The QPSK baseband signal is composed of real (I) and imaginary (Q) components, or complex values. Using a Quadrature modulator, you can combine these onto one sideband. Like the SSB signal of FreeDV, it is a complex signal. But creating an analog SSB signal is what makes VHF SSB so expensive. If you could do all of this digitally, and then modulate with a complex signal, I would imagine expense and stability requirements would be reduced. The signal would be half the bandwidth, and no carrier.
If you look at FM radio stations on your SDR, you see the signal duplicated on both sides of the carrier, and also a large carrier when the modulation is low. I guess you could say FM has built-in diversity. Although IQ values have to be calibrated, as any error results in a large carrier and/or alternate sideband. A radar I used to work on, balanced the IQ channels every slant. The beam would go to the top, the IQ channels balanced, the beam would scan down (phased-array), etc. Steve On Fri, Feb 2, 2018 at 11:43 AM, Adrian Musceac <kanto...@gmail.com> wrote: > If you don't mind me asking, why do you need a complex input? The job > of the frequency modulator is to transform a real domain signal into > complex domain. The usual case is you have baseband audio represented > as floats as an input. ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
