Is sampling an analogue signal that produces pulses an encoding? - Page 1
Forum section: electronics / beginners

author: richard1991
author_rank: Contributor
timestamp: December 08, 2017, 05:53:21 pm
content: Hi. We sample analogue signals and the result we get is a series of discrete pulses, the amplitude of which reflects the amplitude of the analogue signal when it is sampled. We can effect that by simply connecting a switch that closes when we wish to sample, and open when we don't.Does this sampling amount to an encoding of the original analogue signal? Thanks.(Note: The train of pulses would, apparently, be a PAM signal (Pulse Amplitude Modulation). Thanks.

author: w2aew
author_rank: Super Contributor
timestamp: December 08, 2017, 06:06:09 pm
content: What you've described is commonly called a "sample and hold" circuit. This is often done as one of the first steps in analog to digital conversion, where the S/H samples are then converted to a digital value by an ADC.

author: Benta
author_rank: Super Contributor
timestamp: December 08, 2017, 06:27:01 pm
content: And with sample/hold, you'll get a sin(x)/x response, which you'll need to take into account as well.

author: richard1991
author_rank: Contributor
timestamp: December 08, 2017, 06:50:00 pm
content: Here is a Wikipedia article.https://en.wikipedia.org/wiki/Pulse-amplitude_modulationNote: "Pulse-amplitude modulation (PAM), is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulse."I'm yet to understand why the output from sampling would amount to an encoding of the signal. And actually, I'm finding it hard to figure why a sampled signal, amounts to a modulated signal.If I'm understanding it, the article is saying a signal consisting of a series of sampled pulses amounts to PAM. But, where is the encoding? Where is the modulation?

author: richard1991
author_rank: Contributor
timestamp: December 08, 2017, 06:59:42 pm
content: Maybe the modulation is this: The sampled pulses will have a different spectrum from the analogue signal. And in that sense we have modulation. But, no new information is transmitted.Still unsure whether chopping up the analogue signal into pulses of various amplitudes, amounts to an encoding.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 08, 2017, 07:17:52 pm
content: You seem to be confusing two different things that superficially seem similar.Sample-and-hold ("S/H") is a widely used scheme in analog to digital conversion.Pulse-amplitude-modulation ("PAM") is one of many schemes for encoding a signal onto a carrier for transmission.The two things have nothing to do with each other. Each is an independent concept unrelated to the other.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 09, 2017, 06:22:14 am
content: Quote from: richard1991 link=topic=100064.msg1369759#msg1369759e date=1512759582The signal at the sampling frequency will now have side bands, the amplitude & frequency of which are dependent on that of the sampled frequency, so yes, new informationistransmitted.All types of modulation consist of the addition of sidebands to the original signal.Don't be taken in by the variation in the pulses amplitude.This display is quite deceptive, being the result of looking at the carrier & sidebands with a wide band device like an oscilloscope.Use a Spectrum Analyser, & you will see the original sampling frequency unchanged in amplitude, surrounded by its sidebands.The classic case of this deception is the traditional "Amplitude Modulation" envelope.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 09, 2017, 06:27:43 am
content: Quote from: Richard Crowley on December 08, 2017, 07:17:52 pmHe never said he was going to do anything other than sample it.No matter what you do with the resultant signal, therawoutput of a sampler is the same as that from a modulator.

author: richard1991
author_rank: Contributor
timestamp: December 09, 2017, 11:34:30 am
content: I come from a background where modulation means impressing information of the nature of audio onto a RF carrier. So, I've had difficulty in grasping why, if you chop up an audio signal, by way of sampling, the result is Pulse Amplitude Modulation (PAM). Because we are not modulating a carrier. But, the pulses are PAM, because the sampling, or the chopping or switching, has produced sidebands. Not in the RF spectrum. but in the audio spectrum, the exact nature dependent on the audio frequencies in the analogue signal, and the sampling or chopping or switching frequency. So, it is now settled in my mind, that the results of sampling is PAM.I am less certain whether this form of PAM signal, the result of sampling, represents a form of encoding. Should encoding mean something particular, which is not covered by sampling of an audio analogue signal? If the answer is yes, then the pulses would not be the result of encoding. The case may be different if a carrier is being modulated. Any thoughts on this? Thanks.

author: dmills
author_rank: Super Contributor
timestamp: December 09, 2017, 01:50:43 pm
content: The carrier can be considered to be (theoretically) a set of dirac delta impulses at the sampling rate which is multiplied by the signal value.Multiplying two signals together is modulating one with the other, hence sampling is mixing.As the sample rate string of dirac delta impulses has a fourier transform that is itself a infinite comb of dirac delta impulses at the sample rate, the mixing product of the sampling and the signal has a infinite set of copies of the signal amplitude modulated around each integer multiple of the sample rate.In reality we tend to use sample and HOLD which changes the fourier transform of the sample clock to be that of a square wave with the appropriate effect on the amplitude of each image, but that is a detail.Regards, Dan.

author: richard1991
author_rank: Contributor
timestamp: December 09, 2017, 02:45:33 pm
content: Sampling of an analogue signal leads to PAM.But, can someone please give me a response as to whether it is correct to say that this kind of PAM signal (by way of sampling) is the result of or has some relation to some kind of encoding? And also, in the case where the result of sampling is used to modulate a RF carrier, whether, in that case, there is to be found something associated with encoding.To my mind, sampling does not involve any kind of encoding. Also, modulating an RF carrier with sampled pulses (taken from an analogue signal) likewise. But, I would like confirmation of this. Thanks.EDIT: Is there anything wrong with this explanation?:https://en.wikipedia.org/wiki/Pulse-amplitude_modulation

author: vk6zgo
author_rank: Super Contributor
timestamp: December 09, 2017, 10:48:48 pm
content: Quote from: richard1991 on December 09, 2017, 02:45:33 pmIf the signal you are sampling is already encoded in some way, the resulting modulated carrier will carry the same encoding.You could, for instance, have simple multi- tone modulation, where each tone modulating the carrier meant something different.Just the act of modulation does not, in my opinion constitute "encoding" as you could, say, modulate the carrier with a 1kHz tone forever---Is this really conveying information?I think you should, initially, think of sampling simplistically, as for a basic understanding, you can just "put aside" the modulation implications.Then, read some good information on Modulation------I would recommend the ARRL Handbook.It is always good to be able to look at things from different perspectives.

author: drussell
author_rank: Super Contributor
timestamp: December 10, 2017, 08:20:58 am
content: Quote from: vk6zgo on December 09, 2017, 10:48:48 pm+1 for the ARRL Handbook being an excellent reference.