[mythtv-users] Single Pass Mpeg-4 Transcoding Quality

Chris Birkinshaw chris at postbox.org.uk
Thu Dec 2 12:14:14 UTC 2004

On 29 Nov 2004, at 09:09, Lachlan McIntosh wrote:

> Yeah reducing the resolution seems to be a bit of an issue.
> I'm recording from DVB-T. So the file comes in as SDTV(?) MPEG-2 
> 720*576
> at about 15000 kps (so I get >really< large files).
> Even if you change the resolution for the transcode setting for mpeg-2
> it seems to keep the resolution of the original
> File. Is this expected behaviour? Or am I doing something wrong?
> I have tried to increase the bit rate significantly, but even at 2200
> Kbps, I still get a lot of jaggies.
> I've done some further research on OpenDivx VBR 1/2 pass and according
> to "www.spin.net.au/~imroy/content/Tips/capture.html" (which you will
> have to read from google cache by the look of it) the attached (below
> for future reference - as the original page is down) Shell scripts can
> help with MEncoder to do a 2 pass VBR encode.
> So this leads to some questions:
> Am I barking up the wrong tree here?
> Will 2 pass VBR give me the quality at the file size that I'm looking
> for?  (350Mb for a 30-40 minute show - two per cd for archive)

If you don't want to see very many MPEG compression artifacts I would 
imagine you will need to go for about 2500 kbits/s at DVB-T resolution. 
  However, I have not found recoding to MPEG4 satisfactory at any 
bitrate for either TV or films. This is not related to the bitrate 
setting of the MPEG4 coding but is caused by re-encoding an MPEG 
stream. There is just no way that with all the requantising you will 
get a decent picture, especially when you consider that it has maybe 
been recompressed several times on the way through the broadcast chain 
(sometimes upto 7 times (!!!), though at higher data rates to reduce 
recompression problems).

I was wondering what country you are in by the way - are you sure that 
your service bit rate is 15Mb/s? They range from 2Mb/s to about 6Mb/s 
for the services on the DVB-T setup here in the UK.

I do think it would be nice to have a double pass high quality encoding 
option for mythtv, maybe this would make recompressing to MPEG4 a more 
viable concept for me. It would be interesting to see if the quality 
was much better.

> Can Transcode currently resize the resolution of a file?
> Has anyone else seen the change from 16:9 to 4:3 after transcode?

Yes, I have seen this, and like you just press the aspect ratio button.


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