AVC or ALC ...or ?

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dacflyer
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AVC or ALC ...or ?

Post by dacflyer » Tue Sep 20, 2005 1:13 pm

ALC=automatic level control ?
AVC=automatic volume control ?
there might be another word or two for these..
but this will take any sound level and turn it up or down to keep any signal at a certain level.<p>i have many cd's and mp3's some are louder than others when they play or not loud enough,so i am constantly adjusting the volume..
same with the radio..the commercials seem to blast in..
is there a circuit out there that i can put in between the audio signal and the amplifier to adjust the signals to where its always at a even level?
i hateit when i am listening to something decent,and the next song will blast in or you cannot hardly hear it... {:>I

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Re: AVC or ALC ...or ?

Post by rshayes » Tue Sep 20, 2005 2:21 pm

Also called:
AGC = automatic gain control<p>"...the commercials seem to blast in..."<p>This has been a problem for about the last 50 years. After the tape recorder was developed, it became the practice for the advertiser to provide a tape of his commercial for broadcast over the air. The advertisers figured out that the modulation level was limited by the peak amplitude of the audio signal but that the perceived loudness was determined by the average level of the audio signal. Reducing the peak level by clipping introduced some distortion but it did not make speech unintelligable. Amplifying the clipped signal gave an audio signal with the original peak level that would sound louder. Some music is recoreded with heavy clipping, either for artistic purposes or to make it sound louder.<p>Try doing a search for volume compressors or expanders. Typically there will be a path with a variable gain stage for the audio signal. Usually there is a rectifier which measures the output signal level and controls the gain of the variable gain stage to maintain a constant output level.<p>The problem is tat determining the average level of a signal takes a fair amount of time. When the signal characteristics change, it may take several seconds for the circuit to readjust itself. If the time constant is shorter, the circuit begins to reach to short gaps, such as the spaces between words.<p>Radio stations have been trying to solve this problem for about 50 years and, as you have noted, they haven't been very successful.

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Re: AVC or ALC ...or ?

Post by Robert Reed » Tue Sep 20, 2005 4:01 pm

Years ago, when the FCC provided real service to the common citizen, they put a cap on the relative amplitude of commercials versus progam level. But the FCC has long gone by the wayside, now lodged firmly in the back pockets of BIG companies. It s all about MONEY.
The cure for the problem you are having is a research project in itself. For example consider the program content of classical music-demands the widest dynamic range of all, Now consider heavy metal rock- no dynamic range at all just solid 120DB level. To make these two (and everything in between) sound at the same level and without introducing distortion would be beyond the capability of most cicuitry. It has to do with sound density and the human ear is unexelled at detecting this ,since sound is a human sensation that takes place in the ear and not an electronic quantity.

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Re: AVC or ALC ...or ?

Post by Enzo » Tue Sep 20, 2005 5:26 pm

They don't do it by clipping the signal, they do it by compressing the signal. That doesn't add all the distortion of clipping. The commercials are "all the way up", while the show itself is only all the way up on the loudest sounds.

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Re: AVC or ALC ...or ?

Post by Dean Huster » Tue Sep 20, 2005 5:34 pm

Actually, distortion is ANY change to the original signal at all and compression fits into that category. The ear just doesn't hear it .... well, actually, the golden-eared audio wackos can hear it .... they can hear 0.002% THD.<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

Robert Reed
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Re: AVC or ALC ...or ?

Post by Robert Reed » Tue Sep 20, 2005 9:17 pm

The normal way to handle audio source variances is by the use of "companders". These are precalibrated by outputting all inputs to a constant level output. These require optimum attack and decay times for the program material of interest. However, just keeping the audio within certain bounds will not neccessarily present the same "loudness to the human ear (sound density coming into play here). In a given situation, these perform wonderfully. One would be a radio transmission system with multiple console inputs. All the different level inputs would be levelled at the output, thus presenting a constant level of audio to the modulator.these will and do produce distortion (%10). But due the constant level and almost monotone quality (and this is desirable for this purpose) this distortion goes unoticed. Now music and program material present a whole new set of problems in regards to dynamic range and fidelity. Difficult to design attack-decay times over the whole spectrum of program content. Even more difficult to design for constant "loudness" or "quietness". And extremely difficult to maintain dynamic ranges and still make all these various programs sound the same level to the listener.<p>[ September 20, 2005: Message edited by: ROBERT REED ]</p>

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Re: AVC or ALC ...or ?

Post by terri » Wed Sep 21, 2005 5:30 am

".... well, actually, the golden-eared audio wackos can hear it .... they can hear 0.002% THD."<p>How snide, Dean. How very, very snide. And they also can "hear" whether the 0-gauge speaker wires are silver-plated or not. Sheesh!<p>(This is not to start a sub-thread, just to express a little amusement. Talk among yourselves.)<p>[ September 21, 2005: Message edited by: terri ]</p>
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Re: AVC or ALC ...or ?

Post by jimandy » Wed Sep 21, 2005 6:46 am

Years ago I built a "compander" circuit which used an ordinary flashlight bulb as a nonlinear resistive element to control the output amp stage gain. The side benefit was the lamp also provided a visual indication of the level of compression.<p>[ September 21, 2005: Message edited by: jimandy ]</p>
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Re: AVC or ALC ...or ?

Post by dacflyer » Wed Sep 21, 2005 1:05 pm

so basically this is a mission impossible for the average Joe ? kinda ecplains why my neighbors
SMART SOUND doesn;t work worth a hoot on their magnavox tv.<p>
nonlinear resistive element to control the output amp stage gain....huh? splain it for this dummy<p>i have used lamps as tweeter protectors before.
is that what you ment?<p>output amp stage gain?? whats this mean anyway :p

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Re: AVC or ALC ...or ?

Post by terri » Wed Sep 21, 2005 1:26 pm

Jimandy:<p>So what'd you do, feed the power stage through the bulb such that if the input power was too high, the bulb's resistance would go up, thereby cutting the signal to the power stage?<p>(Bulbs are handy little devices for all sorts of unexpected things.... there used to be a section in the Ham Handbook on the wierd uses for them... like, even for SWR indicators.)
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Externet
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Re: AVC or ALC ...or ?

Post by Externet » Wed Sep 21, 2005 1:37 pm

Hi dacflyer.
There is a large number of possible causes the commercials sound louder in broadcasting, as audio is processed with equipment with many, many adjustable parameters that are not always respected.<p>If the normal program material is fed from the mixing console at a lower level than the commercial audio source, it will happen.<p>The best you can do is call the offending radio station(s) engineer/manager and let kindly, politely know how DISGUSTING their commercial levels are. Almost sure they will take immediate action.<p>Maybe less than 1% of stations do not have an audio processing unit like this one in their chain :
http://www.orban.com/
And for sure, far more than 1% are not properly operated. Even properly calibrated, if source levels are not equalized, the differences you complain about will show up.<p>Miguel
- Abolish the deciBel ! -

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Re: AVC or ALC ...or ?

Post by jimandy » Wed Sep 21, 2005 2:02 pm

dacflyer, terri sort of answered your question about the non-linear resistance of the light bulb. And, terri, I don't think it was as simple as the power amp (for the speakers) running through the bulb. I built the thing when I was a '"yute" so I don't remember the details but I think there was an intermediate power amp that drove the bulb with a current sensor that followed which, in turn, controlled a transistor gain circuit. But, in theory, a bulb directly in the speaker circuit might work. In such a case I think it would be more of a "limiter" than a compressor.<p>BTW, dacflyer, the fact that an incandescent bulb's resistance goes up as the filament heats up is used in some sine wave oscillator circuits and makes for a very pure waveform.
"if it's not another it's one thing."

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Re: AVC or ALC ...or ?

Post by terri » Wed Sep 21, 2005 4:19 pm

Well, I meant between the driver stage and the final amp stage. I also thought maybe you stuck them directly in the speaker lines, but I figured that was pretty unlikely.<p>[ September 21, 2005: Message edited by: terri ]</p>
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Re: AVC or ALC ...or ?

Post by Chris Smith » Wed Sep 21, 2005 5:45 pm

A small mic pick up and a VU circuit plus the Lm3914,15,16etc,...can be used to feed back and cut back the volume. Its all analog but it easy to do. <p>I built one in the early 80s for my 4x4 truck in reverse. <p>The mic picked up the tire noise and boosted the stereo output to compensate for the 4x4 tire noise. <p>Blaupunkt has these in their stereo systems, and I think they held the patent for many years.

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Re: AVC or ALC ...or ?

Post by terri » Wed Sep 21, 2005 9:40 pm

"I built one in the early 80s for my 4x4 truck in reverse."<p>Do I interpret this correctly? You don't mean you had it hooked to the reverse gear so it would cut the volume when you put it in reverse, but rather that you built one that boosted the volume instead of cutting the volume as we had been discussing. That is, you reversed the process.<p>Clarification, please? Just trying to interpret Smithese. :)
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