[mythtv-users] Building a library.....chews up space!!

[Andre]

On 29 Mar 2011, at 00:55, Tom Runner wrote:

> 
> Encoding h264 from an already maximally compressed mpeg2 (US HD) source isn't going to give you equivalent quality going to the same bitrate/filesize as the original recording! Thankfully most broadcast is live encoded and has bit stuffing to make up bandwidth also multi audio tracks, subtitles and other data that can usually be discarded.
> 
> I find that for mpeg2 HD broadcasts I can get equivalency at about 80% of the original recording size and acceptable to almost all at 50% size. For UK H264 HD recordings the size stays about the same, sometimes a little more.
> 
> My setup box send 1080i signal, whatever the chanel (HD or SD). I transcode to 720p the file generated by the HDPVR (SD chanel only). I get an equivalency at 50% of the size.

The comments I made don't have much relevance to HDPVR recordings, I was talking about direct recordings. Re-compressing a HDPVR recording is a really bad idea as it's already a re-compression and of an analog source. Unless the HDPVR is a really bad encoder (I don't have or want one) which i don't believe it is you will get better results by setting the HDPVR to encode to the rate and format you actually want rather than adding another encode generation.

Of course if you want 720p and your broadcaster transmits 1080i then you are at the mercy of your STB's de-interlacer or the HDPVR but I don't believe it has that option.

If you are stuck with 1080i recordings from the HDPVR then set the encode rate as high as you can and re-encode, that way you don't care about the file size as it is only temporary and that will give the best possible quality or looking from the other side, the smallest file for the same quality.


> Now, for sure the transcode takes forever, an average of 8-10 hours per movie.

Yup, I just re-encoded a 3hr HD recording of the Australian GP, that took all night on a 2.8Ghz i7.

> Reason for that, I do not have the space to put extra drives close to the TV. NAS over a homeplug network is not very efficient.
> 
> 
> What is worth a go is pure remuxing, no image quality is lost, just those padding bytes, teletext, eit, subtitles and a lot of timing packets that are needed for live broadcast but not for file playback (if you stream they are re-created anyway), you can save 30% by remuxing to a simpler ts file or a little more if you go to mkv or mp4, no nuance of the video is lost as you never alter the encoded video.
>  
> Which tool are you using to do so. HDPVR generated files are not so much welcomed by most software. But I'm curious to see whether I can gain something by remuxing them.

HDPVR recordings are totally different to off air recordings, effectively they are already muxed more efficiently with no extraneous stuff. 

For anyone else who finds this I've been using tsmuxer most of the time, it's a cost free Linux/windows/OSX download but not open source, it's biggest gotcha is it needs files named *.ts or it doesn't parse properly, a hangover from it's Windows port.

Otherwise you can use ffmpeg with -vcodec copy -acodec copy and output to .mp4 .mkv or .mpg

Either way be prepared to fiddle with audio sync offsets :-(

Andre