Preemphasis and Deemphasis Filtering In Fig. 6.4, the noise PSD at the output of the demodulator in PM is flat within the message bandwidth. However, for FM, the noise power spectrum has a parabolic shape. This means that FM performs better in low-frequency components of the message signal and PM performs better in high-frequency. So we want to design a system that performs FM for low-frequency of the message signal, and works as a PM for high-frequency. Thus, we would have a better overall performance compared to each system alone. This is the idea behind preemphasis and deemphasis filtering.
Preemphasis and Deemphasis Filtering The objective in preemphasis and deemphasis filtering is to design a system which behaves like an ordinary FM in the low frequency band of the message signal, and like a PM in the high-frequency band. Since a phase modulator is simply the cascade connection of a differentiator and a frequency modulator, we need a filter, in cascade with modulator, that does not affect the signal at low frequencies and acts as a differentiator at high frequencies. A simple highpass filter is a very good approximation to such a system. Such a filter has a constant gain for low frequencies; at higher frequencies, it has frequency characteristics approximated by K|f|, which is the frequency characteristic of a differentiator.
Preemphasis and Deemphasis Filtering At the demodulator side, low frequencies have a simple FM demodulator and high frequency components have a phase demodulator, which is the cascade of a simple FM demodulator and an integrator. Therefore, the demodulator needs a filter that has a constant gain at low frequencies and behaves as an integrator at high frequencies. A good approximation to such a filter is a simple lowpass filter.
Preemphasis and Deemphasis Filtering The modulator filter, which emphasizes high frequencies, is called the preemphasis filter. The demodulator filter, which is the inverse of the modulator filter, is called the deemphasis filter. Frequency responses of a preemphasis filter and a deemphasis filter are shown in below a) Preemphasis filter b) Deemphasis filter
Preemphasis and Deemphasis Filtering Another way to look at preemphasis/deemphasis is to note that due to the high level of noise at high-frequency of the message in FM, we want to attenuate the high-frequency of the demodulated signal. This results in a reduction in the noise level, but it causes the higher frequency components of the message to be also attenuated. To compensate for the attenuation of the higher components of the message, we amplify these components at the transmitter before modulation. Therefore, we need a highpass filter at the transmitter and a lowpass filter at the receiver. The net effect of these filters should be a flat frequency response. So, the receiver filter should be the inverse of the transmitter filter.
Preemphasis and Deemphasis Filtering The characteristics of the preemphasis and deemphasis filters depend largely on the PSD of the message process. In commercial FM broadcasting of music and voice, 1st order lowpass and highpass RC filters with a time constant of 75 s are employed. In this case, the frequency response of the receiver (deemphasis) filter is given by fo = 1/(27510-6) 2100 Hz is the 3 dB frequency of the filter. To analyze the effect of preemphasis/deemphasis on the overall SNR in FM broadcasting, we note that since the transmitter and the receiver filters cancel the effect of each other, the received power in the message signal remains unchanged, we only consider the effect of filtering on the received noise.
Preemphasis and Deemphasis Filtering Of course, the only filter that has an effect on the received noise is the receiver filter, which shapes the PSD of the noise within the message bandwidth. The noise component before filtering has a parabolic PSD. Therefore, the noise component after the deemphasis filter has a PSD given by where we have used (Eq. 6.2.15) for Sno(f) of FM. The noise power at the output of the demodulator can be obtained as
Preemphasis and Deemphasis Filtering Since the demodulated message signal power in this case is equal to that of a simple FM system with no preemphasis and deemphasis filtering, the ratio of the output SNR's in these two cases is as following: where we have used (Eq. 6.2.16) for Pno of FM. Hence, above equation gives the improvement obtained by employing preemphasis and deemphasis filtering. In commercial FM broadcasting (Example 6.2.3), in dB,
Comparison of Analog-Modulation Systems Now, we present an overall comparison of different analog communication systems. The systems that we have studied include linear modulation systems (DSB-SC AM, conventional AM, SSB-SC AM, VSB) and nonlinear modulation systems (FM and PM). Here we present a comparison of these systems based on three important practical criteria: 1. The bandwidth efficiency of the system. 2. The power efficiency of the system as reflected in its performance in the presence of noise. 3. The ease of implementation of the system (transmitter and receiver).
Comparison of Analog-Modulation Systems Bandwidth Efficiency The most bandwidth efficient system is the SSB-SC with a transmission bandwidth equal to the signal bandwidth. This system is widely used in bandwidth critical applications (voice transmission over microwave and satellite links, some point-to-point communication systems in congested areas) Since SSB-SC cannot effectively transmit DC, it cannot be used for the transmission of signals that have a significant DC component, such as image signals. A good compromise is the VSB system, which has a bandwidth slightly larger than SSB and is capable of transmitting DC values. VSB is widely used in TV broadcasting and in some data communication systems. PM, and particularly FM, are the least favorable systems when bandwidth is the major concern, and their use is only justified by their high level of noise immunity.
Comparison of Analog-Modulation Systems Power Efficiency A criterion for comparing the power efficiency of various systems is the comparison of their SNR at a given received signal power. We have already seen that angle-modulation schemes, and particularly FM, provide a high level of noise immunity and. therefore, power efficiency. FM is widely used on power-critical communication links, such as point-to-point communication systems and high-fidelity radio broadcasting. Conventional AM and VSB are the least power-efficient systems and are not used when the transmitter power is a major concern. However, their use is justified by the simplicity of the receiver structure.
Comparison of Analog-Modulation Systems Ease of Implementation The simplest receiver structure is the receiver for conventional AM, and the structure of the receiver for VSB system is only slightly more complicated. FM receivers are also easy to implement. These three systems are widely used for AM, TV. and high-fidelity FM broadcasting (including FM stereo). The power inefficiency of the AM transmitter is compensated by the extremely simple structure of hundreds of millions of receivers. DSB-SC and SSB-SC require synchronous demodulation and their receiver structure is much more complicated. These systems are, therefore, never used for broadcasting purposes. Since SSB-SC is only slightly more complicated than DSB-SC, DSB-SC is hardly used in analog signal transmission, due to its relative bandwidth inefficiency.