That's a good description of MSK, but there is a second order discontinuity when it switches tones.? The slope of the curve makes an abrupt change there.? GMSK makes the transition more gradual, but still centers the transition on the zero crossing like MSK.
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A modulation scheme such as Olivia can choose from dozens of different tones sent one at a time, not just two different tones.? With enough different tones to choose from, you can probably have enough bandwidth to meet the needs of Codec2 voice transmissions and still meet the US FCC's 300 baud max symbol rate.? But I'm guessing the total bandwidth occupied would not be much less than a two tone signal switched at a fast enough rate to give the same bandwidth.? Switching between many different tones can probably be done using the GMSK method, but all the basic descriptions of GMSK I've seen so far only tell us about the two tone version.
Most Codec2 transmissions on HF take it a step further, and send multiple tones simultaneously to get even more bandwidth out of that 300 baud max symbol rate.? These transmissions must go through a linear amplifier.
Jerry, KE7ER
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On Mon, Oct 18, 2021 at 08:59 AM, Ashhar Farhan wrote:
Gmsk is really very simple.
First, it picks two frequencies that the carrier switches between. Second, the frequency is switched preciselt where the signal hits the zero voltage. Thus, it avoids the abrupt ending of one frequency waveform and start of the next.
- f
