We have talked about the most compact way to transmit this information there are four possible patterns 00 01 10 and 11 however we have to be careful here because what we have done and referring back to what I said previously about compression methods what I have done here is devised a code which has no redundancy at all we talked about how to squeeze redundancy of text files and pictures, which is what compression is its getting rid of unnecessary information there is nothing unnecessary here every one of those bits is vital and you better hope they don't get lost or distaughted on the way unfortunately they might get lost or distaughted on the way so the next thing we have to say is these codes are beautiful but they rely on you having clean noiseless communication across your telephone wire, what would go wrong if that wasn't the case just occasionally one of the zero bits that is signified by a short (sound) for a dot might get turned into a 1 because part way to Sacramento suddenly an electrical storm occured and hit a telephone wire so what was received at Sacramento was a different sound a longer sound and the people thought it was 01 its sunny in Francisco no it isnt its foggy so you can see very quickly there is no possibility for recovering from error in these if you get an error people will receive a good pattern but for the wrong weather condition how could you add something extra to these to give them a bit of protection? against noisy telephone lines the simplest way is being used in telecommunications and indeed within computers for years and years now you put in a parity check bit so here's a code with a check bit and all I am going to do here is take every two bit code and turn it into a 3 bit code but the rule will be that in the overall 3 bit code i must have an even number on one bit so this is called an even parity system so here we have 00, there are no ones in that so I will add the parity check bit and because this is an even parity its a zero there is a numberphile video questing if zero is an even number im sorry im just a computer scientist ok how about this one 01, well overall its got to be an even parity which means and even number of 1s so my bit parity is 1 here for cloudy it says 10 becomes 101 rainy is easy there is an even number of 1s already so I put a zero now those codes with the check bit help alot to detect whether an error has occured just suppose that when this 000 was sent it got distorted again an electrical disterbance happened to chew up one of the 0 bits 