# PAM clarification

Discussion in 'Electronics Homework Help' started by max_torch, Dec 8, 2014.

1. ### max_torch

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Feb 9, 2014
Here it shows a circuit that performs pulse amplitude modulation where there are gaps between the pulses:

Here in wikipedia it shows a circuit block diag that does not have a discharge switch and there are no gaps in between the pulses:

So I guess there are really two kinds of flat-top PAM, one where there is a gap between the pulses and one where there is no gap, and if yes what are the two called (I could not find a term to distinguish the two)? which kind of sample and hold PAM is used exactly in T-carrier, E-carrier system?

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Am I right in suspecting this is homework or an assignment?

3. ### max_torch

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Feb 9, 2014
this is our topic in school but it is not a homework assignment i am taking the initiative to read and understand the lesson and do my own research so I guess this belongs in electronics chat because it is not an assignment?

Last edited: Dec 8, 2014
4. ### Harald KappModeratorModerator

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Nov 17, 2011
If that's so, o.k. it just looks a bit suspicious.
You may want to place this kind of question in the homework section anyway. This way others will see that you are trying to learn and understand and their responses may be phrased accordingly. This in turn may help you to better grasp the concepts discussed.

5. ### max_torch

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Feb 9, 2014
It's just that I have seen and read that t-carrier and e-carrier are using PCM or pulse code modulation and in all the pulse code modulation articles it shows that there is the sample and hold block diagram but I have seen that there are different kinds of sample and hold circuits that can be in that block, so I'm just wondering what kind of sample and hold do they really use in the T-carrier and E-carrier system?

6. ### Harald KappModeratorModerator

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Nov 17, 2011
Sorry, that is ouside the scope of my experience.

7. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
In PCM, the analogue signal is sampled and converted to digital form at regular intervals using an ADC (analogue to digital converter). A sample-and-hold circuit may be needed at the input of the ADC depending on the type of ADC used. I can't see any reason for the "discharge switch" and the "bar graph"-style waveform in the first part of your post. I can't think of any reason to discharge the holding capacitor in between samples.

8. ### max_torch

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Feb 9, 2014
When I do a google image search for flat-top pulse amplitude modulation I find so many other images of this "bar-graph" style, also I looked at another article and it has the same bar graph style. The only reason it would be that way I could imagine is because of the discharging of the capacitor.

9. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Right, it would. But why discharge the capacitor? AFAIK the capacitor is only needed to hold a steady voltage for the ADC to convert. I can't see any reason to discharge it. Can you see any explanation of why the capacitor is discharged?