Charles -

What they meant above 1KHz was the generator had to apply pre-
emphasis, or all the harmonics would be rolled off by the tuners de-
emphasis. Most test gear did not have pre-emphasis built in,
including the ST-1000A. I just bought a nice Meguro that does though,
and it is handy to have. The sidestep of course for mono is to use
the detector output, which will give an honest answer in mono at
frequencies above 1kHz.
I believe this is the reason 400Hz was an option on the ST1000A, the
2nd and 3rd harmonic would have very little attention.

The harmonic distortion again is certainly available up to 10kHz in
mono, again from the detector output, as there is no 19kHz pilot or L-
R signal from 23 to 53 kHz to interfere. But stereo is another
matter.
Bob



--- In FMtuners@..., "charlesppeterson" <charlesp@d...>
wrote:

--- In FMtuners@..., "pzwong" <pzwong@y...> wrote:

My interest is actually in sorting out what the so-called THD

data

meant in the test reports or manufacturer spec sheets. My guess,

and

only a guess, is that they actually all measured it the same way

you

did: set the THD+N. If they were looking at "harmonic distortion"
only, then nobody would see much at high frequency because of the

19

kHz filter. In reality, every THD vd f graph shows the number

rising

at higher f. I had always in my mind ignored the H and took that

to

mean the "total distortion," and not necessarily harmonics at 2f,
3f,... etc.

[...]

You who know more about this than I, please correct my mythology.

Until around 1970 or so, the equipment for measuring individual
harmonics would have been expensive laboratory stuff. I happen to
have one of those antiques, a Genrad 1900-A wave analyzer.
I hesitate to guess what this would cost to build today (of course,
we have different methods now, but high resolution spectrum

analyzers

are still expensive) since it makes a 10B look like a nice remote
control.

And measuring harmonics with such a thing is fairly labor intensive.
Well, it's easier if you have the matching chart recorder (which
I don't...) but still slower and more labor intensive than good old
THD+N meters, and didn't they call those "Distortion Meters" back
then?

Of course, the manufacturers, in there ever greater quest for
specsmanship could always ship a choice unit out to a laboratory
to get the better numbers, if they didn't already have good enough
equipment in house (as a few certainly did, and a some others
certainly did not).

But I don't think this sort of thing was done by magazines or most
technicians. They just had thar "distortion meters" and that was
that.

I have one actual fact now, I'm quoting from the IHFM-T-100, adopted
in 1958 (later superceded, I'll dig up the later one later), here's
what it has to say on our subject. Remember that it's talking

about

mono FM only:

".......This equipment may measure each frequency component
individually or may measure all frequency components
collectively......."

I suspect this is saying that either a THD or THD+N measurement was
OK, just call it "distortion." But it does say later:

"In making distortion tests at the higher frequencies, special
apparatus and special test methods (such as simultaneous

application

of two modulating tones) are required."

Then it says somewhat cryptically:

"Harmonic distortion measurements are useful and significant up to
mudulation frequencies of approximately 1000 cps. At higher
frequencies the de-emphasis characteristic of the tuner will
attenuate
the higher distortion products severely and give rise to

considerable

errors in measurement."

Funny, because I think people have done such measurements...in

fact,

Greg just did them. Maybe what the IHF, an industry trade group,
didn't like were the high numbers they were reading. But anyway,

it

later says:

"...Here the distion product of most interest is the difference

tone

obtained when the carrier is modulated by two audio frequencies
differing by less than 500cps.

It later proscribes using for the IM test 15,000 and 14,600 Hz at
identical frequencies at 100% modulation and measuring the 400Hz
referenced to 400 Hz at 100% modulation.

IM is a good test, but it still is ignoring all the spurs and
garbage of the cheap bad old THD+N test, which aint so bad after

all,

except for specmanship.

Charles