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I understand how the number of symbols used can be increased with things like quadrature phase-shift keying or quadrature amplitude modulation, and so on. I have thought for a year that it would be interesting design to increase bits-per-symbol by using the analogy of chords in music, superimposing many frequencies, and then demodulating them into the individual frequencies. I ask here if anyone knows of such techniques used historically and/or in the present, or if there is any clear reason why it would be a bad technique, or if it might be good but just has not been tested yet.

Update: Only thing I miss now is more examples of where the baseband wave is multifrequency (and for numerical data. ) Such examples would give a sense of how that technique compares to alternatives like QAM. "Touch tone telephones" is one example although pretty low-tech (combining audio tones rather than just two individual electromagnetic frequencies. )

Examples of carrier waves done in parallel has been clearly answered with DSL and PEP, and is clearly used there to increase bits-per-symbol (and has also been answered for running multiple connections over same medium, like in radio or telephone wires. ) But I am not aware of many examples at baseband wave level, in DSL the examples I have seen (elsewhere, not this question) use QAM on each individual channel. I would assume it is superior at that level, because market selected for it.

I added this update for context of where I was coming from with the question. The question has been answered, but I am interested in techniques used historically and/or in the present and do not mind that expanded on. Very thankful for the help regardless. Peace.

BipedalJoe
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    "Touch-tone" telephones encoded four bits per symbol by combining two audio tones. The technique is called DTMF (dual-tone multi-frequency). – Elliot Alderson Sep 11 '22 at 13:11
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    Yes, of course it is possible. It has been done for decades already. OFDM was invented about 60 years ago, and FDM about 100 before that. – Justme Sep 11 '22 at 14:00
  • In the world of analog phone-line modems (1990s), Telebit's [PEP](https://en.wikipedia.org/wiki/Telebit#PEP_and_the_TrailBlazer) scheme took this idea to an extreme level, transmitting up to 6 bits at a time at 6 baud on each of up to 512 carriers at once, for a total throughput of about 18 kbps. – Dave Tweed Sep 11 '22 at 14:50
  • Great! Yeah all good points. I did not see the forest for the trees there (but I am not that good at electronics yet. ) Obviously it is everywhere like radio, and like Andy aka mentioned channels in DSL is a good example. And PEP is a very good example (since the FDM should be used for a single data stream run in parallel and not multiple different ones like radio stations in what I was wondering about. ) – BipedalJoe Sep 11 '22 at 16:21
  • @BipedalJoe are we done here? – Andy aka Apr 09 '23 at 12:59

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Imagine a radio frequency band of many usable channels. Maybe hundreds of channels separated by 25 kHz (quite often used) for each carrier. Each individual channel is a radio "system" in its own way and, is protected from adjacent channel interference by tight filtering systems.

I ask here if anyone knows of such techniques used historically and/or in the present

Then imagine that the whole of the radio frequency band is dedicated (all the channels) to one transmitter and one receiver. That appears to be what you are asking in your question. One such system that springs to mind is ADSL2 telecom transmission: -

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An extension is VDSL: -

enter image description here

And, of course, it can be a radio medium, an optical medium or a copper medium.

Andy aka
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  • Thanks! Right the channels in DSL, good example. Radio too, are you aware of examples of radio using multiple channels to transfer more bits per symbol? Some mobile phone tech that uses that? – BipedalJoe Sep 11 '22 at 16:05
  • @BipedalJoe and all - This comment thread got flagged, so mods are required to get involved. My decision is that it's reasonable to ask, as in this case, if an answer's author can improve their answer by adding additional related examples. They don't have to do so. || Remember, everyone, to [be nice](/help/behavior) and to follow the [Code of Conduct](/help/conduct) which requires everyone to avoid language that appears unfriendly. Thanks. (Several comments have been deleted. Take any further discussion about what is allowed to [Meta] if wanted. Further meta discussion is not allowed here.) – SamGibson Sep 11 '22 at 17:36