i'm confused.  the wikipedia article says that it's pulse position modulated [<a href="http://en.wikipedia.org/wiki/Pulse-position_modulation" rel="nofollow">http://en.wikipedia.org/wiki/Pulse-position_modulation</a>], but manchester encoding (at least as i understand it from your link) is not pulse position modulated.<p>[edit: perhaps the key uses differential pulse-position modulation and assumes a reasonably constant insertion rate after some initial header?]