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To clarify the question: As far as I understand, a ADC (Analogue to Digital Convertor) will sample and therefore quantize an analogue signal. Does a ADC therefore output a Pulse Amplitude Modulation (PAM) signal? And if not, why not?

If you look at Pulse Amplitude Modulation, on Wikipedia, you see that PAM "..is a form of signal modulation where the message is encoded in the amplitude of a series of signal pulses". Therefore, the mere sampling of an analogue signal results in PAM. The article goes on to say "Demodulation is performed by detecting the amplitude level of the carrier at every single period." However, when an analogue signal is sampled, the information is encoded. No actual need for modulating a carrier. It seems.

P.S. The question may appear rather odd, but I'm coming from the world of AM and SSB amateur radio, where modulation conjures a certain image of what modulation amounts to.

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    \$\begingroup\$ hm, why do you think it would output a PAM signal? This is a rather interesting assumption to make, it's not right, but it's coming from somewhere. RF-typical modulation types and the way digital data is put out by ADCs are not inherently very related topics. \$\endgroup\$ – Marcus Müller Dec 8 '17 at 12:57
  • \$\begingroup\$ If you look at Pulse Amplitude Modulation, on Wikipedia, you see that PAM "..is a form of signal modulation where the message is encoded in the amplitude of a series of signal pulses". Therefore, the mere sampling of an analogue signal results in PAM. The article goes on to say "Demodulation is performed by detecting the amplitude level of the carrier at every single period." However, when an analogue signal is sampled, the information is encoded. No actual need for modulating a carrier. It seems. \$\endgroup\$ – user46097 Dec 8 '17 at 13:35
  • \$\begingroup\$ I'll take the liberty of adding that background info to your question, as it illustrates where your train of thought arrives from. \$\endgroup\$ – Marcus Müller Dec 8 '17 at 13:50
  • \$\begingroup\$ So, now I have to ask: If you, yourself, notice that there's no carrier to be modulated, why are we comparing what an ADC does to a modulation type? \$\endgroup\$ – Marcus Müller Dec 8 '17 at 13:51
  • \$\begingroup\$ First, I've just(sort of) "accepted" or "internalised", that no carrier has to be modulated for PAM to be created. Also, if I were to ask a question again, I would make no mention of ADC's. I'd have asked: If you create a pulsed output from an analogue signal, using a switch, is that PAM? Anyway, seem to have got the answer to that. \$\endgroup\$ – user46097 Dec 8 '17 at 16:39
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Sampling and quantization are two entirely separate concepts that you seem to be somehow conflating.

If you sample a continuous-time signal without quantizing it, you do indeed end up with a PAM signal. But it is not a digital signal.

An ADC both samples its input signal and then converts the amplitude of each analog pulse to a digital word. The word is a quantized measurement of the pulse amplitude.

A DAC can convert that sequence of words back into a series of analog pulses (PAM signal again, but still quantized), and this must be followed by a "reconstruction filter" (e.g., low-pass filter, sometimes included in the DAC itself) in order to get a continuous-time signal that closely approximates the original.

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  • \$\begingroup\$ Good answer. Because there has been no stated requirement that a PAM signal requires the modulation of a carrier. Which I think is correct. \$\endgroup\$ – user46097 Dec 8 '17 at 13:57
  • \$\begingroup\$ This answer is saying, I think, that we can achieve PAM by encoding. But, we can also achieve PAM by modulation. That would involve a carrier signal. My current understanding. \$\endgroup\$ – user46097 Dec 8 '17 at 14:27
  • \$\begingroup\$ I'm not entirely sure whether achieving PAM by say using a simple switch, to produce the pulses, is a matter of encoding. Seems like for sure, such an operation would amount to sampling. \$\endgroup\$ – user46097 Dec 8 '17 at 16:45
  • \$\begingroup\$ In PAM, the train of pulses IS the "carrier". The information is encoded in the amplitude of the pulses. Those amplitudes can be either continuous (before quantization) or discrete (after quantization). \$\endgroup\$ – Dave Tweed Dec 8 '17 at 18:10
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Does a ADC therefore output a Pulse Amplitude Modulation (PAM) signal?

No, not all ADCs do.

For example Sigma-Delta (same as Delta Sigma btw) ADCs don't, they output a bitstream which is a clocked (at the sample rate) stream of ones and zeros (so binary data). For some SD ADCs this signal looks more or less like a PWM signal.

In the end it depends on the architecture of the ADC what kind of signal comes out.

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  • \$\begingroup\$ OK. So, if a type of ADC outputs a PAM signal, that means the DAC demodulates that PAM signal. That must be correct. I think I'm understanding things. \$\endgroup\$ – user46097 Dec 8 '17 at 12:07
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    \$\begingroup\$ No, not at all. Sigma-Delta is in no way similar to PWM in terms of how it represents a value. Most ADC are digital bus output. D-S type is edge case only. \$\endgroup\$ – Paul S Dec 8 '17 at 12:22
  • \$\begingroup\$ @PaulS Arguably a very common edge case these days. \$\endgroup\$ – pipe Dec 8 '17 at 13:49
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    \$\begingroup\$ @PaulS Sigma-Delta is in no way similar to PWM in terms of how it represents a value Well, I know plenty of examples where it is. And similar to a PWM signal, we can just low-pass filter it to recover the original signal. We do that at work and it matches SD modulator theory. Maybe you have not seen this type of SD ADC before. \$\endgroup\$ – Bimpelrekkie Dec 8 '17 at 14:34
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No, the output from most ADCs (SAR for example, but also Flash and some sigma-delta) is properly called PCM (Pulse Coded Modulation).

A few (notably some sigma-delta ADCs, output PDM (Pulse Density Modulation) which shares some features with PWM, namely that averaging can approximately reconstruct the waveform.

Many DACs on the other hand, do output PAM.

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ADC devices take an analog signal in and output a digital signal typically consisting of a multitude of bits. For example a 10 bit A/D will convert analog input into a 10 bit value, so output of ADC will be digital value ranging from 0-1023 (2 to the 10th power). 12 bit ADC will output value from 0-4095 etc. An 8 bit ADC will output from 0-255, etc.

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  • \$\begingroup\$ This would be a case then, where to ADC's output is not a form of modulation. However, where a ADC simply outputs pulses of varying amplitude, according to the instantaneous amplitude of the analogue signal - I am taking that to be a case of modulating the analogue signal, by way of sampling. Thus ADC outputs PAM. Whether such ADC's exist in practice, I do not know. \$\endgroup\$ – user46097 Dec 8 '17 at 12:25
  • \$\begingroup\$ Downvoting, doesn't answer question on PAM. Explains ADC operation instead. \$\endgroup\$ – TonyM Dec 8 '17 at 12:39
  • \$\begingroup\$ It's a strange world where people come on here, and get down-voted, when folks are only trying to grasp the working of something. \$\endgroup\$ – user46097 Dec 8 '17 at 12:48
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    \$\begingroup\$ @user50499 nah, I'm pretty sure Paul will understand; he's posted an answer, but not to the question. I could write a fantastic answer about why some plants die in winter, and it should be downvoted, as it doesn't contribute an answer to the original question. That's quality control for you: don't take it personal! \$\endgroup\$ – Marcus Müller Dec 8 '17 at 12:49
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    \$\begingroup\$ not your comment (you can't downvote comments. You can only upvote some to put a highlight on them), Paul's answer is (please read the help page to learn about how this page works). You're currently in the comments of that answer that doesn't deal with PAM. I think you're a bit confused. \$\endgroup\$ – Marcus Müller Dec 8 '17 at 13:14

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