The Three Basic Impairments And How They Affect The End-User

There are three basic impairments found in all telecommunication transmission systems. These are:

· Amplitude (or attenuation) distortion

· Phase distortion

· Noise

Amplitude Distortion

The IEEE defines attenuation distortion (amplitude distortion) as the change in attenuation at any frequency with respect to that of a reference frequency. For the discussion in this section, we’ll narrow the subject to the (analog) voice channel. In most cases a user is connected, through his/her metallic subscriber loop to the local serving exchange. This circuit is analog. Based upon the CCITT definition, the voice channel occupies the band from 300 to 3400 Hz. We call this the passband.

Attenuation distortion can be avoided if all frequencies within the passband are subjected to the same loss (or gain). Whatever the transmission medium, however, some frequencies are attenuated more than others. Filters are employed in most active circuits (and in some passive circuits) and are major causes of attenuation distortion. Figure 3.3 is a response curve of a typical bandpass filter with voice channel application.

As stated in our definition, amplitude distortion across the voice channel is measured against a reference frequency. CCITT recommends 800 Hz as the reference; in North America the reference is 1000 Hz.2 Let us look at some ways that attenuation distortion may be stated. For example, one European requirement may state that between 600 and 2800 Hz the level will vary no more than −1 to +2 dB, where the plus sign means more loss and the minus sign means less loss. Thus if an 800-Hz signal at −10 dBm is placed at the input of the channel, we would expect −10 dBm at the output (if there were no overall loss or gain), but at other frequencies we can expect a variation at the output of −1 to +2 dB. For instance, we might measure the level at the output at 2500 Hz at −11.9 dBm and at 1100 Hz at −9 dBm.

When filters or filter-like devices3 are placed in tandem, attenuation distortion tends to sum. Two identical filters degrade attenuation distortion twice as much as just one filter.

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

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 wave front of one frequency arriving before the wave front 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

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.