I purchased my DVR-633H in January and have had the problems described here roughly every 3 to 4 weeks since. I just decided to browse this forum to see if I was unique... sadly I'm not. But after spending a few hours reading this thread, I note a few common questions that I may be able to help with. I have been a chief engineer for a PBS station for 25 years, so that puts me in a position to answer those questions. This is my first post to AVS Forum, so bare with me.
First, some history. Many years ago PBS recognized the value of the largely unused VBI (Vertical Blanking Interval). PBS can be received by more Americans than any other network. And the technology already existed to deliver low bandwidth data via the VBI. And as others have mentioned, PBS stations can always use some extra money! So... PBS created an organization known as National Datacast Inc (NDI) to market this bandwidth to organizations interested in a national distribution system. Individual PBS stations signed a contract with NDI making a certain number of VBI lines available to this project. In turn, NDI distributed the profits of this venture to them. In some markets, there is more than one PBS station widely available. In such cases, you can't make the assumption that NDI has a contract with each of these stations. And in any event, signing the contract was a voluntary act upon the part of the station. In a few cases, for reasons of their own, stations didn't choose to participate.
The first customer of interest here was Starsight. They provided an EPG to users who purchased hardware that supported it. The business model was based upon users paying for a subscription. For whatever reasons, this venture never became popular with the public (or equipment vendors). Meanwhile, there was a competing product, Guide+, that NEVER had anything to do with PBS. BUt it resulted in much confusion with the public, retailers, cable companies, etc.
Then TVGuide/Gemstar purchased Starsight... and basically killed it, thus ending the confusion between the competing products. And they contracted with NDI to deliver the TVGuide product. Unlike Starsight, the business model was based upon license fees from equipment vendors and as "free" to the consumers (if you don't count the increased cost of your DVR, etc). Also unlike Starsight, Gemstar contracted with other stations in addition to PBS. In my community, they consider PBS to be the "primary" provider, and the local ABC affiliate is the secondary provider. I have no idea if anything in the data makes this destinction, but I suspect it does.
On to how it is implemented. PBS/NDI's only involvement is in the administration of the contract, arranging for equipment to be sent to the PBS stations, etc. NO EPG DATA comes from PBS. Each PBS station that chooses to be part of this endeavor receives the equipment necessary to provide the service. A phone line is attached to the equipment and each day it gets a call from Gemstar and the EPG data is delivered. The box has a video input jack that the broadcast station is SUPPOSED to connect to a receiver (demodulator) output. That allows the box to monitor what is being broadcast, and let the provider (Gemstar these days) know if there is a problem. In short... the signal IS being monitored 24/7. I have personally received a call from NDI (who got a call from Gemstar) asking if we had a problem. "My" Gemstar box has worked without any issues. I think MAYBE there was ONE time when I had to reboot it. Note that there is no way for Gemstar to know if the broadcaster connected an off-air video signal... or just a sample of video taken PRIOR to delivery to the transmitter. In the latter case, Gemstar wouldn't know if the transmitter was off the air or if there was some other difficulty that prevented it being broadcast.
One final issue... time signals. Many years ago, PBS worked out a deal with an organization (I believe it was the Consumer Electronics Association) to broadcast a time-of-day signal. Basically they paid for the hardware to be installed at PBS stations which agreed as a public service. It is the responsibility of those stations to make sure the time is accurate. The equipment includes the ability to synchronize its time with time sources commonly available in broadcast stations, so it shouldn't be a problem. Because the time-of-day signal uses a protocol quite similar to closed captions (and is in similar area of VBI), if you can receive reliable closed captions, you should almost certainly receive the time-of-day signal. This is SEPERATE from the TVGuide data. I have no idea if there is also time data sent along with the TVGuide data... if so, the PBS stations have no control over that.
On to possible transmission problems. One... the transmitter can be off the air for various reasons... including LONG power outages (over a day) to mountain top sites. If Gemstar has a secondary source in your market, in theory, you should still get your EPG.
Another issue is that a growing number of cable companies are using DTV receivers to receive the station's digital signal, then feeding that to an analog modulator to feed their analog customers. The upside is the customers may get better picture and sound quality. The downside is that some auxiliary services such as SAP and TVGuide get lost. In my market, as soon as the cable company figured out this problem, they decided to go back to using an analog receiver for ONE of the two TVGuide sources. The upside is that their customers still get TVGuide. The downside... there is no longer redundancy.
IF a cable company is using an OLD DTV receiver (or mis-configured) to receive a DTV broadcast and converts it to analog for their viewers, BOTH the captions and the time-of-day data may be lost. This is because early DTV receivers didn't provide ANY VBI data on their composite video outputs. By law, they now have to.
There is NO provision in the DTV standards for passing VBI data other than closed captions, so it is IMPOSSIBLE for a broadcaster or DTV receiver manufacturer to pass TVGuide data for cable customers in the scenario described above. There are some convoluted technical solutions to that, but they are expensive. I don't know if those solutions have been implemented anywhere.
I hope this long message provides some clarity on some of the issues raised in this forum.