Hey Ax - Nerd question
-
I'm not sure whether that's part of your speciality, but let me try:
I know a little about analog modulation of radio signals. I have a clue how AM, FM, SSB etc. work.
I don't know much about digital modulation techniques. Here's something I'd like to understand:
According to my layman understanding of modulation, if you want to transmit N bits per second, you'd need a bandwidth of at least N Hz.
But there seem to be methods to transmit way more with a small bandwidth, e.g., even the good old 56K modem, which worked over a phone line with just 3Khz bandwidth or so.
How the hell does this work? I read a little about spectral efficiency, Nyquist rates etc., but it's a little over my head.
In particular, I wonder how much data one could transmit with very low frequency signals (say, <1 Khz) that could be used to communicate with submarines and the like.
-
This is a great overview - may be a little too basic:
Link to video -
@Klaus , see “spectral efficiency” (e.g., https://en.wikipedia.org/wiki/Spectral_efficiency )
There are multiple ways to achieve multiple bits per second per Hertz. For modern high speed digital radio frequency communications, many technologies take advantage of the Quadrature Amplitude Modulation (QAM) scheme. So if you want to study a modulation scheme to see how one can achieve multiple bits per second per Hertz, QAM can be a good place to start. Good luck!
