Using ffmpeg to halve frame rate

[RS]

On 15/04/18 17:47, Anthony Kehoe wrote:
> I actually worked on this on Friday. I do a lot of work with ffmpeg to
> get encodes working nicely with my 4K telly along with putting things
> in iTunes for the AppleTVs.
> 
> I added a new option for my copy of get_iplayer, akfps, that changes
> the ffmpeg resolutions should a 50fps version be downloaded.
> 
>          # Recording
>          akfps           => [ 1, "akfps!", 'Recording', '--akfps', "AK:
> Turn on ffmpeg override for 50fps to 25fps conversion"],
> 
> Down in sub postproc, I modified the section where @codec_opts builds
> the video/audio codec options. I looked at a few BBC encodes and it
> seems like they use a bitrate around 2285 with a maxrate of 3500 for
> hls streams:
> 
> Bit rate mode                            : Variable
> Bit rate                                 : 2 335 kb/s
> Maximum bit rate                         : 3 500 kb/s
> Width                                    : 1 280 pixels
> Height                                   : 720 pixels
> Display aspect ratio                     : 16:9
> Frame rate mode                          : Constant
> Frame rate                               : 25.000 FPS
> 
> Therefore, I used a target bitrate of 2285 with a maximum of 3500.
> 
Thanks, that's useful. -b:v=2285k does indeed give me the bit rate I was 
expecting.

For the moment I want to try to reduce the frame rate by dropping 
alternate frames as that should avoid any loss of quality in 
re-encoding.  Also I want to be able to do it in a reasonable time on 
this ancient desktop which has a 2.3GHz Core 2 duo.

I gather -r in ffmpeg does not work with -c copy.  According to this 
article there is a workaround by concatenating a dummy file at the 
beginning to set the PTS value.  I'll give it a try.

https://superuser.com/questions/1088382/change-framerate-in-ffmpeg-without-reencoding

Best wishes
Richard