RADIO TRANSMISSION SYSTEM FOR TELEVISION 649 



described in various other terms: "multiplex radio," "double modu- 

 lation," "the Hammond system," etc. The output of a 30,000-cycle 

 oscillator was modulated with the speech signal. The resulting 

 carrier and sidebands were selected by means of a suitable filter 

 passing frequency components ranging between 25,000 and 35,000 

 cycles and impressed on the input terminals of the radio transmitter 

 along with the 10 to 20,000-cycle signal from the television apparatus. 

 A suitable low-pass filter was employed in the line to the latter in 

 order to preclude "crosstalk" due to 25, 000-35, 000-cycle energy 

 working back into the final amplifier stages. The input to the radio 

 •transmitter thus consisted of a band extending from 10 to 20,000 

 cycles together with a 25,000 to 35,000 band, with a particularly 

 strong component at 30,000 cycles representing the low-frequency 

 carrier. 



In order that it might be capable of handling this wider band without 

 discrimination, further modifications in the radio transmitter were 

 required. In the case of some of the radio-frequency circuits, which 

 were required to pass a 70, 000-cycle band, it was found to be necessary 

 to insert resistance to reduce the sharpness of resonance. On account 

 of lack of time, it was not possible to obtain a complete series of 

 characteristic curves for the transmitter under these conditions. 

 Isolated measurements with a single-frequency input of 35,000 cycles 

 indicated, however, that components of this order could be trans- 

 mitted without serious loss and the subsequent performance of the 

 system as a whole confirmed this conclusion. 



It is well known that if a sinusoidal alternating current i = lo sin co/ 

 is modulated with a signal of frequency/ = $/2x, the resulting modu- 

 lated current may be represented by the expression: 



i — lo sin cot -\ — r- sin (co + ^)t -\ — r-sin (co — $)/, 



where ^ is a fraction indicative of the degree of modulation. In other 

 words, a modulated current, or wave, may be resolved into three 

 components: (1) a steady component, known as the "carrier," which 

 has the amplitude and frequency of the original unmodulated current, 

 (2) an "upper sideband" which is equivalent to the signal spectrum 

 with each individual frequency increased by an amount equal to the 

 carrier frequency, and (3) a "lower sideband" which is an inverted 

 reproduction of the signal spectrum, that is, each individual signal 

 component is laid oft in the downward direction from the carrier 

 frequency, or subtracted from it. Hence, assuming a carrier fre- 

 quency of 1575 kilocycles and a signal input to the radio transmitter 



