hello people i need your help again guys, i am facing this project that came out from no where. its about a device that other devices with varying audio level connects to, and then it connects to the tv's audio input to provide a constant level of audio. so it will be like :- [[device (dvd/mp3/satellite receiver/..etc )]] ------->> [[attentuator/amplifier]] --------->> [[tv]] the device i have to do is to take the input measure it and if it passes a certain threshold it attenuate the signal till its reach just about the threshold. And when the signal is below the level it amplifies it. simple ??!!! it looks so, but i have so little to start with although been searching google like crazy all i can come up with is some stuff about AGC i think my project has something to do with it, but am not sure and am not too good in analog electronics. anybody got an idea of how i can do this??? thanks and wish you all a happy new year everybody
Send your input to a peak hold detector (op amps), then feed that signal to a comparitor (op amp), then use the results of the comparitor to pass the signal or send it to an amplifying circuit (op amp) which will either decrease or increase the gain by a set amount. Pass this signal back to a comparitor and repeat the process until the signal is where you want it. With analog, this could get to be a pretty big circuit unfortunately. If you can add some digital circuitry such as mosfets and AND/OR gates, you'd greatly simplify things.
To stop the level getting too high you may want a compressor: essentially above a certain threshold any increases in volume are attenuated by a certain factor. So if you imagine a graph of input volume (X) vs. output volume (Y) you'd have a straight line with a kink in the middle. Here's an example from a winamp plugin called Ozone, where the threshold is 20dB and the ratio after it is 1.8:1 (before it is simply 1:1). You can also get them with a smooth curve. An alternative which might allow both sides of the problem to be tackled simultaneously would be some kind of level detection device for the output. Something that gives you a voltage that you could drive a VUM meter with would work, as you essentially know what voltages would relate to 'too quiet' and 'too loud' [from the spec sheet of the device, presumably], however something with a long term average might be better otherwise the volume will waver and sound bad -- e.g. if you have a suddenly silent or quiet passage of music, the VUM method will react instantly and stick the gain up, then if the music suddenly goes loud again, it will clip until the circuit and swing the gain the other way, and so on. Thus to sound better it would need to ignore relatively short quiet passages; however at the same time if you're watching a film like Pirates which has loud LOUD bits and quiet bits, you'd want it to catch up quick enough that you didn't miss speech, etc. in the quiet bits. Catch 22. Because of this a lot of active gain control equipment have what is called 'Attack' and 'Release' which you control with knobs, and essentially control how fast the AGC reacts to sudden increases - how quickly it attacks - and then how quickly it releases control when the loudness has passed. (This is one area where valves actually have some 'nice qualities', they don't get so easily saturated as transistors by things like sudden bass... on the other hand they require lethal power supplies and are expensive to build equipment with). You may also find that certain frequencies pose more of a problem than others -- the compressor in that winamp plugin actually only applies to bass frequencies. Edit: Building on what jkeyser said; you could [buffer, then] half-wave rectify the signal (send it through a diode), peak-hold this (big capacitor and resistor in parallel, between ground and the output of the rectifier diode) use a difference amplifier to difference this and a preset voltage from a potentiometer (audio pot; linear ones are rubbish for audio), then use this 'difference' voltage to control something like a JFET Voltage Controlled Resistance which is used to set the gain of a final amplifier, out of which comes your neat signal. The hard part is probably matching the output of whatever your "it is too loud by X amount" module is, with whatever the gain control is (I use JFEt VCR as an example). The entire circuit might be quite complex but the individual parts aren't especially complex, if you avoid the maths, at least.
This is going to sound very pessimistic, but I think unless you're going to have a massive number of devices then an individual set of amps with independent gain control would be easier to make tbh. You could even just make it into an amp-type affair with a large project box and a whole boat-load of pots. Then again, you could just alter the output signal of each device in the normal way.
If you build one - do me one too. I could use it to keep the volume down on all the tv adverts! I hate that.
