Bit Rates
and Sound Quality
Selecting
Bit Rates for Encoding
Bit
rate refers to the speed at which a bit stream (compressed audio data in
this case) will travel, or the amount of bits per second. With
MP3, you usually refer to it in terms of kbps or how many thousands of
bits per second. Just like the way film works with a higher number
of frames producing a better picture, the more audio data that flows in
a time period, the smoother and clearer the sound. "So what
real difference is there in sound quality?" you might ask. Check
out this MP3
bit rate page
(courtesy of MP3-Tech.org,
a good tech resource) where Gabriel Bouvigne ran a comparison of one
track recorded at different bit rates and noted the difference.
Many MP3 enthusiasts will convert to MP3 at 128 kbps (kilobytes per
second) as a good medium in sound quality and file size. However,
I recommend 160 kbps or VBR encoding, especially if you are a musician
or if you have a sensitive ear to musical quality.
Streaming
MP3 Audio and Different Bit rates
One
other useful tool is this chart
courtesy of
Audioactive, makers of
Audioactive
Production Studio,
a high-quality MP3 encoder. Using the chart provided, you
can select a bit rate that meets your sound quality needs while being
sensitive to the file size drawbacks of the higher rate if you are
streaming the audio file over the Internet.
Constant
Bit Rate Encoding
Constant
bit rate encoding is the standard method used by most encoders.
With CBR encoding, the same number of bits are added to each frame of
the audio data regardless if there is silence or a wailing guitar
solo. This method is good to use if you need to predict the size
of the encoded file. It is simply calculated by whatever bit rate
you have chosen to encode with multiplied by the length of the song.
Average
Bit Rate Encoding
Average
bit rate encoding (ABR) lets you choose an average bit rate and the
encoder adds bits where necessary.
Variable
Bit Rate Encoding
Variable
bit rate encoding (VBR) is a method that seeks to keep the quality of
the sound file high throughout the encoding process. Software with
this technology makes a decision when to add bits to the file if the
stereo separation is ever too far apart, producing a much clearer sound.
The end file size will vary after encoding depending on what decisions
the software has made. Simple parts of songs, including moments of
silence, will not need the same amount of bits as more difficult parts
and VBR encoding is able to make an intelligent decision regarding where
the bits are needed most. Use this method in encoding if you want
the best quality possible and are not real concerned about the file size
(usually pretty close to that of regular CBR encoding, sometimes
smaller) . YAMP,
AudioActive
Production Studio, Xing
MP3 encoder, LAME,
and AudioCatalyst
are a
few software products that use this technology.
For
more good reading on bit rates and sound quality in the encoding
process, read Fraunhofer's
encoding basics
or "enter bit rates" on chapter 2 of MP3:
The Definitive Guide.
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