PN SEQUENCE (zero values)
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From: Jerry Avins on 4 Feb 2010 18:36 robert bristow-johnson wrote: > On Feb 4, 3:19 pm, "Avier" <shahanwark...(a)hotmail.com> wrote: >> just for check >> >> IS there any way to produce a PN sequence from all zero intial state. >> >> that is all zeros in shift regiters > > long ago (in the eighties, and it was on a 68K, like my first Mac) i > did a sorta MLS in which i inverted the logic whether to XOR the > accumulator or not. i think i XORed if the bit shifted out was a 0, > not a 1. > > and i ran experiments to see that it did 2^N - 1 states before getting > back to all zeros. that meant that there was some non-zero state that > it never did, and i think i found it, but i don't remember if the word > representing the skipped state was something i could derive. All ones. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Mark Curry on 4 Feb 2010 19:05 In article <T8ednaRDyYKmqvbWnZ2dnUVZ_vadnZ2d(a)giganews.com>, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >John wrote: > >> On Feb 4, 3:19 pm, "Avier" <shahanwark...(a)hotmail.com> wrote: >> >>>just for check >>> >>>IS there any way to produce a PN sequence from all zero intial state. >>> >>>that is all zeros in shift regiters >> >> >> I believe the all-zeros state is never legal because it would cause >> all subsequent outputs to be zero. > >You can modify LSFR by sticking an additional invertor into it. This >will take care of all zero state however the sequence will stil have 2^N >- 1 "normal" states. You can also make the sequence max length (all 2**n) states with a little logic. It makes the sequence non-linear - but it's pretty much the same sequence. You modify the "feedback term" by comparing the least signficant N-1 bits with all zero. Take this bit and XOR it with the feedback line to generate a modified feedback term. This modifies the sequence from: MSB bit high, rest of bits zero -> All zeros -> Continue with sequence If this is done in hardware, this changes the critical path from a single XOR to an (n-1) bit compare with all zeros. Rereading that not sure it's clear. If you need more clear examples, let me know... Regards, Mark
From: glen herrmannsfeldt on 4 Feb 2010 19:24 robert bristow-johnson <rbj(a)audioimagination.com> wrote: > and i ran experiments to see that it did 2^N - 1 states before getting > back to all zeros. that meant that there was some non-zero state that > it never did, and i think i found it, but i don't remember if the word > representing the skipped state was something i could derive. It should be easy, though I don't think I ever tried. Write down the recursion function and set the new state equal to the previous state. -- glen
From: Avier on 4 Feb 2010 19:56 one way as i see if the XORed result is further XORed with a constant 1 then there are some cases if every time the sequence is XORed then again all 1 state will go inactive. so if we XOR our XORed result alternately or maybe after 3 or 4 chips then we can have all states what do you say ????
From: Tim Wescott on 4 Feb 2010 20:08
On Thu, 04 Feb 2010 12:45:18 -0800, John wrote: > On Feb 4, 3:19 pm, "Avier" <shahanwark...(a)hotmail.com> wrote: >> just for check >> >> IS there any way to produce a PN sequence from all zero intial state. >> >> that is all zeros in shift regiters > > I believe the all-zeros state is never legal because it would cause all > subsequent outputs to be zero. PN yes. PN from a linear shift register, no. -- www.wescottdesign.com |