I just had an idea for an even simpler RLE
compression algorithm. I got the idea from the
old .PCX RLE system, which were good for
compressing 1bit data (b/w images) doing
one funcion for each data byte.

ENCODING:
* if bitstream data has no long strings
just save 7 bits:
        First bit is a flag that shows
        data is 7 separate bits (x=0).
        incoming:  111010110110
        stored:   x1110101
* if bitstream is for this byte is all
"1"s or "0"s, this is a run, so tally the
total length from here until the run ends.
Then express the remainder of the run as
a 7bit length.
        First bit flag is (x=1) showing the other
        7 bits are a run-length code.
        incoming: 111111111111 -> continues to 41 places
        stored:   x010 1001     (binary for length=41)
The bit type, a "1" or "0" is determined
from the last bit of the previous byte,
so it will compress runs from 9 onwards,
depending on where the run crosses the byte
boundary.

This compression system is VERY simple to encode
or decode, one simple calc per 8 bits.
A PIC could definitely encode high quality
audio bitstream in real time.
-Roman




Roman Black wrote:
> Hi, are you talking an ENCODING delay?
> Encoding just generates the bitstream using the
> RC filter model vs the desired analogue waveform,
> then the encoder does the simple RLE compression
> system in one pass. No encoding problems.
> Depending on the simplicity of your RC filter
> model you might even get an encoder on a PIC,
> it only requires one calc per incoming "bit"
> sample.
>
> DECODING is by one bit per PIC interrupt
> (for instance) and if the PIC gets 5 consecutive
> "1" or "0" bits, it sets a specified period
> based on the value of the next 8 bits, giving
> ACCURATE periods from 5 to 260 bits.
> Since the bitstream data is loaded as bytes
> you only need the current byte you are decoding
> bits from and the next byte already loaded.
> I suppose that is a lookahead buffer but
> a very easy one. That way you always have the
> "next" 8 bits handy if you detect a compression
> command.

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