TIME DIVISION MULTIPLEX SYSTEMS 213 



The vestigial method is required only when strong sidebands very near 

 the desired ones must be removed. Modifications of the time division 

 process exist in which vestigial filters are unnecessary because very little 

 energy appears at unwanted side frequencies. It is clear that if we regard 

 the problem as one of producing certain sidebands on carriers of definite 

 .phases, we are not restricted to commutating devices only but may make 

 use of general modulator technique. Further details concerning specific 

 circuit arrangements are described in U. S. Patent 2,213,938, W. R. Bennett; 

 and U. S. Patent 2,213,9-41, E. Peterson. As a general guide the following 

 table of carrier phases for an .V-channel system {N even) is furnished: 



Table of Phase Shifts for ^V-Channel System 

 (iV/2 Carrier Frequencies Required) 



Transmission Requirements 



Practical success of a time division multiplex system requires the main- 

 tenance of a satisfactory ratio of wanted signal to crosstalk. In order to 

 accomplish this, the transmission link must preserve the amplitude and 

 phase relations of a group of sidebands. A physical picture of the relations 

 involved may be obtained from Fig. 4, which is drawn for the particular 

 case of a 5-channel system of the on-and-off switching type. For this 

 example the theory previously developed shows that five sidebands of 

 equal amplitude are sufficient, namely — the signal itself (which may be 

 regarded as a sideband on a carrier of zero frequency), the upper and lower 

 sidebands on the switching frequency and on the second harmonic of the 

 switching frequency. If we take the phases of the switching fundamental 

 and its second harmonic as applied to the first channel as a reference, the 

 proper phases of fundamental and second harmonic respectively for the 

 other four channels are given by the following table: 



In Fig. 4, we have assumed a single-frequency signal component as input 

 to the first channel. If the line has distortionless attenuation and phase 

 characteristics, the five resulting side frequencies are received in the first 

 channel as the five in-phase vectors of equal amplitude shown in (a). Re- 



