User:Jugandi/sandbox
Example: 4-ASK[edit | edit source]
ASK (amplitude shift keying) quantizes the magnitude of a carrier wave. This technique is used in:
- analog telephone loop modems
- RF carriers
- optical carriers
The nearest 'digital' equivalent of this technique is 2B1Q.
There is a very subtle difference between the analog and digital implementations of multi-level signalling; a polarity reversal in a carrier system affects the carrier phase, but not the amplitude.
2B1Q[edit | edit source]
In 2B1Q, 2 binary bits are encoded as 1 quaternary pulse. This format is used in the North American version of ISDN at the BRA interface. This effectively reduces the baud rate by one-half. The European equivalent is 4B3T, where 4 binary bits are encoded as 3 ternary pulses.
Let us use the following scheme:
- "00" = +5V
- "01" = +1.66V
- "10" = -1.66V
- "11" = -5V
we can see now that we can transmit data twice as fast using this scheme, although we need to have a more complicated receiver, that can decide between 4 different pulses (instead of 2 different pulses, like we have been using).