Timeline for How does PAM-4 encoding of a digital signal relate to theoretical *pulse* amplitude modulation?
Current License: CC BY-SA 4.0
6 events
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| Nov 2, 2023 at 21:21 | comment | added | MBaz | Well, strictly speaking it is a sum of pulses, if you define $p(t)$ as a rectangular pulse, right? You can choose any shape for $p(t)$, but some are better than others. | |
| Nov 2, 2023 at 21:03 | comment | added | JMC | Sometimes PAM-4 is even depicted like a "stairs" function as in this figure: tek.com/-/media/sites/default/files/media/image/… The only way I can reconcile this with your mathematical description as a sum of pulses is if the pulses have the shape of the box-filter/box-distribution and are exactly as wide as the interval between them. Why is this then still analyzed as a sum of "pulses"? | |
| Nov 2, 2023 at 20:56 | comment | added | JMC | Thanks, that makes sense. In my comparison to the first sense of PAM I was only just thinking of the width of the pulses rather than the amplitudes. So my last question still stands: In the PAM-4 waveform and eye diagram it looks like the pulses are so wide that they completely blend into eachother, which looks almost like there never were any pulses in the first place, but instead as if the voltage instead just went immediately from one level to the other like a smoother "staircase" function where the heights of the stairs are the 4 levels of the encoding. Why is the p(t) function so wide? | |
| Nov 2, 2023 at 13:28 | comment | added | MBaz | "If the p(t) function was a train of ideal impulses, i.e. zero every except in t=0, it would look like the first concept, right?" No, it wouldn't, because in 4-PAM the sample values have been quantized and encoded. For example, assume a sample is 3.8556783, it is quantized to 3.86 and encoded to binary 100101. Then, the pulse amplitudes would be (say) {0.5, -0.5, -0.5, 0.5, -0.5, 0.5}. In the original, older sense of PAM, the pulses have the exact same amplitude as the sample; that is, you'd transmit one single pulse with amplitude 3.8556783. | |
| Nov 2, 2023 at 0:05 | comment | added | JMC | If the p(t) function was a train of (impossible) "ideal" impulses, i.e. zero every except in t=0, it would look like the first concept, right? So the fact that instead of pulses we see wider levels is because the actual pulse function used is "wide" in relation to T, no? If T was sufficiently long, would we see the signal "return to zero" inbetween the information-carrying pulses? | |
| Nov 1, 2023 at 22:40 | history | answered | MBaz | CC BY-SA 4.0 |
