[FFmpeg-user] frame rates

[Moritz Barsnick]

On Mon, Oct 04, 2021 at 09:15:49 +1100, Tim Lloyd wrote:
> I don't know whether I am expecting too much. I would like to lower the
> frame rate and hence the bit rate while keeping other video aspects the
> same. I decided to ramp down to 25 fps.

Depending on the content, that may not reduce the bitrate much at all.
But worth a try.

> The first attempt changed height &
> width so I added the "-vf" qualifier. This is me clutching at straws so time
> to seek assistance.
>
> ffmpeg -i file -r 25 -vf "scale=960:540" output
>
> This gives me:
>
> Duration                                 : 58 min 59 s
> Bit rate                                 : 854 kb/s
> Width                                    : 960 pixels
> Height                                   : 540 pixels
> Display aspect ratio                     : 16:9
> Frame rate mode                          : Constant
> Frame rate                               : 25.000 FPS
>
> I was expecting the bit rate to drop but that seems a huge drop. Is this to
> be expected? Is there a recommended path to lower the frame rate?

This is because ffmpeg encodes at its own defaults, in this case
probably (we would know if you showed us the complete console output)
to H.264 with CRF=23, preset medium. If the original was encoded for
more quality, it will have had that much more bitrate.

If the visual result of the conversion is okay for you, you can
consider yourself lucky, and enjoy the saved bandwidth/space. If you
would like more quality, explicitly set the crf to a lower(!) value,
and see what it changes.

Cheers,
Moritz