human voice / music radio signal without RF moduation

Question

This question is about digital voice modes that are used with AM / SSB / FM radios. Dstar , FreeDV , etc.

assumptions: pulse width modulation amplifiers can take a digital signal as input.

a software defined radio(SDR) is a sensor that can recive any kind of signal within a certain frequency range.

SDR demodulation is just software and can theoretically process any signal.

My question is , could traditional rf modulation techniques be removed entirely ?

Scenario: microphone in to computer such as microcontroller / raspberry pi I2C digital connection direct to PWM amplifier PWM amplifier transmits raw signal at RF frequencies in exact frequency range SDR set to receive any signal in exact known frequency range PC software detects PWM signal in frequency range and converts to digital audio signal PC outputs audio

There is no audio conversion except what is normally done by a PC when you use a microphone or speakers .There is no separate RF modulation like FM / SSB. The "modulation" is whatever the pulse width modulation of the amplifier is.

This would be like a class D audio amp with digital input from a pc except that the amplifier is connected to an antenna instead of a speaker.

Answer 1

A radio without any modulation would simply take the input signal, amplify it, and feed it directly into an antenna. In other words, if $x(t)$ is the signal to be transmitted, and $y(t)$ is the transmitted signal, then we're looking for:

$$ y(t) = x(t) $$

There are some radios that could work somewhat like this. For example, the part of the radio spectrum from 0.3 to 3 kHz, which overlaps with the frequency range of human speech, is called ultra low frequency. But there are extreme practical challenges with transmitting a signal at such low frequencies: the antenna much be extremely large, and also the fractional bandwidth would have to be extremely large.

Now, if you wanted to "just" shift the baseband signal (say, an audio signal of your voice) up to RF and transmit that, that is possible: it's called upper-sideband modulation. Not only is it possible but it's very common. However, just shifting the frequency up or down still counts as modulation.

Answer 2

Yes. In fact lots of digital signals already work this way. That's what creates broadband RFI.

Hold a cheap AM radio (ferrite antenna) fairly close to a running USB connection, ethernet connection, modulated switching power supply, or most any other fast digital signal wiring, etc., and you will hear the buzz sent by radio waves directly created by the digital drivers for those signals, no FM or SSB modulator required.

However, it's really inefficient. Note that you have to hold that AM radio fairly close to pick up the digital buzz. (Maybe, with low background noise, a good receiver and antenna can pick it up that RFI a few hundred meters away?) Whereas that same cheap AM radio can tune to properly modulated AM stations 100's to 1000's of miles away, due to the AM modulations more efficient use of bandwidth.

If you amplify a digital RFI signal to get more range, you will likely violate many radio laws or regulations (FCC part 15 in the U.S.?) in your particular legal jurisdiction.

Added: Yet another way to think about it is that unmodulated digital signal are very broadband, and there are likely tons of digital signals in your building. Modulation allows one to efficiently squeeze the information into a relatively tiny slice of spectrum where all those other digital signals in your environment are less likely to interfere and drown out your intended transmission.