WIRE TRANSMISSION SYSTEM FOR TELEVISION 



631 



and demodulating the received currents from the two circuits. The 

 250-cycle outputs from the two sets of receiving apparatus were 

 connected in opposition to a pair of telephone receivers through a 

 low-pass filter. Potentiometers Pi and P2 were provided for adjusting 

 the relative intensities of the two 250-cycle output voltages and a 

 condenser C2 was arranged so that it could be used to change the 

 phase of either of the 250-cycle voltages. It is evident, then, that 



+2TU 







-2TU 



Kilocycles 

 Line No. 2 with Dry Weather Equalizers 



20 



I4 .^-^^— -- 



Kilocycles 



Line No.2 after 3 Days Rain with Dry and 

 Wet Weather Equalizers ^ 







-2TU 



o i: 4 b o 10 \d w lb 10 i;u 



Kilocycles 

 Fig. 9 — Measured attenuation characteristics of line circuits plus equalizers 



by making suitable adjustments the two voltages could be adjusted 

 to exactly the same intensity and opposite phase so that no sound is 

 heard in the telephone receivers. As long as the value of Co is small, 

 the envelope delay of one line at the carrier frequency with respect 

 to the delay of the other line at 5100 cycles is proportional to the 

 value of C2. 



The condenser Ci shown at the sending station is for the purpose 

 of introducing a phase shift in the 250-cycle current of either channel 

 relative to the other in order to compensate for the differences in 

 delay of the apparatus itself at the two frequencies. The value of Ci 

 was determined by experiment before moving the sending apparatus 

 to Washington and was adjusted to its calibrated value for each 

 frequency when the oscillator frequency was adjusted. 



The measurement of the phase shift of the 250-cycle current, which 



is transmitted by means of a carrier over a circuit as described above, 



is actually a measurement of the difference between the phases of 



the two received side-band currents situated 250 cycles either side of 



the carrier. The envelope delay is equal to A/S/Aco where Aw equals 



27r times 500, and A/S equals the measured difference in phase of the 



two side bands in radians. 

 41 



