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,
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.
Creator : Patrick Lindecker (F6CTE) in 2005
Baud rate : 62.5 for PAX and 125 for PAX2
Speed : 5.86 characters/sec so 59 wpm output 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.
Note: 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 : no
Convolution code : no
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 : 0.76
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.
+Good for networks
+More robust compared to 300 baud packet
PAX and PAX2
Mode Audio Sample
FFT Window Screenshot
PAX and PAX2