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Topic: Multi-Pass Encoding (Read 4939 times) previous topic - next topic
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Multi-Pass Encoding

Something I've noticed with the DivX Pro digital video codec is that it offers "multi-pass" encoding options that can improve the quality of the final output. Is there any particular reason as to why digital audio encoders (such as LAME, for example) don't offer this kind option for encoding VBR MP3s? I can't think of a single audio encoder that has an option for multi-pass VBR. Is this really the case?

Multi-Pass Encoding

Reply #1 –
Windows Media 9 offers 2 pass audio encoding. If the encoders analyzed the entire audio file before encoding it, then it should be able to produce higher quality files ? Like divx looks better in 2 pass.

Multi-Pass Encoding

Reply #2 –
Quote
Is there any particular reason as to why digital audio encoders (such as LAME, for example) don't offer this kind option for encoding VBR MP3s?

I don't know much about video coding, but about audio, multi-pass does not improve the VBR files. It would improve the ABR files though.

IIRC, what video codecs do on first pass is doing a complexity analysis, and from this information, distribute the bitrates of the frames to attain a certain bitrate.

The only thing that could improve the encoding of VBR files would be encode->decode->analysys->decide if ok or not and try a different combination if needed.
(This is a bit hardcore in the end, because if you don't trust your psychoacoustics algo, how can you trust the analysys algo?)

Multi-Pass Encoding

Reply #3 –
Why does Windows Media encoder 9 allow you to make 2-pass CBR audio files, if it offers no improvement?

Multi-Pass Encoding

Reply #4 –
2-pass you need if you want tell the encoder the exact average bitrate (ABR) you want to get. In the first pass it analyzes the file, that it can setup the quantizer to get the wanted average bitrate when encoding the file in the second pass. Multipass-encoding (more then 2 passes) allocates the bits more smartly. But for CBR or VBR it makes no sense.

Multi-Pass Encoding

Reply #5 –
The neccessity behind two-pass encoding in video is that you have to be able to predict the filesize of the result quite precisely.  You have to because the video files, although compressed, still tend to be pretty large which makes storing them on media more difficult. As most people encode audio track by track, this dilemma seldomly occurs in audio encoding.

[Edited: typos]

Multi-Pass Encoding

Reply #6 –
For example you have a movie which is two hours long. You want to encode in MPEG2 for DVD. A DVD has 4.38GB space available. If you want optimal quality you have to use the whole space available on a DVD. So you need 2-pass-ABR (or Multipass) to get a MPEG2-file which has together with the audio a filesize as near as possible to 4.38GB.

Multi-Pass Encoding

Reply #7 –
I understand that, but why does WMA 9 offer 2-pass CBR encoding if it should be of no benefit ?

Multi-Pass Encoding

Reply #8 –
It depends on the coding algorithm definition of 'CBR'

I've read about AAC encoder projects that could do 2-pass encoding, but in most cases bit-reservoir control is adapting bit allocation very good, and 2 pass encoding would offer only small improvements.

Multi-Pass Encoding

Reply #9 –
Quote
I understand that, but why does WMA 9 offer 2-pass CBR encoding if it should be of no benefit ?

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