Audio Teleconferencing

Although videoconferencing is making headway, audio teleconferencing continues to deliver on the promise of long-distance communications for education and business.

When my mother used to ask my brother questions about baseball, Bob would invariably begin his answer saying, "Mom, nine men are on a baseball team." In a similar vein, this discussion of teleconferencing hybrids must start with a short explanation of the two-wire plain old telephone service.

The standard analog telephone connection between a residential or commercial site and the local switching office is a two-conductor pair. Because both sides of the conversation travel over the two-conductor pair, some means must be provided to separate the transmit and receive signals. One two-wire to four-wire conversion is done on the switching-office side to facilitate sending to and receiving from distant switching offices. The other conversion happens at the customer's handset. The separation of the transmit signal from the receive signal is not accomplished perfectly. In particular, when the customer speaks into his handset, he hears himself in the earpiece. This is known as sidetone and gives a handset the familiar live sound. This is a classic case of "If you can't fix it, feature it."

Signal leakage reduction * The analog approach: Not more than a few years ago, if you wanted to connect a sound system to an analog phone line, you would use an analog hybrid. An analog hybrid uses passive techniques to minimize the amount of the local transmit signal that leaks through to the local receive signal. Because the impedance of the telephone line is complex and can change during a conversation, an analog hybrid can only achieve about 10 dB to 15 dB reduction in the transmit signal leakage to the receive output. This is an obvious problem in a teleconferencing application because this leakage is clearly audible from the local loudspeakers. Aside from being annoying, transmit leakage can cause reduced intelligibility or feedback in the local teleconferencing sound system. * The digital approach: With the advent of modern digital signal processors, a much more effective approach to transmit leakage reduction became feasible. Using a digital adaptive filter, the digital hybrid can realize a much better approximation to the telephone line impedance and can also track changes in the line impedance over time. Fairly typical for a digital hybrid is 30 dB to 40 dB of transmit leakage reduction. As an added bonus, once the signal is in the digital domain, other desirable signal processing can be performed by the digital signal processor.

Echo suppression There is perhaps no better known catch phrase in teleconferencing than full-duplex operation. Full duplex literally means that the transmit and receive paths are always fully open all the time. But because all digital hybrids use some form of echo suppression, none can have truly full-duplex operation. In practical terms, full-duplex means the ability for participants who are physically remote from one another to hold conversations. Whether this goal is fulfilled has to do both with the performance of the digital hybrid and with other aspects of the system implementation.

Echo suppression refers to manipulating the transmit and receive signal path gain to achieve even lower transmit-to-receive leakage. In addition, echo suppression can minimize the effects of low-level echoes caused by line reflections in the phone system.