>On Monday, November 19, 2012 3:53:54 AM UTC-8, gongdori wrote: >> ... >>=20 >> Dale, >>=20 >> Thank you for your reply. what you talked about in your previous postis>> what I was not sure. If Polyphase channelizer is used, it extracts all >> channel, down-samples "enough" so that it can meet the Nyquistcriterion.>> In other words, all channels are critically sampled. Thus, I amwondering>> what benefit we get by using WOLA chnanelizer, where we can have higher >> sampling rate in each channel. >>=20 >> Gongdori > >The polyphase channelizer and the WOLA channalizer are examples of DFTchan=>nelizers that work by windowing a data sequence and generate subchannelswi=>th frequency response determined by the coefficients of the window. Suchsy=>stems have three independent parameters when applied to streams of data:wi=>ndow (size and coefficients), transform size and stride. The windowdetermi=>nes the frequency response of the channels. The transform size determinest=>he number of channels generated (but not necessarily used). The stride ist=>he number of samples by which the window moves along the data streambetwee=>n subsequent implementations of the channelizer. The stride is thedesampl=>ing ratio. > >The people who speak of 'critical sampling' seem to assume a rectangularwi=>ndow the size of the transform and a stride of the same size. This is nota=> necessary assumption. The difference between polyphase and WOLAchannelize=>rs is that the channelizer described as WOLA has a window longer thaninste=>ad of equal to the transform size. > >There are applications where the rectangular window does not produce anade=>quate channel frequency response and so longer windows and non-unityweight=>s may be required. The 'critical sampling' assumptions produce a channelfr=>equency response that is a sinc function. This gives a worst-case stopband=>rejection of only about 13dB and a gain at adjacent channel crossover of-3=>.92 dB. Some applications don't find this acceptable and require largerwin=>dows to achieve greater stopband rejection or flatter gain across thechann=>el. Application determined channel responses can require strides withvalue=>s smaller than the transform size (smaller desampling) to preventsignifica=>nt aliasing. > >Dale B. Dalrymple >Dale, Thank you so much for your informative reply. There was one thing I was not sure about in your reply and that was the part you mentioned about rectangular window. Assuming we have polyphse- channelizer, which outputs critically-sampled channel data, I don't understand why it has only 13 dB separation. Doesn't it depend on the filter which is realized as polyphase structure? In frequency domain channelization where data is transformed to frequency domain and only the channel of interested is selected and converted back to time-domain by performing IFFT, I understand that it would suffer from rectangular windowing and sinc roll-off. If what you meant was the frequency domain channelization, is there any technique we can use to efficiently compensate the roll off? Gogndori

# Question on channelizer

Started by ●November 16, 2012

Reply by ●November 20, 20122012-11-20

Reply by ●November 20, 20122012-11-20

On Tuesday, November 20, 2012 3:44:30 AM UTC-8, gongdori wrote:> ... > > Dale, Thank you so much for your informative reply. > There was one thing I was not sure about in your reply and that was the > part you mentioned about rectangular window. Assuming we have polyphse- > channelizer, which outputs critically-sampled channel data, I don't > understand why it has only 13 dB separation. Doesn't it depend on the > filter which is realized as polyphase structure?The fourier transform of the window is the filter realized by DFT channelizers. If the window coefficients are all 1.0, the frequency response is a sinc.> In frequency domain channelization where data is transformed to frequency > domain and only the channel of interested is selected and converted back to > time-domain by performing IFFT, I understand that it would suffer from > rectangular windowing and sinc roll-off.In the DFT channelizers, the DFT outputs are interpreted as time domain samples. Each DFT produces one time domain sample for each channel. In the DFT/weight/IFFT filter structure as in the IEEE SP Mag article john referenced, you get an additional filter effect from the coefficients you apply to the DFT outputs you select for each IFFT. If you didn't explicitly apply a coefficient, the coefficient value you have selected is 1.0. You can't operate on only a finite block of data without that block selection being a window.> > If what you meant was the frequency domain channelization, is there any > technique we can use to efficiently compensate the roll off > > GogndoriFor the DFT channelizer, you can choose a different window size and different window coefficient values to get a response different from the sinc. For the DFT/weight/IFFT filter structure you get an additional opportunity to modify the output frequency response when you select the frequency domain weights (and, of course, another place to create aliasing if you aren't careful). Dale B. Dalrymple