MT63

MT63 works with 64 carrier frequencies which are phase modulated. This mode is not sensitive to fading and, due to its interleaving, is considered as a very robust mode. The minimum Signal-to-Noise ratio is at 10 bauds is - 5 dB.

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

    

Created by : Pawel Jalocha SP9VRC in 1997
Description :
Baud rate : 5, 10 or 20
Speed :  5, 10 or 20 characters/sec so 50, 100 or 200 wpm
Modulation : DBPSK on 64 carrier tones. the tones are separated by 7.81 Hz for the 500 Hz bandwidth, 15.625 Hz for the 1000 Hz bandwidth or 31.25 Hz for the 2000 Hz bandwidth mode. The data is encoded using a Walsh-Hadamard transform to provide high degree of redundancy.
Note : the matrix dimension used is 128x64dimension, where the first 64 lines are the 64 standard orthogonal combinations of the Hadamard matrix. The next 64 lines are their opposite. Therefore any couple of lines are either orthogonal (zero scalar product) or bi-orthogonal (negative scalar product).
Receive mode : only USB
Character set : ASCII 7 bits (128 characters)
Bandwidth : 500 Hz (500 to 1000 Hz), 1000 Hz (500 to 1500 Hz) or 2000 Hz (500 to 2500 Hz)
Synchronization : automatic using the signal
Correction code : no
Convolution code : no
Interleaving : three levels (very short over 8 bits, short over 32 bits or long over 64 bits)…which when associated with the different baud rates give a delay of decoding between 0.4 to 12.8 seconds. The codes used for interleaving respond to two problems connected with diversity :
    - a) give the biggest distance between two interleavings on two adjacent frequencies (for example, we do not use a code that increases the interleaving position by unity with every increase of the frequency,
    - b) never have the same distance for the 64 frequencies (the 64 possible positions must be occupied).
Pmean/Ppeak : 0.1
Lowest S/N : – 8 dB for 5 bauds,  - 5 dB for 10 bauds and -2 dB for 20 bauds (without sampling frequency offset).
Note : the standard mode for calling is the 1000 Hz wide with long interleaving.

Additional  specifications by F6CTE

The MT63 specifications require a precision of +/- 0.01 % for the sampling frequency. However Windows MT63 softwares accept +/-0.5 % offset and many sound cards have an offset of up to 0,7 % or even more for the nominal sampling frequency of 8000 samples/second, which does not allow the decoding of MT63 (as the offset of RX and TX can reach 1,4 %). To correct this situation, Multipsk automatically determines the offset between receive and transmit up to 1.5 % (only with good signals).

On the other hand, the chosen idling character is not CHR(0) one (which is the standard in MT63 and which is the worst for this problem) but the choice is for CHR(6).  And not to lose the tracing of offset calculation:

* the characters CHR(32), CHR(96) and CHR(112) are preceded by two idling characters,
* the characters CHR(48) and CHR(80) are preceded by one idling character.

A simplified example of how a character is sent in the 1000 Hz mode (without consideration of digital techniques used in the source code: inverse FFT, decimation, interpolation, analytic signal...:

* going through the Hadamard transform of the ASCII code of the letter, determination of the frequencies affected with a 180 degrees phase shift, and taking the phase shift into consideration,
* generating even frequencies for the duration of 0,256 second, but with a phase angle increase corresponding to 0.1 second interval (for 10 bauds) and saving in a buffer,
* filtering of the buffer and adding the result to a collecting buffer of 0.256 second capacity,
* sending to the soundcard (after scaling) of samples corresponding to the first 0,05 second of the collecting buffer,

* shifting the collecting buffer by 0,05 second,
* generating odd frequencies for the duration of 0,256 second, but with a phase angle increase corresponding to 0,1 second interval (for 10 bauds) and saving in a buffer,
* filtering of the buffer and then adding the result to a collecting buffer of 0,256 second capacity,
* sending to the sound card (after scaling) of samples corresponding to the first 0,05 second of the collecting buffer,
* shifting the collecting buffer by 0,05 second.

Decoding MT63 is equal to decoding of 64 BPSK signals in parallel. The main difficulties are in determining:
    * the offset in frequency on transmit,
    * the difference between the sampling frequencies of the corresponding soundcards.

Recommanded frequencies for  MT63 (USB)
1822, 1838, 3580, 3590, 3635, 7035, 7037, 10140, 10145, 14106, 14109, 14114, 18100, 18105, 21130, 24925, 28130 Khz