Intercom over a network
To broadcast audio signals by means of a network is not as easy as it seems to be. Usual network technology has not been developed neither to synchronize audio sources nor to realize conference connections. With conventional intercom devices the incoming voice channels are added in a special central unit. This central unit has to be built for a maximum number of stations and conferences.
The idea of the described intercom device is: Replacement of the central unit by a network. Thus the digital voice channels are no more processed by the central unit, but they are switched through directly to the receivers. During a conference different voice channels from the network node (netnode) are switched temporarily one after another on a receiver line (Time Division Switching Network). The sum up and handling of these channels is accomplished in the subscriber stations. The process is comparable with crossbars applied in studio technique, but with the extension that several sources may be switched to one destination. Instead of asynchronous networks (Ethernet, ATM) we developed a synchronous network, which is optimized to audio signals. Using standardized 2Mbit ISDN data streams makes it possible to combine with ATM- or telecommunication links.
The advantages of these "intelligent" stations are: Wide area distribution of signals over a standard telecom network. Each unit is equipped only with a computing capability necessary to the existing number of stations. A later enlargement of the unit is possible without great efforts like fibre-optic cables... A netnode can currently handle up to 32 (64) stations. Interconnection of several netnodes makes the unit expandable whereby the failure of one netnode does not affect the others.
- The distance between netnode and subscriber is up to 2 km according to the used cable.
- A 19", 3HE big netnode combines 32 subscriber stations and contains optional a redundant power supply and a remote power supply for the stations. It may be enlarged by addition of interface cards. Several netnodes can be connected by subscriber lines.
- The transmission is star configuration like, using 4-wire twisted-pair cables. Existing telephone cables may be used. Remote power supply for the stations may be realized through these data cables.
- The data rate is flexible within limits. In the intercom mode the transmission is compressed, that corresponds to a resolution of about 16Bit and 16kHz data rate. Then each station may receive up to 15 channels at the time, while the channels are assigned dynamically. As an option the transmission of a high-quality stereo channel with 48kHz data rate is also possible and you can synchronize with an external studio clock.
- Each station is dual-channel designed thus it may be used by two persons or for stereo purposes.
- It is planned to use the existing signalprocessors in the subscriber stations also for other purposes like e.g. echo cancellation or noise suppression.