On 02 Oct 2007 13:16:53 GMT, John Phillips
wrote:

On 2007-10-02, Don Pearce wrote:
On 02 Oct 2007 12:33:43 GMT, John Phillips
wrote:
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.

Yes - that was a concern I had and FWIW I think you must be right.

I was also concerned over checking properly the ability of a system to
accurately reproduce low-level signals in the presence of high-level
signals and whether this result could be deduced from simple two-tone
IMD tests (but with full graphic results - not single summed figures).

That also comes out of the basic linearity test. Any system that can't
do that will reveal it in an IM test.

Some systems do this deliberately of course. MP3 codecs work this way
using the phenomenon called masking. Even moderately large signals
close in frequency to very large ones can be deleted inaudibly.

d

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