The Measurement of a Telephones' Minimum Sidetone Impedance - Zs0 a New Method

L INTRODUCI'IO N. 250 is the abbreviation for the complex Impedance, which when connected across the two wire port of an analogue telephone, results in complete suppression of local electrical sidetone.Because sidetone plays an important part in the subjective performance of a telephone connection and is sensitive to line impedance, it is possible to improve the pater-name range of a given telephone by specifying and controlling this quantity during manufacture.Some telephones exhibit values of hi) which have negative real componans of impedance.This makes the physical realisation of a Zso network impossible.In any case the measurement is tedious.The method described here synthesise: the required impedance by using an agile sourcedalta natingemf.Bythismeansi tisposslblet osuppress thesidetone byup to 80 :13 over all four quadrants of the complex impedance plane.Once this condition has been achieved it is a simple matter to calculate the value of 2.90.Four methods are presently in use [1].All these methods have the disadvantage that the measurement and subsoquent calculation are not made at the point of zero sidetone.Thus errors an creep in due to nonllnearities in the telephone.The equipment described here provides the actual circuit termination condition for no sldetone for any quadrant of the complex plane.Since 'l'lGGER [2] systems provide the kernel for telephone transmission objective measurements, it is a logical step to integrate the zw measurement into this facility, with all the advantages that compatibility and uniformity bring.-'2. THEWHEATSI 'ONB BRIDGE...A QUICK RECAP.Simple two wire telephones can be represented by. a .Wheatstone bridge shown in gure 1 below. 2.90