FSK vs Manchester Coding – How to Distinguish Between Them

FSK is difficult to visualize as you increase in order. The reason for this is when you are using FSK, you have orthogonal frequencies that essentially add an extra dimension to your plane. You can visualize up to 3d (3 frequencies) as shown below (pardon the hand paint drawing), but once you get greater then that FSK just can't be represented this way. However, just because we aren't able to plot it on a graph, the matrix math still applies equally the same, just with added dimensions.

There are two main reasons for using a preamble to start a packet of manchester encoded data:

1. Let the data slicer settle. Manchester is often used over radio links and physical links where there is no direct connection and the difference between a high and low level not explicitly known up front. This usually means a anlog signal is presented to a data slicer, which detects and passes on the sequence of digital highs and lows.

   One property of manchester encoding which makes it useful for such links is that it averages to 1/2 high and 1/2 low over short intervals. Each bit has the same average level of 1/2. This makes data slicing relatively easy because it can be a simple comparator against the 1/2 level. However, that means you have to know what the 1/2 level is. Old receivers would do this in hardware by low pass filtering the manchester stream. Such filters deliberately don't react much over a single bit time, so take a few bits to settle. The preamble contains throw away bits that the receiver is not intended to properly detect while its data slicer is finding the 1/2 level.

2. Indicate start of packet. Since most manchester receivers essentially have automatic gain control via the 1/2 level used for data slicing as described above, the receiver will interpret background noise as a series of high and low levels just like the real signal. There is generally no such thing as no signal to the digital stream interpretation part of the receiver, only that the stream makes sense when there is real signal.

   Manchester contains some redundant information so that some sequences of levels can be detected as invalid directly without higher level interpretation. For example, there needs to be a transistion in the center of every bit. Three half-bits of the same level is illegal, so is two-one-two, for example.

   While resetting to start of packet whenever a violation like above is detected helps, there is still enough chance of random junk making it further into a packet that this needs to be dealt with in most cases. You don't want the higher levels deep in the packet decoding logic from noise when a real packet comes along that then just looks like more data for the bogus packet. Eventually the bogus packet will presumably fail the packet checksum test, but you still loose the real packet.

   A good strategy is then to make the preamble a unique sequence that is not valid in the rest of the the packet. When this sequence is detected, the higher level packet interpretation logic is reset to start of packet regardless of what it thought it was doing at the time.

   I usually do this by using a stuff bit scheme. If the real data contains 7 0s in a row, for example, then the transmitter must add a 1 stuff bit after the 0s. The receiver knows this and strips out the 1 bit following 7 consecutive 0 bits. This means that there are never 8 consecutive 0 bits, which would be a bit stuffing violation. If a bit stuffing violation is detected, then the packet interpretation logic can be safely reset to start of packet since it definitely wasn't in a valid packet at the time. The preamble deliberately contains such a stuff bit violation to force the interpretation logic to be reset to start of packet.

   This stuff bit scheme and resetting to start of packet is not standard manchester, but something I like to use to make manchester more reliable over links like radio.

There is more to manchester encoding when you really think about the details. You can do a much faster responding data slicer in a digital processor, for example, in which cases you can use other means than bit stuffing to reliably detect start of packet, but that is a whole topic onto itself.