(approx.) sections. Voice: Log-law 64kbits/s 2 x 10-5 28.2.2 Bit-Error Ratio (BER) Voice: ADPCM 32kbits/s The bit-error ratio (CCITT, 1988a) sometimes referred to as the Video: Linear PCM 60Mbits/s 2 x 10-7 bit-error rate, is defined as the number of bits received in error Video: Inter-frame 2Mbits/s 10-10 (approx.) divided by the total number of bits transmitted in a specified time coding interval. Within the specified interval, it is numerically equal to the bit-error probability. 16kbits/s 10 7 (approx.) Studies have been conducted to quantify the BER requirements 600Mbits/s for various digital services (Yamamoto and Wright, 1989). Some typical objectives, expressed in terms of a long term mean BER, are given in Table 28.1. able audible clicks but are still tolerable if they do not occur too However, the long term mean BER will usually be an insufficient often. Errors in a digital video signal will impair quality (CCIR, performance measure by itself. For many digital services the degree 1986), and again a viewer will usually be happier with the effects of of burstiness of the errors, that is their distribution, will also be random rather than burst errors. With video however, additional important. Figure 28.8 shows a typical result where, for a given long care is needed as excessive bursts of errors can result in a sudden term mean BER, the BER over shorter measurement intervals is and catastrophic loss of picture synchronisation. A similar sharp seen to exhibit considerable variation with time. During some measurement intervals no errors are recorded whilst at the other bound on acceptability may also occur with data if error control extreme bursts of interference can cause bursts of errors and thus a coding is used to protect the data from transmission errors. Occa- high short term BER. sional errors in an encoded block or even bursts of limited duration Within the illustrative limits quoted in Table 28.1 speech, video may be acceptable (in a retransmission scheme for example). How- and data services will ha