At University I studied some stuff about modulation (only two lessons, so my knowledge is very limited) and as far I've understood, the FM signal is most reliable in audio than a AM signal, and it is more power efficient than AM modulated signal.

I would like to know which is the most efficient digital mode in FM for text and data transmission. Most efficient in term of bitrate and so data transmitted with minor errors, in order to use all the NFM channel bandwith

I suppose that AFSK is the only ""sub-modulation"" (is afsk a sub modulation of a fm signal ?) that I can use in FM signal, in particular with cheap fm handheld radio.

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    $\begingroup$ "I would like to know which is the most efficient digital mode in FM" What does "efficient" mean to you? Energy efficiency doesn't matter for FM-modulated audio (the over-the-air-power is constant), so that's not it. Generally, the question "what is the best?" never has a single answer, you'll really need to define the conditions under which something has to perform and the metrics with which you measure its "goodness". There's a lot of reasonably good in-FM modulations (doing in-FM modulation is mathematically bad, but you might not have another choice, so, hm),but none of them is optimal $\endgroup$ Nov 20 at 13:55
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    $\begingroup$ And we can mathematically prove that none of them is optimal for some kinds of channels. Generally, using the AM mode on your handheld and using that to do low-IF or baseband modulation is probably better in terms of spectral efficiency, so you might completely be off to a wrong start. Could you really edit your question and tell us exactly what you want to achieve, maybe taking care not to mix your requirements and your own suggestions? $\endgroup$ Nov 20 at 13:58
  • $\begingroup$ The most efficient / best mode in any circumstance is the magical one that hasn't been invented yet. Without choosing parameters (like bandwidth vs. power or something), efficient has no meaning, and "most efficient" without scoping limiting conditions has even less meaning. $\endgroup$
    – user10489
    Nov 21 at 6:30
  • $\begingroup$ I've modified my question in order to explain what I was meaning when I say "efficient". So, in term of quantity of data transmitted/errors $\endgroup$
    – Gigi
    Nov 22 at 9:35
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    $\begingroup$ Note also, in the US, symbol rate for wavelengths above 33cm are legally limited, so you can't have a symbol rate higher than that. Note that generally symbol rate == baud rate, but baud rate != bit rate. So by that definition, the most efficient mode for wavelengths larger than 33cm is the one that can pack the most bits per symbol while still staying in the audio channel bandwidth and not exceeding the legal symbol rate. $\endgroup$
    – user10489
    Nov 23 at 5:29

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