CARRIER SYSTEMS ON TELEPHONE LINES 



619 



equal to it in width, located on the opposite side of the carrier fre- 

 quency. In addition to these products of modulation there are 

 produced other frequency bands, the important ones occupying side- 

 band positions about the harmonics of the carrier frequency. See 

 Figure 38. 



The first requirement on the band filters is imposed by the need of 

 suppressing the unwanted sideband to prevent distortion when the 



Carrier 



Voice 

 Band 



Lower 

 3ide Band 



Other 

 Modulation Products 



Upper 

 5ide Band 



Frequency — ^ 

 Figure 38 — Frequency range of products of modulation 



carriers are out of synchronism. The tests mentioned above in 

 connection with the oscillator frequency stability were made with 

 but one sideband transmitted. If both sidebands are transmitted, 

 the carriers must be exactly in synchronism or a "wobble" due to 

 the demodulation of both sidebands can be detected. One sideband 

 must be suppressed by an increasing amount as this carrier difference 

 increases. For a carrier frequency difference of about 20 cycles it is 

 necessary to suppress the unwanted sideband about 40 TU, thus re- 

 ducing it to about 1/100 of the strength of the wanted sideband in order 

 to eliminate completely this type of distortion. This requirement can 

 be met by providing the necessary attenuation in either the transmitting 

 or the receiving band filter, or by making the sum of their attenuations 

 equal to 40 TU. 



The suppression of the unwanted sideband is necessary for another 

 reason in a multi-channel system in which the transmitted sidebands 

 are close together. The unwanted sideband from one channel overlaps 

 the wanted sideband of an adjacent channel, and would be demodulated 

 and appear as "crosstalk" into this channel if it were not suppressed 

 by the transmitting band filter. The suppression needed is determined 

 by the amount of interference which can be tolerated from one channel 

 to another. It has been found that to meet this requirement the 

 transmitting band filter must suppress the unwanted sideband about 

 60 TU. The other modulation products mentioned above must also 

 be reduced by the transmitting band filter to a value which will not 

 cause interference in any channel into which they might pass. The 

 discrimination requirement for these frequencies is less severe because 

 the magnitude of these modulator products is not so great. 



