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Analog signal compression

Discussion in 'General Electronics Discussion' started by Riveay, Jun 27, 2010.

  1. Riveay

    Riveay

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    Jun 27, 2010
    Hi everyone,

    I'm into the audio effects for the guitar and I want to understand how to compress the guitar signal to make a compressor.

    I've found several diagrams and I know they'll work but can't understand how they work.

    For compression I understand that the large wave length is reduced to a smaller one and the short wave length is increased. In other words low frequencies get high and high frequencies get low.

    This is because if you compress the amplitude you turn down the volume so thats what I know for now, everything I have found is about coding and compressing digital signals.

    Thanks in advance.
     
  2. Externet

    Externet

    710
    145
    Aug 24, 2009
    Audio compression is a form of distortion that reduces the dynamic range. To explain, if an audio amplitude signal (music) varies from 0.1 V to 1 V has a dynamic range of 0.9V; compression can make it vary between 0.5 and 0.9V instead, where the new dynamic range is 0.4V.
    The level at which the circuit handles the high passages downcompression is the knee treshold.

    Low level audio can also be upcompressed by being amplified more than the high passages.

    It is mostly used in broadcasting, cinema and recording. Compression is not raising a small signal or lowering a high level signal; it is usually both.

    It is not related to manipulating amplitudes of audio frequencies/wavelenghts, (that would be equalization) it is about their volume levels.

    When you watch a television program, audio is greatly compressed, where dialog is nearly at the same level as scenes with shootings. Not real life at all, very artificial.
     
    Last edited: Jun 28, 2010
  3. Riveay

    Riveay

    7
    0
    Jun 27, 2010
    So it has nothing to do with the frequency?

    And that would explain why they are called sustainers too, when the signal gets below the range it is amplified to meet the range value until there is no signal at all to amplify (when the guitar string stops vibrating).

    Am I right?
     
  4. Externet

    Externet

    710
    145
    Aug 24, 2009
    If someone calls them sustainers also, is another story. But a compressor is a compressor and works with dynamic amplitudes of a signal. Many times you will find who names something one way because do not know what exactly is.
    There is dozens of effects, combination and cascade of them can produce compression to only a portion of the audio spectrum as also hundreds of permutations.
    If a compressor circuit is fed from a band pass filter circuit, selective sustaining could be had because you can make a compressor behave as a automatic volume control until as you say there is no strings vibrating.

    ---> http://en.wikipedia.org/wiki/Effects_unit

    If you connect a passband filter, then clip its output and apply echo to the highs and expansion to the lows of the non clipped spectrum left, it may be called wachamapitch, or whatever you want, fit all circuits in a box and comercialized as a 'unique' effect designed by you, that may end attractive or disgusting.

    This is exactly what I have in my home stereo, you will see there is no control on the frequency :
    ---> http://www.musicshed.net/equipment/processors/DBX_160X.jpg
     
    Last edited: Jun 28, 2010
  5. Riveay

    Riveay

    7
    0
    Jun 27, 2010
    Thanks a lot now I understand how it works.

    I'll leave aside the frequency control and I guess I should read more about transistors to get the attenuation.

    Last question:

    In the diagram of this circuit:

    DOD 280 compressor

    The attenuation is made by Q2?

    Thanks again.
     
  6. Externet

    Externet

    710
    145
    Aug 24, 2009
    Sorry, have no access to that link.
     
  7. Riveay

    Riveay

    7
    0
    Jun 27, 2010
    Hi,

    I have uploaded the file.:)
     

    Attached Files:

  8. Externet

    Externet

    710
    145
    Aug 24, 2009
    Sorry. Unable to open the file. A warning of corrupted file pops up on my compfuser.
     
  9. Riveay

    Riveay

    7
    0
    Jun 27, 2010
    There's and image of the diagram. :)
     

    Attached Files:

  10. Externet

    Externet

    710
    145
    Aug 24, 2009
    From what I can discern, the GAIN of the amplifier IC1a is lowered when its pin 1 output saturates Q2 with too much level on + peaks.
    The other half IC1b seems to do the same to - peaks with Q1, decreasing the gain of IC1a.
    So, both Q1 and Q2 do the same symmetrically. Low levels are amplified, up to a certain amount and high levels are less amplified
    Seems the unit turns on with the insertion of a monophonic jack input, the AC adaptor is not grounded.
     
    Last edited: Jun 29, 2010
  11. Riveay

    Riveay

    7
    0
    Jun 27, 2010
    I get it now but still I'll read more about transistors.

    Thanks for everything Externet.
     
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