Encoding circuit for a SECAM color television transmission

An encoding circuit for a SECAM color television transmitter in which two line-sequential color difference signals modulate a color subcarrier signal generated by a frequency-modulatable oscillator, the generated subcarrier signal being at a first quiescent frequency during scanning line periods of the first color difference signal and at a second quiescent frequency during scanning line periods of the second color difference signal, the modulated subcarrier signal being at the quiescent frequency corresponding to any scanning line period during a blanking interval thereof in which the relevant color-difference signal is zero, such encoding circuit comprising:

• means for generating a first reference signal at a frequency which is a reference for said first quiescent frequency and a second reference signal at a frequency which is a reference for said second quiescent frequency;
• a frequency demodulator adapted to derive at its output a demodulated voltage of a value dependent on the frequency of a signal applied to its input;
• a multiplex circuit connecting said reference signal generating means and said frequency-modulatable oscillator to the input of said frequency demodulator;
• pulse generating means adapted to supply switching signals to said multiplex circuit which cause it to apply to the input of said frequency demodulator predetermined sequences of said modulated subcarrier signal and said first and second reference frequency signals during successive scanning line periods;
• a first control path connected to the output of said frequency demodulator and responsive to further switching signals supplied by said pulse generating means to a produce a first control voltage by subtracting the demodulated voltage derived from said first reference frequency signal during predetermined scanning line periods from the demodulated voltage derived from said modulated subcarrier signal during the blanking intervals of predetermined scanning line periods when such signal is at said first quiescent frequency;
• a second control path connected to the output of said frequency demodulator and responsive to further switching signals supplied by said pulse generating means to produce a second control voltage by subtracting the demodulated voltage derived from said second reference frequency signal during predetermined scanning line periods from the demodulated voltage derived from said modulated subcarrier signal during the blanking intervals of predetermined scanning line periods when such signal is at said second quiescent frequency;
• and means connected to each of said control paths for applying said first and second control voltages to said frequency-modulatable oscillator to modify the first and second quiescent frequencies thereof to correspond with said first and second reference frequencies;
• the switching signals supplied by said pulse generating means being sequenced so that there is at least one scanning line period between those during which are derived the demodulated voltages which are subtracted to produce each of said control voltages.