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System Overview AM64/128A User Manual
7
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.