Hi Brian, The BPSK pilot is used for acquisition (e.g. estimating the initial frequency offset) and frame sync. More power is allocated to the BPSK pilot to make those two functions fast and reliable (an important requirement for PTT digital voice). As there is only one BPSK carrier, the overhead in terms of total power is about 1dB IIRC.
The FreeDV 1600 waveform (the FDMDV modem) was an open source implementation of an earlier waveform (see the History section on http://freedv.org) that was known to work well for this application. So it was something of a starting point for FreeDV, and a stepping stone to later waveforms. There are Octave and C versions of the modem ( https://github.com/drowe67/codec2/blob/master/README_fdmdv.md), plus various blog posts, if you would like to experiment with the modem or inspect it's implementation. Cheers, David On Sun, 2021-08-22 at 22:59 +0100, Brian Morrison wrote: > I'm looking for an explanation of something that was probably > considered during the design of the 1600 mode but is implemented > differently from other systems that use pilot tones. > > Looking at the spectrum and the scatter diagram I see that the pilot > tone amplitude is much larger than the QPSK carriers and I wondered > why. The phase of these pilot tones (BPSK?) is the same as 2 of the > QPSK points, in other systems I'm familiar with the BPSK modulated > pilots are at a 45 degree offset from the QPSK points and have a > lower > amplitude because there is both phase and amplitude difference from > the > QPSK symbols to assist demodulation. This allows the use of a lower > PAPR in the modulation, which is beneficial because it allows a > larger > amplitude for the QPSK bits and a less linear transmitter can be > used. > > No criticism is intended, I'm just trying to understand the trade > offs, > I suppose that there are only 4 symbol phases to worry about whereas > with offset phase BPSK there are then 6, which really means 8. > > No rush to get an answer, this is just an itch I wanted to scratch > when > I was testing with the recorded files that come with freedv and what > I > saw got me thinking. > > Cheers > _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
