Variable Bitrate MP3 Encoding
[ No. 57 - July 2001 ]
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To those with trained ears, much of what passes for Internet Audio
is defined by its limitations. Some sanguine engineers, realizing
the value of balancing speed with pristine sound, have incorporated
an oft-overlooked but eminently useable MP3 encoding technology
called variable bitrate (VBR). VBR is used for many other encoding
systems and codecs, especially video. Part of the MPEG standard,
VBR was first implemented for MP3-encoded files by Xing Technologies
(the developer of AudioCatalyst - now owned by Real Networks) and
has been available long enough that all recent MP3 players support
it. Our context here is for MP3 files.
Simply put, VBR technology allows better sounding MP3 files in the
same space taken up by your garden-variety constant bitrate (CBR)
encoded song. To those who value small file size more than sound
quality: think of it as providing the same sound quality in a
smaller file or, conversely, better sound quality in a similar file
size.
Let's take a closer look at how VBR works. 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. 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.
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, users are likely to have a more consistent listening
experience with frequent bitrate changes in a VBR file than with
CBR. 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).
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. To
help you figure things out, we've included a comparison between
some common encoders for Windows and Macs. For each setting available
we list what we'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. We assume you Unix users can figure this
out yourselves!
- 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
(see above audio description):
- 10% = ~95kbps
- 25% = ~105kbps
- 50% = ~128kbps
- 75% = ~170kbps
- 100% = ~220kbps
- 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)
- Very High - 224kbps
- Ultra High - 256kbps
- Note: AudioCatalyst (Mac version only) has a glitch where if
you have "CBR Quality" set to 24kbps or less, it overrides the
"VBR Quality" regardless of which you have selected in "MP3
Mode".
- iTunes
<www.apple.com/itunes/>
- VBR setting: Quality
- Options available: Lowest, Low, Medium Low, Medium, Medium High,
High, Highest
- iTunes has a preference called "guaranteed minimum bit rate"
to control the VBR encoding. We couldn't find detailed
documentation on how iTunes' VBR works, but trial and error
showed the minimum bitrate setting drives the encoding. Setting
it to 8kbps resulted in 8kbps files at all quality levels. We
suggest using one of the tables from the other apps listed here
as a reference for your preferred settings.
Note: MusicMatch wins two prizes this month: (1) The "Cross Platform
Award" for adding Macintosh and Linux support
and (2) the "Bad Boy of Media Types Award" for setting itself
as the default player for all audio formats during regular
installation (even after letting us specify not to).
For Windows encoders, we used a PIII/600 with 256MB RAM and for
Mac encoders we used a G4/450 with 256MB RAM.
Encoding Speed: Across all encoders, the difference between VBR
and CBR 128k was only about one second. The fastest encoder was
AudioCatalyst for Windows (averaging 5.9x realtime) and the slowest
was AudioCatalyst for the Mac (due to its loading the track from
the CD first, and encoding afterwards).
File Size: Here you can see the difference in file size for both
clips at a few different VBR settings and how they compare to the
typical 128k CBR. The number with each VBR listing is the "base
bitrate" mentioned above.
Quality: We feel the 96k VBR files are roughly equivalent in sound
quality to the 128k CBR files. This is because when the music needs
it, VBR-encoded files bump up to 128k (or more). Most encoders tend
to increase rather than drop bitrates, so "base bitrates" we've
shown indicate (especially in iTunes' case) the lower-end used
throughout the resulting file.
Other Observations and Notes: The file size chart demonstrates the
effect of a wide dynamic range on VBR technology. Using standard
CBR encoding, file sizes were (of course) identical for both File
A and File B. However, in VBR, file size difference becomes noticeable
with File A (complex classical music) requiring more bits to
represent the same sonic experience than File B (standard rock).
VBR is a good idea and now makes us want the MP3 players to have
their groovy visualization plug-ins change as the music changes.
If the bitchin' graphics could react in some way as they detect
bitrate changes in VBR-encoded files, it would be cool. We noticed
that Winamp does have a handy feature on VBR playback - you can
see the bitrate changing as the song plays.
A Caution: Some older MP3 players may not be able to play VBR MP3
files. However, they are somewhat rare and there are a wide variety
of current free players that work fine. Make sure the links you
provide to MP3 players on your web site work properly.
Even modern players that play VBR might still have problems displaying
accurate track lengths and seeking to the right place in the file.
It's a result of the method used to figure out how long a song is
and doesn't affect playback quality of the song. With CBR files,
players can perform simple division of file size by bitrate, but
that becomes more complex with VBR. A small detail, and not likely
to cause any real grief.
Of course, quality is in the ear of the beholder, so we encourage
you to do your own tests, or check out our resulting clips for
yourself.
You can listen to our results yourself by clicking on the sizes
in the table above.
Visit us: <www.fezguys.com>
