Difference between revisions of "Configuring HDTV"

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=== DVB-T For Terrestrial (Antenna) ===
 
=== DVB-T For Terrestrial (Antenna) ===

Revision as of 20:20, 2 January 2007

HDTV Help Pages

Setting up MythTV to fully use HDTV can be rather complicated. There are different HDTV formats for North America and Europe. Parts of this information may apply to other countries that use HDTV as well.

Understanding HDTV

HDTV (High Definition Television) is sent as stream of binary data encoded in the radio spectrum. A digital tuner works much like a modem extracting the original digital data. The digital TV data is sent as MPEG-2, so if you can get the raw data, the compression is already done for you.

HDTV comes in three flavors: Broadcast, Cable, and Satellite. The encoding for each is different, sometimes requiring different hardware.

North America

Antenna HDTV broadcast

This is called OTA (Over-The-Air) ATSC. You can get a tuner card that will extract the raw transport stream for a given channel. Antenna Broadcasters sometimes include multiple subchannels, all of which are part of the same transport stream. In some areas, subchannels include encrypted pay channels.

HDTV on Cable

Its standard is called QAM ATSC. Many of the OTA Tuner cards like AirStar HD-5000 or PcHDTV work with Clear (unencrypted) QAM. It just takes a little bit more time and energy getting it to work.

HDTV and Satellite TV

All channels off of Satellite are digital but not all of them are in HDTV Format. Most are Encrypted but not all of them (For example LyngSat Free OTA).

Other Methods

There are other Methods of grabbing data from the above Sources. Fortunately, for cable and satellite some of the converter boxes include a firewire port where the raw MPEG data is sent. In theory, MythTV can record unencrypted channels from this port. At this time, this is something that a few people have experimented with, but it is not yet ready for general use.

Europe

Europe uses DVB (Digital Video Broadcasting) for HDTV and digital standard definition video.

DVB-C For Cable

Cards Exist

DVB-T For Terrestrial (Antenna)

Cards Exists

DVB-S For Satellite

Cards Exists

Recording HDTV

As discussed above, OTA ATSC and Clear QAM ATSC HDTV can be recorded at this time.

Hardware

Several cards are now available for HD capture in Linux. The pcHDTV HD-2000, pcHDTV HD-3000, pcHDTV HD-5500, Air2PC, Technisat AirStar HD-5000, AVerTV HD A180, and ATI HDTV Wonder are just a few that have Linux drivers available. There is little reason to worry about the differences between the HD-2000, HD-3000 and HD-5500 cards, except that the HD-5500 is being sold, while the HD-2000, HD-3000 and the Air2PC have been discontinued.

Here is a listing of ATSC cards supported in Linux and their capabilities: http://linuxtv.org/wiki/index.php/ATSC_cards.

The CPU requirements are quite minimal for recording because it's simply a matter of copying the transport stream from the card to the hard drive. I believe that people have reported success with less than 1GHz processors. While Playback is a completely different Story.

Another popular option is FireWire capture of digital streams directly from a cable box. As with the ATSC tuners mentioned above, the CPU requirements for this capture method are minimal, as the backend machine is simply copying an MPEG video stream to disk.

Drivers

The drivers are a bit temperamental; it can take some effort to get them to work correctly.

FIXME: Explain the different drivers, kernel versions, and such.

Playing HDTV

Hardware

Most people use nVidia cards to play HDTV, although ATI cards are capable as well. The minimum recommended nVidia card is usually the FX 5200 although it has some odd display issues when playing video at 1920x1080(i or p) via DVI even when not using XvMC. Interestingly, underscanning by 3% in the horizontal direction within MythTV solves the FX 5200s problems. The GeForce 6200 does not exhibit these problems and since there are now fanless GeForce 6200s on the market for well under $50, it would probably be wise to purchase one of them if you are going to buy an nVidia card for a HD-HTPC. The minimum recommended ATI card is the Radeon 9600, which is also fanless, found for about $60. You should have at least 128 MB of video RAM. If you have an integrated graphics chip on your mobo, make sure to dedicate 128 MB of RAM to video, if possible. This will likely decrease RAM available for the OS. Even if you are going to output an HD stream to an SD TV, over S-video or composite, you will still need the minimum recommended cards, as the full video stream still needs to be processed.

The CPU requirements for playing HDTV are significant. While XvMC support (nVidia only) offloads some of the work to the video card, most people have reported better results with just using Xv acceleration.

The actual minimum requirements are hard to pin down, as people have reported conflicting results. Relevant factors may include the bus speed, memory speed, CPU speed, kernel version, Linux distribution, and compiler version and optimizations. Compiling X and MythTV with a recent gcc and optimized for your exact CPU may provide an important boost, so you'll hear some people talk about the importance of running Gentoo Linux for the extra performance. Some recent experimentation seems to indicate that the most common hardware bottleneck in P4-based frontend systems is memory bandwidth.

I have it working with an AMD 2500+ CPU and Gentoo. If you are using an ATI card, use at least a Pentium 4 2.8 or equivalent.

Software

If you have an nVidia video card, you may want to try XvMC along with nVidia's proprietary drivers. If you have an ATI card, be sure to use their proprietary "fglrx" drivers, and XvMC will not be available.

FIXME: Discuss setting up MythTV, including adjusting the minimum signal strength.

Transcoding HDTV

HDTV recordings typically consume slightly under 9GB per hour. Unless you have tons of storage, this will restrict you to a fairly small number of recordings unless you transcode.

As of 0.19 a "lossless" mpeg2 transcode has been implemented. This method is used for the default transcode. This method will allow you to cut commercials without losing any of the HDTV quality as only the frames necesary to be reencoded to maintain proper playback will be encoded. For commercial cutting these frames are usually all black, with a possible station logo, and so no quality loss should be seen at all. This transcode also has the added benefit of audio sync correction similar to what ProjectX does, as well as converting the MPEG2 TS stream to a PS stream with up to a possible 20% savings in file size on top of any cuts made.

FIXME: I haven't figured out how to do this yet. The editor function works to set a cutlist, but there is no option to set up a transcoding profile specifically for HDTV recordings. I suspect that further development effort is required before MythTV can transcode HDTV. Several people have reported on the mailing lists methods of using external programs for transcoding.

mencoder works well for xvid encoding. I've got a script that will take any 30min or 1hr show and convert it to what "the scene" terms an HR HDTV encode. It automatically uses the correct IVTC/frame removal filters and properly sizes the output files which I can post here if there is interest. I'm also thinking of looking into a modification to the script that would convert the file using the same input Q for output encoding with the IVTC/frame removal filters which should result in a good deal of space savings while maintaining the proper HDTV quality.--Steve Adeff 17:29, 17 May 2006 (UTC)
Steve, I think that there's lots of people out here that would be interested in that script. Would you mind posting it somewhere?

FIXME: There should be a discussion of the alternatives for keeping the original sound stream. How much space does a full surround sound stream take per hour? If you don't have a surround sound setup, you might as well convert to stereo; is there a way to preserve the original stereo stream only?

AC3 for HDTV is usually 384Kb/s and for a 1 hour show (approx 44mins) comes out to something along the lines of 120MB. This can be cut further down in transcoding but the space savings is ill worth the time and possible quality loss.--Steve Adeff 17:29, 17 May 2006 (UTC)