Parallel tone method
The parallel tone process is a term used in analog television reception technology in which the intermediate frequencies for picture and sound are processed in parallel, i.e. in two separate intermediate frequency amplifiers.
Technical background
The receiving part of an analog television receiver works on the principle of the heterodyne receiver , so that after the mixer stage of the tuner, intermediate frequencies (IF) of the video and audio signals are present that have the same frequency difference as the transmitter - for single-channel audio 5.5 MHz ( CCIR ) or 6 , 5 MHz ( OIRT ) - have. With the parallel tone method, separate IF amplifiers are used from here on for the video and audio signals. This has the advantage that picture and sound do not interfere with each other, but it requires a high frequency stability of the mixer oscillator and a higher component cost.
In the intercarrier process, which was only used later , however, the image and audio intermediate frequencies are first filtered and amplified in a common IF amplifier, and only then in the video demodulator does a difference frequency (DF) arise from the frequency difference between the image and sound carriers, which is converted into a DF -Limiter amplifier is processed for frequency demodulation .
Quasi-parallel tone method
The quasi-parallel tone process is a mixture of the intercarrier process and the parallel tone process. Before the picture IF filter amplifier (38.9 MHz), the audio IF and the picture IF are sent to a separate mixer, which generates the difference frequency of 5.5 MHz and 6.5 MHz for the sound. This DF is prepared for further processing in an amplifier.
Since the DF is not generated in the video demodulator as is the case with the intercarrier method, the mutual influence between image and sound is not possible here either.
In addition, the audio IF is filtered out in front of the video demodulator in the picture IF amplifier so that the video demodulator only receives the picture IF (the video signal frequency band).