VoIP Voice and Fax Signal Processing

6.1 TALKER AND LISTENER ECHO IN PSTN VOICE CALL

Figure 6.1(a) represents functional PSTN-based voice conversation. In the figure, a voice conversation is happening between two phones—A and B. In a PSTN call, the telephone interface is terminated at the central office (CO) or digital loop carrier (DLC) through a two-wire TIP-RING interface. The DLC hybrid as part of a subscriber line interface circuit (SLIC) creates a two-to-four-wire conversion. Analog-to-digital converter (ADC) and digital-to-analog converter (DAC) are referred to as a subscriber line access circuit (SLAC), also known as hardware CODEC. The DLC-to-PSTN CO and CO-to-CO interfaces work as four-wire conversions with separate send and receive paths. In the local PSTN call, the CO completes the call directly or through another DLC. In the long-distance call, the PSTN CO will send voice and signaling bits to the destination CO. The destination CO completes the call through its local DLC and telephone interfaces. The interface between the DLC and the telephone is a two-wire TIP-RING in most countries. Inside the telephone, a handset works equivalent to a four-wire interface, but the telephone interface going to the DLC is of two wires. A hybrid inside the telephone creates a two-to-four-wire conversion. Echo is created at every two-to-four-wire hybrid of DLC and telephone. A telephone hybrid echo level of 6 to 14 dB is more dominant than a DLC electronic circuit hybrid echo of 18 to 24 dB. In Fig. 6.1(a), echo is created ...