[mythtv] DVB, (new) recording profiles, and transcoding

[Mike Wilcox]

Geoffrey Hausheer wrote:
> On Fri, 3 Oct 2003 09:32:22 +0100, "Edward Wildgoose
> Edward.Wildgoose-at-FRMHedge.com |mythtv/1.0-Allow|"
> <pfnahiui2x0t at sneakemail.com> said:
> 
>>For the UK at least over DVB-T (ie free to air).  The mpeg is usually in
>>720x576 with an embedded aspect ratio of 16:9 (implies that maintaining
>>embedded aspect ratio is important)
> 
> I don't really understand the aspect ratio stuff (since your capture
> resolution is still effectively 4:3), so I have no idea what is required
> to keep it.

I think the key to the difference is remembering that the pixels aren't 
always square - the rectangular shape of the pixel can vary - and the 
aspect ratio of the whole image is an indirect way of telling you what 
shape the pixels are.

The resolution just tells you how many pixels there are in the overall 
rectangle, but doesn't say anything about the size of the whole picture, 
or the relative dimensions of the sides.

If the capture is 720x576 with a 4:3 DAR (display aspect ratio), then we 
could create a 40cmx30cm image. Each pixel would be 0.55mm wide and 
0.52mm high - almost square (roughly 81:75)

Same 720x576 capture at 16:9 DAR. We could create a 53cm x 30cm image, 
and the pixels this time would be 0.74mm wide but still 0.52mm high - 
nowhere near square (roughly 7:5)

It is kind of important to keep that Aspect Ratio value...

>>Thanks Geoff.  I will have a play with the transcode stuff real soon.  If
>>I understand the discussion correctly it seems that (as a DVB viewer) I
>>just have one profile that I need to adjust, the MPEG2 option under the
>>Transcode group.  Seems that I can ignore resolution, etc and basically
>>the only configurable params are codec, bitrate and audio rate?  Is this
>>about right?
> 
> that's about it.  the 'Auto scale' switch for bitrate should be set as
> well, since you don't necessarily know what the input resolution will be.
>  I'll probably clean up the recodingprofile stuff sometime to disable
> switches that make no sense when using the transcoder.  As I said, the
> transcoder igonres resolution, settings, and just uses the resolution of
> the original recording, since we have no method of manipulating it.  And
> I'm not sure how to deal with your aspect-ratio stuff once transcoding is
> done (since I don't even understand how it works yet)

Yup. Those extra parameters that are there to be altered, but have no 
effect, all helped to confuse me :-(

Howwever, I guess if you're not changing resolution, you probably have 
no need to do anything about the aspect ratio.

If the value isn't stored in the DB or the stream for all recordings, 
then I guess the best thing to do is have a variable selection of 
settings under the W key, and let the final video player output a 
variety of scaling & zoom settings.

 From other messages, it looks like this has started to happen...

Mike