Polyphase filter outputs

Started by nikolatesla20 July 14, 2009
>>nikolatesla20 wrote: >> >>>>When interpolating, you multiplex (interlace) the phase output >>>>sequences to build the high-rate sequence. When decimating, a >>>>polyphase FIR filter is pointless but you would indeed add the >>>>phase outputs in the case of IIR filtering. >> >>> Could you answer one more question. By "multiplex" the outputs, >>> do you mean that basically the outputs become the values that >>> replace the zero filling in the new buffer? >> >>Yes, if I interpret your formulation right. Each input value is >>turned into a block of successive result values that is just the list >>of phase outputs. >> >> >>Martin >> >>-- >>Quidquid latine scriptum est, altum videtur. >> > > >Thanks for your answer Martin. > >ACK! I do have to say though that the polyphase is extremely confusing.
>haven't found anything on the internet that explains them well. > >For example, I found the articles at dspguru where he shows an "example" >polyphase that has 16 taps and interpolation ration of 3. Then he shows
>table of how the values are calculated. But the table is confusing! They >way he shows it seems like the data is being "shifted" into the delay >line..which I could understand, but then he shows each equation for each >"poly" channel.. > >Basically I trying to do this in code. I can just copy my entire buffer >and step through it for now to avoid looping buffers until I get it >working. > >My interpolation ratio is 5. > >But do I step ONE sample at a time or 5 samples at a time? And if I step
>sample at a time, then I'm assuming I will have 5 polyphase filters, so >then do I add the result of all of them and put that in as the new >calculated sample, or do the 5 outputs create the new values for
>1-5? Ugh. I just need a nice clear step by step explanation, not more >equations...I get how the math makes it faster, but I want to understand >how do I implement the darn thing! > >-niko >
Ok I got 'er working. Here's my code for 35 taps, interpolation of 5. // "polyphase filter" int filtercount = 0; for (int ii=30; ii < outsamples.length; ii++) { short tempval = 0; // calc new value based on the 5 poly filters tempval = (short)((coefs[filtercount] * outsamples[ii-filtercount]) + (coefs[filtercount+5] * outsamples[ii-(filtercount+5)]) + (coefs[filtercount+10] * outsamples[ii-(filtercount+10)]) + (coefs[filtercount+15] * outsamples[ii-(filtercount+15)]) + (coefs[filtercount+20] * outsamples[ii-(filtercount+20)]) + (coefs[filtercount+25] * outsamples[ii-(filtercount+25)]) + (coefs[filtercount+30] * outsamples[ii-(filtercount+30)])); newdata[ii] = tempval; filtercount++; if (filtercount > 4) filtercount= 0; }