no more hslv format ?

[RS]

On 02/05/18 23:21, Owen Smith wrote:

> 
> I've been mystified for a while why people talked about "dropping every other frame" as if it were trivial to do, and an email earlier in this chain looked like someone was trying to do that again. I was explaining why that simply is not possible in the general case.
> 

If I have caused confusion by talking about dropping alternate frames I 
apologise.  I had come across some posts in another forum which 
suggested it could be done, but I now recognise I was wrong.  What makes 
it worse is that I have since come across a thread in this listserver 
from two years ago where I was asking exactly the same questions, and 
Vangelis pointed out I was wrong and directed me to a Wikipedia article 
on H.264.

It is not possible to change the frame rate using -c:v=copy in ffmpeg; 
it is necessary to re-encode which is why it takes so long.  Inevitably 
there will be losses, added to which the codecs available to us may be 
inferior to those used by the BBC.

I think it was Nick Payne who said he had experimented with re-encoding 
in HEVC (H.265) and found that the file size was the same for 25fps as 
it was for 50fps, which led him to conclude that frames were being 
duplicated to achieve 50fps.

The broadcast signal (at least on satellite) is 1920x1080i at 25fps 
usually with two audio streams, AC3 and NAR.  If the broadcast signal is 
recorded, the resultant file size is about 3GByte/h.

The BBC'S explanation of what it does with the broadcast signal is, "The 
Elemental encoders are used to convert the 1920x1080 interlaced content 
to 960x540 for progressive encoding at 50fps."

It also says, "The 50fps, 1280x720 profile, however, will be available 
to those with 5Mbit/s broadband connections." but it does not explain 
where it comes from or why it cannot generate a 1280x720p 25fps profile.

Best wishes
Richard