Originally Posted by dr1394
That signal looks fine to me. How old is your receiver? Anything that's more than a few years old won't have the best demodulator chip in it.
I would have said that was pretty ugly with all that ripple compared to most signals I've seen here.
By comparison, here's a signal (KICU-DT) with much more distortion. It decodes perfectly (no errors) on a newer demodulator, but sometimes won't even lock on an older demodulator.
Looking at a spectrum analyzer doesn't show you how well the demodulator is handling the distortion. A much better indicator is the constellation diagram after the demodulator channel equalizer.
True. It is hard to tell looking at the spectrum analyzer how bad multipath is. KSBW had a 10 dB dip in the middle of their signal here when they were on channel 10 and it had no affect on reception. It's gone now that they're on 8.
So I have to ask if you know what the SNR of KICU was in that picture? No errors could be as low as 16 dB.
A nice strong signal (KNTV-DT):
A weak signal (KCNS-DT):
Although the display is different, it's similar to the PSK constellations I'm familiar with. I'm sure in your images the dots should not be crossing the lines. Do you know what the actual signal strengths and SNRs were for those two stations? I think those displays actually show SNR which could
mean KNTV was not strong but a high SNR (low multipath) and KCNS was strong but a low SNR (high multipath).
Signal strength and multipath are too interrelated to be able to look at just one instrument.
Unfortunately, although the constellation diagram output is available on demodulator chips, no receiver manufacturer exposes it.
Isn't the SNR number the same as the constellation diagram without the nice picture? IOW, if you're familiar with both, you know what the constellation diagram will look like if you have the SNR? That was my experience with PSK testing.
A lot of TVs give an SNR. I'm not so sure how many give you real signal strength. If you have both shown by a TV you should be able to deduce whether your problem is too weak of a signal or too much multipath.
My main point on the spectrum analyzer is that it is a way to check the signal strength independent of a TV. If you can eliminate signal strength as a problem then the next most likely cause of non reception is multipath.
BTW, how did you get those constellation diagrams?