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How does an RMS detector work?

Discussion in 'Electronic Basics' started by MRW, May 30, 2007.

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  1. MRW

    MRW Guest

    I'm currently delving into audio stuff. I was wondering how exactly
    does an RMS detector work. I can picture how a peak detector works,
    but if you send a speech signal to an RMS detector, how does it
    compute the RMS value?

    Also, does attack time dictate the time period for RMS calculations?
  2. It squares the signal (converting it to a unidirectional
    signal by the transformation), averages that with a low pass
    filter, and then takes the square root of the low passed
    signal. RMS means square Root of the Mean of the Square of
    a signal. Mean is a way of saying average over some time.
  3. BobG

    BobG Guest

    The RMS volts is the same as the DC volts that gives the same watts.
    In fact, the teletronix RMS limiter used an incandescent light bulb as
    the rms calculator. There is certainly a 'time constant' associated
    with it... get an old speaker voice coil, hold it between your fingers
    and run music thru it and turn the volume up and down and it gets
    hotter and cooler. The 'crest factor' is the peak to RMS ratio. I was
    surprised to see that the RMS seems to track the avg but 3dB higher
    for most of the music I've run thru my rms and avg calculator program,
    so either you can use an RC which gives a good avg and just add 3dB to
    get the rms, or my program is messed up. Anyone else want to run a
    wave file thru an rmsser and an averager and see what comes out?
  4. Charles

    Charles Guest

    One way is to dump it into a resistive load and measure the temperature rise
    of the load. By the way, equal dc and rms voltages cause the same
    temperature rise in a given load.

    Also, check this out:
  5. Phil Allison

    Phil Allison Guest


    ** Where did you come across an "rms detector" in audio gear ??

    Something made by dBx ??

    If so, their use of the term is 100% bollocks.

    ** A " true rms to DC converter " has a slow response time when used for
    audio band signals, ie 100mS at least.

    Not must use in audio, unless maybe you are trying to compute the heat
    dissipated in a speaker's voice coil.

    ........ Phil
  6. Chris

    Chris Guest

    A really great and thorough response to your question is in Analog
    Devices' "RMS-to-DC Application Guide", Second Edition. It's
    available in .pdf format at this page:,2886,773%5F866%5F15010,00.html

    The whole thing is over 10MB in .zip file format. Most of your answer
    will be in secion 1: theory:

    Take some time, and read it. You'll definitely learn the answer to
    your question, as well as a lot of other stuff. It's presented in the
    old Analogue Dialogue method, so even students and newbies can get a
    lot out of it.

    Good luck
  7. MRW

    MRW Guest

    Wow! Thanks, Chris! I've been looking for something like that. Time to
    hit the books.
  8. Phil Allison

    Phil Allison Guest


    ** Shame how it does not answer your original question.

    How about you tell us what the heck you are on about.

    Is it a " dbx " comp / limiter - or not ?

    ........ Phil
  9. Jasen

    Jasen Guest

    one way is to digitise and then arithmetically compute the RMS

    another is to use a logarythmic amplifier

    another is the feed it to a resistor and see how hot it gets.

  10. Correct. Take a MC1496 4quadrant multiplier and build your own RMS
    Take care for the crest factor!
  11. Phil Allison

    Phil Allison Guest

    ** LOL !!

    Love to feed * Jasen the Fuckwit Jerkoff * a 120 watt soldering iron
    where the sun don't shine.

    ....... Phil
  12. BobG

    BobG Guest

    So I'm starting to get the picture... the Aussies dont like the Kiwis,
    the Norskis dont like the Finns, the French dont like the Bosch, the
    Yanks dont like the Wetbacks. Thus is the wonderful peacful
    civilization of humans.
  13. MRW

    MRW Guest

    Can't we all just get along? I thought the Internet was a place of
    homogeneity, but I guess our IP addresses deceive us.
  14. me

    me Guest

    Not only off topic, but glaringly obvious throughout human history....
  15. Jasen

    Jasen Guest

  16. Marra

    Marra Guest

    0.707 of the peak value ?
  17. redbelly

    redbelly Guest

    Except during "National Brotherhood Week" . . . :)
  18. redbelly

    redbelly Guest

    It seems that standard practice is to filter out or somehow exclude
    the DC value when doing rms measurements. I.E., a DVM or
    oscilloscope set to "AC" will read Zero when measuring a constant DC

    Is there anything like a standard, agreed-upon cutoff frequency used
    by DVM's and 'scopes for AC/RMS measurements? I'd guess mainly you'd
    want it well below the lowest audio frequencies of ~20 Hz, so that
    audio and higher frequencies are included in the measurement.

  19. Phil Allison

    Phil Allison Guest

    ** ROTFL !!

    Love to feed * Jasen the Autistic Fuckwit Jerkoff * a 120 watt soldering
    where the sun don't shine.

    ....... Phil
  20. Jasen

    Jasen Guest

    yeah, it makes sense to filter out the DC when doing an AC measurement,
    and it hard not to filter out the AC when making a DC
    certainly rolloff would want to be below mains frequency.

    probably anything that's too high in frequency to read on the DC scale should
    appear as AC.

    ideally the sum of the AC and DC readings should be the RMS of the
    whole signal (assuming that the DC is advertised as mean and not RMS
    reading) , I guess that puts the cutoff frequency down below 3Hz somewhere

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