Frequently Asked Questions
Constant and Variable Bit Rate Encoding
- What is Constant Bit Rate encoding?
- How does Constant Bit Rate encoding work?
- When should I use Constant Bit Rate encoding?
- What are the benefits of Constant Bit Rate encoding?
- What is Variable Bit Rate encoding?
- How does Variable Bit Rate encoding work?
- When should I use Variable Bit Rate encoding?
- What should I know before using Variable Bit Rate encoding?
- What is the difference between Varaible and Constant Bit Rate?
- I have encoded music with Variable Bit Rate. Why won't it play on my MP3 player?
- What is Constant Bit Rate encoding?
- How does Constant Bit Rate encoding work?
- When should I use Constant Bit Rate encoding?
- What are the benefits of Constant Bit Rate encoding?
- What is Variable Bit Rate encoding?
- How does Variable Bit Rate encoding work?
- When should I use Variable Bit Rate encoding?
- What should I know before using Variable Bit Rate encoding?
- What is the difference between Varaible and Constant Bit Rates?
Constant Bit Rate (CBR) encoding is an encoding method that varies the quality level in order to ensure a consistent bit rate throughout an encoded file.
In order to maintain constant bit rate throughout the file, difficult passages (for example, passages containing a relatively wide stereo separation), may be encoded with fewer than the optimum number of bits. When encoding easy passages, (for example, passages containing silence or a relatively narrow stereo separation), RealJukebox uses more bits than necessary in order to maintain the constant bit rate. Consequently, difficult passages may experience a decrease in quality, while easy passages may include unused bits.
Use Constant Bit Rate encoding when you need to limit the size of the MP3 file or produce consistent and predictable file sizes.
Constant Bit Rate encoding produces MP3 file sizes that are easily predicted by multiplying the bit rate by the duration. For example, a 128k (16k/sec) bit rate audio file that is 30 seconds in duration produces a 480k MP3 file.
Variable Bit Rate (VBR) is an encoding method that ensures consistent high audio quality throughout an encoded file by making intelligent bit-allocation decisions during the encoding process. VBR encoding produces an overall higher and more consistent quality level than Constant Bit Rate encoding.
When RealJukebox encounters difficult passages in the audio source (for example, sections containing a relatively wide stereo separation), it allocates additional bits to that section, ensuring the audio quality. When RealJukebox encounters easy passages (for example, sections containing silence or a relatively narrow stereo separation), it allocates fewer bits to that passage, reducing unused bits. As a result, your MP3 file consists of variable bit rates throughout the file because RealJukebox allocates the bits where they are most needed.
Variable Bit Rate encoding was developed for use when consistent audio quality is the top priority and constant or predictable encoded MP3 file size is not critical.
Because a Variable Bit Rate (VBR) encoded MP3 file consists of variable bit rates throughout the file, the MP3 file size cannot be easily predicted. VBR encoding should not be used when you need to predict and/or limit the size of the MP3 file. Many MP3 players may inaccurately report the duration of VBR-encoded files. If the MP3 player cannot accurately determine the time, seeking in the file may also be unpredictable.
Some audio passages are more difficult to encode than others. To maintain Constant Bit Rate (CBR) during encoding, difficult passages (where you need more bits for the complexity) and easy passages (that conceivably need less bits) encode at the same fixed bit rate. By maintaining a constant bit rate, you force the quality of the CBR operation to vary according to the complexity of the audio file.
However, with that same difficult audio passage, the Variable Bit Rate (VBR) operation forces the quality level and allows bit rate to vary instead. Difficult passages encoded using VBR allows a higher than average bit rate and during easy passages VBR allows a lower than average bit rate. The net result is that VBR produces an overall higher, more consistent quality level compared to CBR at similar bit rates.
In other words, if you set your files to encode with CBR and the files need more or less bits to vary with the complexity of the piece — you vary quality. If you set you file to encode with VBR the bit rate will vary and the quality won't.