648 BELL SYSTEM TECHNICAL JOURNAL 



If attention is confined to the interval during which scanning takes 

 place along the line A A, it is evident that the television signal will 

 have the form shown. Amplitudes above the dotted line indicate the 

 current through the photoelectric cell. Since transformer-coupled 

 amplifiers are employed in the television apparatus, however, the 

 direct component is eliminated and the zero axis for the input to the 

 radio transmitter is the solid line. Sketch II shows the modulated 

 output of the radio transmitter. The received signal, shown in III, is 

 assumed to consist of two components, the larger due to the "ground 

 wave," and the smaller due to reflected energy from the Heaviside 

 layer. The latter lags somewhat because of the greater length of the 

 transmission path. The resultant of these two components will 

 necessarily depend on the relative phase of the two carriers at the 

 receiving point. Two cases are considered: when the components 

 are exactly in phase, and when they are exactly out of phase. The 

 effect at intermediate positions may be readily evaluated from these 

 examples. With the components in phase, the detector output is 

 proportional to their sum which is shown in IV. It is evident that this 

 will result in a major image and a secondary positive image. If the 

 components are out of phase, the rectified signal shown in V results. 

 It is simply a matter of subtracting amplitudes. This resultant 

 consists of the desired signal with the amplitude somewhat reduced 

 which will produce a gray background. The secondary image will 

 be formed by the two small peaks shown and will be lighter than the 

 background, in other words a negative. 



A pattern frequently observed was the diamond with a cross through 

 its center due to a secondary image. This represents a change in 

 framing of approximately one half line. With 17.7 pictures per second 

 and 50 lines per picture, this corresponds to a difference in trans- 

 mission time of 1/17.7 X 1/50 X 1/2 or 5.65 X IQ-* seconds. A 

 rough computation of the height of the reflecting layer based on this 

 figure and a distance of 22 miles between transmitting and receiving 

 stations gives 100 kilometers, which is substantially in agreement with 

 determinations made by other methods. 



Transmission of Voice and Image with a Common 

 Carrier Frequency 



Following the demonstration, a brief series of supplementary tests 

 was arranged to obtain some appreciation on experimental grounds 

 of the problems involved in transmitting both voice and image with a 

 single radio transmitter. The system employed may be considered 

 as the extension of carrier current technique to radio, but has been 



