At 01:16 PM 8/3/02 +0200, you wrote:>Hi,
>
>Maybe this is documented somewhere, if so, please send me a link to the
>documentation.
>
>My question is: how to set up different (VBR, CBR, ABR) modes when
>calling the Ogg Vorbis API?
>
>Currently I do:
>
>CBR:
>
> ret = vorbis_encode_setup_managed( &vorbisInfo,
> getInChannel(),
> getOutSampleRate(),
> -1,
> getOutBitrate() * 1000,
> -1)
> || vorbis_encode_ctl( &vorbisInfo, OV_ECTL_RATEMANAGE_AVG, NULL)
> || vorbis_encode_setup_init( &vorbisInfo);
This is a fully VBR mode (as for the VBR mode below, but selected by
bitrate). True CBR is more or less completely useless (most other
codecs don't even offer it - even mp3 just fakes it using the bit
reservoir), but can be configured if you want by setting min/max/nom
to the same value. Expect poor performance if you do.
>
>
>ABR:
>
> ret = vorbis_encode_init( &vorbisInfo,
> getInChannel(),
> getOutSampleRate(),
> -1,
> getOutBitrate() * 1000,
> -1 );
>
>VBR:
>
> ret = vorbis_encode_init_vbr( &vorbisInfo,
> getInChannel(),
> getOutSampleRate(),
> getOutQuality() );
>
>
>Are these calls correct?
The latter two are correct.
You can also set hard min (not very useful) and hard max (potentially
very usefu) bitrates for quality-based modes using
vorbis_encode_ctl(&vi, OV_ECTL_RATEMANAGE_SET, ai) (where ai is a
struct ovectl_ratemanage_arg *) with the struct members set
appropriately (pretty obvious - just look at the struct definition in
vorbisenc.h)
<p><p>>>What effect do the max_bitrate, min_bitrate parameters have in the CBR
>and ABR modes?
In managed modes (ABR/CBR are subsets of that), min and max bitrate do
the obvious thing - they set a minimum and maximum bitrate for the
encoder to use.
>
>When measuring performance, it seems that the ABR mode takes a lot more
>(about twice) processing power than VBR of even CBR. why is that?
>
Managed modes are much slower. The encoder has to do a lot more
work to figure out an encoding whilst also meeting the bitrate
constraints. The reason 'CBR' seemed faster to you was that you
weren't
using a CBR mode at all. It's a choice the user has to make - if they
need the hard constraints (which is possible, though rare, for streaming
purposes, and maybe for some other purposes too), then it's worth
using > twice the cpu.
Michael
<p><p>--- >8 ----
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