[mythtv-users] How to raise priority of ticket 13459?

James Linder jam at tigger.ws
Wed Nov 13 08:42:56 UTC 2019


> Just change the name of the file to something the TV will accept and
> see what happens.
> 
> The recordings that mythbackend makes from digital TV sources (DVB-T,
> DVB-S, ATSC, ...) are MPEG2 transport stream containers, with the
> streams inside simply copies of the streams that were broadcast.  For
> a long time, the recording files were stored with a .mpg extension,
> which normally is for MPEG2 program stream files, not MPEG2 transport
> stream files.  When mythbackend finally fixed this by changing to
> using a .ts extension, the content of the files was not changed at
> all.
> 
> The difference between transport streams and program streams are not
> huge, but TVs are strange in what they do and do not accept.  It is
> possible to convert a transport stream file to a program stream file
> without transcoding the streams inside the file.
> 
> The word "transcoding" gets misused a fair bit.  It actually means
> running the data in the streams inside a multimedia file through a new
> codec and re-encoding the data using that codec.  The input stream is
> run through a decoder (from the old codec) to decompress it and
> produce the full data for each frame, using the codec for that stream.
> Then the full frame data is sent to the output codec's encoder for
> compression using that codec.  Uncompressing the input stream does not
> lose any of the data in the input stream, but when the output codec is
> the "lossy" type, the compression through that loses data that is
> present in the input data to that codec.  So the output from a lossy
> codec is *always* lower quality than its input.  So every time a
> stream is transcoded using a lossy codec, it gets degraded.  Hence it
> is never a good idea to transcode anything using a lossy codec if it
> can be avoided.  The best idea to retain as much quality as possible
> is to only ever use one lossy codec on a multimedia stream.  So if you
> want H.265 encoding of a video that you already have an H.264 version
> of, you go back to the original (hopefully uncompressed) master
> recording and compress that with H.265.  If you use the H.264 version
> as the source, the H.265 output from that will be worse than the H.264
> version, despite H.265 being a better codec.
> 
> There are other things you can do to multimedia files that are not
> "transcoding".  Video files come in many different container formats,
> such as AVI, MPEG1, MPEG2 program stream, MPEG2 transport stream, MP4,
> MKV.  Each of these container formats has different ways of storing
> the streams inside, and each container format only supports certain
> types of streams.  For example, the MPEG1 format can not contain any
> video streams other than the MPEG1 video stream format (and note that
> the same name "MPEG1" is used for the container format and the video
> stream format, but they are two completely different things).  Later
> container formats tend to support more stream formats, and MKV
> containers can support virtually any stream format.  You can convert
> the container format of a multimedia file without having to transcode
> the streams inside it.  This can involve changing the stream data to
> match the new container format, but it does not involve "transcoding"
> where the data is run through a decoder and encoder.  It just involves
> the changing format the data is presented in, not its content.  People
> tend to still call this conversion process "transcoding", when it does
> not actually involve transcoding at all.
> 
> For your particular problem with your TV, the first thing to do is to
> just change the .ts file extension and see if that is all the TV is
> having a problem with.  It often is.  If the TV can still not play the
> files, then try converting from an MPEG2 transport stream to an MPEG2
> program stream.  Only if that does not work try actual transcoding.
> Most TVs will list what they accept somewhere in their manual. Working
> out exactly what is meant by the usually sparse information provided
> can take a fair bit of detective work though.  Often TVs say something
> like "AVCHD", for example, which means that the TV will accept MPEG2
> transport streams, but with certain constraints so that only a subset
> of all MPEG2 transport stream files that match the AVCHD constraints
> will be playable.  And they are usually very picky about the file
> extensions they will accept and will not play a file with what they
> consider to be the "wrong" extension even if it is actually a format
> they can play.  This is quite unlike software video players like VLC
> and MythTV, where the content of the file is all that matters and the
> file extension is usually completely ignored.

For the aulder folk this is lovely:
https://www.youtube.com/watch?v=updE5LVe6tg

Since 'olden days' I’ve made a habit of reading vi’s man pages and learning one new thing several times a year.
Likewise I ‘knew’ most of the discussion on traanscode but still learned a few things. Thanks for posting Stephen.

James



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