[MPlayer-users] Mencoder: are the displayed bitrates accurate?

[D Richard Felker III]

On Tue, Oct 28, 2003 at 09:54:48AM +0100, Levente Novák wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> >
> >
> >Of course. A bitRATE is a rate, which means it involves bits and time.
> >MEncoder's numbers are for the whole file, i.e. total bits/total time.
> > 
> >
> Yes, I understand what a rate is, but a frame can also have a bitrate: 
> the number of bits sent per the duration of the frame. And sadly enough, 
> standalone DVD or VCD players do not care about averaged bitrate (so 
> that mencoder displayed values are maybe not totally useless, but close 
> enough in this case), they expect a certain maxrate to be meet. If you 
> have a higher bitrate even for only one frame, the player won't play it.

Not true. You usually go well over the target bitrate for intra
frames. What matters is that you don't overflow the player's (fairly
small) buffers, i.e. that you keep time (duration) bounded below for
the number of bits that fit in the buffer.

> In my opinion (but AFAIK this is shared by many others), mencoder is 
> close to useless in producing streams for DVD or (S)VCD authoring if it 
> is impossible to set a maxrate lock. It would be a pity since its 
> quality, speed and the number of postprocessing options should make it 
> the best encoder out there.

-lavcopts vrc_maxrate=XXX:vrc_buf_size=XXX

> >Computing a bitrate for a single frame is very silly, since obviously
> >some frames will take many more bits (easily up to 100x more) to
> >encode than others.
> >
> It is not silly at all. If you have to set a maximal rate, these frames 
> should be quantized with higher value quantizers then, but it should not 
> exceed the maxrate. It is mandatory in many cases, otherwise the DVD or 
> CD won't play in a standalone player.

No, if you do that on a per-frame basis, the whole movie will look
horrible (due to all P frames being predicted from an I frame with
bad quantization) and even if they manage to clean up the ugliness
(likely if there's low motion), you'll have a periodic oscillation in
quantization noise matching the keyframe interval, which will be very
noticable and ugly to the eye. No one actually masters (S)VCD like
this.

Rich