On May 12, 5:20 pm, "Green Xenon [Radium]" <gluceg...@excite.com> wrote:> Aliasing is a digital entity. What is the analog equivalent of aliasing?I regard aliasing as a form of information loss or information ambiguity. If you sample below half the bandwidth, you lose information about whether a certain frequency represented by the samples might be above or below Fs/2 (unless you pre-notch the lower frequency, as in under-sampling). You can get the same thing in the analog domain by using an analog mixer to mix down a wideband source and then adding back the original baseband. You will then no longer know whether a signal in the result was from above or below the mixer frequency. Sampling is just a easy way to mix down stuff above Fs/2, add it to the baseband, and similarly confuse the result (unless you bandlimit above or below first). . IMHO. YMMV. -- rhn A.T nicholson d.0.t C-o-M http://www.nicholson.com/rhn/dsp.html

# What is the analog equivalent of aliasing?

Started by ●May 12, 2008

Reply by ●May 13, 20082008-05-13

Reply by ●May 13, 20082008-05-13

On Mon, 12 May 2008 21:16:01 -0700, "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote:> >"Green Xenon [Radium]" <glucegen1@excite.com> wrote in message >news:4828ded3$0$5698$4c368faf@roadrunner.com... >> Hi: >> >> Aliasing is a digital entity. What is the analog equivalent of aliasing? >>(snipped by Lyons) Hi, Good point Fred. You made me think of the backward-spinning stagecoach wheels in the old black & white cowboy movies. The wheels, whose positions are repetitive over time, were being sampled by the the camera's shutter. [-Rick-]

Reply by ●May 13, 20082008-05-13

Dnia 13-05-2008 o 06:36:36 Green Xenon [Radium] <glucegen1@excite.com> napisa�(a): [...]> But won't an analog device just smoothly cut-off a frequency that is too > high -- i.e. at a certain point the cut-off gradually beings and the > higher an incoming frequency is, the more it will be attenuated -- > without any aliasing?[...] Aliasing means that signal appears in the device's bandwidth. -- Mikolaj

Reply by ●May 13, 20082008-05-13

"Green Xenon [Radium]" <glucegen1@excite.com> wrote in news:4828ded3$0$5698 $4c368faf@roadrunner.com:> Hi: > > Aliasing is a digital entity. What is the analog equivalent of aliasing? > > > Thanks, > > RadiumAliasing is not a digital entity. It occurs during analog amplitude modulation if the carrier frequency is not high enough. -- Scott Reverse name to reply

Reply by ●May 13, 20082008-05-13

On May 13, 12:36�am, "Green Xenon [Radium]" <gluceg...@excite.com> wrote:> But won't an analog device just smoothly cut-off a frequency that is > too high ... without any aliasing?Stop thinking of signal processing as analog or digital. Signal processing is mathematical. Analog or digital is a detail of its implementation. As Fred Marshall said, spectral aliasing is an artifact of temporal sampling. A sampled temporal signal results in a periodic spectrum that is continuous in frequency. Is that analog or is it digital? A periodic but continuous temporal signal results in a discrete- frequency spectrum. Or, looking at it the other way around, a sampled spectrum can result in temporal aliasing, the dual of spectral aliasing. Is that analog or is it digital? Now, to confuse things even further, what is commonly called "analog" is really "continuous time". But as I pointed out above, a continuous time signal can have a discrete frequency spectrum. Similarly, what is commonly called "digital" is really "discrete time". But as I pointed out above, a discrete time signal can have a continuous frequency spectrum. So exactly what do you mean when you say, "Aliasing is a digital entity"? Greg

Reply by ●May 13, 20082008-05-13

�Similarly, what> is commonly called "digital" is really "discrete time". �But as I > pointed out above, a discrete time signal can have a continuous > frequency spectrum. �So exactly what do you mean when you say, > "Aliasing is a digital entity"? > > GregThere are singals that are continuous in time and singals that are sampled and discrete in time. There are voltages that are continuous in amplitude and voltages that are quantized and represented as digital values. Timne sampling and voltage quantization are two different issues. FM MPX stereo is a good example of a common system that uses time sampling and can suffer aliasing but is not digitized i.e. is "analog". http://transmitters.tripod.com/stereo.htm Mark

Reply by ●May 13, 20082008-05-13

On May 13, 9:44�am, Mark <makol...@yahoo.com> wrote:> Timne sampling and voltage quantization are two different issues.Exactly. Digital has become the label for what is really "discrete time / quantized amplitude" (and, often, "discrete time / discrete frequency / quantized amplitude in both domains"). It really should refer only to amplitude quantization. But that's not the way it's commonly used. When "digital" is taken to mean "quantized amplitude", the original statement that "Aliasing is a digital entity" becomes even less meaningful. Greg

Reply by ●May 13, 20082008-05-13

>Digital has become the label for what is really "discrete >time / quantized amplitude" (and, often, "discrete time / discrete >frequency / quantized amplitude in both domains"). It really should >refer only to amplitude quantization. But that's not the way it's >commonly used.When I first learned the term "DSP," it actually meant "discrete-time signal processing." It has since been morphed to "digital signal processing," though with reason, since it is difficult to do anything with discretely sampled continuous waveforms unless you first digitize them. Digital signal processing is essentially quantized DSP. Perhaps someone will start to call it QDSP one day and it will stick? :) Mark

Reply by ●May 13, 20082008-05-13

On Tue, 13 May 2008 10:55:59 -0500, "markt" <takatz@pericle.com> wrote:>When I first learned the term "DSP," it actually meant "discrete-time >signal processing." It has since been morphed to "digital signal >processing," though with reason, since it is difficult to do anything with >discretely sampled continuous waveforms unless you first digitize them.Check out switched capacitor filters; perfect example of sampling without quantization. The issue of difficulty is in the eye of the beholder but it's certainly a lot cheaper to do digital signal processing in an FPGA coupled with a suitable ADC/DAC then it's to design your own SCFs.

Reply by ●May 13, 20082008-05-13

Hi Radium, A very good question, because the answer gives a much deeper understanding of the sampling theorem. Say you start with an analog signal, x(t), and convert it to a digital signal, x(n). Mathematically, this process is carried out in two steps. In the first step, we sample the signal by multiplying it by an impulse train. An impulse train is an analog signal that is always zero, except for periodic spikes (also known as impulses or delta functions). The product of these two signals retains the amplitude of the original signal at the spike locations, but inserts a value of zero in between. In other words, it has wiped out all information contained in the original analog signal, except at the "sample" locations. In the second step, we measure the height of each of the successive spikes in the analog product signal, forming a list of numbers. Ignoring quantization, this gives us x[n]. Now we make an interesting observation. From the standpoint of information content, step 2 does nothing. That is, the analog product signal and the list of numbers, contain exactly the same information. By this I mean that one description can be used to exactly generate the other, without any error. This equivalence is very important, it provides a deep understanding of how the analog and digital worlds relate to each other. To answer your original question, since step 2 does nothing, all of the changes to the information content happen in step 1. In other words, aliasing can be viewed as being 100% analog. For instance, this is what happens in the RF mixing examples provided by the other posts. Here's a link if you want to see graphics of the above description. Regards, Steve http://www.dspguide.com/ch3/2.htm http://www.dspguide.com/ch3/3.htm