Make cheap ATSC (or DVB) RF-modulators


As I continue to play with HDTV, I found I had a horrendous time getting good output from my computer running the MythTV open source PVR into my TV. DVI, the uncompressed digital standard, just wouldn't work from the video card I had to the TV. The TV has Firewire/1394, which would allow me to stream mpeg-2 to it, and that would be really great, but as yet no software supports it because few TVs have such inputs.

Here's another idea, one that reverses old thinking. The earliest VCRs did their outut on an RF modulator to channel 3, the only way to get into older TVs. Now we of course recommend non-RF methods, such as composite video, S-video or best of all component video/VGA, or in the digital realm, DVI and HDMI.

But in fact with TVs being mandated to have ATSC tuners, could it make sense to go back to RF? This gives digital decoding in the TV, in theory the highest quality if the TV has a good decoder. The cables are easy and super cheap, and carry digital video and 6 channel sound -- something even DVI doesn't do. You can run them as long as you want.

Plus, you can have tons of multiple inputs, just on different channels. Put your cable box on channel 3, your PVR on channel 4, DVD player on channel 5 and so on -- no need for the plethora of inputs and mass of cables.

(Don't get me wrong, I think a single ethernet jack would be better than any of these methods, but the TVs don't have them and they do have the tuners now.)

There is one big issue, however, which is on-screen display and comptuer generated menus. The RF sends a compressed mpeg stream. On the
surface, that's great because the boxes handling the video can be slow and
cheap -- they are just slinging bits they don't understand. But once you want
to overlay text on the video, you suddenly have to decode the stream (hard enough) and then re-encode it, which is close to impossible with today's hardware. On the other hand, it should be possible to do non-transparent overlays, where you take over a region of the screen (perhaps the bottom is easiest) and replace it with your generated text.

The ideal solution to this would be to modify the protocols to allow sending a second stream to be overlayed, with an alpha channel, on the main one. This is true no matter how you send compressed video -- RF, ethernet or firewire. However, we don't get to change the protocols, the idea here is to make use of something already out there.

Generating Mpeg from computer menu displays on the other hand is something that should be within the capabilities of today's CPUs. They can do it at smaller resolution (720x480 DVD res is fine) but more to the point unless there are fancy animations, it's static and machine generated and easy to set up for quick conversion.

There's no encryption, which might cause pressure to balk at this but it has a lot of advantages worth considering as kludges go.


It's not exactly cheap yet, but can't GNU Radio's new $450 gizmo do this? It's supposedly able to receive ATSC, and the hardware's able to transmit as well. I doubt that all the software exists for the ATSC transmit path, but it's almost certainly less complex then the ATSC reception code, because you don't need to be able to decode arbitrary transport streams.

Why would you need to decode the stream ... if you could build a hardware device that you turn vga to astc then all you would need to do is get a atsc tuner card then pleasing whatever content on the screen ... and pass it out the box to the tv where it would act like a cable box does today ...

actually I really don't think it would be hard to do. I would assume most video cards could be adapted to create a base ASTC Stream that could be directly modulated. In theory an old RF modulator would work as it uses the same type of modulation for the video. The FM audio would have to be clipped out. The video modulator would have to be re-tuned to move the pilot down to the base of the channel and allowed a full 6 Mhz bandwidth.

The real reason you will not see any is simple. ATSC is copyrighted and like a 20,000 dollar royalty is required. It is for TV station only. You see they are afraid you might do something that is legal-- record the program.

on a final note, I feel ATSC's days are limited. Reason, the audio and video codecs are proprietary and can not be changed;such as, using mpeg-4. congress did not make provision to allow the media to be updated. As all of us Computer users know, to watch a new type of video codec we only need to download and install the new codec. ATSC-OTA could have allowed just that, the codec could even float in the stream like the EPG and clock from station.

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