipates the signal level and adjusts the gain such that it keeps the audio signal within a reasonable level as defined by the AGC-T control.  If you have this control set at 100 (full tilt), it will amplify everything to the target of -2.5 dBFS including the noise.  This is annoying for typical operation, but may be of use when hunting weak signals.

By contrast, when dropping the AGC-T control, this puts a cap on how much gain the AGC system can add.  This means that all signals over a certain level will sound the same, but signals below that start to sound quieter.  It will still keep a strong signal from going above -2.5 dBFS, but weak signals may sound quieter depending on the setting.
(Edited)
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ai6re

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I agree that these controls can be readjusted after the fact to moderate the audio level. Unfortunately, it is hard to know until the strong signal is present what combination is right for every situation. I use these controls effectively on a wide range of signals.

Maybe the way to understand the problem is a safety setting that can be set for each individual system like the addition of the maximum power setting that was recently added to the radio setup to keep us from inadvertently over driving our amplifiers.

In practical use of this 6K radio system with several Slices running, each with its own AGC-t and Audio level control, making sure they are all properly adjusted has proved to be difficult and as you might see from the other comments it is something many users would welcome.

I have had my Flex 6500 for 2 years and find it almost impossible to use my other radios as I feel very handicapped without the Flex. I made this request 1 year ago and just like the Automatic Volume Limiter I wish I had to limit the level of TV Commercials it is still just an idea.
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Eric - KE5DTO, Official Rep

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I totally agree on the TV commercials!

I think what I would suggest may actually seem counterintuitive.  I would suggest that in order to protect your ears, you actually want to push the weak signals UP to the same limited signal target as the strong signals by turning the AGC-T level UP (rather than setting AGC to off).  You may then need to adjust your Slice volume and/or your master volume down, but I think this will give you the protection you are looking for.

Give it a try and let me know whether that helps.
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ai6re

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I was not the person that suggested turning the AGC-T off. I always use it just like you suggest and it is very effective. For our 40M Net each morning there are signals of every level and I usually run the AGC-T at 50 to 60 which does make the radio noisier but does level the weak and strong signals and then the Noise Reduction (NR) helps reduce the noise.
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George Molnar, KF2T, Elmer

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Might a "hearing protector" function like iPods, etc. have, help? Just a "max output" value defined to not affect signal processing, but just limit delivered speaker power?
(Edited)
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Eric - KE5DTO, Official Rep

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I believe this would just limit the volume setting on either a particular Slice or the Master volume.  Those controls already exist today.
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KC2QMA_John

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The reason who the is more of a problem than with traditional radios is the Flex SDR has an awesome amount of dynamic range and that is good but because of this it is harder to strike a good audio volume level between weak and strong  signals.

If you set the AGC-T as recommended your are forced to unnecessarily listen to more noise floor.

So have having A “Audio limiter” after or post “ACG-T” would allow the user to set the AGC-T control as recommended but with the added automatic audio limiting (AVC) to tame the “VOLUME” level instead of trying to use the RF signal gain (AGC-T).

So let's forget everything and just imagine setting the flex as recommended and then hooking up an audio limiter/compressor to the audio output to automatically control the overall Audio volume level.

I think what some of us would like is to Add audio limiter to the last stage of each slice just before audio is output or just the main output.

73/KC2QMA

John

(Edited)
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Ted, NX6C

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Well done.  You have exactly described the situation and the solution!
Ted
NX6C
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Eric - KE5DTO, Official Rep

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You already have such a control. In fact, you have 2 of them. The Slice Audio Gain slider and the Master volume slider. Both behave exactly as you describe (sets a limit / attenuates to some max level) and is the very last thing in the signal chain well after the AGC.
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Ted, NX6C

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Eric,
   The missing process in the chain you are describing is compression.  The dynamic range of the audio output goes between a carefully adjusted AGC-T, which pulls the signal out of the noise level, and the maximum audio setting limited by the Slice and Master volume controls.   The volume level between the low end and the high end can be quite large.   Adjusting the Master or Slice volume downward will also reduce the weak signal, often making it disappear.
  Does the way I'm describing this make sense?
   Ted
NX6C
(Edited)
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Eric - KE5DTO, Official Rep

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Actually, a properly adjusted AGC-T setting for listening to weak signals would put weak and strong signals at about the same volume level.  This is going to be higher on the 0-100 scale and yes, there will be more amplification of the noise with this setting.  This is the cost of trying to listen to weaker signals that are near the noise floor.  If this adjustment isn't done, then the AGC is going to limit the gain applied to weaker signals thus leaving them much quieter than the stronger signals.  The compression you are looking for is accomplished by adjusting the AGC-T control to the right.

Our AGC system performs this "compression" role dynamically by constantly adjusting the gain to compensate for the input signal and anticipating the gain needed to keep the audio level near a target.  This is true with weak or strong signals and is only kept from achieving the target amplitude by the AGC-T threshold.  It actually does this in 2 time domains to ensure that short impulses don't capture the AGC the way a longer strong signal would.  

Adjusting the gain down at any stage (Slice or Master volume) will always reduce all signals equally including the weaker signals.  However, I'm not aware of another way to "limit" strong signals after the AGC in a linear fashion that doesn't introduce distortion.  Certainly we could just clip the larger signals, but this would be very displeasing to the ear.
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Lee, Elmer

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with agc off listening to a -123 dBm signal when a -70dBm signal fires up is also quite displeasing.  When I run no agc I keep a macro at the ready that will immediately switch me from no AGC to fast AGC with the AGC-T set to some nominal value thereby saving my ears.  

The reason some of us run with agc off is because it works best in certain situations like on 160 listening for 6 dB under the noise coherence with thunderstorms and static crashes rolling in from the Atlantic Ocean.  Its utility is determined empirically.  AGC off is either better or it is not better in being able to copy a weak signal in a particular noise/qsb environment.  The problem is 160 pileups are often simplex and 160 players often run "big big signals"  so you have super weak qsbing little jokers among the wailing atom smashers all within 25 hz of each other.  I haven't tried messing with the 2 vol controls so that may work 

73  W9OY
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Ted, NX6C

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Lee,
   You have accurately described what I was trying to say.

Eric, 
    Thank you for explaining the difficulty in designing a 'smart' volume control.

Ted
NX6C
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KC2QMA_John

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If we can not have an Audio Limiter could we just have (Attack, Hold & Decay) controls for the "AGC-T".

The AGC-T works as is should if you don't mind hearing a lot more band noise to hear strong and weak signals at about the same volume level.

73/KC2QMA
(Edited)
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Mike - N8MSA

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I think that having more adjustment is the answer, as the proposed AVLC is really nothing more than an AGC mode.

73,

Mike - N8MSA
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Steven G1XOW

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To me this is a good idea to offer max audio level (for ear protection, not automatic)...why?  

Recently I have been using AGC-T set to "off" so that I can hear some of the very very weak South Pacific DXpeds. They have often been just s0-s1 here on SSB (almost directly over the pole). Turning AGC-T off can make all the difference between just noise and just being able to copy them. Having it on at 100% (fast), or lowered to the noise floor method doesn't achieve the same result. I'd say there is still about 3db more to be had by turning it off in those ultra-weak situations.

So, why is it a problem: well, when you have AGC-T off and working split things are fine and you can squeeze that that last bit of juice out of the flex. As a keen DX chaser, I am used to straining to hear what is going on at the noise floor. But, then some muppet calls on the DX freq at s9+20 without split, and thus blows your eardrums in to the next county. I have actually had this happen twice now, and the result is severe ringing in the ears all day curtesy of some IQ0 not knowing how to use split.

Cheers, Steve G1XOW

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Ted, NX6C

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Eric,

  Your suggestion:

"I think what I would suggest may actually seem counterintuitive.  I would suggest that in order to protect your ears, you actually want to push the weak signals UP to the same limited signal target as the strong signals by turning the AGC-T level UP (rather than setting AGC to off).  You may then need to adjust your Slice volume and/or your master volume down, but I think this will give you the protection you are looking for."

This works for me when listening for weak signals when occasional strong signals occur.

Thanks,
Ted
NX6C
(Edited)
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Lee, Elmer

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I was thinking what m..ight work it to have a db differential between the noise floor and some value, say 30 db. The value could be set by the user. If the value was exceeded the agc would turn on. You would reset to agc off to reset the limit

73. W9OY
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KC2QMA_John

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The obvious solution is the addition of a AVC (Automatic Volume control), Audio Limiter, Compressor what every you want to call it but it is separate from the AGC-T control.

When I have time I will grab one of my Audio limiters connect it to the Audio output on the back of the flex then out to the speakers and demonstrate how this works and is DIFFERENT FROM USING THE AGC-T CONTROL.