In FM Broadcasting, the effect of noise is more intense on higher frequencies than on low frequencies. Therefore, in order to have high signal-to-noise ratio(low noise), the high frequencies are amplified at the transmitter side (pre-emphasis) and for compensation, deemphasis (decreasing the amplitude of those boosted frequencies ) is done at receiver.
The amount of pre-emphasis and de-emphasis used is defined by the time constant of a simple RC filter circuit. In most of the world a 50 µs time constant is used. In North America and South Korea, 75 µs is used. This applies to both mono and stereo transmissions. For stereo, pre-emphasis is applied to the left and right channels before multiplexing.
Here, we present a simple pre-emphasis circuit, which can be used in mono FM transmitters or it can also be used to pre-emphasize the left and the right audio channel in any FM stereo generator.
The circuit is actually a high pass filter because pre-emphasis actually refers to a filter designed to increase the magnitude of some higher frequencies with respect to the magnitude of lower frequencies.The proposed pre-emphasis network characteristics are shown on the next figure.
The presented pre-emphasis circuit uses 50us pre-emphasis because it was built and tested in Europe (Greece). However, it can be easily changed to 75us by simply changing C3 to 2.7nF.