See this link

[Link: remotecentral.com]

Parity bit: Some remote controls change their codes each time the signal is sent. For instance, if you press the number "5" the code would end in a "0". The second time you press it the code ends in "1". The problem with this is that a learning remote control can learn only one of those signals – not both. Thus, such devices typically will not respond to a universal remote when you send the same code twice in a row (for instance the channel "55"). For more help on this problem click here. Also known as a toggle bit. See also: learning.