Re: VBR Format

Srikanth wrote:
> Hi
>
> Can anyone tell me or point me to a link from where I can get the
> format for VBR (variable bit rate) encoded files? I tried searching a
> lot, but i could never find out where the bit rate is specified. My
> main question is how does a player know what bit rate to play the song
> at? Where is this information stored? Since the bit rate is changing,
> which part stores this info?
>
> Thanks
> Srikanth.

Hi,

Well, the main difference between the Standard and VBR is that Standard
MP3 files are encoded at a single bitrate through the entire file
(hence the CBR moniker). In contrast, VBR encoding looks at the audio
file and chooses what bitrate to encode based on how much audio
information is present at any given moment. A song that begins quietly
or with a single musical instrument will, during that section, be
encoded at a lower bitrate then the middle of the song when all the
instruments are playing together and the volume and frequency range is
high. As a result, most songs will be encoded at several different
bitrates corresponding to fluctuations in dynamic range. The key
conceptual difference between CBR and VBR is that where in CBR encoding
you specify your compression by space, with VBR you specify it by
quality. With CBR, sonic quality is consistently reduced to maintain
the bitrate you specify. With VBR, the bitrate is changed to meet the
quality level desired. Furthermore, CBR is inefficient in that 10
seconds of silence encoded at 128kbps requires the same file size as 10
seconds of full-on opera. VBR encoded files of the same audio would
result in a very small file for quiet sections and larger sized files
for loud sections (depending on the quality you chose). Like CBR, VBR
encoding parameters are set at the time the song is encoded.

As for storing the information itself, note that the MP3 standard is
designed so that information about the encoded bitrate level is
included throughout the file. This makes it easy for VBR-enabled MP3
players to seamlessly decode and play VBR files. In fact, the users are
likely to have a more consistent listening experience with frequent
bitrate changes in a VBR file than with CBR. Since in a CBR file, if
you suddenly reach a more dynamic piece of a song, you're stuck with
the same bitrate. This can result in audio artifacts (most noticed when
encoding to lower bitrates - 64kbps and less).
As a compression you should note that VBR encoding offers the ability
to compress audio more efficiently by specifying the desired sound
quality level rather than by setting a flat, numeric bitrate. Sounds
great, yes! Alas, there is no standard for how to select this "quality"
in your encoder and that results in different settings for each
encoding application. But the way the encoder identifies the VBR
format is very simple - in case the bitrate is changed during a track
you got a VBR file.
To help you figure things out, I've included a comparison between two
common encoders for Windows. For each setting available I have listed
what I'll call the "base bitrate"-- the starting point that VBR uses to
encode. Compared to the same rate used for CBR files, a VBR-encoded
file will sound better because it switches to higher bitrates as
needed:

AudioCatalyst <www.xingtech.com> VBR slider setting: "Quality" Options
available: Low - 96kbps (Near CD Quality - Acceptable for portables)
Low/Normal - 112kbps (CD Quality - Best for portables) Normal -
128kbps (CD Quality - Best for most) Normal/High - 160kbps (Archival
Quality - High-end stereos) High - 192kbps (Archival Quality -
Highest-end stereos)

MusicMatch <www.musicmatch.com> VBR setting: "Custom Quality"
percentage slider Options available: 1%-100%, samples follow: 10% =
~95kbps 25% = ~105kbps 50% = ~128kbps 75% = ~170kbps 100% =
~220kbps

In case you need some implementation understanding you can always look
at one of the open-source project which implements VBR, suchlike:
Speex.

- Nir

.

Relevant Pages

  • Re: Ansermay of the Det Weber Bassoon Con Halaerts SRO 1968
    ... >> why vbr. ... And what is the bitrate for aps ... As you encode you see onscreen he ... > Mehta and the LA Phil, with our principal clarinettist Michele Zukovsky, ...
    (rec.music.classical.recordings)
  • Re: MPEG Mainconcept best quality
    ... I was planning to encode directly from the timeline, ... Unfortunately, I'm quite disappointed by the quality I get, compared to ... Minimum bitrate: 4Mbps ... Quality, VBR 2 pass? ...
    (rec.video.desktop)
  • Re: Ask EU - .wma files
    ... what is the difference between CBR and VBR? ... CBR varies the quality to keep a near ... they don't all have difficult to encode scenes at the same time. ...
    (uk.media.radio.archers)
  • Re: iPod sound quality
    ... > kirk@xxxxxxxxxxxxx (Kirk McElhearn) appears to have caused the following ... >>> the encode. ... In the old days MP3 VBR files were a big problem for many players, ... Prev by Date: ...
    (rec.music.classical.recordings)
  • Re: iPod sound quality
    ... >> I don't know why VBR would affect playback - the VBR stuff is all on ... >> the encode. ... I ask you to judge me by the enemies I have made. ...
    (rec.music.classical.recordings)