Traduction anglaise.

PAX is a robust MFSK mode which derives from Olivia. The minimum Signal-to-Noise ratio is about - 10 dB. It is also a protocol closed to AX25 (the one used for the Packet). Hence, it permits frame exchanges in non-connected mode (Unproto) and the transport of APRS (Automatic Position Reporting System) frames,


Click on the copy of the screen in order to listen to the corresponding audio signal.

    The PAX2 is the PAX mode but modulated twice rapidly. The minimum Signal-to-Noise ratio is about - 7 dB. It allows a quick communication. The protocol is the same as PAX's one.


Click on the copy of the screen in order to listen to the corresponding audio signal.

    This mode is included into the software MULTIPSK by Patrick F6CTE, there is the description of this mode and its variants as well as the specifications coming from his software :

Creator : Patrick Lindecker (F6CTE) in 2005
Description :
Baud rate :
62.5 for PAX and 125 for PAX2
Speed :
5.86 characters/sec so 59 wpm ou 11.71 characters/sec so 117 wpm
Modulation :
FSK 8 tones (3 bits arranged in a Gray form), with a shift between tones of 62.5 Hz (500 Hz bandwidth) for PAX and a shift of 125 Hz (1000 Hz bandwidth) for PAX2. A block is composed of 32 symbols of 3 bits (in other words it is a matrix of 32 columns (following the time) on 3 lines). Each of the 3 block lines of this block corresponds to a character which has been encoded, on a 32 bits vector, using a Walsh-Hadamard transform to provide a high level of redundancy.
: the implicite used matrix is 64x32 dimension, the 32 first lines are the 32 standard orthogonal combinations of the Hadamard matrix. The 32 next lines are their opposite. So any couple of lines are either orthogonal (scalar product=0) or biorthogonal (scalar product<0).
Reception mode :
sensible to the side (USB or LSB), USB is recommended
Shape of pulse :
specific shape (between a Hanning window and a rectangular window)
Character set :
ASCII 6 bits (64 characters)
Bandwidth :
500 Hz for PAX and 1000 Hz for PAX2
Synchronization :
automatic using the signal
Correction code :
Convolution code :
Interleaving :
yes, spreading of 3 bits "vertically" in the block,
Scrambling :
yes, with a 32 bits sequence applied on each line of the block but with a delay of 13 bits between two successive lines.
Drift tolerance :
30 Hz/mn (depending on signal the signal-to-noise ratio)
Pmean/Ppeak :
Lowest S/N :
10 dB for PAX and – 7 dB for PAX2
Note :
there are modes directly derived from the 8 tones, 62.5 bauds Olivia mode (Pawel Jalocha SP9VRC) but with a 32 bits vector instead of a 64 bits vector, a different set of characters and a double speed for PAX2 (125 bauds). Moreover, any AF frequency (200 to 4000 Hz) must be able to be used.

Other PAX/PAX2 characteristics:

the transmission begins with the transmission of the extreme low tone for a time which depends of the value of the parameter TXDELAY and the type of frame (minimum 0.5 second), to be able to manually tune on the frequency, if needed,
- the reception and the transmission can be done everywhere on the spectrum,
- the search of the PAX/PAX2 central frequency is done when clicking on the spectrum. It takes about one second. Afterwards, when decoding, there is no automatic search but a simple frequency drift following, this to avoid abrupt frequency variations on events as QRM or QSB,
- an offset between sound cards sampling frequencies inferior or equal to 1 % is withstood, but it is hardly advised if not indispensable to determine the sound card sampling frequency (see Determination of the sound card RX/TX sampling frequencies). In fact, if the speed offset is too high, the synchronization could be too long and if would be possible not to detect the start flags.

PAX/PAX2 protocol in non-connected mode ("Unproto"):

All the communication is done through "frames" (limited set of characters). There is, in non-connected mode, only one sort of frame called UI, which permits to send either messages or APRS frames. The Unproto PAX/PAX2 protocol is similar to the AX25 Unproto protocol (Packet).
Each frame is composed of:

* at least 3 flags ("flag": special and unique character),
* a PID field (protocol identifier) + Version (of the protocol),
* a control byte defining the type of frame (UI here),
* an adress field: destination then source then possible repeaters (2 max),
* a "length" byte giving the number of information characters,
* an information field where data is located,
* a FCS field composed of 2 bytes where is located the CRC ("Cyclic Redundancy Check Sum") calculation on all the frame data until this field (except flags). A difference between calculated CRC and received CRC makes the frame invalid.

the PAX/PAX2 "bytes" are 6 bits long (instead of 8 bits).

It is possible to pass through 2 PAX/PAX2 repeaters (with possible use of the alias ECHO).

PAX/PAX2 protocol in connected mode

The connected PAX/PAX2 mode is an ARQ mode type (ARQ for "Automatic Repetition reQuest" by acknowledgment ACK or non-acknowledgment NACK) inspired from AX25 (Packet) and Pactor1 but asynchronous (Pactor 1 being ARQ synchronous) because the acknowledgments and non-acknowledgments are not done at fixed interval (this possibility not being available under Windows) but inside a maximum duration.

Each transmitted I frame is subject to an acknowledgment or a non-acknowledgment by a RR or I frame.

It is possible to pass through 2 PAX/PAX2 repeaters. Without repeaters, the transmission can be "full-duplex" type (protocol level (OSI level 2)). With repeaters, the transmission must be "half-duplex" type (protocol level (OSI level 2)), each one in turn.

This protocol permits free error transmissions. APRS transmission can also be transmitted.

Note :
the complete PAX/PAX2 protocol is not available at the moment (the author considering the protocol as experimental, and reserving the possibility to do all the necessary modifications, before issuing it).

Recommanded frequencies for
PAX (USB in HF, FM in VHF)
The following frequencies are suggested:

3590, 7042, 10148, 14075, 144620 Khz.

Patrick F6CTE proposes to focus calls at the following frequencies and times :

* 14075 Khz (AF around 1000 Hz), at 17h00 UTC if few traffic,
* 10148 kHz (AF around 1000 Hz), at 22h00 UTC if few traffic,
* 144620 kHz (AF around 1000 Hz), at 20h00 UTC if few traffic.

Recommanded frequencies for (USB en HF, FM en VHF)
The following frequencies are suggested:

3610, 7042, 14112, 18112, 21112, 24932, 28302, 144620 Khz.

Back to th liste of modes included in MULTIPSK