Keeping volume level constant

Discussion in 'The Projects Forum' started by DAK, Aug 6, 2010.

  1. DAK

    Thread Starter New Member

    Nov 25, 2009
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    I would like to be able to keep the volume level on my TV constant within a somewhat broad range. As you probably already know that commercials tend to be much louder than the original setting. For some reason some DVD’s that I watched have also fluctuated. Is there a way to use some basic type of sound level indicator circuit to lower and raise the volume whenever it exceeds a predetermined level? I have a good understanding of most basic circuits and components. I’ve built many circuits from schematics that I have found on-line and wouldn’t have any problems doing so. Is there such a circuit and schematic out there that may accomplish this. Any information would greatly be appreciated.

    Thank you in advance for your help.
    DAK
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    It can be done, even some of the earliest radios had an AVC/AGC feature built in between the detector and the audio amplifier circuit.

    Problem is with modern TVs virtually the entire set is built out of a couple of hybrid analog+digital ICs and there's rarely a way to control the volume aside from sending code to the microprocessor.

    If you've got line outputs on the set and you're using a separate stereo/surround receiver as your amp you could do it, it's a simple matter of taking a sample of the sound level, amplifying then rectifying it into a DC voltage that varies with the average volume level it sees. You then use this voltage to adjust the gain of a buffer amp between the line out jacks to the audio equipment.

    http://en.wikipedia.org/wiki/Automatic_gain_control

    Should be able to find some simple circuits with a search.
     
  3. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    I can understand why you want the "average" sound level to be the same. Like commercials being the same level as the TV show but it would have to be pretty slow acting. If it was fast acting it would detract from the real changes you would have in a TV show like explosions and such.

    PS. Commercials are definitely not at a different level.....Just ask the TV stations. They will tell you that it is against the law and they would never do it. :D OHHHHH is that cynicism I here? hehehehe
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Commercials are at the permissible level. It is illegal to transmit more than 100% modulation.

    Like you have noticed ... everything is at a different level. Tracks from a CD, commercials, movies, tv shows.

    Had they releveled the audio with the maximum peak at 100% of the allowable level this discussion would be moot.

    Back when I was working at an AM radio station, care was taken to ensure things were recorded at 0 dBm. I know, that is not a contemporary standard for digital services. But the releveling arguement is still valid.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    It's true, they can only go on the source material and how what's reproducing it is set up. Old movies had terrible modulation, modern commercials go to extremes to grab every last bit of the 100% they can and in many cases employ elaborate compression techniques.

    Rcently a proposed law started circulating around the FCC that would rquire cable and air providers pay more attention to balancing their source material levels because, ideally, that's where it should be done anyway.

    In the VERY early days of cable in the small college town I lived in they just used modulators to produce CH2 - 13 to combine into a cable feed. I had a key to the building where it all came from and it wasn't unusual for me to drop by at night and balance things out since nobody else seemed to understand how the crude system worked to begin with.

    The town wasn't too tiny but I was always the one the visiting rock bands hunted down when they had a PA amp blow on a Friday night, or a local Policeman's basement TV broke down on Saturday afternoon before the big game. I was never hard to find and kept a very large parts inventory at my shop since I worked on most anything.

    "So what you got and what happened?" It's a Crown DC300A and smoke started pouring out of it, or it's an older Motorola TV and the picture just quit. An educated guess, a drop by the shop to fill a bag with the most likely parts I'd need and off I'd go with a pretty good success rate that I'd be able to get them back running without having to scrounge for or order parts.

    Worst case was when the somewhat newer UHF transmitter went out at the local Police Station and my roommate with the only FCC license in town was, well, out of town for the weekend. I got the owner of the shop he worked out of (which I had previously worked at for a couple of years before I started my own) to open up for me and let me at the parts. Simple case of the output tube filiament going kaput, retuned it as best I could by monitoring cathode current to the manual specs. I left them with the warning that by not having it on the bench I couldn't verify all the other tests you were supposed to perform regarding spurious radiation , harmonic output etc and since I was breaking FCC rules by working on it, if they had anyone else in the area complain about interference they'd just have to pull the plug until my roommate came back. Turns out I did miss some adjustments but didn't have any way to move it to the shop where the equipment was nor any way to move the equipment to where the transmitter was. It got them through the weekend with no complaints and my roommate took care of the rest of it when he got back on Monday. He carried around a sort of half-pissed I worked on it / half-glad I was able to get it back up attitude for about a month until he was finally convinced I wasn't going to go through certification and try and go into competition with him.
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You could have went through the certification and just been his "standy-by" so he could go out of town for whatever he went out of town for ... :D
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Didn't have enough of the questions I'd face memorized, if it's like most certification tests there are trick questions throughout that can easily trip you up. Didn't really have a need for the license either.

    I did get nailed by the FCC once a long time ago. Between a friend and I (and some help from his engineer father) we combined some salvage parts from a very old TV, some hand-wound coils, air variables, a long wire antenna and a crystal we had cut by what at the time was a local firm to produce a transmitter on the commercial AM band. His father had a big reel-reel tape unit in his hi-fi setup so we'd record hours of our favorite music non-stop at the old 1-7/8 IPS speed and this provided us with our own radio station we could hear on pocket AM radios for farther than we ever rode our bikes to. If I recall we used the vertical output tube from the TV to modulate a rather rare tube - 6DZ7 - which was in effect two audio output pentodes in a single envelope. How it worked around 700 KHz I'm unsure but it did.

    I don't know how we got caught but since we were only 9 years old at the time all they did was make us take it apart and confiscated our crystal.
     
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