Defeat the AGC with 15KHz tone

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    • #37726
      Avatartoddboyle
      Participant

      One of my camcorders has AGC, making it pretty useless for the work I do (recording lectures in large, formal settings)

      What if I get a tone generator and mix a 15Khz sine wave with the audio signal I’m getting from the Lecture Hall, before putting the audio into the camcorder?

      Will this fool the AGC into thinking theres a constant, strong audio level, thereby preventing it from going into Hyper-Magnification of the background noise everytime the speaker pauses???

      I will gladly giveup the high end of the audio spectrum by filtering everything above 10khz with an equalizer, in post production

      Todd

    • #167173
      AvatarJaimie
      Participant

      It seems like you are proposing a complex solution for a simple problem. The obvious answer is to turn off the AGC and use the camera’s manual volume controls. If you can’t do that, maybe it’s time to upgrade your camera.

      Addressing your suggestion, your proposal may be more difficult than it seems. Basically, you are saying that a combination of a 15KHz tone and the mic audio would keep the AGC from raising its gain during quiet periods. That means that during quiet periods the 15KHz tone will be recorded at full volume on the tape. Note that the voice isn’t exactly being held constant, the sum of the mic audio and the tone are being held constant by the AGC adjusting the gain.

      To try this you will need a good low distortion sine wave generator and a high quality mixer. Low distortion is essential because distortion of the sine wave will give rise to harmonic components that may be audible after going through the digitation process inside the camera. After all, during quiet periods, the 15KHz tone is being recorded at full volume because the AGC has brought it up to that.

      The low pass post filter to remove the 15KHz tone from the audio output of the recorded program is probably not too critical in that 2nd or 3rd order filter with a 7KHz knee would probably reduce the 15KHz tone enough to prevent it from overloading downstream equipment.

      By the time you buy a low distortion sine wave tone source, a low distortion mixer and a filter, you are well on your way to the price of a new camcorder with adjustable audio levels.

      Good luck!

    • #167174
      Avatartoddboyle
      Participant

      Thanks Jamie. It seems to me, the 15 or 20 khz tone can be imperfect, as long as it has no low-frequency component, right? There would certainly be harmonics and combinations with the audio signal but they would all be above this “carrier” frequency… right? so I’d be looking around the ham radio lists, etc., to build a little oscillator, with a low-cut filter…

      I guess I’ll just try it and see. Even if this stupid thing creates some noise, it could hardly be worse than the roar of hyper-amplified background noise caused by the AGC. I’m not doing music or professional, paid productions. I am recording lectures and political speeches, where the audio is pretty lousy anyway.

    • #167175
      AvatarJaimie
      Participant

      Todd,

      I am interested to see how it works out. I had a reverse experience some years back where I was using a directional mic to pick up some voice. Worked great in rehearsal, then a live combo showed up for the real performance and although they weren’t loud, they were loud enough to keep the voice from being heard. Luckily, I corrected the oversight in the second performance. In your case, the combo is the 15KHz tone and it is “masking” the background noise.

      The reason I stressed that the 15KHz should be a low distortion sine wave is to avoid the possibility of aliasing. For those who may not be familiar with aliasing, it is an undesired effect that occurs in digital signal transmission or recording when the recorded frequency exceeds one-half of the sampling frequency.

      Camcorders sample audio at 48KHz (audio CDs sample at 44.1KHz) which means that a camcorder could, theoretically record audio up to 24KHz. But, if a frequency of 24,001 Hz were to be recorded, it would not appear as 24,001, but as 1 Hz. Likewise, if you tried to record 24,100 Hz, it would appear as 100 Hz! These new frequencies are called aliases and are as strong as the original frequencies that caused them. They represent serious distortion.

      You don’t hear them because every sort of digital signal processor, including camcorders, includes a sharp cut-off low-pass input filter to greatly attenuate higher frequencies. That, coupled with the fact that most mics don’t have much response at those frequencies means that you normally doen’t experience aliasing. But your case may be different.

      When the speaker is silent, the AGC in the camcorder will bring the gain up to the point where your 15 KHz tone is being recorded at full volume. Any harmonics due to distortion will also be present. IF – that’s a big “if” – the camera’s anti-aliasing input filter is not good enough to greatly attenuate these harmonics, you will get aliasing.

      Your best bet is to try it and see what happens as it is pretty expensive to try to measure harmonic distortion and calculate it effects. If you get funny distortion or noise look for sine wave distortion caused by your oscillator or an overdriven amp. Also, don’t forget a lowpass filter on the output to get rid of the 15KHz when you play back because young people, especially ladies, can hear 15KHz and find it irritating.

      Good Luck!

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