What Is a Variable Bitrate?

Variable bit rate (VBR) is a term used in telecommunications and computers that refers to the bit rate used in audio or video coding. VBR files change the amount of output data for each time period. Compared to CBR files of the same data, it produces a better quality-space ratio, but encoding may take more time. Opus, Vorbis, MP3, WMA and AAC audio files can optionally be encoded in VBR.

Variable bit rate (VBR) is used in telecommunications and computers involving sound or video

Opus, Vorbis, MP3, WMA and AAC audio files can optionally be encoded in VBR. Variable bit rate encoding is also commonly used for MPEG-2 video, MPEG-4 Part 2 video (Xvid, DivX, etc.), MPEG-4 Part 10 / H.264 video, Theora, Dirac, and other video compression formats. In addition, variable rate coding is inherent in lossless compression schemes, such as FLAC and Apple Lossless.

The choice of a variable bit rate (VBR) method only affects the encoding process. In all cases, decoding of the VBR stream is performed regardless of how the encoder chooses to allocate bits.

Variable bit rate multi-channel coding and single-channel coding

VBR is created using so-called single-pass encoding or multi-pass encoding.

Single-pass encoding analysis and "on-the-fly" encoding of data, and it is also used for constant bit rate encoding. Use single-pass encoding when encoding speed is most important for real-time encoding. Single-pass VBR encoding is usually controlled by a fixed quality setting or bit rate range (minimum and maximum allowable bit rates) or average bit rate settings.

When encoding quality is most important, multi-pass encoding is used. Multi-channel encoding cannot be used for real-time encoding, live broadcast, or live streaming. Multi-channel encoding takes longer than single-channel encoding because each channel means passing a large amount of input data at once (usually through the entire input file). Multi-channel encoding is only used for VBR encoding, because CBR encoding does not provide any flexibility to change the bit rate. The most common multi-pass encoding is two-pass encoding. In the first pass of the two-pass encoding, the input data is analyzed and the results are stored in a log file. In the second pass, the collected data from the first pass is used to achieve the best encoding quality. In video encoding, dual-channel encoding is usually controlled by the average bit rate setting or bit rate range setting (minimum and maximum allowed bit rates) or the target video file size setting. ^[2]

Variable bit rate fixed quality

One method of VBR coding is fixed quantizer or fixed quality coding. It is usually a single pass encoding. The user specifies a given subjective quality value, and the encoder allocates bits as needed to achieve a given quality level. This ensures that the output stream will have consistent quality. The quality level usually has an associated bit rate range. The disadvantage of this encoding method is that the average bit rate (and therefore the file size) is not known in advance, and it takes trial and error to reach a certain average bit rate. This usually cares more about audio than video because the file size is larger and encoding can take longer.

Variable bit rate bit rate range

This VBR encoding method allows the user to specify a range of bit rates-minimum and / or maximum allowed bit rates. Some encoders extend this method with an average bit rate. The minimum and maximum allowed bit rate set boundaries, the bit rate of which may vary. The disadvantage of this method is that the average bit rate (and file size) is not obtained in advance. Bit rate ranges are also used in some fixed-quality coding methods, but usually do not change a specific bit rate. ^[3]

Variable bit rate average bit rate

Average bit rate (ABR) encoding can be used to ensure a predictable long-term average bit rate for the output stream. This is usually achieved using multi-channel encoding, where data is collected on one or more initially passed streams, and eventually uses the data to achieve uniform quality at a specified average bit rate.

Either you can perform ABR with smaller blocks of output, or you can use periodic averaging by responding to fluctuations in ABR by increasing or decreasing overall quality. These can achieve ABR in a single pass, but do not produce the same degree of uniformity as multi-pass ABR. Some encoders use "ABR encoding" and "multi-channel encoding" to refer to single-channel and multi-channel ABR encoding, respectively.

Some encoders also allow the user to specify the maximum allowed bit rate or maximum quality value. This is sometimes called constrained variable bit rate (CVBR), and is often applied to ABR algorithms.

The disadvantage of single-pass ABR encoding (with or without CVBR) is the opposite of a fixed quantizer VBR-the size of the output is known in advance, but the quality obtained is unknown, although still better than CBR. Specifying a higher average or maximum value may simply make the file larger, without discernable quality effects, and the increased maximum bit rate may introduce noise when streaming the file. However, lowering these standards too low will eventually result in a considerable quality loss. The effect on the video is usually an increased block effect because the frames are no longer in a fully detailed state in their rendering.

Multi-channel ABR encoding is more similar to a fixed quantizer VBR, because higher averages will greatly improve quality. [9]

In video encoding, ABR does not have the ideal "one size fits all" setting. For low-resolution (320 or 640 lines) video encoded using MPEG-1 or MPEG-2, the average bit rate can be as low as 1000 kbit / s and still achieve results. For high-resolution videos such as 1080, this average may need to be 6000kbit / s or higher. The main factor in determining the minimum video bit rate is the coding efficiency of the video. Using more efficient video encoding (such as MPEG-4) will help increase the lower bit rate, while large amounts of motion or white noise will require a higher bit rate to encode without visible artifacts. Finally, users may have to achieve a minimum file size for a given video stream by using trial and error by encoding at a given bit rate and then watching the results.

Variable bit rate file size

VBR encoding using file size settings is usually multi-pass encoding. It allows the user to specify a specific target file size. In the first pass, the encoder analyzes the input file and automatically calculates a range of possible bit rates and / or an average bit rate. At the last pass, the encoder allocates the available bits throughout the video to achieve uniform quality.