Yep and it shows that at around 7.5 it's down 10db - and not symmetrical


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? The inner spectral detail is interesting, especially
since the lopsided spectrum did not occur later in the day on
different program material (see http://n2.net/k6sti/later.jpg), but
it's the spectral boundaries that tell the story.


So is the -25db roll-off by 7.5khz - this indeed looks like good
conformance to NRSC-1.


That spectrum was recorded during classical music with little
high-frequency content, hence the sloping spectrum. What's interesting
is where it suddenly vanishes at 10 kHz.

For preemphasis shape, take a look at http://n2.net/k6sti/noise.jpg .
I recorded this later on the same station during a quiet piano
passage. The spectrum beyond 3 kHz is background noise. It shows the
expected preemphasis shape right up to 10 kHz where the spectrum
suddenly vanishes.

I did discover two local signals whose spectrum did not extend to 10
kHz. Both rolled off very rapidly at 8 kHz. When I realized that both
transmitters were in Mexico, I thought I was on to something. I found
a third strong Mexican signal, but its spectrum extended right out to
10 kHz like the US stations.

Brian