26 November 2015

CIS-128 and its ACF discrepancy

Looking at a CIS-128 signal heard this morning, I came across the discrepancy about its ACF value and the the right way to calculate it.
The signal exhibits the well known values of this waveform (64+64 channels, 20.9Bd speed and 23.5Hz step) but the modulation seems a BPSK with the add of 4 service tones: unfortunatelly, it's quite difficult to separate a single tone in order to confirm this mode. As it is common for CIS-128, a special char is sent each 5 symbols (Pic. 1) so that the expected value for the ACF should be ~239ms ...but looking for that value we get strong ~477ms spikes in SA module (Pic. 2): it's just the double, or else 10 symbol periods!
In my opinion, the reason for this double ACF is in the way the special chars are sent. 
By looking at the whole 477ms length frame in Pic. 3, it's visible - straining a bit the eyes - the right ACF period that occurs each 5 symbols (~239ms) just as it was expected: but the spec chars sent each 5 symbols are in some way alternately reversed as illustrated by the white-circled bits (Pic. 3). This way, these special chars seem to appear to the ACF-tool eyes as a sort of sequence like 

:: :: :: :: XY :: :: :: :: YX :: :: :: :: XY :: :: :: :: YX :: :: :: ::
and then the repetition period, according to its point of view, is logically 10 rather 5 symbols. Something like an optical illusion. 

Pic. 1
Pic. 2
Pic. 3
The same situation can be found analyzing the 6000Hz 'broadband version' of this waveform (Pic. 4).
In this case the ACF is ~238 ms BUT here the speed is double, so the special chars, which are sent each 5 symbols, should make ~119ms spikes: using the CCF measurement these chars are visible inside the red-marked circles (Pic. 5) and white-marked in the structure of the frame (Pic. 6).

Pic. 4
Pic. 5
Pic. 6

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