Automatic Volume Limiter

Anyone have a schematic for something to limit the volume from line level sources? I hate watching a tv show or movie and then getting blasted by a commercial at twice the volume.

Adam

What I'd like to find is something in the VBI stream that identifies "Commercial", to make the VCR "pause" and skip the bloody things.

Then again, the advertisers do keep TV free, "the way God intended it to be."[1] ;-)

Cheers! Rich

[1] Dave Letterman

Rich,

I've looked at the VBI data - no go. It varies from station to station but not really durring commercials.

I have an approach to killing commercials, but it requies a 'video out' from the TV. Its just like my intrution detector....

You take an array of 8 by 8 samples of the video at very low resolution. This forms a pattern that rarely occurs by accident.

When you see a comercial you really hate, you hit a button to store an image near the start of the commercial. When it detects this pattern in the future, it kills the sound for 1 minute.

Hmmm, I may make one just to prove it out.

Luhan

There are some schematics on the

website. They use proprietary devices, but are arguably one of the best ways to do the job.

Another good way (something that a hardware hacker could make work) would be to use a photoresistive device along with a LED (various companies used to make those.) The response time of the photoresistive cell is slower, but that can actually be advantageous WRT the effects on perception. (A similar effect can be done by using the RMS detector and VGA scheme, but the idea of attack/release time constants should be generalized so as to get the best audible effects.)

If you are interested only in mono, you could use a JFET in a voltage variable resistor mode. Matching problems would make stereo a little less fun.

John

in Australia i think there is a limit on loudness... however they get around this law using a compressor,

So the aduio is compressed up to the limit.

One attack of the problem:

Comparitor LM311 Sign --!>--------------------------------------------- ! ------ ! ! ------- ------ ! High ! ------- --- In -----+--! ABS() !---! ln() !--! pass !---! exp() !---! X !-- Out ------- ------ ------ ------- ---

One advantage this has is that the ln() and exp() operations can be quite well matched and tend to track with temperature. Also, the high pass can have a few poles in it without stability problems.

Other method:

Any normal feedback based one, needs a quite linear multiplier to sound any good. You may be able to take advantage of the fact that for normal volume levels you don't want to change the sound at all. I'm thinking of something like this:

Audio quality buffer !\\ -\\/\\/\\---+---------! >-----------------+--------------- Out ! !/ ! ! -------- ! J1 U1 LM324 ish ! -ABS() ! --! /+!--GND -------- !!--/\\/\\--+ ! ! ! R2 ! GND +---!!----+---/\\/\\/----- C1 ! ---/\\/\\-- +Vref R3

The Jfet J1 is selected for having a high Vgs off.

D1 is a 1N914 like diode.

The output of U1 normally rests at the (-) supply rail. Op-amps like the LN324 don't have diodes across their inputs so C1 will charge up to more than just the rail voltage. Vref, R3, R2, D2 and R1 sets the amount of "integrator wind up".

When someone shoots the bad guy, there is bang. If it isn't followed by abunch more noise, R1 and D1 won't be able to discharge C1 far enough to start the output of the LM324 going positive, so you won't effect the sound.

When the soap ad comes on, The output of U1 (LM324) will swing positive and reduce the sound. A fair bit of distortion will be added but you may not care.

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kensmith@rahul.net   forging knowledge

I suggest you look here for starters.

The 'audiohm' couplers are available from Farnell and I can attest they they work well. In fact I've been using them for around 10 ? yrs in my designs.

Nice thing about a CdS cell is that it's typically more linear than any solid state multiplier chip and has no DC conditions that require set up. A breeze to use and not even expensive.

Graham

Oops ! Missed the link last time....

A VCR makes a very good limiter. I have tried this before. The internal record holds the volume constant.

I have made automatic volume controls using op-amps and a lamp-photoresistor combo. The photoresistor changes the gain.

Terk makes a nice inexpensive unit for abot $30-$40. It even has two frequency channels.

I find the devices very good for changing channels and maintaining volume. I had too much of a problem though with the Hughes surround sound system, where the volume changes as the surround sound changes.

greg