The binary codewords generated by the transform coder form the input to a transmission buffer. The function of the transmission buffer is to keep the output rate of the encoder fixed. If the buffer starts filling up faster than the transmission rate, it sends a message back to the transform coder to reduce the output from the quantization. If the buffer is in danger of becoming emptied because the transform coder is providing bits at a rate lower than the transmission rate, the transmission buffer can request a higher rate from the transform coder. This operation is called rate control
The change in rate can be affected in two different ways. First, the quantizer being used will affect the rate. If a quantizer with a large step size is used, a larger number of coefficients will be quantized to zero. Also, there is a higher probability that those not quantized to zero will be one of the those values that have a shorter variable-length codeword.
Therefore, if a higher rate is required, the transform coder selects a quantizer with a smaller step size, and if a lower rate is required, the transform coder selects a quantizer with a larger step size. The quantizer step size is set at the beginning of each GOB, but can be changed at the beginning of any macroblock. If the rate cannot be lowered enough and there is a danger of buffer overflow, the more drastic option of dropping frames from transmission is used.
The ITU-T H.261 algorithm was primarily designed for videophone and videoconferencing applications. Therefore, the algorithm had to operate with minimal coding delay (less than 150 milliseconds).
Furthermore, for videophone applications, the algorithm had to operate at very low bit rates. In fact, the title for the recommendation is “Video Codec for Audiovisual Services at p×64 kbit/s,” where p takes on values from 1 to 30. A p value of 2 corresponds to a total transmission rate of 128 kbps, which is the same as two voice-band telephone channels. These are very low rates for video, and the ITU-T H.261 recommendations perform relatively well at these rates.