Rather technical cable TV question

Factual Questions
1

This question requires a rather long setup/explanation, so please bear with me.

Where I live, the cable TV has two wires (coax cables), an A side and a B side. Each side has about 40 channels. So, for example, channel 2 on the A cable is KTVU, channel 2 on the B cable is CNN, 3A is KNTV, 3B is Fox News, etc. Normally, you run the two wires into an A/B switch to select which side goes to the TV.

Now, the cable company also offers digital cable, which I recently got. So now I have a set-top on, well, the top of the TV. Both the A and B cables are connected to the set-top. The channels that you tune the set-top to, however, have no notion of the A and B; the B-side channels are on the set-top’s channels 52 and up. In other words, If I tune the set-top to channel 2, I get KTVU (2A); if I tune to channel 52 I get CNN (2B). The entire lower range (channels 2 through 99 on the set-top) are clearly not digital in origin, they’re just the old analog channels that you get when you connect the A and B cables directly (this is clear because the true digital channels have a digital artifacting when changing to them, and the analog channels do not).

I’m getting closer to the question, honest. The set-top is (near as I can tell) a standard model General Instruments DCT 2244/1161. To facilitate the input of the A and B cables, a small silver box has been added to the back. This box is a “Dual A/B-RF Bypass”, seen here: http://www.gocontec.com/products/it110001.html.

Ok, almost to the question, one more key point: The silver box has inputs for the A and B cables, but only a single output, which is fed to the input of the set-top.

The question: How and where are the B side channels converted to the 50-and-up channels that the set-top tunes to?

More detail: Given that, when tuning the set-top, the 50-and-up channels are clearly analog, and that there is but one input from the silver box to the set-top, my initial assumption was that the silver box is simply converting the B input channels by just “adding 50” and then feeding them to its output. The silver box comes off of the set-top easily. So I took it off and fed its output directly to the TV. The A side comes through as expected; i.e. tuning the TV to channel 2 gets me KTVU (2A). Surprisingly, though, tuning the TV to channel 52 gets me . . . nothing, snow, empty channel.

So now I’m baffled and that’s why I came here. The silver box must be converting the B cable channels to something, right? I mean, if it just sent both the A and the B out its output, unaltered, then when I connect it directly to the TV and tune to channel 2, I would get a munged signal that was the combination of both 2A and 2B, wouldn’t I? (And so would the set-top, for that matter.) So the silver box must be doing SOMETHING, right?

But then, if the silver box is not converting the B side to channels 50-and-up, what IS it doing? And how, then, does the set-top “know” to take whatever the silver box does and put the B channels on the 50-and-up channels? I mean, the set-top has the single input. One might assume, then, that it expects that input to contain a series of analog channels and some digital ones. And on the analog side, tuning the set-top to channel 52 would cause it to simply tune in channel 52 analog. But, apparently, that’s not what’s happening because the silver box is not outputing a channel 52!

Can anyone explain this?

2

I only have a little RF experience, so this is just a WAG…

Perhaps the cable box sends a signal over the coax into the “output” connector of the silver box to control which input is selected? A small dc voltage would not interfere with the signals, and would be enough to power an internal rf switch. The box does say it is a switch, and that it is for a specific model converter, so I’m pretty confident in my explanation, if not the technical details of how it knows to switch.

If you have a volt meter, set it to DC volts (~12 volt scale) and tune to a low channel, then a high one to see if there is a difference.

MC$E

3

MC$E’s answer was what I first thought of. Heres a test to try. Connect cable B directly to the set-top, and tune to 52; do you get 2B? If so MC$E’s answer is almost certainly correct. You could also switch cables A and B, and see what happens.

4

Thanks, MC$E. I had thought of that as a possibility. But if that’s true, it means that the cable box has to somehow “know” that it’s connected to the silver box, and also be equipped to send the signal that tells it to switch. Not to mention that then the cable box has to have the capability to convert channel 2 to channel 52. Which all adds up to the conclusion that the makers of the cable box had to have planned for the possibility that it would be connected to an A/B system via the silver box. Now, if you were building a cable box, and knew that that was a possibility, why wouldn’t you make it easer on everyone and just build it with two RF inputs for the A and B sides in the first place? I will check the voltage tonight, though, to see if that’s what’s happening.

5

Ah, that’s an experiment that I hadn’t thought of trying (the first one, I mean). I’m pretty sure that I tried switching the A and B as they went into the silver box, and the result was that tuning the set-top to channel 2 now got 2B, and tuning it to channel 52 got 2A, but I’ll double-check that again tonight, too.

6

Just some educated guesses from a STB engineer:

1). The frequency lists for cable A & B are different. Cable A could have (for example) cable HRC frequencies and cable B could have cable IRC frequencies. That would put channel 2 at 54002700 hz for cable A and 55262500 hz for cable B.

2). This means the STB is doing some channel translation (taking the IRC cable and adding 50 to the displayed channel number), which isn’t difficult.

3). And since as of right now there is no national cable standard, each cable company does its own thing. And since STB maker sells a similar but not exactly duplicate STB to multiple cable companies, he/she builds it so it works for the most customers at the lowest manufacturing cost (which means 1 RF input), but with modified software to know how to handle the dual input.

7

I’m pretty sure this is not how the cable system works here. Putting the set-top aside, the standard A/B switch for analog-only service is totally dumb, it just sends either the A cable or the B cable down the output. If one side was HRC and one was IRC, it would mean that an older TV that wasn’t capable of auto-switching between the two would only be able to see one side or the other at a time, wouldn’t it? I can double-check this too, but I’m pretty sure that I have just such an older TV upstairs (no, not the one hooked to the digital box), and if I set it to HRC (I think), it gets both A and B, but if I set it to IRC, it gets nothing.

Yeah, I’m starting to believe that the most likely explanation is that the silver box just acts as a switch, and as you say, the set-top has some “knowledge” and ability such that, when you tune the set-top to channel 52, it tells the silver box to switch to B and then just sends channel 2 to the output, labeled as 52. I guess my main reason for being dubious of this is the notion (based on no facts) that there probably aren’t many cable systems that actually use the A/B system, and so there wouldn’t be much incentive for set-top manufacturers to support it in any way.

I’m still going to try the experiments tonight and report back, for anyone who wants to know.

8

Ok, MC$E got it right on the button. The set-top sends a continuous voltage out it’s RF input. Tune the set-top to 1 thru 49 and it sends 9.2 volts. Tune to 51 thru 99 and it sends 8.2 volts. Take off the silver box and connect just the A cable, and tune the set-top to channel 2 and you get KTVU. Tune it to channel 52 and you get . . . KTVU. Connect the B cable and tune to channel 2 and you get CNN, tune to 52 and you get CNN.

Now the followup question (you can probably see where I’m going): What are the chances that if I buy a DCT 2244/1161 off Ebay, and get one of the A/B bypass boxes from Contec that it would work as expected? Given the above, it seems to me that the set-top could be “hard-coded” to always send out the voltages; if the silver box isn’t there, no harm done. But then, it also has to “know” to tune to channel 2 when the user tunes to channel 52. On a cable system without the A/B nonsense, the set-top would have to tune to 52 when the user tunes to 52. So it probably does require special programming, which means buying one off Ebay wouldn’t work (at least for getting channels 52 thru 99).