Gerhard Fiedler wrote:
> Scott Dattalo wrote:
>   
>> You may wonder how Manchester encoding may be more robust against
>> multiple transmitters or how it can allow the distance between the
>> transmitter and receiver be increased. Well first, a Manchester encoded
>> data stream has state information. For example, if you look at a
>> Manchester stream there are narrow pulses and wide pulses. Only certain
>> combinations of these pulses are allowed. So if interference damages a
>> particular pulse then there's a significant probability that the
>> Manchester state decoder will encounter an illegal state. 
>>     
>
> Same for PWM. If you sample 3 times per bit, the first sample must be 1,
> the second is the bit state, the third must be 0. Anything that's not 1x0
> is noise. 
>   

Gerhard,

You're confusing pulse width validation with pulse stream state. For 
example, it's possible for interference to perturb a skinny pulse into a 
fat pulse or vice versa in both Manchester encoded data streams and PWM 
encoded streams. In a PWM encoded stream, you can only detect this error 
via your error code -- and to the degree it's robust enough to catch 
these types of errors, then there's no problem. However, in Manchester 
coding there is state information in the data stream itself. Regardless 
of the message, there are certain combinations of skinny and wide pulses 
that are illegal. For example, a skinny pulse must either be preceded or 
followed by another skinny pulse. So if you see a wide pulse on either 
side of a skinny one, then that's an error.

However to be fair here, interference very seldom perturbs a single 
pulse. Pulse width validation is adequate protection against 
interference. A more likely scenario is one in which there's variability 
among the transmitters. For example, 1/3 of a bit time may differ from 
one transmitter to another (due to things like the transmitter's 
hardware or it's distance from the receiver). This requires a less 
stringent check on the pulse validation which in turn makes the system 
more susceptible to interference.

> OTOH... Why would Microchip use this as basis for their KEELOQ products if
> decoding Manchester is as easy as decoding PWM and results in better
> performance?
>   

I'm not sure for their reasons for choosing a PWM encoding scheme over a 
Manchester encoding. But KEELOQ is a whole lot more than just the 
physical layer.

Scott
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