tv loud during breaks agc

Discussion in 'The Projects Forum' started by tibbles, Sep 26, 2009.

  1. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    hello all

    i posted this last year and they said it couldnt be done...tv breaks at far higher volume particularly during documentaries.and different channels having different volumes
    the answer was quite simple, most digi boxes have an aux audio output. this i connected to a 3 op amp stage level compressor. with a fet doing the limiting.
    i of course lose my remote volume control function, but ive been using this circuit for 3 weeks now and ive only needed to change the volume a couple of times.heaven.
    but to get to the point, i have reached the limit of my expertise, so could i please call upon the experts to help me finalise the circuit.
    i am getting a slight distortion when i drop the volume to almost nil.
    due i think to a gross mismatch between the output from the digi box and the first stage of the agc.
    the first stage of the agc is a non inverting op amp with the input divider of two 220 k ressisters giving an input impedance of 110 ohms.i think.the circuit needs a maximum input of 200 mv.

    the output impedance from the pace digibox is unknown, presumably line, which i understand to be around 775mv at 1000 ohms, but i can get no info on this.
    i have cobbled together an L pad which helps, but any suggestions on this would be much appreciated ,

    regards
    dougal
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You might want to post up a circuit, as that is the only way we can see what you have made.

    Are you sure about using an L-pad? Those are used to control individual speaker volumes, as they divert some power while presenting an almost constant impedance to the amplifier.

    If you're dealing with a line audio output, the input is rule of thumb good with 47K through a capacitor, sized to suit the low frequency roll-off.
     
  3. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    input stage
    new to this so might not work
     
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  4. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    i tried an audio transformer 3.2 ohms to 1kohm, which i thought might have done the trick but it didnt seem to work very well .
     
  5. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    thanks beenthere,
    re L pad. i think the circuit i have used was probably designed for low level input, so the high level from the digibox was swamping the agc.
    the crude attenuator seemed to do the trick up to a point.
    i have tried measuring the output from the digibox with little success.-no reading on ac 200v scale.(the lowest range on my meter) and 5 mv dc.
    in fact the first stage which has a gain of 10 could probably be deleted.
     
  6. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    as i increase the volume the quality is supberb,im using a good quality computer sound system for the output amp at the moment.

    moderators, got the circuit from an old book so there may be copywrite issues if i post the whole circuit, am i right in thinking that if i redraw the circuit ,copyright will then not apply?.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    With attribution, there will not be a copyright problem. I'm curious about the audio transformer function.

    You can't measure audio on a DC scale. The signal is AC.

    Most AM AGC circuits got put in IF strips to manage the level of signal present. They may not do well with straight audio - IF frequencies are at 455 KHz.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your input level is too high. Reduce it with two resistors making a voltage divider.
    A FET AGC creates distortion when the FET has a signal level exceeding about 50mV peak.

    FET AGC circuits are all over the web so post the entire compressor circuit.
    Many Wien Bridge generator circuits use a FET for amplitude stabilization.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    I hear what you are saying....... FOX network is absolutely the worst for this. Shows like "House", "24", "Fringe" and others use a very low sound level for the program and deafening levels on the commercials. It's infuriating. I record any show I want to watch on FOX so I can zip through the commercials. Somebody should take a tire iron to those morons, they are destroying people's hearing to make sure you hear the commercials.

    It is possible to make a dynamic range limiter with a peak level clipper but I am not sure it would sound very good. A crude way to do it would be to build a pre-amp stage and adjust the gain so that the level where the pre-amp clips (limits because of the rail voltages) is the maximum volume you can tolerate. What will happen: you adjust the volume to comfortable level for program content then the commercials will clip the audio (which will sound distorted) but you won't care because you don't want to hear it anyway.

    A more complex way would be to build signal processing circuitry that puts out the average signal level like I use for my VU meter, then AGC around the average level.
     
    Last edited: Sep 27, 2009
  10. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    This is my personal understanding of the situation.

    Contrast to common beliefs, the commercials are not at a higher signal voltage level but instead absolutely stuffed full of content in the signal itself. It is this nature of signal that "sounds loud" to the hearing.

    Thus a standard voltage level operated type AGC will not make the overall perceived sound level sounds acceptable to our usual hearing comfort.

    Sadly this is also the common practice of compressing the sound/music and then level limiting it to make it sound loud, as in most CDs produced today.

    I have yet to see a good circuit to deal with this problem.
     
  11. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    thanks for your replies on this,
    i have solved the problem from the other end, by setting the avc at a slightly higher level and lowering the volume control on the the amp/ speaker system.
    the system now works perfectly, the sound quality is supberb. better than the supposedly high quality tv sound, the computer sound system im using is an altec lansing so perhaps thats not surprising.
    i will rethread re IR control.

    many thank again
    dougal
     
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