Principles of Operation (AlphaCom)

Stentofon's intercom exchange AlphaCom is based on a new technology compared to previous systems. The two major changes in AlphaCom compared to Touchline is the speech coding used, and the fact that all actions in the exchange is based on messages on a 16 Mbit/s data bus. This makes the exchange fully digital internally. The purpose of this document is to describe the basic principles of the exchange and to give an overall view of the system. The different boards in the exchange are identified and described.

System Overview

Exchange

The AlphaCom intercom exchange is built up by one ore more modules. Each module can have a maximum of 138 subscribers. This number is heavily dependent on the number of other boards than subscriber line boards in the module. When the term exchange or AlphaCom is used, it refers to one or a number of interconnected modules.

Module

The module is based on a backplane carrying digital signals. This backplane interconnects the various parts of the system.

A module is the backplane fitted into a 19" cabinet. There are 26 slots (connections) to the backplane. The signals on the backplane can be divided into three sections.

Audio channels. There are 16 TDM buses, thus giving a total of 256 timeslots in the system. These 16 buses are referred to as the audio bus.
Data channels. There are two 16 Mbit/s serial data channel, the display data channel and a set of control and sync signals. These signals are referred to as the data bus.
Power leads. A set of supply voltages.

The module can be equipped with different boards, each with a number of features and resources.

The boards of the module

Module Controller AMC

The AMC board is the key board in the system. This board is literally controlling the entire module. All connections, switching and routing is controlled by AMC. All the internal data-transfer and communication between the boards in the module is performed on one of the 16 Mbit serial data channels. The processor on AMC is linked to this serial bus by the Master Bus Interface (MBI).

Both the SBI and the MBI will be described separately. The other blocks are:

Tone Filters

All steady tones in the module are generated on AMC. The tones are generated by the MBI and by the DUARTs. Typical steady tones are tones that are available on the Audio Bus like Busy tone and Dial tone.

In addition to these tones there are a Attention Signal (GONG) generated by the DUARTs. All these signals are generated as square waves. These signals must be filtered to sine waves before they are modulated and switched to the Audio Bus by the SBI.

DTMF Generator

Some external equipment like pagers require DTMF signalling from the Module. The DTMF signals are modulated and switched to timeslots in the Audio Bus.

Watch Dog

The Watch Dog circuitry supervises the operation of the 68000 processor, as well as monitoring the power supply to the board.

Real time Clock

The Real time Clock has a Battery Back up. Used as a time reference in the module

DUART - Line drivers/receivers

4 Dual Asynchronous Receivers and Transmitters makes up the 8 serial ports to the module. Dependent on the drivers these ports will be RS232, RS422 or RS485.

Memory

The Memory Block consists of 5 different elements:

Standard Program Memory	This is 1Mbit EPROMs with the standard SW for the 68000
RAM	Parts of the RAM are battery backed, making it a Non-Volatile RAM. (NV-RAM). All subscriber information resides in RAM.
Two Port Ram	The communication between the 68000 and the MBI goes through a Two Port Ram.
Back-up memory	Flash-EPROM witch contains a subset of the information in the NV-RAM.
Plug-in-SW	Customer specific SW that operates on a defend set of resources in the module.

Subscriber Line board ASLT

The ASLT board connects the subscribers to the exchange. The letter T in ASLT states "Touchline compatible". This means using the four wire interface to subscriber equipment. All signalling to and from the station uses different current-level on the a-b and c-d wires. Since the back wiring is digital, the audio signals are modulated by the subscriber circuitry and subsequencly multiplexed and switched by the Slave Bus Interface SBI.

There are 6 subscriber connections on each ASLT. In addition there are two speech channels.

Current Scaning

The line current scan circuitry measures the current levels on the a-b and c-d wires of all subscribers lines.

Digit receiver

Like the line current scan, the digit receiver is connected to the analog part of the subscriber block. Based on the information from the current scan, the μcontroller will detect when a digit key is pressed, and assign the digit receiver to this subscriber.

Buffer

The σcontroller provides 6 programmable digital outputs. These outputs are buffered before they are fed to the edge connector.

Program and Clock APC

The APC board contains the system clock, i.e. the timing reference for all other boards in the module.

In addition, APC contains circuitry for 8 program-feed channels. A program-feed channel is a audio program with a known signal level. This program is fed by any analog source e.g. a radio, a tape-recorder or similar.

Stored Voice Message Playback ASVP

The ASVP board stores pre-recorded messages. These messages are coded using ADPCM. The bit rate is 32 kbit/s. On command from AMC, a specified message is decoded. The decoded signal is subsequently -Delta coded and routed to the correct timeslot by the SBI. The ASVP has 8 channels, enabling simultaneous playback of 8 messages.

PB ACPCM

Each play-back block consists of four elements. First a FIFO, to which the 68000 is transferring data from the Stored Voice Memory. Then follows the ADPCM decoder (MSM 5218). The output of the decoder is filtered before it is Delta Coded and switched to the Audio Bus by the SBI.

Stored Voice Memory

The memory for the encoded speech is based on EPROMs. The maximum amount of memory available is 16 sockets for 1Mbit/2Mbit/4Mbit EPROMs. Based on a 32 kbit/s coder, the theoretical maximum speech time is 34 min.

Program Feed APF

The APF board is the analog interface to different program sources. The board is equipped with 16 balanced inputs. These inputs are sampled and routed to the audio bus.

ASICs in the system

Slave Bus Interface SBI

The key element of the exchange is the SBI.The SBI interface to both the audio bus and the data bus.

The SBI performs a number of actions. The most important features are the following: Interface between the -Delta Coded signal from each subscriber and the back wiring. The SBI is a Time Space Switch, capable of switching any signal to any of the 256 timeslots on the audio bus.

Master Bus Interface MBI

The other important ASIC of the system is the Master Bus Interface MBI. MBI interface the data bus, but have an interface to SBI's on the same board as the MBI that is independent of the serial data bus. The MBI interfaces the 68000 processor to the SDI and SDO, this making up the connections to all the SBIs in the system.

System architecture

Modes of operation

Simplex Mode

In Simplex Mode the speech direction is controlled by the M-key on one or both stations.

Voice Switched Duplex mode (Handsfree operation)

This is the normal loudspeaking mode of the stations. In this mode only one loudspeaker is connected at the time. One microphone signal is only connected to one demodulator (or loudspeaker). The switching is performed by AMC, based on information gained from the two measuring or speech channels involved.

Open Duplex mode

In this mode all stations involved must have lifted their handsets. In this case the loudspeaker in the handsets are fed with the other party's microphone signal. In this mode the duplex algorithm is disabled.

Program mode (Listen mode)

ASLT is capable of connecting the loudspeakers to timeslots with other sources than subscriber connections.

These are

Program Sources
Voice Paging

Both these sources can be connected simultaneously to the loudspeaker.

Microphone signal

When a station is engaged in an conversation, the microphone signal is always assigned to a timeslot on the audio bus. Timeslots will be referred to as TSn, where n is a number between 0 and 255. The other party will have its microphone signal assigned to another timeslot. The conversion from an analog microphone signal on the c-d wires to -Delta coding is performed by circuitry on the ASLT. The software on the AMC will allocate a timeslot for the signal, and set up the SBI to route the signal to this timeslot. All microphone signals from stations engaged in a conversation are always connected to the audio bus.

Loudspeaker signal

There are no timeslots with dedicated loudspeaker signals, since the SBI is capable of routing any timeslot to the decoder. The output of the decoder is feed to the output amplifier on the ASLT. A timeslots can carry a microphone signal, audio-program source or other audio signals like fixed tones and DTMF-tones, all generated on the AMC board.. Unlike the microphone signal, the output amplifiers are not permanently connected to the audio bus through the SBI. This will be discussed under the different modes of a connection.

Audio bus

All boards in the system are connected to the audio bus (DAB[15:0]). Some of the boards only feed the audio bus (like program feed boards (APF), tone generators (AMC), playback boards (ASVP)), while other boards have are capable of both feeding and tapping the audio bus. In conversations, the microphone signal will be fed into a timeslot. The other party will tap this timeslot and the signal being fed to the loudspeaker. There can be only one signal feeding a timeslot, while there is no limitation to how many SBIs that can pick up from a timeslot. This is utilized in program distribution.

Address-map of the exchange

The 68000 processor on AMC sees the entire module through the MBI as a part of its memory (Memory mapping). The address range of the memory map is 20 bits, and gives a range of addresses from 0x00000 to 0xFFFFF (1 Mbyte). Each slot in the backwire ABW represents the address range from 0x0000 to 0x7FFF ( 32 kbyte). Each SBI is addressed from 0x000 to 0x1FF (512 bytes).
The first SBI is always located at offset 0 in each slot. Each slot (board) can have a maximum of 8 SBIs.

Addressing examples

The first SBI on board in slot #0	0xE00000 to 0xE001FF
The first SBI on board in slot #3	0xE01800 to 0xE019FF
The second SBI on board in slot # 12	0xE60200 to 0xE603FF

Data bus

Serial data bus and control signals

On the backplane there are two 16Mbit/s serial buses and a number of control signals. These signals are:

BPOS[4:0]	Board position. The physical address to each slot in the backplane is coded in the backplane, giving each slot a unique address from 0 to 25 (actually 31)
MRES	Master Reset. This signal will reset the entire module.

MCK1

MCK2

Master Clock1.

Master Clock 2.

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2 phase 16 MHz clock with a 90 degrees phase shift.