The invention relates to a television receiver incorporating a processing section for processing stereo/dual sound signals having a first sound carrier which is modulated by a first sound signal and a second sound carrier modulated by a second sound signal and also by a pilot signal which is modulated by a stereo-dual sound identifying signal, said processing section comprising a synchronous demodulator in which the pilot signal is demodulated and to which an output signal of a Phase-Locked Loop circuit is applied, the phase-locked loop circuit comprising a frequency-controllable oscillator and a phase discriminator which compares a signal derived from the oscillator with a reference signal which, as regards frequency and phase, is in a fixed relationship to the pilot signal. Such a television receiver is obtained when the known integrated circuit TDA 4940 marketed by Siemens is used.
As is known, two frequency-modulated sound carriers are used, according to the German standard, for the transmission of stereo/dual sound television signals, the second, for instance weaker, sound carrier being frequency-modulated by a pilot carrier which, in the case of stereo or dual sound transmission, is amplitude-modulated by an identifying signal which characterizes the stereo or dual-sound transmission mode and which is required in the receiver to enable the required switching actions to be effected automatically. In a television receiver comprising the prior art circuit, the identifying signal is obtained in that the modulated pilot carrier is multiplied by the output signal of a PLL circuit whose oscillator oscillates at a frequency equal to 28-times the line frequency and whose frequency is controlled by a phase discriminator which compares the frequency-divided oscillator signal with the line frequency.
The fact that, in accordance with the relevant German standard, the line frequency is in a fixed frequency and phase-relationship to the pilot carrier frequency is utilized, and the pilot carrier frequency is precisely 3.5-times the line frequency. As a result thereof, a signal which is phase-locked onto the pilot carrier can be recovered from the ocscillator signal by means of a 1:8-frequency divider. Then only the modulation product of the pilot carrier, that is to say one of the two identifying signals, is then only present at the output of the multiplier circuit.
An advantage of this circuit is that it has a high identifying signal reliability and sensitivity. A disadvantage is that it always requires the presence of a signal of the line frequency which is phase-locked onto the pilot carrier frequency. This signal is, however, not always available. Novel receiving and display concepts provide, for example, the possibility to connect the receiving section of a television receiver to a video recorder and the display section to a video disc player. However, for recording with the video recorder, the identifying signal must already have been demodulated. The line-frequency signal required therefore is however only available in the display section of the television receiver and is derived from the video disc, and consequently has no fixed phase relationship with the pilot carrier. The prior art circuit is not suitable for such a receiver concept.
To recover the pilot signal, from the antenna signal it is necessary for the carriers contained in the antenna signal and on which the sound signals are frequency modulated, to be first converted into a frequency corresponding to the frequency spacing of the two carriers from the picture carrier on which the video signal is modulated. For the German standard, two sound carriers are obtained in this manner at 5.5 and 5.742 MHz, respectively. In a known television receiver published in Funkschau 2, 1982, pages 76 to 79, these two sound carriers are separated from each other by two filters and demodulated by two demodulators, which produce the two sound signals and also the modulated pilot signal.
As in the dual-sound mode, the two sound signals are independent from each other as regards their content, a very high cross-talk attenuation, for instance better than 60 dB is necessary between the two sound channels. For that reason and as the frequency spacing of the upper side band of the sound carriers to the lower frequency of the lower sideband of the sound carriers having the higher frequency--relative to the sound carrier frequencies--is very small, the filter arrangement must be formed from very expensive high-selection filters. For that purpose, the known arrangement utilizes ceramic filters. Also the FM-demodulators require resonant circuits which, in the known receiver, are also ceramic filters. All the filters and resonant circuits must be balanced. Consequently, the processing section for processing the dual-sound stereo signals is very expensive.
A further disadvantage is that with such a receiver, only signals in accordance with the German standard can be received. If signals in accordance with a different standard, that is to say with an other frequency of the sound carrier or a different frequency spacing between the carriers for the picture and the sound information, must be processed, additional filter, resonant circuits, etc. are required.