rover8898 said:
Hello,
filter does not ABSOLUTELY 100% assure that there will be no aliasing present. I am obviously imagining a worst-case scenario were there are unforseen unwanted signals (perphaps higher order harmonics) that might have coupled themselves onto the desired analog input signal. If these parasitics are located beyond the Fs/2 (over)sampling frequency/2 that is used to retrieve the data samples prior to passing them through the low-pass decimation (digital FIR) filter and subsequently downsampling them, then there will aliasing.
However, an analog lowpass filter with a very high cuttoff frequency (ideally Fsmax/2 of the DSP/hardware), placed before the A/D conversion should do the trick. Thus, I am thinking that perphaps a broad analog filter is also used in tandem with the multirate signal processing scheme. It is the only certain method to prevent aliasing in my opinion. Is this accurate? If not, how is it possible that the multirate signal processing scheme can garantee that the bandwidth of the input signal does not exceed Fs/2 (as high as that may be)?
-Roger
I am quite new on digital filters and DSP, I also have some doubts concerned with the subject, so I think this discussion has been very good, and I hope I can learn more from this.
In engineering we have to consider among other, some aspect: the problem at hand, specifications, the desired result, the availables tools, and the most important COST. When we discuss theory, we are leaded to forgeting some aspects, mostly the last one . What it has to do with the discussion:
First, In spite of the fact that we love math, in practical life we canot be sure 100% of nothing, because we dont live in a deterministic world, so what do we do ? we put in our project redudance, safety coeficients, safety margins and other stuff , in order to get confidence in the results, so as much as we are close to 100%, as much as the cost of the project increase.
When we are working with low frequency is pretty hard to build analog filters with a reasonable transition band. One of the greatest advantages of digital filters is dealing in low frequency and getting acurate transitions bands, this feature has a great contribution in biomedical advances, so for instance, how we should have a analog filter to prevent aliasing in low frequencies? How this filter would gonna work ? The DSP has the advantages of changing parameters with a simple changing in their algorithms, so why we should incorporate an analog filter in the input of DSP hardware before the A/D converter ? It wouldn't introduce constrains in the design ? Imagine that we are working in low frequency, with available A/D speeds , what should be the value of the sampling frequency ? How many times it should be possible with no lost in processing performance, considering the higher known and desired signal frequency ? With a very high sampling frequency what should be the probability of parasitics frequencies to cause aliasing ? woul be cost and size advantage to put an analog filter to play the antialiasing role in this context ?
Obviously that this considerations has other answers when working with high frequencies context, considering microstrip technologies and specific applications as for instance cell phones handsets, the use of an antialiasing filter before the DSPing is more cost efective if we use an microstrip analog filter ? What shoud be speed of processing if we try to use multirate processing ? It would be cost efective?
So, I think the magic of studying the theory is try to fit it on given applications. Sometimes we are lead to find an unique answer to a question, when the answer is just: It depends on the application. That was what I did in my first answer, I thoght in low frequencies, and probably that was what other pall did when we wrote that it is common practice to put an analog filter before the A/D converter, Probably he thought in high frequencies. Anyway, I can be wrong in this point of view, and if so I would appreciated if someone point me the way. But In my oppinion you should focus on applications and get the conclusion to each of these.