Vocoder output

[jbv]

I have a question :

Looking at the synthesizer section of the block diagram of the ETI
vocoder, I noticed that the outputs of bands 3,5,7,9,11,13 are 
sent to an “inverting mixer” before being fed into the final 
output mixer, along with outputs 1,2,4,6,8,10,12,14.
Of course, my question is : why is it so ? Is there any phase 
problem between contiguous bands ? And why is it different 
in the Elektor vocoder, in which all bands outputs are sent 
to one single mixer ?

jbv