---Original Message-----
From: William Chops Westfield <billw@CISCO.COM>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Wednesday, 23 July 1997 09:59
Subject: Re: PIC RF link software..



>    To implement compression for GENERAL data in a PIC sounds complicated,
but
>    would be possible (but it would take long to write the software). If
you
>    are going to transfer text for example, it would be better to write a
very
>    simple routine that specifically transmits text more efficiently. Or if
you
>    will transfer simple images you can use the method used in fax and
>    IFF-ILBM. The big question is if you really need to go through all this
>    work just to shrink the data size down to not much less than half? The
only
>    efficient compression methods are MPEG for video and audio, and JPEG
for
>    photo images, but they are of course very complicated to implement.
>
>MPEG and JPEG are both lossy compression schemes unsuitable for data (I'm
>pretty sure.)  The sort of compression implemented by WINZIP and related

It would depend on data.  If it's code, its certainly unsuitable, but
datalogging ?

>tools (including gzip, which has available source) (this includes the
>compressions like V.42bis used in modems) can easilly achieve 2:1
>compression on typical text and binary files, they are highly array-based
>and sometimes recursive , which would make them a pain on PICs.

If you know your data's maximum rate of change, say you're logging level of
water in a tank, then RLL or a delta system may be ok.

>We're on our third generation of compression for object files here at
cisco,
>having done unix compress, lharc, and gzip styles...
>

I sure wouldn't want to do a dictionary based compression system on a PIC.
An AVR - maybe.

MikeS
<mikesmith_oz@relaymail.net>