View source for 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:
{| border="1"
|-
|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 &sigma;controller 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). <br>
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:

{| Border="1"
|-
|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.
|}



[[Image:Databus - mck2.JPG|thumb|left|200]]
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{| Border="1"
|-
|'''MCK1'''
'''MCK2'''
|''''Master Clock1.'''
'''Master Clock 2.'''
| '''}'''
|'''2 phase 16 MHz clock with a 90 degrees phase shift.'''
|}



{| Border="1"
|-
|REFCK
|'''1 MHz reference clock for synchronizing MCK1 and MCK2 to a'''
'''Module Interconnect board AMIS/AMID'''
|-
|FRSYN
|256 kHz Frame Synchronizing signal. Synchronizes the SBI's to the audiobus
|}



[[Image:Databus - SPSYN.JPG|thumb|left|200]]
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{| Border="1"
|-
|SPSYN
|'''8 kHz signal with 75-25 duty cycle. Every 80'th pulse has a 50-50 duty cycle generating a 100 Hz synchronization for the''' '''duplex scanner and all other routines which run every 10'th ms.'''
|}



==== Memory - Mapped Data Bus ====
{|
|-
|SDI
|Serial Data Channel In. 16 Mbit/s serial data to the AMC board
|-
|SDO
|Serial Data Channel Out. 16 Mbit/s serial data from AMC board.
|-
|BERR
|Bus Error. If an error is detected on the bus, this signal will reset
all bus interface devices.
|}



==== Interrupt and Board Status Bus ====

{|Border="1"
|-
|Brint
|Buffer Ready INTerrupt. Active when SBI is ready to accept a message.
|-
|PMINT
|Primary Message Ready Interrupt. Active when a message in a SBI is ready to be read by the Master.
|-
|AINT
|Auxiliary INTerrupt. Interrupt from board resources external to SBI
(microcontrollers, interface devices...)
|-
|BST
|Board STatus. Indicates that the board in the given position is in operating
mode, that is presentt and running.
|}

==== Display data ====
Since the station interface is analog, the transmission of display data is done using a data over voice technique. The coding of the display data is coded using ''Frequency Shift Keying'' FSK using the two frequencies 48 kHz and 64 kHz. The bit rate is 9600 baud or kbit/s. The display data is generated on the AMC board. First the signal is modulated using FSK, then the signal is Σ-Delta Coded. The display data signal is a 16 Mbit/s signal, and occupies two wires in the backplane. 



{|Border="1"
|-
|DDA
|The active data line, where the Σ-Delta Coded FSK signal is transmitted.
|-
|DDP
|A reference line where a steady Σ-Delta Coded 48 kHz signal is transmitted.
|}



The SBI is connected to DDA and DDP. There are 8 display data outputs on the SBI. The display data output can be turned of and on to each individual output, which gives the system a powerful way of addressing groups of subscribers to receive display information. This function can be used to add new functions to the Touchline Station. This due to the fact that the processor in the stations are able to receive messages on DDA/DDP that controls output pins on the processor. These can be used for LED-handling, Volume setting override... (on stations with these capabilleties)

==== Subscriber scan ====
Every 10' ms ( clocked by SPSYN ) the subscriber connection are scanned. The scan is performed by the μcontroller on ASLT. The scan is separated into two sections: Duplex scan and line current scan.

The duplex scan measures the decoded output of the two (freely chosen) timeslots set up by AMC. The values of these microphone signals are quantisized (see PCM) and transferred to registers in the SBI. Another routine checks for presence of consonants by detecting pulse trains. The status of this routine is also contained in an SBI register.

The content of these registers are  subsequently transferred on SDI to AMC .

The line current scan measures the current levels on the 6 subscriber connections. If a key has been pressed or if there is any change in privat/open. When a change of state is detected,the AMC board will be notefied.

==== Speech channel ====
Since the system is based on loudspeaking stations, the loudspeaker signal of the talker must be muted. Both microphone signals levels are measured in order to calculate which microphone that should be active. The circuitry performing these measurements are called the speech channel or scanner. Based on the measurement of two microphone channels the AMC will perform the appropriate action. This would typically be connecting and disconnecting the demodulator from the audio bus. This would turn of and on the loudspeaker.

In AlphaCom the speech channel circuitry is on the ASLT board. The core element in this is the μcontroller on the board. Each ASLT board contains two separate speech channels. To stations involved in an conversation do not have to use speech channels on the same ASLT board. They must however use two speech channels connected to the same AMC. See ''Module interconnect''. The reason for this is that the Duplex Handler resides on AMC. The Duplex Handler is a software routine that takes data from two speech channels. Based on the input the Duplex Handler decides when to switch the speech direction.

=== Signalling ===
==== Subscriber line signals ====
The DC signalling on the a-b and c-d wires are similar to the Touchline system. The different states and signals on the a-b and c-d wires are:

[[Image:Signalling - (1).JPG|thumb|left|250]]<br>

States on the a-b wire:

AB1     Low-speaking (off-hook)

AB2     Normal loudspeaking

AB3     M-key

AB4     C-key

[[Image:Signalling - (2).JPG|thumb|left|250]]<br>

States on the c-d wire:

CD1    Digit

CD2    Private

CD3    Open

CD4    Conversation

CD5    Button press in conv
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===== Substution keys, DAK keys =====
In addition to these states or signals, there are some signals that combine the states of the a-b and c-d wires. In order to give certain features to keys on the station, the digit is combined with a current level on the a-b wire.

[[Image:Signalling - (3).JPG|thumb|left|250]]<br>

Digit +current level.

DG1 = CD1 + AB1 
This mode is also referred to as D20.

DG2 = CD1 + AB2
This is the normal mode of the station. 

DG3  = CD1 + AB3
This mode is also referred to as D10 (M +digit).

DG4  = CD1 + AB4
This mode will be called D30 in AlphaCom (C +digit).
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===== Digit during connection =====
[[Image:Digit during connection.JPG|thumb|left|250]]
One special case must be described. When a station is engaged in a conversation, there are some services available when a digit is pressed. This digit under conversation feature works the following way:
The first state is the current level when the station is engaged in a conversation. Then a key on the keyboard is pressed. The current is reduced to a very low value. This is detected by the exchange, which in turn reverse the voltage on the c-d wire. This leads to a reversed current flow in the station. The digit oscillator is started, and the digit is received in the exchange. When the key is released, the current will be reduced to either private or open level, depending on the station. 
The exchange will then change the polarity on the c-d wires back to conversation mode.
In the figure transitions caused by the station are marked with filled arrows. The other transitions are generated by the exchange.
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==== Data bus signals ====
All data to and from the different board are routed through the SBI except for AMC, where the MBI are connected to the serial bus.

Since the AMC controls the data bus, this board is also called Master

'''The format of the output serial data (SDO) is:'''

<start> <Lock ><Address[19:0]>  <read/write> [<Data[7:0]>] <stop>

{|Border="1"
|-
|start
|start bit. Always 0
|-
|Data[7:0]
|8 bit data to/from register inside (or outside) the SBI. Present either on SDO or SDI in read and write cycles, respectively.
|-
|Address[19:0]
|20 bit backplane register address
|-
|Lock
|Signal for locking or reserving the SDO/SDI to a master.
|-
|read/write
|if this signal is read, the data of the addressed register will be returned on the SBI. If it is write, Data[7:0] will be written to the addressed register.
|}

'''The format of the input serial data (SDI) is:'''

<start> [<Data[7:0]>] <stop>

== Related articles ==

[[Technical description 7000 series TouchLine station]]




[[Category:AlphaCom E System]]