This invention relates to time assignment speech interpolation systems.
Because of the extremely high cost of communications transmission facilities, e.g. satellite channels and undersea transmission links, the prior art has sought various means to maximize the efficiency of existing transmission facilities. One such system is known as a time assignment speech interpolation (TASI) system. In a typical TASI system, calls from n callers are transmitted across, for example n/2 transmission facilities to a remote location. At that location, the n/2 facilities are connected to n output speech channels. TASI systems operate on the assumption, verified as a statistical fact, that at any given time not all callers will wish to talk simultaneously. In fact, as a general rule, callers are actively talking less than half of the time the talker and the listener are interconnected. Accordingly, TASI systems may be defined as switching systems which interconnect talker and listener only when the talker is actively speaking, provided there is a transmission facility available at that time.
"OVER-ALL CHARACTERISTICS OF A TASI SYSTEM" by J. M. Fraser, D. B. Bullock and N. G. Long, The Bell System Technical Journal, July 1962, pages 1439-1473 describes a TASI system. Such systems have been successfully used on undersea cables, for example, where a relatively large number of transmission facilities are available. Typically, thirty-six transmission facilities are available to transmit signals from seventy-four speech channels. Another transmission facility, referred to as the order wire, or control channel, is used to transmit disconnect signals to the remote location. FIG. 3 of the aforementioned article shows such a system.
It is necessary to use a separate transmission facility as an order wire in the prior art in order to disconnect an unused output speech channel from a facility at the remote location before connecting that facility to a new output speech channel. The prior art systems transmit a tone signal preceding the speech burst. This tone identifies the output speech channel to which the speech burst is to be connected. If the transmission facility is not disconnected from the old output speech channel before arrival of this tone, an unpleasant noise is applied to the old speech channel.
It is desirable to apply the principles of time assignment speech interpolation to systems in which a smaller number of transmission facilities are available. For example, it might be desirable to apply the TASI principle to private line communications systems having as few as four transmission facilities. In order to make the most efficient use of such a system, it is desirable to eliminate the order wire.
It is an object of the present invention to eliminate the order wire in a time assignment speech interpolation system.
It is a further object to eliminate the order wire without introducing the disadvantage of hearing the tone which precedes a speech burst. The present invention disconnects a speech channel from a transmission facility before the arrival of a tone identifying a new output speech channel can be heard.