Originally Posted by mabuttra
The PAL prefers your CBS station simply because you entered a zip code that matches the DMA of the TVGOS data that is transmitted by that station. When you enter a Baltimore zip code, it will then switch to the Baltimore station for TVGOS data because it matches its DMA.
More boring info about how TVGOS works...
When a TVGOS device searches for TVGOS data it will set the clock with the first station it finds that is sending TVGOS data. Then it will wait for a zip code packet from the station. If the zip code packet matches the zip code you entered, it stays put, but if it doesn't match, it then continues searching for TVGOS data. It will stop at the next station sending out TVGOS data, set the clock, and then wait for a zip code packet. This process repeats until it finds TVGOS data that matches the zip code you entered. Note that it is possible even with a Baltimore zip code, that it will find WUSA first, and set the clock wrong, but after several minutes, it should find channel 13, and then the clock will be corrected.
My CM-7000PAL never seems to look solely at the zip code setting to decide what CBS affiliate to use for TVGOS data.
I had repeatedly tried switching to a Baltimore zip code, but my clock skew (AKA error) never improved. Then, since the DVR doesn't explicitly announce which station it's using for TVGOS, I tried to figure this out by deleting from the channel list the local CBS station and/or the more remote Baltimore CBS station. Whether my zip code was set to Wash DC or Baltimore, as long as the local CBS station (WUSA Ch 9) was in the listing, my clock was inaccurate. And again regardless of zip code, if I deleted WUSA, the clock became perfect. If I deleted BOTH of the CBS stations, the clock was again perfect, and it no longer said it was using TVGOS for the clock (as expected), so I now know that PSIP must be alive and well here in Maryland.
So the question was, how to get the DVR is use the Baltimore station for TVGOS, while at the same time have the local CBS station in the channel list for recording (the two CBS stations don't always broadcast the same material). Just changing the zip code did NOT accomplish this.
It was recently mentioned that signal strength was a possible factor in determining which CBS station is used for TVGOS. So yesterday I tried a test by fiddling with our antenna rotation, looking for a compromise bearing between Wash DC, Baltimore, and Annapolis (the latter is a whole other story, but there's a major PBS station there, one that's desirable to watch). At the same time, I set our zip code to Baltimore. I found a solution by having the antenna pointing more or less between the two remote cities. And in so doing, it's now receiving all our local stations (including WUSA) off its back side. Though the antenna has a very good front-to-back ratio, evidently inverse square law works and now the Baltimore stations are about as strong as the ones in Wash DC. IOW, the Baltimore stations are stronger, the Wash DC ones are weaker, but now they both are approaching a mean strength of around 90 on the signal strength bar graph.
The Baltimore CBS station is now usually stronger than the one in Wash DC, and with the Baltimore zip code being used, all of a sudden the clock shifted to being perfect. Since the zip code change never did a thing previously, this recent "fix" appears to have come from juggling the signal strengths. BTW, I should have said earlier that in all cases the DVR indicates it's using TVGOS for the clock setting (the only time this was not the case was the short interval earlier on when I deleted both CBS stations).
Now I'm watching the clock very closely. My concern is that these two CBS stations now have VERY similar signal strengths. So what will happen if propagation or day/night variation, rain, or who knows what causes the Wash DC station to become stronger? Will the CM-7000PAL sometimes switch it preference to the Wash DC CBS station which gives lousy clock data (regardless of zip code)? I'm now waiting to see how this turns out, but right now all is well here!