"Paul Guy" wrote in message
news
On Thu, 19 Aug 2004 11:03:32 -0400, "Arny Krueger"
wrote:
.....some stuff deleted.....
I'm
sure Arny has the equipment more readily available - and may even
have .wav files for your listening pleasure, so you can hear for
yourself what the effects are.
Slightly different context, but its all FM:
http://www.pcabx.com/technical/jitter_power/index.htm
What would be really nice, is to frequency shift
a chunk of music with different delta-freq, and different modulation
frequencies, i.e., varying M with different conditions. Multi-tone
and real music should the preferred way to check this out.
I tried listening to your jitter samples in a less than optimum
environment. You have some castanet samples castanets-060.wav
(unjittered) and castanets_060_jit-20FF2.wav (-20 db 60 Hz jitter). I
can barely tell them apart. To my ears, the jitter version is slightly
duller, but the difference is so tiny, I could easily be fooled. All
the other samples are far too similiar to the reference.
I agree. The actual amounts of FM distoriton seemed high, but the audible
effects seemed to be pretty innocious.
It actually was quite a bit of work to prepare those samples, and I never
returned to the situation.
Your piano selections (piano1_1644.wav [unjittered] and
piano1_1644_-20FF2.wav [-20db 60 Hz jitter])are indistinguishable to
me. I noticed that they are both distorted somewhat, nowhere as nice
as your reference piano_nlref.wav file.
Ironically, those samples were taken from a ADC/DAC vendor's site. At this
time I have gigabytes of better-sounding piano samples at my disposal.
Either my ears are totally wrecked (not likely), but the jitter
(FM) page you have really makes the case that it is not a very big
deal. From your spectral analysis, most of the crud is very close to
the fundamentals, and as such will be largely masked.
Something like that.
Have you
synthesized higher or lower frequency jitter components to see their
audibility?
I picked 60 Hz because my PCAVTech work suggested that this was a very
common, perhaps the most common jitter freqeuency.
I know know quite a bit more about the psychoacoustics of FM distortion, and
were I to revisit the topic I would shift the jitter frequency down.
In rough terms FM distortion is most audible for low modulating frequencies,
kind of plateaus from 1 to 5 Hz, and then falls off at about 6 dB/ocatave.
This is rough paraphrase of Zwicker and Fastl's comments in the matter. It
also agrees with the design of the old NAB wow and flutter weighting curve.
What is the prevaling opinion about the jitter (or FM "distortion")
samples you put on your site?
Nobody hears nuttin even though the amounts of jitter are vastly in excess
of what one sees in digital gear, even the crap.
From my own testing, the sidebands need to be more like -10db (or
-10 db jitter as you specify it) before they begin to be audible.
That's pretty disgusting! 30% crud!
That would depend on modulating frequency, of course.
Masking theory does confirm what
my ears tell me, namely that junk very close to the fundamental is
very well masked i.e., inaudible.
There is actually a separate case for low frequency modulation. Zwicker and
Fastl mention both, but in different places, as I recall.
It interesting that conventional
spectrum analyzers have the same difficulty.
1 million point FFTs don't have similar difficulties, to say the least!
The ear does have much of
the behaviour of a poor dynamic range (30db) spectrum analyzer, with
strange post processing and AGC.
Agreed.
Readers of this newsgroup would be well advised to read up about the
ear (especially the cochlea) to understand masking and other
mechanisms the ears uses as "garbage cleanup".
Agreed. I have often decried the fact that EEs & studio workers aren't
routinely taught much about psychoacoustics, or other forms of perception.