Michigan Telephone, VoIP and Broadband blog

It’s another in my (very) irregular series of “why hasn’t anyone made this yet?” posts…

The idea on this one is simple. Since all the television stations in the United States are now broadcasting in digital format, and since you can buy DTV converter boxes for as little as about $50, I’m assuming that decoding a DTV channel’s data stream isn’t a real big deal.  So why not take that one step further and allow that signal to be remodulated onto a different channel?  The difference between a converter box and what I have in mind is simply this:  A converter box takes a digital signal and converts it to another channel, but the output is analog.  I’m saying do the same thing, except make the output a digital signal, on a different channel than the original.

And WHY would you want to do this?  Well, for any of several reasons:

• Your antenna is at a distance from your house (or from another building that you’re feeding a signal to), and a station you want to get is way up on UHF, and coaxial cable is typically quite lossy at UHF frequencies.  So, at the bottom of your tower, you convert that channel to VHF (say channel 2, which is now unused in most areas) and now you can send it even a few hundred feet down RG-6 (or RG-11) and still have a usable signal.
• You have two adjacent channels that you want to receive that are almost equally strong, but in different directions (say for example, channel 8 from the south and channel 9 from the north).  You want to put up a separate antenna to pick up the one station that’s in a different direction from the others, but you can’t just combine the signals using a reverse splitter because on some channels the two antennas might cancel each other out.  Nor can you use a JoinTenna because they are not recommended for use on adjacent channels. So you run the one antenna in a digital demodulator-remodulator and change only the one channel you want to a low VHF channel that’s unused in your area.
• Cable channel substitution – say your cable company gives you a channel your wouldn’t be caught dead watching (the gimme-your-money religious and shopping channels come to mind) but they don’t give you a channel you want to watch that you can receive with a rooftop antenna. A deluxe model of this unit could actually do drop-in channel substitution – say you receive the over the air channel on 9 but you want to replace a station on 24, you could do that.

Give me enough time and I could probably think up a few more uses – point is, this is a DTV problem solver.

In my opinion, an excellent unit of this type Would have two coax inputs and two coax outputs:

• An input that carries the channel you want to convert (often from a “secondary” antenna)
• An input that carries the signal that you want to insert the converted channel into (the cable line or “primary” antenna feed)
• An output that carries the combination of both the second input and the converted channel from the first input
• A “passthru” output for the first input, that passes it through unchanged

The reason for the last output would be so you can “stack” boxes, in case you have multiple signals coming off that “secondary” antenna.  You’d bring that antenna into the first box, have that one convert the weakest signal, and then pass the line to box 2, and have it convert another channel.  Your primary feed (cable or main antenna) would pass through both boxes, and each box would insert a channel.

Options: Each box should have a way to do basic setup. Options that would need to be set are as follows:

• Channel you want to convert (specified by channel number or frequency)
• New output channel (also specified by channel number or frequency)
• Option – primary and channel to be converted are on same input

The last option would be used in the case where you have only one line coming in but you still want to convert a channel, say from UHF to VHF.  In this case, the converted channel would still appear at its original postion, but would also be downconverted to the lower channel and inserted back into the feed.

An inexpensive unit might only allow a limited choice of output channels, say VHF only (or even low VHF only, but please not just 3 and 4!).  A deluxe unit might allow the type of channel replacement I mentioned above (actually removing a digital channel from the primary feed and replacing it with another channel).  A VERY deluxe unit might have a network connection that would present a web interface browsable on the TV or the local network, allowing selection of video files off the local network or perhaps from online sources, and could modulate those files onto a particular local digital channel, in addition to (or instead of) remodulating an off-the-air signal.  In addition, the deluxe unit might have the ability to convert more than one channel from the second antenna or source.

But the main point is, in no way should this be an analog conversion, and absolutely no RF should be allowed to cross paths – each input and output should be totally isolated from the other inputs and outputs, except of course for the “passthru” output, which should carry ONLY the unchanged signal from the secondary antenna. When a channel is changed in frequency, it should be done by extracting the data stream from the original signal, then regenerating that digital stream and remodulating it on the conversion channel.

Finally, I’ll even throw one other idea out there, if someone wants a real challenge – have a “replacement signal” mode.  How would that work?  Well, extending the previous example, let’s say your digital channel 8-1 and your digital channel 9-1 are both CBS affiliates.  And let’s say that neither is 100% reliable.  So what you might do is, in some way indicate to the box that both digital channel 9 from the secondary antenna and digital channel 8 from the primary antenna are to be converted to digital channel 3, but you want the channel 8 signal to take precedence.  The receiver would constantly demodulate both signals, but if for any reason the channel 8 data stream is interrupted, it would smoothly switch over to the channel 9 stream until the channel 8 signal comes back to an acceptable level.  This feature, of course, would probably only work well during times when both stations are showing the same network programing, and even then might be a little disconcerting, but not nearly as annoying as having a program disappear completely for thirty seconds or a minute. And, it would seldom or never work right for the secondary channels (e.g. 8-2 and 9-2 in this case).

(I might also add that it could look at the SAME channel on the two different inputs, in case you’re trying to use “diversity reception” techniques – in essence it would then pick whichever data steam was the most usable at any given moment, and try to “fill in the blanks” from the other, converting the best data stream to the remodulated channel frequency).

Now, I know that you can buy equipment designed for use at a cable company head end that can do this sort of thing, but what home viewer has two grand to drop on something like this?  That’s why I say, make a consumer grade unit and keep the price reasonable – the lowest end units should not cost more than about $75, certainly not more than $100 (and if you charge that much, please include a fantastic tuner!).

Whoever is the first to make something like this will be hailed by consumers and TV antenna installers alike!

Of course there’s also that OTHER device that a lot of people want, but that we may never see — a device that will take the HDMI video output and digital audio output from some source, and modulate it onto a channel for local distribution. No, even that would NOT replace what I’ve described above, though, because it would likely only be able to modulate one video source per channel, whereas the over the air data streams may actually contain three or four program streams (which makes my “replacement signal” idea a lot more difficult to do properly).  But my thought is that if you’re going through the trouble and expense of remodulating a channel, you at least want ALL the programming on that channel (the entire data stream, in other words).

Originally posted on Friday, July 3, 2009 at 4:00 AM EDT.