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> Qam 128 Or 64?, What is the main practical difference?
thickglass
  Posted: October 04, 2016 10:01 pm
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Hi guys, I was playing with hdmi qam modulator recently - and I got confused a little. I'm familiar with basics of qam modulations, but what is the practical difference - If I would keep bitrate the same and switch from qam 64 to 128 - Will the quality of modulation change anyhow?
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Sch3mat1c
Posted: October 05, 2016 08:46 am
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Twice as many points in the constellation means twice the sensitivity to noise (well, give or take, depending on the coordinates and the kind of noise being tested against), while constant bitrate means half the baud (symbol) rate, and therefore also half the analog bandwidth. Which is only sqrt(2) as much noise, if the noise is spectrally flat (i.e., proportional to rtHz, not Hz), so SNR is worse by half of half (3dB).

The modulator and detector obviously need to be slightly more precise, as well. (For an ordinary ADC, that's not a problem, they go up to 12 bits with reasonable accuracy pretty well. But with phase detection, depends.)

I think? It's been forever since comms class wink.gif

Tim


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