no more hslv format ?

[Dave Lambley]

On 3 May 2018 at 00:24, RS <richard22j at zoho.com> wrote:
> 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."

Searching turns up this,

http://dpp-assets.s3.amazonaws.com/wp-content/uploads/specs/bbc/TechnicalDeliveryStandardsBBCFile.pdf

... which tells you that 25fps interlaced is what programme makers
will be supplying for HD programmes.

> 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.

They would remove the interlace to bring the framerate to 50fps before
encoding. There are many methods of doing so, ffmpeg can do this for
example.

Dave