Recording HDTV with MythTV

In a comment to the upgrade post, a reader comments:

How are you recording HD channels with Myth? Last time I checked, all the available HD tuners out there (ATI, Hauppauge, etc) only record over-the-air broadcasts.

I’m in Canada and I get my HD via a cable box, but I can’t seem to find a reasonable solution to record it with Myth. Component-video (Y Pb Pr) capture card? Too expensive. Over the air? Too unreliable. Firewire output from cable box? Not available through Rogers. Cablecard? Not supported by Rogers either.

This is a thorny enough topic that I wanted to call it out in a full post, rather than let my answer get buried in the comments.

To answer the first question: I am recording over the air (OTA). OTA recording gives you access to the full-rate video stream, whereas cable/satellite providers will often squeeze the stream into a smaller size to fit more of them into the transmission pipe. This may manifest itself in areas of “flat” color or pixellation. (When I last went to a video store to look at large high-definition sets, they had all of their 50″ and up TVs tuned to a satellite HD feed, which looked pretty bad because it was so compressed. If I were selling TVs, I’d want to have a system that showed off the full-rate picture at its best, but I digress.)

I’m fortunate to be less than three miles from San Francisco’s TV transmitter, which does make my life a bit easier. (You might be surprised at how difficult it can my life, too, since some of the channels I receive come in too strong for tuners to lock up.) I’m using two different HD tuners: the pcHDTV HD-3000 and the AirStar HD-5000. My MythTV system has three tuners because I like to be able to pad recordings out by a couple of minutes, so it’s not uncommon for me to have all three tuners going at once.

Unfortunately, this means that reliability isn’t really that big a concern for me. I have had to set up my antenna very precisely, but if I were willing to get on my roof, I think that would be less of a concern.

CableCARD. I’m not all that familiar with CableCARD, but it is not consumer friendly. According to Dave Zatz, the cable companies are the reason why there is no Tivo-to-Go on the Series 3. Microsoft did strike a deal to get Cable Labs certification for Media Center PCs, but that’s only likely to help large Media Center PC suppliers, not MythTV do-it-yourselfers.

Component capture. I don’t know of any consumer-level device for this. Over-the-air broadcasts run within the 19.2 megabit/s (2.4 megabyte/s) broadcast limit. Uncompressed, the stream takes a significantly higher amount of capacity. I did some back-of-the envelope calcuations on the stream capacity, and I came up with the following nubmers. (Note that these are all based on 60 fields per second, which is the North American standard, and that there will be additional overhead to store the container for the data frame.)

Standard Resolution bits/pixel megabytes/sec
720p 1280×720 8
1080i 1920×1080 8
1080p 1920×1080 8

I don’t know of any consumer-level storage solution that can write 50-150 megabytes per second. Maybe there is somebody working on a component-capture card that can do the MPEG-2 encoding, but I don’t know of one. MPEG is asymmetric, so encoding takes a quite bit more processing punch than decoding.

(This doesn’t even get into the DRM surrounding HD component capture, wherein some high-def devices may not supply you full resolution without implementation of a protection scheme.)

Unfortunately, there is no good answer to this question. Over-the-air broadcasts are the only way to efficiently get HD into MythTV. The EFF is on the case, though that is probably cold comfort to people outside the U.S. living with legal “innovations” like the DMCA.

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