Yes, that all sounds right.?
You can do a lot with 1 carrier, but the more you do the more susceptible it is to noise.
I am curious how much more bandwidth 2 tone GMSK needs over something using?
the most sophisticated multiple tones, multiple phases, multiple amplitudes scheme
for equal performance in the presence of typical HF noise.?
I'd guess not much, but could be wrong.? Regardless, on HF we would be forced
to do that stuff if we must follow this absurd max symbol rate of 300 baud.
D-Star on VHF gets by with choosing between just two tones, has a bandwidth of 6khz
for fair voice quality.? Bitrate is higher than what Codec2 can do,
Codec2 should get it down to 3 or 4 khz for the same voice quality.
At 1500 bps, I'd guess Codec2 is just barely intelligible,
everybody sounding like a 50 year old TV robot.
Jerry, KE7ER
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On Mon, Oct 18, 2021 at 10:58 AM, Ashhar Farhan wrote:
The codec2 with packet overheads will come to approximately 1500 bps. With the 300 baud limit, we are looking at 1500/300 = 5 bits per baud.?
We slice this many ways. The easiest way is to have five tones. One for each bit. Further, you can reduce it to half, or lets say 3 bits with each bit having four values at 0, 90, 180 and 270 degree phase shift from the previous phase, thus conveying two bits. Further, you can add two amplitude levels double from four to eight different? values, bringing the bits per tone to eight.
These are usually plotted on a phase/amplitude graph as a constellation.
These higher bits per baud accordingly need better cnr to resolve the exact phase or amplitude.
Wspr/jt set of modulation schemes limit themselves to a single frequency modulated tone. It achieves remarkable relience by averaging the carrier frequency over tens of cycles
- f
