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| From | rickman <gnuarm@gmail.com> |
|---|---|
| Newsgroups | comp.dsp |
| Subject | Re: How to downsample this signal? |
| Date | 2017-05-16 15:35 -0400 |
| Organization | A noiseless patient Spider |
| Message-ID | <offk46$fi4$2@dont-email.me> (permalink) |
| References | (4 earlier) <ofbdtb$vg$1@node1.news.atman.pl> <ofcf3l$6o8$1@dont-email.me> <87h90mqfwv.fsf@garnerundergroundinc.com> <ofe7r4$oc4$1@dont-email.me> <8760h0pw02.fsf@garnerundergroundinc.com> |
On 5/16/2017 1:11 PM, Randy Yates wrote: > rickman <gnuarm@gmail.com> writes: > >> On 5/15/2017 11:49 AM, Randy Yates wrote: >>> rickman <gnuarm@gmail.com> writes: >>> >>>> On 5/15/2017 1:21 AM, Piotr Wyderski wrote: >>>>> rickman wrote: >>>>> >>>>>> Are the 248 words for data or also program? >>>>> >>>>> Data only, there are dedicated code memories. The control part >>>>> is totally crazy: there are two separate code memories working >>>>> in parallel (they take both paths simultaneosly in order to >>>>> provide instructions with no delay in the case of a branch). >>>>> But there is no such thing as a program counter, the code >>>>> memories store instructions at consecutive addresses without >>>>> any deeper structure. A run of instructions is called a block >>>>> and is ended with the presence of a jump instruction. There >>>>> is also a third control memory which stores the information >>>>> where the i-th block begins and to which j-th state the FSM >>>>> should go in the case of a branch. In short, hardware basic >>>>> blocks. The data path is a VLIW with exposed pipelining, >>>>> which adds fun. >>>>> >>>>>> That should be plenty of room for coefficients. >>>>> >>>>> But doesn't the FIR require a lot of cells for >>>>> the past data values? >>>> >>>> Like I said, I wrote this a long time ago, so I am not the resource to >>>> be asking. I can't picture how it works, but I specifically remember >>>> NOT needing to store previous data inputs. I am thinking you store >>>> outputs which will be fewer because of the decimation. As the input >>>> data comes in the output samples can be built up and when one is >>>> complete it is outputted and replaced by a new one. >>>> >>>> But read a proper reference. If I wasn't busy today I'd dig this up >>>> for you. >>> >>> Yes you do need to store N-1 previous inputs (plus 1 current input) for >>> a length N polyphase FIR decimation by M. Just think first principles: >>> for any output y[k], you need N inputs to compute it, even if k = n*M. >> >> I am pretty sure that conclusion is not correct. When I was asked to >> implement a polyphase filter that was the part I had trouble getting >> my head around but finally understood how it worked. At the time this >> was being done on a very early DSP chip with very limited memory, so >> storing the N-1 previous samples was not an option. The guy >> presenting the problem to me was the classic engineer who could read >> and understand things, but couldn't explain anything to other people. >> So I had to take the paper he gave me and finally figure it out for >> myself. >> >> I will dig around when I get a chance and figure out exactly how this >> worked. But the basic idea is that instead of saving multiple inputs >> and calculating the outputs one at a time (iterating over the stored >> inputs) - as each input comes in it is iterated over the stored >> outputs and each output is ... well output, when it has been fully >> calculated. Since there are fewer outputs than inputs (by the >> decimation factor of M) you store fewer outputs than you would inputs. >> >> This is not hard to understand if you just stop thinking it *has* to >> be the way you are currently seeing it. (shown here with simplified >> notation since it is too messy to show the indexes with decimation) >> >> y(k) = a(0)x(i) + a(1)x(i-1) + a(2)x(i-2) + ... >> y(k+1) = a(0)x(i+1) + a(1)x(i) + a(2)x(i-1) + ... >> y(k+2) = a(0)x(i+2) + a(1)x(i+1) + a(2)x(i) + ... >> y(k+3) = a(0)x(i+3) + a(1)x(i+2) + a(2)x(i+1) + ... >> >> Instead of thinking you have to calculate all the terms of y(k) at one >> time (and so store the last N*M values of x), consider that when x(i) >> arrives, you can calculate the x(i) terms and add them into y(k) >> (which is then output) and y(k+1), y(k+2)... This allows the storage >> of fewer values by a factor of M. It does require more memory fetches >> than the standard MAC calculation since you have to fetch not only the >> coefficient and the input, but also the intermediate output value >> being calculated on each MAC operation. >> >> Maybe I won't have to dig it up. I'm pretty sure this is how it >> worked. It ties in nicely with the "polyphase" aspect to allow a >> minimum number of calculations on arrival of each input since not all >> the coefficients are used on each input. So the coefficients are >> divided into "phases" (N/M coefficients) with only one phase used for >> any given input sample. > > Counter-example: Consider M = 8, N = 8 (8 coefficients and decimating by > 8). Then each output y(k) depends on x(k*M + n), 0 < n < M - 1, and thus > the sets of input values for different outputs are disjoint. Yes, so what? You still don't need to store any inputs to calculate the output. -- Rick C
Back to comp.dsp | Previous | Next — Previous in thread | Next in thread | Find similar
How to downsample this signal? Piotr Wyderski <peter.pan@neverland.mil> - 2017-05-13 09:23 +0200
Re: How to downsample this signal? Evgeny Filatov <filatov.ev@mipt.ru> - 2017-05-13 15:42 +0300
Re: How to downsample this signal? bitterlemon40@yahoo.ie - 2017-05-13 07:53 -0700
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-13 11:48 -0400
Re: How to downsample this signal? Randy Yates <yates@digitalsignallabs.com> - 2017-05-13 12:58 -0400
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-13 16:04 -0400
Re: How to downsample this signal? Randy Yates <yates@digitalsignallabs.com> - 2017-05-14 14:19 -0400
Re: How to downsample this signal? Piotr Wyderski <peter.pan@neverland.mil> - 2017-05-15 00:42 +0200
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-15 00:46 -0400
Re: How to downsample this signal? Piotr Wyderski <peter.pan@neverland.mil> - 2017-05-15 07:21 +0200
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-15 10:51 -0400
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-15 11:49 -0400
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-15 17:20 -0400
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 02:59 -0400
Re: How to downsample this signal? eric.jacobsen@ieee.org - 2017-05-16 15:59 +0000
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 15:36 -0400
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 13:11 -0400
Re: How to downsample this signal? eric.jacobsen@ieee.org - 2017-05-16 18:08 +0000
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:35 -0400
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:45 -0400
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:50 -0400
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 21:36 -0400
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 15:35 -0400
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:36 -0400
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:40 -0400
Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 22:06 -0400
Re: How to downsample this signal? Randy Yates <yates@digitalsignallabs.com> - 2017-05-17 01:25 -0400
Re: How to downsample this signal? Evgeny Filatov <filatov.ev@mipt.ru> - 2017-05-15 14:51 +0300
Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-15 10:37 -0400
Re: How to downsample this signal? makolber@yahoo.com - 2017-05-15 06:27 -0700
Re: How to downsample this signal? Piotr Wyderski <peter.pan@neverland.mil> - 2017-05-15 20:39 +0200
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