Originally Posted by richart
Someone refresh my memory, please. My DVR+ shows one channel with a signal strength of 60 but a quality of 0. I cannot receive the signal. Someone had mentioned that this was an indication of something, but I can't recall what.
The signal strength is an arbitrary representation of how much coherent RF signal is present in a 6MHz channel, as compared to incoherent noise; so it bears some (unspecified) relationship to the signal-to-noise ratio. On most TVs and DVRs, a value of 60 is pretty marginal, which means the signal is pretty weak/noisy.
The signal quality is determined by how successfully the tuner is able to demodulate the 8VSB digital signal and come up with enough correct bits to reconstruct the various video and PSIP streams. At the low end (0), enough bits are incorrect that the tuner cannot sync at all and the bitstream is useless. At the high end, all the bits are correctly demodulated and the decoding of the streams can proceed without a problem. In between, errors are being detected in the bitstream; some can be corrected and some cannot; if most can be corrected, a video stream will appear normal; if most cannot be corrected, the tuner will suppress the picture entirely; in the middle ground, various visual and sound glitches will be noticable and the picture may or may not be watchable.
At a signal strength of 60, plain garden variety noise may be degrading the signal quality down to zero. At higher signal strength, the quality may still be intermittent or zero due to multipath (which complicates demodulation, since there are two or more copies of the same bits (or more correctly, symbols of 3 bits) are being received at slightly different times, and even if the tuner can discern both copies, it must be able to lock onto one or the other and ignore the interference. More recent tuners have algorithms that do a better job of this than older tuners. Or at higher signal strength, some nearby man-made noise source (like a microwave oven, a switching power supply, an ethernet cable, a computer or monitor, etc.) may throw in enough intermittent noise to prevent successful demodulation.
Incidentally, it is this last issue that has turned AM radio, once a primary source of audio entertainment, into an almost unusable medium in urban areas.
Re-orienting your antenna to get that signal strength up 10 or 20 units, or to null-out a multipath signal, may be what you need in this circumstance. In the old days of analog TV, it was easy to "see" multipath, as there were literally two images displaced slightly in the horizontal direction; no such tool with digital, you just need to rotate your antenna by fractions of a degree and see how it affects your reception.