Hello, I've been studying polyphase decomposition, decimation, interpolation and fractional decimation. I think I have a handle on what I'm doing, but there's one thing I don't understand... Do you have to have greater than an L*Mth order filter in order to use polyphase decomposition to put the compressor and expander in their most efficient places (first and last, respectively)? For example: Let's say I want to reduce my sample rate to Fs'= 0.8333Fs. L = 5 and M = 6, so they're relatively prime, which is good. But then I do Type II decomposition and get R0(z^5), R1(z^5)...R4(z^5). What if all I needed was a 3rd order FIR filter in the first place? Then wouldn't R4 =0? Whats more, when I go to do the TypeI decompsition, I'm going to get R00, 01, etc, but won't all but the first row of the R filter coefficient matrix equal zero? So, either (A)it's okay to have zero-terms in the polyphase elements, or (B)the minimum order of the original filter, H(z) has to be L*M (per the example, N=30). Which is correct? Thank you for your kind consideration of my question.

# Fractional decimation

Started by ●March 1, 2009

Reply by ●March 2, 20092009-03-02

On Mar 1, 5:41�pm, "tharris00" <ted.harris.h...@gmail.com> wrote:> Hello, > > I've been studying polyphase decomposition, decimation, interpolation and > fractional decimation. I think I have a handle on what I'm doing, but > there's one thing I don't understand... > > Do you have to have greater than an L*Mth order filter in order to use > polyphase decomposition to put the compressor and expander in their most > efficient places (first and last, respectively)? > > For example: Let's say I want to reduce my sample rate to Fs'= 0.8333Fs. L > = 5 and M = 6, so they're relatively prime, which is good. > > But then I do Type II decomposition and get R0(z^5), R1(z^5)...R4(z^5). > What if all I needed was a 3rd order FIR filter in the first place? Then > wouldn't R4 =0? > > Whats more, when I go to do the TypeI decompsition, I'm going to get R00, > 01, etc, but won't all but the first row of the R filter coefficient matrix > equal zero? > > So, either > (A)it's okay to have zero-terms in the polyphase elements, or > (B)the minimum order of the original filter, H(z) has to be L*M (per the > example, N=30). > > Which is correct? > > Thank you for your kind consideration of my question.I find the polyphase terminology in books to be a huge distraction from the very simple and obvious concept of avoiding unnecessary calculations. If you start with a concatenated interpolator and decimator and elimatate wasted work, you'll end up at the right place. The only trick is the book-keeping. John

Reply by ●March 2, 20092009-03-02

Not 100% sure (would need to think for that :-)). But here is what I (don't) think. Since polyphase structure is exactly (theortically and practically) equivalent to the normal interpolation/filtering/decimation structure without polyphase, having some terms equal to 0 should be OK if the filter is very small order. But, maybe I would really think over it more seriously sometime. Regards Piyush On Mar 2, 3:41�am, "tharris00" <ted.harris.h...@gmail.com> wrote:> Hello, > > I've been studying polyphase decomposition, decimation, interpolation and > fractional decimation. I think I have a handle on what I'm doing, but > there's one thing I don't understand... > > Do you have to have greater than an L*Mth order filter in order to use > polyphase decomposition to put the compressor and expander in their most > efficient places (first and last, respectively)? > > For example: Let's say I want to reduce my sample rate to Fs'= 0.8333Fs. L > = 5 and M = 6, so they're relatively prime, which is good. > > But then I do Type II decomposition and get R0(z^5), R1(z^5)...R4(z^5). > What if all I needed was a 3rd order FIR filter in the first place? Then > wouldn't R4 =0? > > Whats more, when I go to do the TypeI decompsition, I'm going to get R00, > 01, etc, but won't all but the first row of the R filter coefficient matrix > equal zero? > > So, either > (A)it's okay to have zero-terms in the polyphase elements, or > (B)the minimum order of the original filter, H(z) has to be L*M (per the > example, N=30). > > Which is correct? > > Thank you for your kind consideration of my question.