On Friday, March 2, 2018 at 4:39:43 PM UTC-8, Steve Pope wrote:> Richard (Rick) Lyons <r.lyons@ieee.org> wrote: > > >On Friday, March 2, 2018 at 1:19:22 PM UTC-8, Steve Pope wrote: > > >> Actually it's true. A discrete-time signal is non-zero only at > >> discrete points in time. So if for example, > > >> X(t) is zero for t not in (0, 2, 4, 6...) > >> Y(t) is zero for t not in (1, 3, 5, 7...) > > >> Then Z(t) = X(t) + Y(t) where > > >> Z(t) = X(t) if t = 0, 2, 4, 6 ... , Y(t) otherwise. > > >> So I would say addition and multiplexing are the same, so long as > >> you haven't mangled the time axis. > > >Hi Steve. I claim that multiplexing two sequences is not the same as > >adding two two sequences. Multiplexing your above X(t) and Y(t) > >sequences yields > > > > Seq# 1 --> 0, 1, 2, 3, 4, 5, ... . > > > >Adding your X(t) and Y(t) sequences produces: > > > > Seq# 2 --> 1, 5, 9, 13, 17, ... . > > You've completely lost me. I claim that you get a sequence that > is equal to X(t) for t = 0, 2, 4, 6 ... and equal to Y(t) for > t = 1, 3, 5, 7.Hi Steve. Yes, I agree that the sequence you describe is the sequence that results when you multiplex your X(t) and Y(t). But I disagree with your claim that multiplexing X(t) and Y(t) and adding X(t) and Y(t) are equivalent operations. (My above dissimilar Seq# 1 and Seq# 2 show that 'multiplexing' and 'addition' are not equivalent operations.) I have an even simpler example of why I believe multiplexing and addition are not the same. If you multiplex two N-sample sequences you'll generate a 2N-sample sequence. If you add two N-sample sequences you'll produce an N-sample sequence. My claim is that the 2N-sample and the N-sample sequences are not equal to each other, thus multiplexing and addition are not equivalent operations. I hope what I've written makes sense. [-Rick-]

# Multiple ADCs and aliasing

Started by ●February 23, 2018

Reply by ●March 3, 20182018-03-03

Reply by ●March 3, 20182018-03-03

Hi Richard, W dniu 03.03.2018 o 00:57, Richard (Rick) Lyons pisze: (...) > Hey Roman, It just "hit me". Isn't it true that the author's Eq. (19) only applies to periodic signals? If that's true then his spectral example in Figure 6 would not be correct for real-world, unpredictable, information-carrying signals. Am I missing something here? > (...) could you clarify why x(t), which spectrum is X(f) in Eq.(19), MUST BE a periodic signal ? Do you agree, that the addition of two upsampled, time-shifted sequences: x1u(n) = x(0), 0, x(2), 0, ... . x2u(n) = 0, x(1), 0, x(3), ... . is equal to multiplexing of two sequences: x1(n) = x(0), x(2), ... . x2(n) = x(1), x(3), ... . ? Kind regards, Roman Rumian

Reply by ●March 3, 20182018-03-03

On Saturday, March 3, 2018 at 12:46:21 AM UTC-8, rr wrote:> Hi Richard, > > W dniu 03.03.2018 o 00:57, Richard (Rick) Lyons pisze: > (...) > > Hey Roman, It just "hit me". Isn't it true that the author's Eq. (19) > only applies to periodic signals? If that's true then his spectral > example in Figure 6 would not be correct for real-world, unpredictable, > information-carrying signals. Am I missing something here? > > (...)Hi Roman> could you clarify why x(t), which spectrum is X(f) in Eq.(19), MUST BE a > periodic signal ?I'll do my best. Let's say I have a ten-second duration analog audio recording of a pure sine wave. First I compute the spectrum, Y1(f), of the first five seconds of the audio signal. Then I compute the spectrum, Y2(f), of five seconds of the sine wave audio starting at the beginning of the 2nd second out to the end of the 6th second. For this periodic sine wave input signal, Y1(f) and Y2(f) will be related to each other as shown in Eqs. (18) and (19) in the cited Razavi. That is, Y2(f) will be equal to Y1(f) multiplied by a phase angle that is a linear function of frequency. Also, in this periodic signal case, |Y1(f)| = |Y2(f)|. Now replace the ten-second audio sine wave signal with a ten-second audio recording of Lady Gaga singing a song (which is not periodic in time) and compute the new Y1(f) and Y2(f) spectra. What I'm saying is the new Y1(f) and the new Y2(f) will be NOT be related to each other as shown in Eqs. (18) and (19) in the Razavi paper. In this non-periodic signal case, spec magnitude |Y1(f)| is not equal to spec magnitude |Y2(f)|.> > Do you agree, that the addition of two upsampled, time-shifted sequences: > x1u(n) = x(0), 0, x(2), 0, ... . > x2u(n) = 0, x(1), 0, x(3), ... . > > is equal to multiplexing of two sequences: > > x1(n) = x(0), x(2), ... . > x2(n) = x(1), x(3), ... .I definitely agree in principle with your multiplexing example. But I would change the description of multiplexing, assuming that x1(n) and x2(n) are finite and equal in length, to following. Let's say that: x1(n) = x(0), x(2), x(4), x(6) and x2(n) = x(1), x(3), x(5), x(7). 1) Create Seq# 1 by following each sample in x1(n) with a zero-valued sample. Seq# 1 = x(0), 0, x(2), 0, x(4), 0, x(6), 0. (Seq# 1 is not simply stuffing zeros in between each x1(n) sample. Seq# 1 must end with a zero-valued sample!) 2) Create Seq# 2 by following each sample in x2(n) with a zero-valued samples. Seq# 2 = x(1), 0, x(3), 0, x(5), 0, x(7), 0. (Seq# 2 must end with a zero-valued sample.) 3) Create Seq# 3 by circularly rotating Seq# 2 forward in time by one sample. Seq# 3 = 0, x(1), 0, x(3), 0, x(5), 0, x(7). (The 1st sample in Seq# 3 is the last sample in Seq# 2.) I claim that multiplexing x(n) and x2(n) equals the sample-by-sample addition of Seq# 1 and Seq# 3 producing: x(0), x(1), x(2), x(3), x(4), x(5), x(6), x(7). Roman, sorry for my long-winded reply. [-Rick-]

Reply by ●March 3, 20182018-03-03

On Saturday, March 3, 2018 at 12:46:21 AM UTC-8, rr wrote:> Hi Richard, > > W dniu 03.03.2018 o 00:57, Richard (Rick) Lyons pisze: > (...) > > Hey Roman, It just "hit me". Isn't it true that the author's Eq. (19) > only applies to periodic signals? If that's true then his spectral > example in Figure 6 would not be correct for real-world, unpredictable, > information-carrying signals. Am I missing something here? > > (...) > > could you clarify why x(t), which spectrum is X(f) in Eq.(19), MUST BE a > periodic signal ? > > Do you agree, that the addition of two upsampled, time-shifted sequences: > x1u(n) = x(0), 0, x(2), 0, ... . > x2u(n) = 0, x(1), 0, x(3), ... . > > is equal to multiplexing of two sequences: > > x1(n) = x(0), x(2), ... . > x2(n) = x(1), x(3), ... . > > ?Hi Roman. I forgot to ask you: Do you agree that the addition of the two sequences, Seq# 1 = x(0), x(2), x(4), x(6) Seq# 2 = x(1), x(3), x(5), x(7) produces the four-sample sequence: x(0)+x(1), x(2)+x(3), x(4)+x(5), x(6)+x(7) ? [-Rick-]

Reply by ●March 3, 20182018-03-03

Richard (Rick) Lyons <r.lyons@ieee.org> wrote:>>> On Friday, March 2, 2018 at 1:19:22 PM UTC-8, Steve Pope wrote: >> >> >> Actually it's true. A discrete-time signal is non-zero only at >> >> discrete points in time. So if for example, >> >> >> X(t) is zero for t not in (0, 2, 4, 6...) >> >> Y(t) is zero for t not in (1, 3, 5, 7...)Okay my notation above was not the best. I intended to write X(t) is zero for t not in {0, 2, 4, 6...) Y(t) is zero for t not in (1, 3, 5, 7...)>> >> Then Z(t) = X(t) + Y(t) where >> >> >> Z(t) = X(t) if t = 0, 2, 4, 6 ... , Y(t) otherwise.>> >> So I would say addition and multiplexing are the same, so long as >> >> you haven't mangled the time axis.>> >Hi Steve. I claim that multiplexing two sequences is not the same as >> >adding two two sequences. Multiplexing your above X(t) and Y(t) >> >sequences yields >> > >> > Seq# 1 --> 0, 1, 2, 3, 4, 5, ... . >> > >> >Adding your X(t) and Y(t) sequences produces: >> > >> > Seq# 2 --> 1, 5, 9, 13, 17, ... . >> >> You've completely lost me. I claim that you get a sequence that >> is equal to X(t) for t = 0, 2, 4, 6 ... and equal to Y(t) for >> t = 1, 3, 5, 7.I get: X(0) + Y(0) = X(0) + 0 = X(0) X(1) + Y(1) = 0 + Y(1) = Y(1) X(2) + Y(0) = X(2) + 0 = X(2) etc.>Yes, I agree that the sequence you describe is the sequence that results >when you multiplex your X(t) and Y(t). But I disagree with your claim >that multiplexing X(t) and Y(t) and adding X(t) and Y(t) are equivalent >operations.>I have an even simpler example of why I believe multiplexing and >addition are not the same. If you multiplex two N-sample sequences >you'll generate a 2N-sample sequence. If you add two N-sample sequences >you'll produce an N-sample sequence. My claim is that the 2N-sample and >the N-sample sequences are not equal to each other, thus multiplexing >and addition are not equivalent operations.I think you are making sense, and if the author made a blanket claim that multiplexing and adding are the same, then that is misleading. They only obtain the same result if: (1) Both sequences are defined for all time indices of the result (2) For any time index of the result, only one sequence is non-zero. Under these conditions -- maybe it is much clearer in Verilog: ( x + y ) == (x ? x : y) Steve

Reply by ●March 3, 20182018-03-03

On Saturday, March 3, 2018 at 2:54:36 PM UTC-8, Steve Pope wrote:> Richard (Rick) Lyons <r.lyons@ieee.org> wrote: > > >>> On Friday, March 2, 2018 at 1:19:22 PM UTC-8, Steve Pope wrote: > >> > >> >> Actually it's true. A discrete-time signal is non-zero only at > >> >> discrete points in time. So if for example, > >> > >> >> X(t) is zero for t not in (0, 2, 4, 6...) > >> >> Y(t) is zero for t not in (1, 3, 5, 7...) > > Okay my notation above was not the best. I intended to write > > X(t) is zero for t not in {0, 2, 4, 6...) > Y(t) is zero for t not in (1, 3, 5, 7...) > > >> >> Then Z(t) = X(t) + Y(t) where > >> > >> >> Z(t) = X(t) if t = 0, 2, 4, 6 ... , Y(t) otherwise. > > >> >> So I would say addition and multiplexing are the same, so long as > >> >> you haven't mangled the time axis. > > >> >Hi Steve. I claim that multiplexing two sequences is not the same as > >> >adding two two sequences. Multiplexing your above X(t) and Y(t) > >> >sequences yields > >> > > >> > Seq# 1 --> 0, 1, 2, 3, 4, 5, ... . > >> > > >> >Adding your X(t) and Y(t) sequences produces: > >> > > >> > Seq# 2 --> 1, 5, 9, 13, 17, ... . > >> > >> You've completely lost me. I claim that you get a sequence that > >> is equal to X(t) for t = 0, 2, 4, 6 ... and equal to Y(t) for > >> t = 1, 3, 5, 7. > > I get: > > X(0) + Y(0) = X(0) + 0 = X(0) > X(1) + Y(1) = 0 + Y(1) = Y(1) > X(2) + Y(0) = X(2) + 0 = X(2) > > etc. > > >Yes, I agree that the sequence you describe is the sequence that results > >when you multiplex your X(t) and Y(t). But I disagree with your claim > >that multiplexing X(t) and Y(t) and adding X(t) and Y(t) are equivalent > >operations. > > >I have an even simpler example of why I believe multiplexing and > >addition are not the same. If you multiplex two N-sample sequences > >you'll generate a 2N-sample sequence. If you add two N-sample sequences > >you'll produce an N-sample sequence. My claim is that the 2N-sample and > >the N-sample sequences are not equal to each other, thus multiplexing > >and addition are not equivalent operations. > > I think you are making sense, and if the author made a blanket > claim that multiplexing and adding are the same, then that is misleading. >[Snipped by Lyons] Hi Steve. On March 2 you claimed that addition and multiplexing are equivalent operations when you wrote: "So I would say addition and multiplexing are the same,... ." And that statement of yours was what triggered my recent posts where I showed that addition and multiplexing are not equivalent operations. I agree with your above last sentence so long as we replace your word "misleading" with the word "incorrect." [-Rick-]

Reply by ●March 3, 20182018-03-03

Richard (Rick) Lyons <r.lyons@ieee.org> wrote:>On Saturday, March 3, 2018 at 2:54:36 PM UTC-8, Steve Pope wrote: >> Richard (Rick) Lyons <r.lyons@ieee.org> wrote: >> >> >>> On Friday, March 2, 2018 at 1:19:22 PM UTC-8, Steve Pope wrote: >> >> >> >> >> Actually it's true. A discrete-time signal is non-zero only at >> >> >> discrete points in time. So if for example, >> >> >> >> >> X(t) is zero for t not in (0, 2, 4, 6...) >> >> >> Y(t) is zero for t not in (1, 3, 5, 7...) >> >> Okay my notation above was not the best. I intended to write >> >> X(t) is zero for t not in {0, 2, 4, 6...) >> Y(t) is zero for t not in (1, 3, 5, 7...) >> >> >> >> Then Z(t) = X(t) + Y(t) where >> >> >> >> >> Z(t) = X(t) if t = 0, 2, 4, 6 ... , Y(t) otherwise. >> >> >> >> So I would say addition and multiplexing are the same, so long as >> >> >> you haven't mangled the time axis. >> >> >> >Hi Steve. I claim that multiplexing two sequences is not the same as >> >> >adding two two sequences. Multiplexing your above X(t) and Y(t) >> >> >sequences yields >> >> > >> >> > Seq# 1 --> 0, 1, 2, 3, 4, 5, ... . >> >> > >> >> >Adding your X(t) and Y(t) sequences produces: >> >> > >> >> > Seq# 2 --> 1, 5, 9, 13, 17, ... . >> >> >> >> You've completely lost me. I claim that you get a sequence that >> >> is equal to X(t) for t = 0, 2, 4, 6 ... and equal to Y(t) for >> >> t = 1, 3, 5, 7. >> >> I get: >> >> X(0) + Y(0) = X(0) + 0 = X(0) >> X(1) + Y(1) = 0 + Y(1) = Y(1) >> X(2) + Y(0) = X(2) + 0 = X(2) >> >> etc. >> >> >Yes, I agree that the sequence you describe is the sequence that results >> >when you multiplex your X(t) and Y(t). But I disagree with your claim >> >that multiplexing X(t) and Y(t) and adding X(t) and Y(t) are equivalent >> >operations. >> >> >I have an even simpler example of why I believe multiplexing and >> >addition are not the same. If you multiplex two N-sample sequences >> >you'll generate a 2N-sample sequence. If you add two N-sample sequences >> >you'll produce an N-sample sequence. My claim is that the 2N-sample and >> >the N-sample sequences are not equal to each other, thus multiplexing >> >and addition are not equivalent operations. >> >> I think you are making sense, and if the author made a blanket >> claim that multiplexing and adding are the same, then that is misleading. >> > >[Snipped by Lyons] > >Hi Steve.>On March 2 you claimed that addition and multiplexing are equivalent >operations when you wrote: "So I would say addition and multiplexing are >the same,... ."What I actually wrote: "So I would say addition and multiplexing are the same, so long as you haven't mangled the time axis." My qualifying language about not mangling the time axis is a little loose, but I cleaned that up in the post you just responded to.>And that statement of yours was what triggered my >recent posts where I showed that addition and multiplexing are not >equivalent operations. I agree with your above last sentence so long as >we replace your word "misleading" with the word "incorrect."Really I was just trying to explain why the original author used this description, and why it is sometimes true. I'm not asserting anything beyond that. I also sort of disagree with your description that summing two discrete-time signals necessitates shifting one of them on the time axis. That is not the natural way to add two signals. In hardware, there would be no such shift unless you deliberately added a delay to one signal. And in this case, it does not give you the signal that you want. I would venture to say it is incorrect to do that. Steve

Reply by ●March 5, 20182018-03-05

On Saturday, March 3, 2018 at 12:46:21 AM UTC-8, rr wrote: Hi Roman. My use of indexing in my March 3rd might cause confusion. I should have written: Do you agree that the addition of the two sequences, Seq# 1 = 1, 2, 3, 4 Seq# 2 = 7, 9, -5, 2 produces the four-sample sequence: 8, 11, -2, 6 Sorry for the possible confusion. [-Rick-]

Reply by ●March 5, 20182018-03-05

On Saturday, March 3, 2018 at 5:48:47 PM UTC-8, Steve Pope wrote: [Snipped by Lyons]> I also sort of disagree with your description that summing two > discrete-time signals necessitates shifting one of them on the > time axis. That is not the natural way to add two signals. > In hardware, there would be no such shift unless you deliberately > added a delay to one signal. And in this case, it does not give you > the signal that you want. I would venture to say it is incorrect > to do that. > > SteveHi Steve. I see what's causing the confusion here. On March 3rd I wrote: "the addition of the two sequences, Seq# 1 = x(0), x(2), x(4), x(6) Seq# 2 = x(1), x(3), x(5), x(7) produces the four-sample sequence: x(0)+x(1), x(2)+x(3), x(4)+x(5), x(6)+x(7)." Those above x(k) samples were meant to merely represent arbitrary sample values, nothing more. What I should have written is: Do you agree that the addition of the two sequences, Seq# 1 = 1, 2, 3, 4 Seq# 2 = 7, 9, -5, 2 produces the four-sample sequence: 8, 11, -2, 6 I wanted to reiterate that, unlike multiplexing two four-sample sequences, adding two four-sample sequences produces a four-sample result. Sorry for the confusion. [-Rick-]

Reply by ●March 5, 20182018-03-05

W dniu 03.03.2018 o 23:43, Richard (Rick) Lyons pisze: (...) > Hi Roman. I forgot to ask you: Do you agree that the > addition of the two sequences, > > Seq# 1 = x(0), x(2), x(4), x(6) > > Seq# 2 = x(1), x(3), x(5), x(7) > > produces the four-sample sequence: > > x(0)+x(1), x(2)+x(3), x(4)+x(5), x(6)+x(7) > > ? Hi Richard, yes, of course. Regards, Roman Rumian