[mythtv-users] 25p vs 50p compression was (Re: OT: LED or Plasma (was Advice on choosing a TV))

Andre mythtv-list at dinkum.org.uk
Thu Jun 16 10:10:40 UTC 2011


On 16 Jun 2011, at 08:57, Paul Gardiner wrote:

> On 25/05/2010 14:37, Andre wrote:
>> It turns out it's actually really easy to send high frame rate video so no halfway  houses are really required, 1080p50 needs no more bandwidth than 
> 1080i50 less sometimes, 1080p300 needs little more than 1080p50, less 
> sometimes, when the frames are clear of motion blur and little change to 
> the next one they compress really well! As a handy little side benefit 
> if there is a lot of sensor noise (which there will be at 300 or 600fps) 
> that changes every frame the real picture doesn't so much, so the first 
> thing that gets removed in compression is the sensor noise, proper noise 
> reduction for free!
> 
> I've now confirmed that myself. I've been experimenting with JYA's new
> mythtranscode with --fifodir --passthrough piping the result into ffmpeg
> using libx264. That for the first time allows me to get good transcodes
> of some 1080i content without losing the interlacing.

I've been following that thread, looks like a useful addition, does this mean you can properly transcode full motion 1080i50 to 1080p50?


> But also I've
> been trying scaling down to 720p25 and 720p50. The interesting thing
> was how little difference there was between the file sizes for the two:
> 
>   720p25   140MB
>   720p50   150MB
> 
> Tiny difference in file size, but huge difference in motion
> quality of course.

You won't see a similar progression to 720p100, 720p300 etc with standard encode settings, the gop needs to stay at the same time duration regardless of the frames included, ie if gop=12 at 720p50 it should be 24 at 720p100 or 72 at 720p300, this also requires a great deal of ram in the decoder, definitely a 1GB vdpau card!

Good luck finding any p100 or p300 to experiment with ;-) I tried to convince BBC R&D to release their high frame rate test material but they weren't interested.

A customer recently trialled some equipment that was capable but although they are a sports broadcaster the production guys were more interested in shooting at 24p than 50,60 or 100,120 or 300!

Andre


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