In article ,
Fred Fred@once wrote:

"Richard D Pierce" wrote in message
...
In article ,
Bob-Stanton wrote:

No it does not. THD gives the sum of ALL non-fundamental
harmonic components, 2dn, 3rd, 4th, and so on. And, when
measured by most analog THD meters, it's actually the sum of ALL
non-fundamental components, including noise, power supply hum,
RFI and so forth.

True, but that is a limitation of the measuring device rather than a proper
definition of THD.

That may be, but until recently ALL THD measurements were THD+N
as a matter of course.

Granted, there are a couple of computer-based measurement system
that sum only the 2nd and 3rd harmonics, but that's NOT what THD
is.

True, and some FFT systems that DO sum all in band harmonics without the
noise.
If you measure ALL other components and subtract the fundamental, then you
are making a S/N+D measurement surely.

Yes, true, and that's what traditional analog-based THD
measurement systems did as a matter of course (common examples
being the venerable HP330, HP334, ST1700, AP System One, and
similar devices by Marconi and the like. The operation was
essentially as described, but in a slightly different order: a
single sine tone was applied to the DUT, then the output was put
through a high-Q notch filter tuned to the frequency of the sine
tone. Whatever was left over was considered distortion.

Now, it may not be a theoretically precise definition of THD, it
is, however, the de facto definition.
--
| Dick Pierce |
| Professional Audio Development |
| 1-781/826-4953 Voice and FAX |
| |