5 AGC-options: What exactly are the differences?

[Arni]

I realized AGC-fast for speech goes down AND up at a decent speed.
AGC-safety clip: it goes down, but when i start at gain +5dB and it should be +15dB for recording radio, it goes up to maybe +6 or +7dB after several hours but not to +15dB gain.
Is the safety-clip setting more kind of LIMITER  (fast down but no or very slow up movement)
AGC would fit perfectly for live-music-recordings IF it would NEVER increase the levels but only decrease them for not clipping.

maybe one of the developers could modify the coding for 1 of these 5 settings...the live-music setting....to prevent from boosting levels?!  
or rename the live-music seting to "concert-taper"-setting and do the modifications mentioned above?!  
thx for your input
arni(vienna)

[Datman]

Could be added settings for attack, release, ratio and similar parameters?

[petur]

AGC-safety clip: it goes down, but when i start at gain +5dB and it should be +15dB for recording radio, it goes up to maybe +6 or +7dB after several hours but not to +15dB gain.
Is the safety-clip setting more kind of LIMITER  (fast down but no or very slow up movement)
AGC would fit perfectly for live-music-recordings IF it would NEVER increase the levels but only decrease them for not clipping.

WHOA!

thanks, you just confirmed my suspicion that there's a bug in AGC Safety. I've seen it increase levels on a very quiet recording. I was beginning to doubt my initial gain setting and blaming it on bad memory.

This is a bug. AGC Safety should *never* increase levels, only lower them if there's a risk of clipping (I think it kicks in when the signal comes above -3dB)

Please create a bug entry in the tracker.

[petur]

should be fixed now, please let me know if it acts up again

[Arni]

should be fixed now, please let me know if it acts up again

ok, will start an 8 hours testrecording from radio now and let you know.
thanx petur!  

ähm... in which build will the fix be released? me dumbhead HAHA!

[pabouk]

It is in the current CVS build: http://www.rockbox.org/cvs.shtml

[Arni]

hi petur,
i just tested the 20061002 build and the saftey-clip mode seems to work perfekt now.
i set the gain on line-in for an fm-recording to -10dB to keep the peak levels very low and during 1h15min it stayed at -10dB, so no boosting of gain anymore!  
thx great work!
arni

[petur]

i just tested the 20061002 build and the saftey-clip mode seems to work perfekt now.
i set the gain on line-in for an fm-recording to -10dB to keep the peak levels very low and during 1h15min it stayed at -10dB, so no boosting of gain anymore!  

I hope you mean +10dB

I wouldn't set line levels much below 0dB as the negative gain is done by the decimator. If your signal clips at 0dB, it will be clipped and a bit smaller when using negative gain.

[Arni]

i just tested the 20061002 build and the saftey-clip mode seems to work perfekt now.
i set the gain on line-in for an fm-recording to -10dB to keep the peak levels very low and during 1h15min it stayed at -10dB, so no boosting of gain anymore!  

I hope you mean +10dB

I wouldn't set line levels much below 0dB as the negative gain is done by the decimator. If your signal clips at 0dB, it will be clipped and a bit smaller when using negative gain.

nope i meant -10dB   to make sure it´s 100% too low, when recording from fm(carbon cable antenna coming out of my wall connected to my standalone-fm-tuner which is connected to the iriver´s line-in) the AGC is balanced(reaches it´s maximum peak )between -5dB and -3dB. when recording FM with the headphones as antenna, it reaches it´s maximum peak at a gain of +13dB as the signal is much weaker and needs more amplification.

[whatboutbob]

petur: any chance you could make it so that AGC never goes into negative gain? (for the reason you stated above)

[petur]

sure, but that would be a bit against the RockBox policy of giving full flexibility. So there should come a minimum AGC gain setting

[Arni]

sure, but that would be a bit against the RockBox policy of giving full flexibility. So there should come a minimum AGC gain setting

and i would have clipping when recording radio from cable-source(not headphones as antenna) where i need -4.5dB for "Bayern 3" for instance as the signal comin out of my wall is pretty strong for this station.
thx for NOT changing anything petur  

[whatboutbob]

sure, but that would be a bit against the RockBox policy of giving full flexibility. So there should come a minimum AGC gain setting

I just can't see any point to applying negative gain...ever...but especially with the AGC (ie when you know you're clipping).  Am I missing something?

It doesn't bother me as a rockbox user, because I understand the concepts and can be careful to avoid the situation...but there are plenty of users who don't understand and think 'well...my peak meters say its not clipping so it mustn't be'.

[petur]

As Arni wrote (and I have verified in the past) you can have a strong signal (almost clipping) with a negative gain and it may still not clip.

How this is possible, I don't know. But I did tests with a signal that clipped at 0dB and didn't at about -5dB.

See here:
http://forums.rockbox.org/index.php?topic=2513.msg44335#msg44335