3.6 Constant and variable bit rates

The bit rate generated by the encoder of Figure 3.19 is called a constant bit rate (CBR), requiring a fixed channel bandwidth. An alternative solution is to use a transmission system which can adapt to the variable bit rate (VBR). For VBR coding, the feedback and the smoothing buffer are no longer needed. The quantiser step size in this case is fixed.

In an asynchronous transfer mode (ATM) system [23], the information is transmitted in the form of fixed length packets or cells and when a cell is full, it is transmitted. This can happen at any time, so that the transmission system has a bit rate capability that matches the encoder. The advantage is realised only when several video channels are multiplexed together. When one channel is generating cells in rapid succession, corresponding to high picture activity, it is probable that the other channels will generate cells at a lower rate. Only rarely will the total bit rate of the multiplex be exceeded and freeze out occur.