Brian wrote:

Take a look at the screen shot again. Don't the extremely steep
spectral walls--the sudden drop of tens of dB at 10 kHz--suggest
something to you?

Yeah - typical processing -- looks like what I would expect with very
good (NRSC) processing.

Let me try and explain it this way: Notice that the signal at 7.5khz is
down roughly 6db from the lower frequency (2-3khz) material. You have to
remember, then -- that makes that 7.5khz material only roughly 1/2 the
volume of the lower frequency (2-3khz) material. The difference here may
not look like much (as it's in db) but it's only HALF as loud. And as
you've noted - it drops off (as best this equipment can do) like a brick
wall past 9.5khz.

The inner spectral detail is interesting, especially
since the lopsided spectrum did not occur later in the day on
different program material

As noted - they probably were really punching talk material. Remember -
you can go way over 100% positive modulation - all commercial
transmitters are designed to do so. You just can't go past 100% negative
(how do you get less than 0 carrier? you don't - you get clipping /
splatter). In fact - most processors won't try to push past -95% -
leaving a "just in case" margin for OOPS!). Sometimes such un-symetrical
modulation causes some strange effects in the sidebands.

The better model optimods can be pre-programmed with many presets - and
chosen as desired. It's all "demographics" now - knowing and appealing
to the (perceived) audience - so changing "the sound" during a broadcast
day is now as routine as sponsors tailoring their ads to suit drive time
commuters, then soccer moms, etc.

(see http://n2.net/k6sti/later.jpg), but
it's the spectral boundaries that tell the story.

Well - let's see -- (relative to 2.5-3khz) they're -6db at 7.5Khz;
rolling on off to about -10db by 10khz - then about another 20db by
12khz. What's your point- looks like NRSC through a pretty good
processor - roll-off pretty much what we've been talking about...

I do note the "spike" in the positive sideband at about 8khz or so - I
feel that is probably a noise spike of some sort - especially since
it's so prominent out of the surrounding "curve"; and also it has NO
correspondence in the negative sideband at all - though it's hard to
tell from a snapshot.

That spectrum was recorded during classical music with little
high-frequency content, hence the sloping spectrum.

Yeah- but even so NRSC pre-emphasis is up to 10db by 10khz - which is
still getting rolled as it approaches the "brick wall".


What's interesting is where it suddenly vanishes at 10 kHz.

Look closer - it's not absolutely a "brick wall". There is some gentler
roll-off before 9khz - then it gets serious. Again - that's what current
state of the art looks like -- consistent with the NRSC spec - and what
I've said about filters / processing.

Again - you don't seem to have a good grasp of what you're looking at in
relation to what you hear. There is no way LA stations (or any other
AM band broadcasts in the U.S.) are going to be "flat" out to 10khz (not
these
days - some (clear channels) used to). It's been noted that the better
optimods (like the 9200) can go way out filter settings (9.5khz) -
Optimods so set would be very useful for ShortWave - and places where
the FCC (or equiv. authority) aren't so strict. I wouldn't recommend
them being so set in LA.

Back to your "music" example - What are they (the station's sidebands)
AT 8-9khz compared to the average at 2-3Khz? That roll-off is the effect
of the filters/processors doing their job in forming (as best they can)
the "brick wall" you do indeed see above 9.5khz.

The picture of "noise" tells us nothing - other than 10khz is down 30db
from carrier. Were it relative to something - that would tell us much.
As it is - it just shows an unknown spectrum of ???? Put a reference in
there (or a known weighted/gated noise source such as specified by NRSC
- like a BruelKjar or equiv.) THEN you can make some solid deductions.

If the material in the 2-3Khz ranges runs at / around -15-20db then
10khz is DOWN roughly 10-15db below that - which is ONLY
(approximately) 1/4 as loud. But since we don't know what the source
material is doing - it's meaningless.

I know dealing in dbs is confusing - but if you're going to deal with
Audio/RF/Spectrum - you HAVE to learn dbs and be familiar with how they
relate to what you hear.


best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
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