Variable bit rate
Variable Bit Rate (VBR) is a term used in telecommunications and computing that refers to the bit rate used in encoding audio or video.
Unlike constant bit rate (CBR), VBR files change the amount of output per time slice. VBR allows you to allocate a higher bit rate (and therefore requires more storage space) to more complex segments of media files, while less space is allocated to less complex segments. The average of these rates can be calculated to generate the average bit rate for the file.
Opus, Vorbis, MP3, WMA and AAC audio files can optionally be encoded in VBR. Variable bit rate encoding is also commonly used in MPEG-2 video, MPEG-4 Part 2 video (Xvid, DivX, etc.), MPEG-4 Part 10 / H.264 video, Theora, Dirac, and other video formats. video compression. Additionally, floating rate encoding is inherent in lossless compression schemes like Apple’s FLAC and Lossless.
Advantages and disadvantages of VBR
The advantages of VBR are that it produces a better quality-to-space ratio than a CBR file of the same data. The available bits are used more flexibly to encode audio or video data with greater precision, with fewer bits used in less demanding passes and more bits used in difficult-to-encode passes.
The downsides are that encoding can take longer as the process is more complicated and some hardware may not be compatible with VBR files. VBR can also pose problems during transmission when the instantaneous bit rate exceeds the data rate of the communication path. These problems can be avoided by limiting the instantaneous bit rate during encoding or (at the expense of higher latency) by increasing the playback buffer.
Additionally, encryption of VBR encoded voice (or other signals, including video) provides only limited privacy, as bit rate patterns can show which language is spoken.
In the past, many hardware and software players could not correctly decode variable bit rate files, in part because the various VBR encoders used were not well developed. This led to the widespread use of CBR over VBR for the sake of compatibility. As of December 2006, devices that only support CBR encoded files are mostly obsolete as the vast majority of portable music devices and software today support encoded VBR files.
VBR support for AAC and MP3 files is found in most modern digital audio players, including those released by Apple, Microsoft, Creative Technology, and SanDisk. Early VBR algorithms sometimes introduced audible artifacts by encoding monotony or minimal tones (such as audiobooks and acoustic music). These displays often mimic the “digital chirp” during the quiet parts of a song, or when only speaking. As VBR improved the encoding algorithms, these problems were addressed in later generations of the VBR standard.
VBR encoding methods
Note that choosing the Variable Bit Rate (VBR) method only affects the encoding process. Decryption of the VBR stream is performed identically in all cases, regardless of how the encoder decides to allocate the bits.
Multi-pass encoding and single-pass encoding
VBR is created using so-called single-pass encoding or multi-pass encoding. Single pass encoding analyzes and encodes data on the fly and this is also used in constant bit rate encoding. Single pass encoding is used when encoding speed is more important, for example for real-time encoding. Single-pass VBR encoding generally controls a fixed quality setting or bitrate range (minimum and maximum allowable bitrate), or an average bitrate settling. Multi-pass encoding is used when encoding quality is most important. Multi-pass encoding cannot be used when real-time encoding, live coverage, or live streaming … Encoding a multi-pass takes much more time than encoding a single pass because each pass means one pass through the input data (usually the entire input file). Multi-pass encoding is only used for VBR encoding because CBR encoding does not offer the flexibility to change the bit rate. The most common multi-pass encoding is two-pass encoding. The first step of two-step coding analyzes the input data and stores the result.
