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From: Tim Wescott on 3 Apr 2010 19:05
Luna Moon wrote:
> Hi all,
>
> I have a vector of real numbers in Matlab. How do I compress them? Of
> course this has to be lossless, since I need to be able to recover
> them.
>
> The goal is to study the Shannon rate and entropy of these real
> numbers, so I decide to compress them and see how much compression
> ratio I can have.
>
> I don't need to write the result into compressed files, so those
> headers, etc. are just overhead for me which affect me calculating the
> Entropy... so I just need a bare version of the compress ratio...
>
> Any pointers?

Find another approach to getting an answer, maybe.

First, most lossless compression algorithms are designed for things like
text, executables, and data bases -- they don't do well with floating
point numbers, tending to see them as "random" even when they're not.

Second, if you measure a bunch of meaningless white noise and put the
result into floating point numbers, then put them into a lossless
algorithm that _can_ handle floating point, it's not going to compress
at all, because the algorithm can't distinguish between white noise and
a signal that's chock-full of information. In effect you'll have
_given_ it a signal full of information, in great detail, about the noise.

I think you're leading yourself down the garden path.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: John on 3 Apr 2010 19:13
On Apr 2, 9:19 pm, "Michael Plante"
<michael.plante(a)n_o_s_p_a_m.gmail.com> wrote:
> Michael wrote:
> >John wrote:
> >>On Apr 2, 3:50=A0pm, Luna Moon <lunamoonm...(a)gmail.com> wrote:
> >>> Hi all,
>
> >>> I have a vector of real numbers in Matlab. How do I compress them?
> >=A0Of
> >>> course this has to be lossless, since I need to be able to recover
> >>> them.
>
> >>> The goal is to study the Shannon rate and entropy of these real
> >>> numbers, so I decide to compress them and see how much compression
> >>> ratio I can have.
>
> >>> I don't need to write the result into compressed files, so those
> >>> headers, etc. are just overhead for me which affect me calculating the
> >>> Entropy... so I just need a bare version of the compress ratio...
>
> >>> Any pointers?
>
> >>> Thanks a lot!
>
> >>Consider the array of numbers in binary form. Rearrange the bits so
> >>all the ones are sequential, and do the same for the zeros. The number
> >>of ones followed by the number of zeros is your compressed file.
>
> >That's hardly optimal (effectively Run-Length Encoding (RLE)), and will,
> in
> >general, result in a falsely high estimate of "information content".  How
> >many PCX images do you see floating around?
>
> Sorry, I should have said "it's throwing away information, and then RLE".
> So it's going to give nonsense.

Nobody in here has a sense of humor
From: robert bristow-johnson on 4 Apr 2010 00:52
On Apr 3, 7:13 pm, John <sampson...(a)gmail.com> wrote:

> Nobody in here has a sense of humor

i got it. your file compression scheme sorta rearranged the order of
data.

but yer right. no sense of humor to be found here.

r b-j
From: Vladimir Vassilevsky on 4 Apr 2010 01:51


robert bristow-johnson wrote:

> On Apr 3, 7:13 pm, John <sampson...(a)gmail.com> wrote:
>
>
>>Nobody in here has a sense of humor
>
>
> i got it. your file compression scheme sorta rearranged the order of
> data.
>
> but yer right. no sense of humor to be found here.

JFYI:

http://en.wikipedia.org/wiki/Burrows%E2%80%93Wheeler_transform


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: John on 4 Apr 2010 07:51
On Apr 4, 12:52 am, robert bristow-johnson <r...(a)audioimagination.com>
wrote:
> On Apr 3, 7:13 pm, John <sampson...(a)gmail.com> wrote:
>
> > Nobody in here has a sense of humor
>
> i got it.  your file compression scheme sorta rearranged the order of
> data.
>
> but yer right.  no sense of humor to be found here.
>
> r b-j

I was one day too late.
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