Topics

Autex mode + EM mode ("conference" modes) #autex


Patrick Lindecker
 

Hello Andy,

 

As you are speaking of Autex, here is a small presentation of the two modes on Multipsk (freeware part), to work on “conference” between several hams.

 

·         The most simple is the “Extended message“. The number of Hams is limited by the AF bandwidth. The mode is sensitive but it is not controlled: there is no guarantee that the others Hams have received your message. So the meeting can be local or not.

·         Most complete is the Autex mode. There is a guarantee that the other Hams have received your message. So beyond 2 Hams, it can only be local, because the risk would be that one Ham blocks the others if his/her conditions are poor.

 

73

Patrick

 

1)      The "Extended Message" pseudo-mode

The EM (Extended Message) is a “pseudo-mode” because it can be used  in parallel of any mode (BPSK31...).

Its main use is to give the possibility to have simultaneous QSO (at standard speed) with different Hams, as messages are displayed on the waterfall. So everyone can see these messages and possibly answer.

 

This system can be used for a small meeting. The number of participants is limited only by the bandwidth.

 

The sensitivity is -14 dB comparable to Olivia or MFSK16, at 47 wpm. It is a free error transmission.

Example of an EM « CQ » message displayed on the waterfall.

 

 

However there is no control about the reception of your message by the other Hams (it’s a one way system not an ARQ sytem).

 

2)      Autex

It is an ARQ system extended to several persons.

AUTEX

The AUTEX modes permits to organize a meeting between 2, 4, 7 or 13 people. Text messages can be transmitted from any person to any other person participating to the meeting, in pseudo full duplex (protocol one, not physical one), each person being sure that his messages will be read by the other participants. So, each one will be sure to read all the texts sending by the other participants to the meeting. It is a mode without errors (if en error is detected, the frame is re-transmitted until complete reception). So, it can be considered as an extended ARQ mode.

AUTEX is composed of 4 different modes (for 2, 4, 7 and 13 people), each one with a different RS ID.

General description of the AUTEX4 mode, for 4 people

Baud rate : 250 bauds for the "Master” (the one who initiates the meeting) and 83.33 bauds for the 3 "Slaves" (users)

Rough speed : 500 bits/s at 250 bauds and 166.6 bits/s at 83.33 bauds or 625 wpm at 250 bauds and 208 wpm at 83.33 bauds

User speed : the maximum, in the best conditions, for each participant, is about 42 wpm (same order as PSK31)

Modulation : QPSK (2 bits per symbol)

Reception mode: only one side (USB or LSB), USB is recommended

Character set : ASCII +ANSI characters (8 bits compressed or not)

Shape of pulse : square root of elevated cosine (index=1)

Bandwidth : 500 Hz in 250 bauds and 167 Hz in 83.33 bauds

Demodulation : coherent,

Synchronization: symbol by using the signal, the phase ambiguity of 90° being solved by using a prefix

Coding: Reed Solomon

Interleaving : no

Scrambling : yes

Memory ARQ: yes, soft one on each I/Q levels

Pmean/Ppeak: 0.5

Lowest S/N (250 bauds): -3 dB
Lowest S/N (83.33 bauds): -8 dB

Before each frame, it is transmitted a (symbol) synchronization sequence, followed by a specific prefix to solve the phase ambiguity and to transmit the way to interpret the frame which follows. The frame contains a preamble for control data, then the message (possibly compressed) and, finally the text which appears in the secondary channel which contains the callsign and various information (sent two characters by two characters in a rotating way). A Reed-Solomon code checks the frame and can fix up to 8 errors.

Moreover, at the beginning of the transmission, the "Master" sends a RS ID identifier for the detection of the meeting by the other Hams and to fix, precisely, at each turn, the central frequency of the transmission (the "Master" one fixing the "Slaves" ones).

Other AUTEX modes

The "Slave" baud rate is always the same for all AUTEX modes: 83,33 bauds

In AUTEX for 2 people, the baud rate of the "Master" is equal to 83.33 bauds, so the bandwidth is equal to 167 Hz.
In AUTEX for 7 people, the baud rate of the "Master" is equal to 500 bauds, so the bandwidth is equal to 1000 Hz (lowest S/N around +0 dB).
In AUTEX for 13 people, the baud rate of the "Master" is equal to 1000 bauds, so the bandwidth is equal to 2000 Hz (lowest S/N around +3 dB).

 

 

-----Message d'origine-----
De : main@digitalradio.groups.io [mailto:main@digitalradio.groups.io] De la part de Andrew O'Brien
Envoyé : dimanche 25 octobre 2020 16:38
À : multipsk@groups.io; main@digitalradio.groups.io
Objet : [digital-mode-radio] Autex mode

 

I took a look again at Autex in Multipsk this morning. I think I remember using it in the past , but not much . I note that Patrick’s documentation suggests using 7080 and 14080 among other QRG. 14080 carries a lot of FT4 traffic these days . So I have moved up to 14082 and will listen there today for any AUTEX activity . As darkness arrives here in the north eastern New York I will listen 7080 . Bradley , when you have used it from South African , who are your partners ? Other ZS hams ?

I note that this mode has a “start a meeting “ button . Perhaps the ham radio version of Zoom :)

 

 

Andy K3UK

 

 

 

 




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