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It seems to me that the most simple way of modulating a digital signal is using amplitude modulation.

When the signal has the amplitude 0 the bit is 0 and when the signal has some amplitude then the bit that is sent is 1.

Why isn't this simple way of modulation used in practical for sending digital data?

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    \$\begingroup\$ Where did you find the wisdom that AM is not used for sending digital data ? Many wireless temperature sensors working at 433 MHz use OOK modulation (google that) which is basically AM modulation with 100% modulation depth. \$\endgroup\$ – Bimpelrekkie Sep 6 '17 at 14:39
  • \$\begingroup\$ @Bimpelrekkie In the Wikipedia article about modulation, at the digital modulation list, AM is not included. \$\endgroup\$ – yoyo_fun Sep 6 '17 at 14:41
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    \$\begingroup\$ ASK modulation is mentioned which is basically the same as OOK modulation but with 100% modulation depth. You expected it to be called "AM" but it is not called that way in the digital modulation context. \$\endgroup\$ – Bimpelrekkie Sep 6 '17 at 14:43
  • \$\begingroup\$ @Bimpelrekkie I see now. I was expecting to be called Amplitude Modulation even if it modulates digital signal. English is not my native language and I really do not understand what does "shift keying" is. I know what a key means and what shifting something means but "shift keying" makes no sense to me right now. \$\endgroup\$ – yoyo_fun Sep 6 '17 at 14:49
  • \$\begingroup\$ You may be conflating amplitude modulation with baseband signalling. The difference is, with AM, you change the amplitude of a carrier wave, whereas with baseband, you simply drive different voltages on the line. \$\endgroup\$ – Solomon Slow Sep 6 '17 at 16:20
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As others have stated amplitude modulation is indeed used for digital modulation.

However, the biggest issue with AM modulation is that you are using a natural phenomenon, the envelope variation, to carry your signal. That means your signal is highly susceptible to interference by outside causes, eg. neighboring frequency transmitters, variability in local field strength, solar effects etc etc.

That's why your AM radio stations sound so much worse, with a ton of "static", compared to your FM stations.

For transmitting the latest Mariah Carey song AM is acceptable, but if you want your 1000 page document to arrive without any errors.. that is a different story.

You CAN use AM to transmit digital information, however, in order to have zero errors, the transmission rate needs to be much slower compared to the likes of FM.

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  • \$\begingroup\$ Envelope is the amplitude right? However, what if the two amplitudes used to transmit digital data are 0 amplitude and some other amplitude. Then the error would be almost impossible. It is either a signal transmitted over an amplitude or there is no signal. \$\endgroup\$ – yoyo_fun Sep 6 '17 at 15:41
  • \$\begingroup\$ @yoyo_fun in an ideal point in the universe devoid of any interference then yes that might work. Unfortunately, anywhere else you care to mention, zero amplitude is a myth. \$\endgroup\$ – Trevor_G Sep 6 '17 at 15:48
  • \$\begingroup\$ but if you only consider the noise then the amplitude of the signal when the transmitter ACTUALLY transmits a signal on that frequency should be much higher then just the amplitude generated by a noise. \$\endgroup\$ – yoyo_fun Sep 6 '17 at 16:08
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    \$\begingroup\$ @yoyo_fun yes indeed, and if the ratio of signal to noise is high then you can do it. The issue with all radio transmission though is at the edge of the transmitter range, the signal falls off down to the level of the noise. The noise level will stay basically the same. As such your receiver will have a hard time distinguishing between "is that noise.. or is that a transmitter some distance away..." \$\endgroup\$ – Trevor_G Sep 6 '17 at 16:12
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    \$\begingroup\$ @analogsystemsrf it's not so much that it's more robust, but rather that the noise is not in the same mode as the signal. Noise wont doesn't normally change the frequency/phase angle. Of course you still need to detect and lock into the carrier frequency and noise can knock you off there too if the signal is weak enough. However the fact that it is continuous helps enormously. \$\endgroup\$ – Trevor_G Sep 6 '17 at 16:17
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What you are referring to does exist, and is called amplitude-shift keying.

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    \$\begingroup\$ en.wikipedia.org/wiki/Amplitude-shift_keying \$\endgroup\$ – Bart Sep 6 '17 at 14:41
  • \$\begingroup\$ Thanks but what does shift keying means? English is not my native language and I do not understand where did those words came from? \$\endgroup\$ – yoyo_fun Sep 6 '17 at 14:42
  • \$\begingroup\$ Read the Fine Wiki Article that I linked to in the comment, I do not know what your native language is, but maybe there is a translation of the page available. Basically it means shifting the amplitude, based on a key (the data you are transmitting). \$\endgroup\$ – Bart Sep 6 '17 at 14:43
  • \$\begingroup\$ My language is not listed in the translation in none of the Wikipedia articles that contain the phrase 'shirt keying' and from the context I do not understand why were those words used. What I understand is 'moving a key'.... I do not know how is this related to modulation or to electronics. \$\endgroup\$ – yoyo_fun Sep 6 '17 at 14:45
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    \$\begingroup\$ Amplitude shift means: a one or a zero shifts (changes) the amplitude of the carrier. Zero amplitude = 0, maximum amplitude = 1. Google "amplitude shift keying" and look at the pictures. \$\endgroup\$ – Bimpelrekkie Sep 6 '17 at 14:46
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Why amplitude modulation is not used for sending digital data?

It is used. In fact, this is all there was in early radio. The carrier was simply switched on and off according to the dits and dahs of Morse code. This was all these early transmitters could do.

Only later did people get clever and encoded a analog signal in the amplitude of the carrier. Then they got even more clever and found other ways to encode analog signals onto a radio carrier, like FM.

Nowadays we use lots of different means of encoding information onto a radio carrier. The main drivers of newer modulation techniques is to use less transmission power for the same received signal to noise ratio, be able to resolve a single transmission from multiple overlapping ones, to be tolerant of ambient noise at a specific frequency, etc. Modulation techniques have come a long way from the original OOK (on-off keying) and its analog equivalent, AM (amplitude modulation).

Today, OOK is still used in some off the shelf radio transceiver modules. Its advantages are that it is easy and cheap to implement.

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I think the point that AM/ASK is still being used is covered, but as far as why it's not more prevalent is that different modulations are more efficient than AM.

I'm not trying to sound snarky, but think of it as the same reason we moved past 8-tracks into cassettes and then into CD's, and so on; technology was able to stabilize digital signals in highly efficient ways.

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