[mythtv] transcoder documentation.

[Geoffrey Hausheer]

Here is updated transcoder documentation to go along with the patch I
submitted (obviously it shouldn't be committed until the patch has been
blessed and applied).  I'll stick to txt format for now, but if Matt or
Robert prefer, I'll try my hand at sgml as promised.  Any comments on
readability would be useful, since I'm not exactly known for my
communication skills.


19.16 What is this transcoder thing, and how do I use it?
The transcoder re-encodes files from one mythtv format to another.  Its
main purpose is to allow users who have either hardware encoders (PVR250)
or systems that can only record in RTJpeg due to performance reasons
(multiple capture cards, slow system, etc) to create MPEG4 streams to
save space.

The transcoder can be used in two ways:
(1) Automatically re encode every file once it has completed recording
(2) After marking commercials, the transcoder can be run manually to
delete the commercials from the file (thus further saving space)

The two methods are independent.  (1) is an automatic process that can be
enabled/disabled through the setup program; (2) is a manual method which
must be invoked on each recording individually.

The second method can be used on already transcoded files (or files which
were are already in the correct format), and only the
frames immediately following a cut section will be re-encoded (thus there
is very minimal quality loss)

To enable automatic transcoding, do the following:
   start the setup program
   under the host-specific settings:
      set the Transcoder Auto-run checkbox

For either manual or automatic transcoding:
   start mythbackend
   start mythfrontend
   select setup, and the Transcoding recording profile.
      now select either RTJPEG or MPEG4 (selecting any of the hardware
      encoders will result in the transcoder not working).
      set the other parameters as you'd like.  For best performance, you
      should match the audio to the 'Default' profile.  If you plan to
      only use the manual transcoder, you can choose to set the video to
      be the same as the default profile too (as long as it is MPEG4 or
      RTJpeg)

Everything should now be setup properly.  If you elected to use the
Auto-Run feature, the transcoder will automatically launch after each
recording is complete (don't worry, it runs niced-down, so it shouldn't
impact any critical tasks, or other recordings)

If you want to manually transcode a program, simply press X while
watching a recording (you should have already finished marking all
commercials).  If you change your mind, hitting 'X' again will stop the
transcoding.

Once the transcode is complete, mythbackend will replace the old file
with the new as soon as it is no longer in use.