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Topic: CBR vs VBR? (Read 3186 times) previous topic - next topic
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CBR vs VBR?

For all codecs and all compressors, I see that many like to use the "smart" VBR. but how do you know that it is really smart? What if they focused their "bits" on the wrong parts, and a balanced CBR would be better? How can it prove that the way it decides which part is more "complexed" will work in reality?

CBR vs VBR?

Reply #1 –
Because the VBR code use the same psychoacoustic model that the encoder itself is using.  In simplified terms, it calculates exactly how many bits are required to "satisfy" the psychoacoustic model.  Overall quality (and therefore bitrate) is controlled by tweaking how sensitive the psymodel is.

If the VBR code was "focusing ... on the wrong parts" than the encoder would be focusing on the wrong parts also, and the music would sound bad whether or not you were using VBR.

(NB: not all encoders use this metholody for VBR, but the open-source encoders that are well-known for VBR - LAME and Vorbis - do)
 

CBR vs VBR?

Reply #2 –
As benski wrote VBR is smart in the sense that it uses the bitrate necessary according to the psy model (and user quality demand), and as the psy model usually is fine, VBR usually is advantegeous.

But for the encoding practice this isn't the whole story. Look at the 2007 64 kbps multiformat listening test (or the 48 kbps test). Readers were astonished that WMA Pro which was used in CBR mode came out on par with its competitors.

What's often overseen is that CBR's constant bitrate addresses the bitrate of the transporting frames in the audio stream. AFAIK all of the usual codecs have the possibility to put part of the audio data that logically belong to a frame into neighboring frames thus effectively yielding a variable audio data bitrate - though in a restricted way. However the way audio data bitrate is chosen is different with CBR and VBR - the CBR mode is still less smart in this respect, but also more robust when it comes to psy model and other encoder weaknesses.
For mp3 it's worth mentioning that bitrate is restricted to 320 kbps anyway so even with a good VBR implementation you get diminishing returns compared to CBR the higher you go with (average) bitrate.

In the end it's all up to encoder implementation. Vorbis and MPC for example are totally based on VBR, and obviously they are doing good. With AAC and mp3 usually VBR and CBR is available, in the case of Lame also ABR which means audio data bitrate control the CBR way but without the CBR restrictions (and so variable bitrate transport frames). My personal impression is for Lame (at high to very high bitrate): with current version 3.98b6 VBR mode is to be preferred, whereas it was the other way around up to 3.97.
lame3995o -Q1.7 --lowpass 17