Rob,
1) the result of data compression depends from algorithm and DATA
2) you are converting a STREAM of data (any buffering  valid for a non 
streaming data compression algorithm is a big problem for a PIC: size, 
time required to fill, etc)
3) lossless algorithm  are computing intensive (ram buffer, cpu clocks)
4) SD cards are cheap and relatively simpler!

            Nic


Rob Susmilch wrote:
> Hello everyone,
>
> First some background. I'm going to be building a simple stand alone data
> logger for personal use (and the fun of creating it of course) since most stand
> alone loggers are hundreds of dollars. I will most likely be using a pic18f4553
> IIRC, and either interfacing to a ADC chip or using the on-board A/D. I also am
> going to be interfacing to SD card since from what I've read they are easy to
> do.
>
> My question is, and I've searched high and low, how to implement compression on
> this little bugger. Granted I suppose getting a jumbo SD card could negate any
> compression done, but I may perhaps go to a simple serial EEPROM instead. All
> the algorithms I've seen on-line seem VERY computationally expensive (which I'm
> not sure if the PIC COULD do, or if it could would kill battery life never
> sleeping) or memory hogs. I think I saw somewhere a person mentioned LZW
> compression may work, with the difference from the current signal and last
> signal. I have seen a few patents where they mention some adaptive data
> compression using in audio but not much else. I understand that any voltage
> signal is a wave form so an audio compression would probably be best? I would
> like to go in a loss-less form. I would be measuring mostly things like temp,
> humidity, sunlight, etc. A weather station would be it's primary focus, but I'd
> like the option of using it for other things (or building a separate one more
> tailored.). Does anyone know of a compression scheme that would work for such
> things that it is worth the trouble to implement and extract some saving in
> space? Maybe I don't need to worry at all? Also I am going to give
> Sourceboost's C a try so C code references are OK. Sorry for the lengthy post.
>
> Rob
>   
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist