WIRE TRANSMISSION SYSTEM FOR TELEVISION 621 



being different, a double image will be produced on the received 

 picture, forming what may be termed visual echo. In the case of 

 telephone lines, the echoes may exist on account of reflections between 

 impedance irregularities in the circuit so that the currents arrive at 

 the receiving point both by way of the direct transmission path and 

 by way of a transmission path which includes an extra loop between 

 two irregularities. If the echo is not greatly attenuated with respect 

 to the main transmission, the result may be quite disturbing on the 

 received picture. It has been found by experiment that'the echo is too 

 weak to be seen if it is more than 25 T U weaker than the main current 

 and, accordingly, care was taken in setting up the New York- 

 Washington circuit to avoid introducing echo paths of lower equivalent 



than this. 



General Choice of Method 



Two general methods are possible for transmitting the currents 

 over the line circuits. One method is to transmit the currents directly 

 without change of frequency. This method involves the transmission 

 of the currents of the frequency range determined upon above, namely, 

 from about 10 cycles to about 20,000 cycles per second. 



The other general method is the carrier method, in which the 

 television currents modulate a carrier current of suitable frequency 

 and are thereby moved to another portion of the frequency spectrum 

 prior to transmission over the line. At the receiving end of the line 

 the carrier currents are then restored to the original frequencies of the 

 television currents. 



Several different schemes of carrier transmission are possible. The 

 simplest is to modulate a carrier with the television currents and to 

 transmit both side bands. This has the disadvantage of requiring the 

 transmission of twice as wide a frequency range as that occupied by 

 the original television currents. Another scheme is to transmit a 

 single side band. A third possible scheme is to transmit both side 

 bands for the lower frequencies and only one side band for the higher 

 frequencies. 



One advantage to be secured by the carrier method is that it lessens 

 the severity of some of the line problems through avoiding the trans- 

 mission of very low frequencies over the line circuit. At these fre- 

 quencies the amount of noise found on lines is usually considerably 

 greater than at the higher frequencies. 



After weighing the relative merits of the carrier and direct trans- 

 mission methods it was decided to make use of the latter because of 

 its simplicity. An important factor in this decision was the successful 

 development, for use in connecting the apparatus to the lines, of 



