Scott Dattalo wrote:

> Manchester encoding has state information PWM encoding does not.
> 
> Take a continuous stream of either encoding and try to start decoding it 
> any point. With PWM the bit boundaries are very explicit. 

Yes, but only if you start decoding (sampling) at the right point. The fact
that this "right point" is easier to find with PWM doesn't mean it's not a
"state information" -- it's just one that's a whole lot easier to find.

> With Manchester, any edge is potentially a bit boundary. With PWM you can
> trivially read off the 0's and 1's. With Manchester you'll get 0's and
> 1's only if you started at the proper bit boundary. 

Sounds to me as if this, by itself, could mean that PWM is more robust.

> Manchester bit boundaries are only ascertainable based on past bits, i.e.
> state information. I'm not sure how this is refutable?

I never said that decoding a Manchester stream doesn't need state
information carried from one bit to the next, so the question whether this
is refutable is a bit moot -- no one's trying to refute it :)  It's you
who's claiming that PWM doesn't. If I understand you correctly, the state
information you're after are the bit start and end points. With Manchester,
you need past bits to find it, with PWM, each bit carries this state
information. But depending on the decoding algorithm, for example when
using preambles to determine the bit timing or the correct comparator
threshold, there's also state information that goes over more than a single
bit. So what is it that PWM doesn't have (not "requires", but "have")?

I also think that there's a difference between "carries state information"
and "requires state information to decode". No single Manchester-encoded
bit carries any state information. You receive a hi-lo sequence or a lo-hi
sequence, and that's your bit (if you know the bit start and end times) --
there's no information about past bits included. The information about past
bits is necessary to find the bit boundaries, but it's not included.

> In my opinion (and experience), the Manchester state information 
> provides another degree of robustness over other encoding schemes.

That may well be, as I've already written. To me it seems likely that the
lower bandwidth requirements are an important part of this.

Gerhard

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist