[mythtv-users] Transcoding and deinterlacing, and other transcode issues

[Steven Adeff]

On 6/15/06, Niels Dybdahl <Niels at dybdahl.dk> wrote:
> > > >        a) Transocde 1080i to 720 lines, for people with 720 line TVs
> > > >        for whom storing the extra megapixel is of limited value.
> > > >            Goal: Reduce size, but lose no quality you would see on
> > > >            your TV.   De-interlace the 1080i of course before
> > > >            resizing.
> > >
> > >
> > > How do you save size with that? 1080x1920x25 is 51.84 Megapixel/second
> while
> > > 720x1280x50 is 46.08 Megapixel/second. There is hardly any difference
> > > between those numbers.
> >
> > Well, my understanding is that since compressors all rely heavily on
> > elimintating redundant information frame-to-frame, frame rate increases
> > do not add to the byte count as much as raw pixel resoution increases.
> > (Size does not increase linearly with pixels or frame rate, but it
> > increases more with pixels than it does with frame-rate.)
>
>
> But 1080i contains two fields which are from different points in time, so
> you have either (1080i) 50 pictures/second of 1920x540 pixels (0.99 Mpx) or
> 50 pictures/second of 1280x720 pixels (0.88 Mpx). So you only save 10%. On
> the other hand moving images (where each 1080i field is different) will be
> sampled up from 540 lines to 720 lines and that probably compresses better
> than a real 720 line image.
>
> > >        d) "Streaming" quality transcodes of SDTV that fit within
> > > >            your upstream bandwidth (384K or 512K for example.)
> >
> > And I should note that long term, a cool tool would be one to
> > act like the slingbox and do this in real time.
>
>
>  Isnt this what MythStreamTV is capable of?
>

1080i is is still a frame resolution of 1920x1080 pixels not 1920x540.
So you can't convert 1080i to 1280x540 without losing a lot of
information.

You could convert 1080i50 to 1080p24 to save space and keep resolution
or you could convert 1080i50 to 720p24 to save size and lower the
resolution.

Either way, IVTC or frame decimation to original source frame rate is
recommended for the best savings in file size.


-- 
Steve
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