Phase Distortion

We can look at a voice channel as a bandpass filter. A signal takes a finite time to pass through the telecommunication network. This time is a function of the velocity of

Attenuation Distortion

300 1000 3400

Figure 3.3 Typical attenuation distortion across a voice channel bandpass filter. Crosshatched areas are response specifications, whereas the wavy line is the measured response.

300 1000 3400

Figure 3.3 Typical attenuation distortion across a voice channel bandpass filter. Crosshatched areas are response specifications, whereas the wavy line is the measured response.

2Test frequencies of 800 and 1000 Hz are not recommended if the analog voice channel terminates into the digital network. In this case, CCITT and Bellcore (now Telcordia) recommend 1020 Hz. The reason for this is explained in Chapter 6.

3Any signal-passing device, active or passive, can display filter-like properties. A good example is a subscriber loop, particularly if it has load coils and bridged taps. Load coils and bridged taps are discussed in Chapter 5.

propagation of the medium and, of course, the length of the medium. The value can vary from 10,000 mi/sec (16,000 km/sec) to 186,000 mi/sec (297,600 km/sec). The former value is for heavily loaded4 subscriber pair cable. This latter value is the velocity of propagation in free space, namely, radio propagation.

The velocity of propagation also tends to vary with frequency because of the electrical characteristics associated with the network. Again, the biggest culprit is filters. Considering the voice channel, therefore, the velocity of propagation tends to increase toward band center and decrease toward band edge. This is illustrated in Figure 3.4.

The finite time it takes a signal to pass through the total extension of the voice channel or through any network is called delay. Absolute delay is the delay a signal experiences while passing through the channel end-to-end at a reference frequency. But we have learned that propagation time is different for different frequencies with the wavefront of one frequency arriving before the wavefront of another frequency in the passband. A modulated signal will not be distorted on passing through the channel if the phase shift changes uniformly with frequency, whereas if the phase shift is nonlinear with respect to frequency, the output signal is distorted with respect to frequency.

In essence we are dealing with phase linearity of a circuit. If the phase-frequency relationship over a passband is not linear, phase distortion will occur in the transmitted signal. Phase distortion is often measured by a parameter called envelope delay distortion (EDD). Mathematically, envelope delay is the derivative of the phase shift with respect to frequency. The maximum variation in envelope delay over a band of frequencies is called envelope delay distortion. Therefore, EDD is always a difference between the envelope delay at one frequency and that at another frequency of interest in the passband. It should be noted that envelope delay is often defined the same as group delay —that is, the ratio of change, with angular frequency,5 of phase shift between two points in the network (Ref. 2).

Figure 3.4 shows that absolute delay is minimum around 1700 and 1800 Hz in the voice channel. The figure also shows that around 1700 and 1800 Hz, envelope delay

Sieden Von Wasser Diagramm

Frequency (kHz)

Figure 3.4 Typical differential delay across a voice channel. 4Wire-pair loading is discussed in Chapter 5.

5Angular frequency and just the term frequency are conceptually the same for this text. Actually, angular frequency is measured in radians per second. There are 2n radians in 1 Hz.

Frequency (kHz)

Figure 3.4 Typical differential delay across a voice channel. 4Wire-pair loading is discussed in Chapter 5.

5Angular frequency and just the term frequency are conceptually the same for this text. Actually, angular frequency is measured in radians per second. There are 2n radians in 1 Hz.

distortion is flattest.6 It is for this reason that so many data modems use 1700 or 1800 Hz for the characteristic tone frequency, which is modulated by the data. A data modem is a device that takes the raw electrical baseband data signal and makes it compatible for transmission over the voice channel.

This brings up an important point. Phase distortion (or EDD) has little effect on speech communications over the telecommunications network. However, regarding data transmission, phase distortion is the greatest bottleneck for data rate (i.e., the number of bits per second that a channel can support). It has probably more effect on limiting data rate than any other parameter (Refs. 3 and 4).

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  • linda
    What is phase distortion with respect to voice channels?
    9 years ago

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