30 September 2017

Chinese PSK-2 ...and errors in its baud rate measurement

Some days ago I had a talk (...email exchange) with my friend KarapuZ about the way to get correct measurements of the baud rate in noisy signals or in uncommon waveforms. I always relied on the tools provided by SA program as the "Auto define param" and mostly the amplitude detectors, but KarapuZ warned me that sometimes they may fail and notably the "Auto define param" tool fails in case of strictly filtered or weak signals.
As a test, KarapuZ sent me a sample (the "x-Bd" wav file in Figure 1) without specify its baudarate and asking me to measure it.

Fig. 1
I used the "Auto define" tool and the modified amplitude detector searching for the lower more bright line and got a baud rate of 1000 symbols/sec in both their outputs (Fig. 2).
Fig. 2
KarapuZ replied: "The speed line of manipulation is 1500 baud unchanged in the preamble! This is a Chinese PSK-2 modem". 
Indeed, in my measurement I simply took in consideration the lower line - as usual - and did not put attention to the discontinuity in the 1000 Hz (999.44) line between preamble and data segments (Figs. 3,4). Really a my hasty measurement (I already had this signal but I forgot).

Fig. 3
Fig. 4
KarapuZ also drawn my attention on the "raster" of the signal that clearly exhibits 15 bits within 10 msec, ie a speed of 1500 baud (Fig. 5)

Fig. 5
Apart from my error in the evaluation of the amplitude detector, why the SA "Auto define param" failed so clumsily?
Quoting KarapuZ "it can be assumed that in the transmitting equipment, filters are used at the output of the signal formation which in some circumstances may influence the determination of the speed of the manipulation of the SA program." So, in order to demonstrate the influence of the filtering in the Chinese PSK-2 signal, KarapuZ synthesized an absolute PSK-2 modulation at 1500 baud with the same 3-bit structure of the Chinese waveform and sent me that file (Fig. 6)

Fig. 6
Then I measured the manipulation speed of the syntesized signal just using the "Auto define param" and it works like a charm.

Fig. 7
 The influences of the filtering is thus well quantifiable (other than visible)

Fig. 8
Things are even more worse since the bitstream  has a relative form and a three-bit structure, visible in raster, which generates many harmonics in the power spectrum! This is China, they love such tricks :)

Fig. 9
Thanks to KarapuZ for the great lesson!

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