n the return loss becomes low (i.e., there is a poor impedance match), there a reflection of the speech signal. That is, speech energy from the talker at her/his distant hybrid leaks across from the four-wire receive to the four-wire transmit side (see Figure 4.15). This signal energy is heard by the talker. It is delayed due to the propagation time. This is echo, which can be a major impairment depending on its intensity and amount of time it is delayed. It can also be very disruptive on a data circuit. We define the cause of echo as any impedance mismatch in the circuit. It is most commonly caused by this mismatch that occurs at the hybrid. Echo that is excessive becomes singing. Singing is caused by high positive feedback on the intervening amplifiers (Figure 4.14). Singing on the analog network could take the network down by overloading multiplex equipment. The possibility of singing on the digital network is very low. The control of echo and singing is discussed in Chapter 8. Figure 4.16 is a schematic digram of a hybrid circuit. 4.5 INTRODUCTION TO MULTIPLEXING 4.5.1 Definition Multiplexing is used for the transmission of a plurality of information channels over a single transmission medium. An information channel may be a telephone voice chan- nel, data channel, or a channel carrying image information. Our discussion below will concentrate on a telephone channel. A telephone channel is a channel optimized for car- rying voice traffic, in this case the voice of a single telephone user. We will define it as an analog channel with occupying the band of frequencies between 300 and 3400 (CCITT/ITU-T definition). Before launching into our discussion, keep in mind that all multiplex equipment is four-wire equipment. If we look at one side of a circuit, there will be a multiplexer used for transmission and a demultiplexer used for reception. The number of channels that can be multiplexed on a particular circuit depends on the bandwidth of the transmission medium involved. We might transmit 24 or 48 or 4.5 INTRODUC