1. Field of the Invention
The present invention relates to an SIF (sound intermediate frequency) signal processing circuit for use in a television set, videocassette recorder, or other.
2. Description of the Prior Art
It is customary to use a quadrature circuit to demodulate an SIF signal, which is an FM signal, especially when the SIF signal is demodulated at a high frequency such as 4.5 MHz. However, the use of a quadrature circuit is disadvantageous because of its high distortion factor and unsatisfactory linearity.
For this reason, it is also customary to use instead a pulse counting method, which offers a lower distortion factor and a better linearity. However, in order to demodulate an SIF signal by the pulse counting method, the SIF signal needs to be first converted to a lower frequency of approximately 500 Khz.
In certain geographical areas where television is broadcast in more than one type of television system, such as the NTSC and PAL systems, television receivers and videocassette recorders are usually designed to be capable of receiving broadcast signals of more than one type of television system.
In conventional SIF signal processing circuits, SIF signals carried by carrier waves of different frequencies of 4.5 MHz, 5.5 MHz, 6.0 MHz, and 6.5 MHz are uniformly converted down to 500 kHz. This is achieved by obtaining oscillation signals of 5 MHz, 6 MHz, and 6.5 MHz from different resonators, each designated for a particular type of SIF signal, and selecting with a switch one signal to be supplied from the oscillator to the mixers.
In this case, the 6.0 MHz resonator can be shared between SIF signals of 5.5 MHz and 6.5 MHz, but still three resonators are required in total. Inconveniently, this not only increases the manufacturing costs, but also increases the space occupied by the resonators.