PipeChat Digest #3157 - Wednesday, October 2, 2002
Re: Intermodulation Distortion
  by "Peter Harrison" <peter@phmusic.co.uk>

(back) Subject: Re: Intermodulation Distortion From: "Peter Harrison" <peter@phmusic.co.uk> Date: Wed, 2 Oct 2002 08:45:06 +0100     ||| Intermodulation Distortion was very much in evidence in the ||| Casavant at the Catholic Cathedral In Erie, PA when I heard and ||| played it, and I discussed this with a piano tuner who displayed it ||| quite vividly in a Steinway grand as well... ||| -- ||| noel jones, aago   The conventional meaning in audio engineering for the term Intermodulation Distortion (IM) is the generation of additional signal components which = are the sum and difference of the frequencies of the original components. For example a pure wave of 440 Hz and another pure wave of 600 Hz should = remain just that - two pure waves. Imperfect, non linear systems will cause a = small amount of signal at 600+440 =3D 1040 Hz and some more at 600-440 =3D 160 = Hz.   In loudspeakers and audio amplifies, the most unsubtle IM distortion often arises when a system is required to handle a powerful low frequency signal and a much gentler high frequency component. In subjective terms, the delicate high ceases to be entirely smooth and becomes modulated by the = low.   In truly linear systems, this intermodulation does not happen however, = even when no audio electronics are involved, everything we hear does pass = through a highly non-linear system - the human ear! Organ builders short of either budget or space have often been thankful of this because if we translate = the example above down just over an octave we come back to that old topic of debate: "are the tones created by resultants real". The answer to this is "yes", in our non-linear heads but "no" when measured with wave anaylsis systems.   IM products can be heard by playing any combination notes, whether on a piano or an organ but this is just part of the sound of that instrument. = We expect to hear these interactions as it is the way we percieve that instrument to sound when those notes are played. Organs can be prone to a very specific form of intermodulation distortion which is related to wind supply. If the wind supply to mid and upper range pipes is totally = isolated from that to lower pipes, the only intermodulation will be the acoustic products described above. If the wind suply to the mid/uper pipes is an = any way influenced by what is happening with more wind-hungry lower pipes, two fresh sources of intermodulation can occur. The first is where the actual note of the low pipe, modulates the wind supply to the mid/upper pipes. = The second is the familiar issue of unsteady wind where it is not modulation = at note frequency but simply the quantity of wind being demanded by the lower pipe that affects the mid/upper pipes.   In the context of the Casavant in Erie PA mentioned by Noel Jones, I can only assume he is describing one or other of these winding issues.     Peter   Peter M Harrison P H M : 48 Moorfield : Edgworth Bolton : Lancs : BL7 0DH : GB fax: +44 (0)1204 853445 : tel: +44 (0)1204 853310 web: www.phmusic.co.uk