Well, its all relative...based on the SNR, I wouldn't call 20 dB for a station 70 miles away poor, especially if it is reliable and stable over time. The fuzzier the constellation is, the more error correction is helping to recover any bits received in error in it's travels over the air interface...
It is very cool to be able to rotate my antenna and see the impact on the received constellation graphically in real time...
For 8VSB, there isn't much need for every signal to be in the 30 dB SNR range where the user is... The way I understand it, if the SNR was measured right there at the transmitter, it would likely be in the mid 30s dB tops. If not, it could be a sign of something unhealthy going on, possibly even resulting in some near out of spec skirts in it's output spectrum.
While reviewing the driver source code, I was also able to find clues in the source code as to when the demodulator sees multipath, so I have altered code a bit further to indicate that in a kernel debug message that ya can see in kernel logs (not passed to user land for anything).. Kinda cool too, but without access to real chipset documentation or test equipment, I have no way to confirm. Just reading the original driver author's comments in the code, and taking a guess as to what he/they meant at the time they wrote it.
The cable signal probably wasn't the best signal available. I see many other QAM channels in the 38 to 39 dB range. I don't normally look at CATV signals,
but I did want to test it to make sure it was able to display the 256 QAM constellation too.
IMO for a wired cable signal that you are paying for, there shouldn't be much variation from one channel to the next. I haven't found a cable signal that was 'marginal' reception wise and probably won't.
At any rate it's not to be confused for real test equipment which none of us will ever have at our disposal, but is not bad for a $70 USB tuner.
All in all, I am really liking this tuner...
BTW, updateDVB is the same application that I use for looking at satellite signals (as mentioned earlier in this thread), and none of the constellation plot capability required any modification to the application side. I only modified the kernel module for the device, and the application worked straight away.
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