Bit Rate – Fixed Quality
A VBR encoder repaired by quantizer or fix quality.
This is usually a single pass encoding. The user determines a given subjective quality value and the encoder allocates bits as necessary to achieve a given quality level. This ensures that the product flow is of constant quality at all times. The quality score usually has a bit rate range associated with it. The downside to this encoding method is that the average bit rate (and thus the file size) will not be known in advance, and reaching a certain average bit rate requires trial and error. In general, this is more of a concern for video than audio, as file sizes are much larger and encoding can take much longer.
Bit rate row
This VBR encoding method allows the user to specify a bitrate range: the minimum and / or maximum bitrate allowed. Some encoders extend this method with an average bit rate. The minimum and maximum bit rate allowed sets the limits over which the bit rate can change. The downside to this method is that the average bit rate (and thus the file size) will not be known beforehand. Bitrate range is also used in some fixed quality encoding methods, but generally without permission to change the particular bitrate.
Average bit rate
The average bit rate (ABR) that encoding can be used to ensure that the output stream achieves a predictable average bit rate over the long term. This is typically accomplished by multi-pass encoding, where one or more initial passes are used to collect data about the stream, and the final pass uses that data to achieve uniform quality at a specified average bit rate.
Alternatively, periodic averaging can be used, either by performing ABR on smaller chunks of output or responding to fluctuations in ABR, increasing or decreasing overall quality. They can achieve ABR in a single pass, but they do not produce the same degree of uniformity as a multi-pass ABR. Some encoders use “ABR encoding” and “multi-pass encoding” to refer to single and multi-stream ABR encoding, respectively.
Some encoders also allow the user to specify the maximum allowable bit rate or the maximum quality cost. This is sometimes called a variable constrained bit rate (CVBR) and is generally applied to ABR algorithms.
The downside of a single pass ABR encoding (with or without CVBR) is the opposite of a fixed VBR quantizer: the size of the output is known in advance, but the resulting quality is unknown, although it is even better than CBR. Specifying a higher average or maximum can simply make the file larger without noticeable quality effect, and an increased maximum bitrate can introduce stuttering when the file is filtered. However, reducing these criteria to too low a level will ultimately lead to rather drastic quality losses. The effect on video is usually squared because the textures are no longer fully detailed in their rendering.
Multi-pass ABR encoding is more similar to fixed VBR quantizer because higher average will increase quality.
There is no one ideal “one size fits all” setting for ABR in video encoding. For low resolution video (320 or 640 lines) encoded with MPEG-1 or MPEG-2, the average bit rate can be as low as 1,000 kbps and still achieve acceptable results. For high definition video like 1080, this average may need to be 6,000 kbps or more. The main factor in determining the minimum video bitrate is the efficiency with which the video can be encoded. Using more efficient video encodings like MPEG-4 will help promote a lower bit rate, while a significant amount of movement or white noise will require a higher bit rate to encode without visible artifacts. In the end, the user may have to use trial and error to reach the minimum file size for a given video stream, encode at a given bit rate, and then review the results.
File size
VBR encoding that uses a file size setting is typically multi-pass encoding. This allows the user to define a specific size for the final file. In the first pass, the encoder analyzes the input file and automatically calculates the possible bit rate range and / or the average bit rate. In the last pass, the encoder distributes the available bits throughout the video to achieve uniform quality.
