American Telecom AM64/128A manual System Overview

System Overview AM64/128A User Manual

3.SYSTEM OVERVIEW

The AM64/128A baseband modem uses the digital transmission system designed and developed by ATL Telecom Ltd. in Cardiff. It is intended for operation on 2-wire local telephone network circuits, such as those meeting BT EPS-9 (2-Wire). It will operate satisfactorily on unloaded lines having a wide range of characteristics; bridge taps can be tolerated, dependent upon their characteristics. Although the system requires a baseband circuit, a continuous DC loop is not required. The system can transmit data at 71 kbps (64k rate) or 142.2 kbps (128k rate) ; the lower rate is useful in situations where the transmission performance is being limited by attenuation or noise.

The line signal comprises AMI encoded data and an embedded synch pattern which is used for timing control; the data consists of user data and a control channel for the systems own use. Echo cancellation is used to eliminate the unwanted reflections of the transmitted signal from the receiver input. The echo canceller can eliminate echoes of up to 16 bit periods duration. To counteract the signal distortion inherent in long lines the system employs adaptive decision feed back equalisation to eliminate trailing inter-symbol interference; this also has a span of 16 bit periods. A fixed linear equaliser is incorporated to control the received pulse waveform and to reduce the effect of the long tails of both echo and transmission responses.

Digital signal processing is used throughout to achieve high performance reproduction. A block diagram of the unit is shown in figure 3.1 above.

The line is connected to the transmission circuit via a line transformer which acts as a balun and provides isolation ; there is surge protection across the transformer line connections. The transmission circuit utilises a custom IC and a number of proprietary components to perform the signal processing described above.

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