Originally Posted by deltaguy
I was under the impression that multipath occasionally actually helps reception. Is there a way to identify useful multipath with the spectrum analyzer?
Not that I know of. A possible exception that sometimes works, is to watch the trace for the best presentation of the pilot signal.
Calaveras has already shown us that you can have a good looking trace, with a flat scan and what looks like a good SNR, for a signal that doesn't decode. The analyzer trace implied a good SNR, but the tuner indicated a bad SNR.
Originally Posted by Calaveras
Even though the signals look strong enough to receive, only 2 out of 9 stations will decode.....
So, what is needed is a way to monitor signal quality, as shown by SNR and uncorrected errors at the tuner. The FEC (Forward Error Correction) is able to correct errors up to a certain limit. Once that limit is exceeded, the signal reaches the "Digital Cliff" causing picture freeze, pixilation, and finally dropout.
Many things can cause an increase in errors (Bit Error Rate/Ratio). Some will show on the scan, and some will not.
Some factors that reduce signal quality and cause a higher BER are:
1. Improper signal level: A weak signal will cause a poor SNR. A signal that is too strong can overload a tuner or preamp, causing spurious signals that raise the noise floor and wipe out the weakest signals. A nearby FM transmitter can also cause overload, which would require an FM trap. These will usually show on the analyzer trace.
2. Multipath reflections, static and dynamic. As Calaveras has demonstrated, they don't always show on the analyzer trace.
3. Power line and impulse noise in the reception area. This will show on the analyzer trace. We find this on VHF, especially VHF-Low.
Many times I have found that the antenna aim for the strongest signal, isn't the best aim for signal quality. I first learned about this with an Apex DT502 which has two signal bars, one for signal strength and one for signal quality (the inverse of BER). I first adjusted the aim for max signal strength. Then I rotated the antenna slightly, and noticed a big increase in signal quality, with only a slight loss of signal strength. This seemed to indicate a multipath problem, and
a way to deal with it.
Later, I used the Diagnostics Screen on my Sony TV that gives signal strength, Errors, and SNR. I have also used Hauppauge 850 and 950 USB tuners that give SNR and errors, but the software isn't user friendly for me. People that use Linux seem to do better with it.
It is possible to buy a signal level meter that will give readings of signal quality, like BER, SNR, MER, and even EVM and echo profile, but I would rather have the tuner tell me what signal quality is necessary since it makes the final judgement anyway.
Multipath Echo Profile:
(see Attachment 3)
Digital Modulation Bar Graphs:
This is a plot of the tap energy versus time delay from the selected channel. The channel is shown as a spike at time t=0, and is represented as 100%. If reflections (echos) of the original signal are present, it will show as another spike at some positive or negative time delay and reduced percentage from the selected channel.
This graph can assist in determining whether signal problems are being caused by multipath problems, or not. The center tap (0μs) corresponds to the main signal and any other echo is interference and noise.
(See Attachment 4)
These bar graphs display the MER, Margin, and EVM for 8VSB and QAM channels. Pass (green)/ fail (red) / warn (yellow) color coded thresholds provide for quick diagnosis. The level thresholds are user definable. This section will be grayed out when measuring an NTSC channel.
MER (Modulation Error Ratio) is the digital equivalent of an analog signal to noise ratio displayed in dB. MER is a ratio of signal power to the power of the impairments within the signal. The larger the MER value, the better the signal quality.
Margin represents how far the MER value is from the threshold of visibility (TOV) / "digital cliff". Margin equals (MER – 15.2 dB) for 8VSB.
EVM (error vector magnitude) is a % RMS value that represents the amplitude ratio of the RMS error vector amplitude to the largest symbol amplitude. This is basically how closely the modulated digital "symbols" are received compared to the theoretical ideal. EVM is calculated from the MER value. The lower the value, the better the signal quality.