928 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



frequency control circuit as a feedback loop/ In this case two factors 

 contribute to loop phase shift. First, the phase angle variation of the 

 oscillator output in response to the control voltage is an integration 

 process. The control voltage changes the oscillator frequency and the 

 resulting phase change can be expressed as the integral mth respect to 

 time of the frequency shift, 



phase, 0, = 27r J ^fdt. (10) 



The integration with respect to time introduces a 90 degree "low-pass" 

 phase shift into the control loop at all frequencies. Second, the averaging 

 low-pass filter introduces phase shift in the same direction so that care 

 must be exercised to avoid instability. In this case a phase stability 

 margin of 45 degrees is provided over a wide range of frequency by de- 

 signing the low-pass filter as a series of resistance capacitance steps of 

 loss. These are staggered in frequency to produce a cut-off rate of 3 db 

 per octave with a phase shift of 45 degrees over a wide frequency band. 



The polarity ambiguity resulting from the squaring process has been 

 demonstrated in the derivation of the phase control voltage. In equation 

 (5) the plus or minus designation indicates that either polarity of carrier 

 signal might be assumed without affecting subsequent expressions. How- 

 ever, in the derivation of the output voltage from the main signal 

 demodulation, equation (3), the output signal polarity reverses if the 

 carrier, equation (2), is assumed with reversed polarity. The carrier 

 polarity estabUshed at any given time depends largely upon initial 

 phase conditions when signal is applied. 



Correct video polarity at the receiver output is established by a 

 new device called a polarizer which follows the demodulator. This circuit 

 recognized video polarity on the basis of standard features in the com- 

 posite television waveform. The particular features used in this case 

 are the vertical blanking discontinuities expected once each sixtieth of 

 a second and the duty factor of sync pulses. These two characteristics 

 taken together form a sufficient condition for the determination of 

 polarity of any composite video waveform independent of picture con- 

 tent. The polarity, once recognized to be inverted, is corrected. 



II .i;\i(>Nic Distortion 



A significant consideration in transmission problems is the generation 

 of distortion by the non-linear amplitude characteristic of the transmis- 

 sion apparatus. In the c^jise of video transmission, non-Unearity results 



