SSB-AM


    Dec 15 2008 | 9:28 pm
    Hi,
    just been looking at the [freqshift~] object...could this be implemented as a patch (as in how to cancelling out/filtering the sidebands)?
    before anyone says it, i am aware the existence of the object negates any need to do this, it's more a synthesis/dsp knowledge-quest
    thanks...

    • Dec 16 2008 | 5:29 pm
      On 15 Dec 2008, at 22:28, robin foster wrote:
      >
      > Hi,
      >
      > just been looking at the [freqshift~] object...could this be
      > implemented as a patch (as in how to cancelling out/filtering the
      > sidebands)?
      yes, this can (and was) implemented as a patch.
      i'd say it consists of two parts: a 90 degree shift and a complex
      modulation.
      the hilbert~ object does the shift. but since this is the most (and
      most interesting) part of the work, you'd have to investigate in this
      direction.
      check out the hilbert~ help file for a freqshift example.
      why this works for canceling the upper or lower sidebands is last but
      not least a 'simple' trigonometry question.
      the hilbert~ transformer produces two signals that are 90 degree
      apart and the complex ringmod adds another 90 degree shift between
      the upper and the lower sidebands. if you add or subtract the two
      modulations, one of the sideband parts will cancel out.
      the 90 degree shift can be achieved by chaining a set of allpass
      filters.
      but finding the right coeffs is something you'd probably not do
      yourself.
      the difficult part here is to achieve a constant shift for a wide
      frequency range.
      long before the freqshift~ and the hilbert~ object, there existed a
      patch from the old ircam days.
      don't know who did it, maybe miller puckette himself.
      hth,
      volker
    • Dec 16 2008 | 5:56 pm
      thanks, there's loads for me to investigate further here, volker.