Hello Phil, Good question, and actually took me a while to remember why the two sync carrier lines are just 25 Hz apart.
FreeDV 1600 uses a symbol rate of Rs=50 symbols/s for each of the 17 carriers, including the central DBPSK sync carrier. Note that we are using Differential PSK (DPSK) - not PSK. The start of a 40ms codec frame is indicated by a change in phase in the DPSK sync carrier, so for example this sequence of phases (in radians) every 20ms: ... pi pi -pi -pi pi pi -pi -pi ..... Note the _change_ in phase every 2 symbols (40ms), indicating codec frame sync. The cosine of the sequence of phases above is ....1 1 -1 -1 1 1 -1 -1 .... which (when up sampled and filtered) approximates a sampled sine wave of frequency Rs/4 (takes 4 symbols to repeat). So using this Rs/4 frequency sequence to modulate the center carrier at frequency fc we get s(t) = cos(2*pi*t*fc)*cos(2*pi*t*Rs/4) s(t) = 0.5*cos(2*pi*t*(fc+Rs/4) + 0.5*cos(2*pi*t(fc-Rs/4) which is two sine waves Rs/2 = 25Hz apart. Cheers, David The BPSK sync carrier sends alternating ...101010...., this a modulating signal that takes two symbol periods to repeat. So it's frequency is Rs/2 or 25Hz. So when you multiply a Rs/2 signal by the central carrier: cos(2*pi*fcentre*t)*cos(2*pi*t*Rs/2) = 0.5*cos(2*pi*t(fcentre+Rs/2) + 0.5*cos(2*pi*t(fcentre-Rs/2) which should look like two sine waves Rs apart On 18/08/16 00:00, Phil Frost wrote: > I've been working on an independent implementation of the FDMDV modem as > a personal exercise. As I understood the description*, the central BPSK > subcarrier should be at a symbol rate of 50 per second, but looking at > the tone spacing it's too narrow: but a 25 symbols per second seems to > line up. Am I overlooking something or is the sync subcarrier really at > 25 sym/sec? > > *: > http://freedv.org/tiki-index.php?page=FreeDV+Specification > http://n1su.com/fdmdv/FDMDV_Docs_Rel_1_4b.pdf > http://www.arrl.org/files/file/FDMDV.pdf > > ------------------------------------------------------------------------------ > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
