Secam chrominance signal demodulator with two red-blue channels and a single oscillator

A video signal receiver comprising:

• a demodulator for a chrominance signal, the demodulator comprising
• a controlled frequency oscillator with a loop input, an adjustment input, and an output;
• a phase comparator with a first input connected to the oscillator output, a second input to receive a chrominance signal with a modulation frequency, and a current output operatively connected to the loop input of the oscillator;
• a fixed current source, also connected to the loop input of the oscillator, the fixed current source providing a fixed current;
• first and second output branches which comprise first and second calibration resistors respectively in series with a common resistor and a common current generator outputting a current equal to twice the fixed current;
• and
• a current mirror to copy a current equal to the sum of the fixed current and a current at the current output of the phase comparator, into the first and second output branches;
• a value of the first calibration resistor being such that a first output voltage measured across the first calibration resistor is equal to a first value determined when the modulation frequency of the signal applied to the second input of the phase comparator is equal to a central modulation frequency corresponding to a first color, and a value of the second calibration resistor being such that a second output voltage measured across the second calibration resistor is equal to a second value determined when the modulation frequency of the signal applied to the second input of the phase comparator is equal to a central modulation frequency corresponding to a second color;
• a reference oscillator to be connected to the second input of the phase comparator to output a signal to the second input at a reference frequency, the reference frequency being different from modulation frequencies of the first and second colors;
• a chrominance channel to be connected to the second input of the phase comparator to transmit a chrominance signal;
• and
• a switch to connect the second input of the phase comparator to the chrominance channel or to the external reference oscillator.

A method for demodulating a chrominance signal using a demodulator including a controlled frequency oscillator, a phase comparator connected to the oscillator, a fixed current source also connected to the oscillator, first and second output branches which comprise first and second calibration resistors respectively in series with a common resistor and a common current generator, and a current mirror, the method comprising the steps of:

• receiving a chrominance signal having a modulation frequency, at the phase comparator;
• providing a fixed current from the fixed current source;
• outputting a current equal to twice the fixed current, from the common current generator;
• and
• copying a current equal to the sum of the fixed current and a current at a current output of the phase comparator into the first and second output branches;
• wherein a value of the first calibration resistor is such that a first output voltage measured across the first calibration resistor is equal to a first value determined when the modulation frequency of the chrominance signal is equal to a central modulation frequency corresponding to a first color, and a value of the second calibration resistor is such that a second output voltage measured across the second calibration resistor is equal to a second value determined when the modulation frequency of the chrominance signal is equal to a central modulation frequency corresponding to a second color.