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

[Paul Gardiner]

On 16/06/2011 11:10, Andre wrote:
>
> 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?

Sure. Although, so as not to overstress my FE, I've been keeping with
1080i50, unless downscaling.

>> 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!

Interesting, although p50 seems really smooth to me. It's 25p and 24p I
don't really like, although loads of people prefer it for being more
film-like.

Cheers,
	Paul.