modem free high base AM data transfer possible?

Question

not a very experienced EE, but I was thinking about a system for high speed data transfer, and it is fairly simple conceptually, so I imagine that there must be serious flaws with the method, because to my knowledge it is not implemented, or at least not widespread. I apologize in advance for my ignorance of the subject matter (I'm just starting my learning).

The way I understand it, binary signals enclosed in a carrier wave are modulated/demodulated through a modem. I understand why this was done in the past - processors weren't that hardcore, and the hardware can only understand binary. I guess I just don't understand why it is still done this way.

If we modulate the amplitude of a wave (I think by providing the oscillator different levels of current), can we not sample this wave with some sort of analog to digital converter and process it on the CPU?

If this is possible, why stick to base 2? If we can have a unique value for each measurable amplitude, data transfer rates would skyrocket. Imagine transferring data with base 1024, or even higher. If we could accurately sample the wave (each oscillation), I don't see why the rate of transfer could be equal to the frequency of the wave times base divided by 2 bits per second (this is probably not correct mathing).

If we have a processor running in the gigahertz, and a signal in the low megahertz, it seems feasible that the processor would be able to sample and translate the data to base 2 (possibly sending to another core for translation). This way, the data rate would be limited by the processor (faster processors would lead to the utilization of higher frequencies for transfer).

Limiting factors that I can think of are how fast the current to the oscillator can be changed (for TX), how fast the analog to digital conversion can be done (read that accurate sampling is possible into the hundreds of megahertz), and the range of measurable amplitudes.

I'm aware that this question probably contains an unusal amount of stupidity, but I want to build this system and I'm wondering why I shouldn't. There has to be something major that I am missing here. What could it be? Thanks.

Answer 1

You've just described two separate and entirely valid technologies used in communication theory today: software-defined radio and (for lack of a good general term that I can remember) multi-symbol/level communication.

If we modulate the amplitude of a wave (I think by providing the oscillator different levels of current), can we not sample this wave with some sort of analog to digital converter and process it on the CPU?

Yes - to a degree. You've just described software-defined radio. The basic idea is what you said: dispense with the majority of the radio frequency equipment and create the modulated sine wave directly from the output of a D/A converter and for the return path use a similarly fast A/D and plenty of DSP processing for both sides. The current problem is that although processor speeds are measured in gigahertz nowadays, the interface with the analog world hasn't yet reached those speeds. This means that direct waveform creation is limited to low frequencies (which, for communications, is still fearfully high compared to frequencies 'normal' analog designers worry about). However, if I read my articles correctly this as still allow removal of some of the intermediate-frequency hardware present in most radios. In the future it may be possible to dispense with more of the hardware.

If this is possible, why stick to base 2? If we can have a unique value for each measurable amplitude, data transfer rates would skyrocket. Imagine transferring data with base 1024, or even higher. If we could accurately sample the wave (each oscillation), I don't see why the rate of transfer could be equal to the frequency of the wave times base divided by 2 bits per second (this is probably not correct mathing).

You're right that it's not perfect but you definitely have the basic idea down. To give an example we'll stick with Amplitude Modulation. When you're trying to transmit 0 or 1 using AM it's called On-Off-Keying (link goes to a site with nice pictures and a description). This works by modulating a pure digital signal - 5v is '1', 0v is '0'. You're right that if you have a number of voltage levels you can send more data at once - this is called Amplitude Shift Keying (another nice description with picture). As you can see, there's multiple levels of voltage for various combinations of bits - 2 bits gives four different voltage levels, 3 gives 8, etc.

The problem with this and other similar schemes is not theoretical but practical - in a communication channel with noise it's very likely you'll have trouble figuring out what exactly was sent. It's just like with analog signals: if my only valid voltage levels are 0 and 5V then if I get 4.3V out I can be reasonably sure it should be 5V. If I have 1024 valid voltage levels then it gets a lot harder to determine.

Also note that you're not limited to Amplitude Modulation - the same techniques can be applied to Phase Modulated signals (similar to FM) or you can step into the realm of Frequency Shift Keying where distinct frequencies represent bits (ie, if you want to transmit '3' in binary that might mean sending a 3KHz sine wave and a 6KHz sine wave, then separating them at the receiving end where sending '1' might just be the 3KHz sine wave).

And these techniques are already in wide use - GSM cell phones use a form of Frequency Shift Keying called Gaussian Minimum Shift Keying. Although I do want to correct one incorrect idea you may have: modulation is still used in all of these schemes. The opposite of a modulated signal is a baseband signal (like a bitstream from a serial port). To communicate at any distance over the air you need modulation, period. It's not going away, but how we generate the modulated waveform will change.

I suggest you take a class in Communication Theory if you can - it sounds like you've got the knack for it.

Answer 2

If you send 1 bit simultaneously you need two different levels (for amplitude modulation). If you want to send 8 bits simultaneously you need 256 levels, which will result in a lot of read errors; a level may change due to noise.
There are ways to send more than one bit simultaneously however, like QAM (Quadrature Amplitude Modulation). Part of the information is in the amplitude of the signal, like in ASK (Amplitude Shift Keying), and part is in the phase of the signal, like in PSK (Phase Shift Keying).

Answer 3

What you are asking for has been done, to some extent or another, and for different transmission mediums. I started to write a short thing about different modulation schemes, but then ran across a Wikipedia page that covers them fairly well. Just scroll down to the section marked "List of common digital modulation techniques".

Many modern systems operate on Quadrature Amplitude Modulation (QAM). Ethernet uses Pulse Amplitude Modulation (PAM), which is not on that page. And many radio based transmissions use some form of Trellis Coding. So, looking at those will give you a good idea on what the common stuff is. Looking at the older AM, PSK, etc will give you an idea about where we came from.

The bottom line is this... Almost every form of computer communications that goes more than about 10 feet involves some level of encoding and modulation. It's basically what you were talking about in your question, but taken to extremes. Much of it is very theoretical and math-intensive. People use this kind of stuff for their Ph.D. thesis.

Answer 4

You probably mean like this?

Having more than 2 signal levels is a very well known technique, the drawback is the lower signal-to-noise ratio. But a good error-correcting scheme can use the extra bits to remove more errors than were added by the decreased signal-to-noise ratio, so this definitely can increase performance.

No clue why you say modems don't do this, they most certainly do. V.90 has a HUGE constellation.

Answer 5

Discrete time sampling and digital signal processing such as you describe is used in telephone-line modems, but on a telephone line one is allowed to output almost arbitrary waveforms in a bandwidth which is quite wide relative to the center frequency (typical range about 300-3,300Hz). By contrast, radio transmissions must fit fit within a fairly small envelope around a center frequency. If you owned the only radio transmission device in the world, you could indeed output quite a bit of data on a 1MHz carrier by modulating every wave, but your transmission would garble any transmissions anyone else might be attempting at many other frequencies. If the transmitter is limited to outputting energy in the range 995,000-1,005,000Hz, sampling the signal a few million times per second and processing everything digitally might allow better reception than using an analog tuner, but there's going to be a pretty tight limit on how much data can usefully be transmitted.

Addendum Amplitude-modulating a sine-wave carrier with another sine-wave signal will generate signals with frequency equal to the sum and difference of the carrier and modulating signals. Amplitude-modulating a sine-wave carrier with a signal that's the sum of two sine waves is equivalent to amplitude-modulating the two sine waves separately on the same carrier and combining the result. The result of amplitude-modulating a sine-wave carrier with a complex waveform may be determined by separating out all the different frequency components of that waveform and figuring the effect of amplitude-modulating each.

If one amplitude-modulates a variety of voice frequencies in the range 0-5KHz on a 1MHz carrier, the result will be a mix of frequencies in the range 995,000-1,005,000Hz. To tune an AM radio broadcast on channel 1000 (i.e. 1,000Khz or 1.00MHz), one should endeavor to have the tuner accept all frequencies in the above range and reject any outside it. If one wanted to tune in channel 990, one should capture frequencies 985,000-995,000. Note that if the broadcaster on channel 1,000 doesn't filter out all audio frequencies above 5KHz before transmission, those would spill onto the channel below (as well as the channel above).