On 02 Oct 2007 12:33:43 GMT, John Phillips
wrote:

On 2007-10-02, Don Pearce wrote:
On Tue, 2 Oct 2007 09:51:32 +0300, "Iain Churches"
wrote:

It seems that the THD itself is not so important as the way
in which that total is made up, i.e. the relationship of
2H,3H,4H,5H etc that is important. It may be this which
gives each amp its sonic signature.

Harmonic distortion itself is really not so important for music, which
is loaded with harmonics anyway. What matters is that distortion - any
distortion, even or odd - produces intermods. These products are
non-harmonic and certainly for music with any degree of complexity
will be dissonant. If the distortion level is moderately low, the
result will just be a sort of "thickening" of the sound, which may
indeed be more interesting than the unadorned music.

I have sometimes pondered (without real cause, I admit) about whether the
"usual" IMD tests are good enough as a predictor of audible IMD effects.

Do you think the normal two-tone IMD tests (from SMPTE, DIN, IEC,
etc.) provide an adequate basis for testing something where (I postulate)
more complex IMDs may be the more audible effect?

Depends what you mean by testing. From a single harmonic distortion
measurement I can predict two, three, four tone intermodulation
performance accurately, given the right data from the harmonic
measurement.

Harmonics and intermodulation are simply the inevitable and calculable
results of a crooked transfer function. So is a normal two-tone IM
measurement sufficient? I would have to say yes, more than enough.

The published result (in % or dB) of a harmonic distortion measurement
is not adequate in itself.

d

--
Pearce Consulting
http://www.pearce.uk.com