stortion on an end to end basis. ratings and the echo loss at the 2- wire to 4-wire conversion unit. Quantising distortion is introduced by analogue to digital coding The echo loss is a weighted average of the power loss at frequen- techniques where an input signal is sampled and encoded into a cies in the voice frequency band and is defined in Recommendation finite set of values. The quantising distortion results from the dif- G.122 (CCITT, 1989c). The values of TELR required to give satis- ference between the recovered analogue signal and the original factory performance to customers are a function of the round trip analogue input. delay of the talker path. For a given TELR value, increasing the Quantising distortion from tandem analogue to digital codes, for delay will worsen customer opinion. example PCM systems interconnected through analogue interfaces, Recommendation G.131 gives an estimate of the recommended is cumulative and can have a significant effect on customer opinion. value of TELR required for a given probability of objectionable It is one of the essential components of the transmission plan of a echo. Figure 30.6 is taken from Recommendation G.131 and shows modern network. values of TELR required for a range of one-way delays. In this End to end planning for quantising distortion has been simplified figure percentages refer to the probability of encountering objec- by the introduction of the quantising distortion unit (qdu). tionable echo and OLR refers to the overall loudness rating of the The quantising distortion introduced by a single 8 bit A-law or talker echo path, approximately equal to TELR. It should be noted u-law PCM system is taken as being 1qdu. Subjective tests have that large values of TELR are required for one-way delays of for 30/6 Transmission planning TALKER-ECHO PATH DIGITAL TRANSMISSION AND SWITCHING Talker Listener Telephone Local Digital Digital Exchange switch line card Figure 30.4 Talker echo path example 250ms. In general these high values of TELR can only be has a more