Manchester encoding: 'signal' and 'bit sent'

Question

I just learned the following about the so-called 'Manchester encoding':

• There is a transition at the middle of each bit period.
• The midbit transition serves as a clock mechanism and also as data: low to high represents 1, high to low represents 0.

The following image is said to illustrate this:

We can see that, in the first 'time interval', the signal goes from high to low, signifying a 0, and during the second time interval, the signal goes from low to high, signifying a 1, as expected. During the third time interval, we see that the signal goes from high to low, signifying a 0, which corresponds to what is in the image, but then, in the middle of the time interval, at the end of the 'bit sent', we see that the signal goes from low to high, signifying a 1, but the next bit sent is still a 0. This latter part seems to contradict what we would expect, since the transition from low to high in the middle of the interval would mean that the next bit would be a 1, no?

Answer 1

You need to see it like this:

• a user bit 0 is line encoded as 10
• a user bit 1 is line encoded as 01

If alternating bits are transmitted, there is only one level transition per bit:

  user           line
01010101 => 1001100110011001
10101010 => 0110011001100110

If same bits are transmitted, there are two level transitions per bit:

  user           line
00000000 => 1010101010101010
11111111 => 0101010101010101

Note the difficulty to detect the exact middle of the two-level symbol where the transition signals the relevant data - all the receiver sees is this:

If the receiver erroneously misplaces the symbol borders it decodes garbage data.

That's why Manchester requires a preamble with alternating bits to synchronize the receiver to the middle transition. The other transitions (between symbols) are simply irrelevant.