Digital Communication. Analog Source. A/D. Binary Bytes. Symbol Decision. Symbol State Modulation. Binary Bytes. Symbol Modulated Carrier. RX. D/A. Recovered Analog. Coding.
Symbol State Modulation
Symbol Modulated Carrier
The previous diagram does not show the data encoding and decoding process. We will discuss that in great depth at a later time. That is where the real advantage of digital communication accrues, because that is where data security (encryption and error correction) is accomplished.
For now, we will be satisfied with a simple system, and survey some of the A/D and D/A techniques in common use.
Multiple data sources can share a communications channel by transmitting data frames at the sample frequency, and each data frame is subdivided into dedicated time slots corresponding to each individual sampled data source.
Within each time slot, the state of the carrier may be determined to represent the value of the sampled data, by several methods.
The state of a channel cannot instantaneously change from one state to another – therefore there is always some remnant of the previous symbol contaminating the present symbol.
This can be mitigated somewhat by utilizing an equalization filter, characterized by rolloff factor “r”
The maximum symbol rate that can be accommodated by a channel having bandwidth BT is:
Fs(max) = 2BT/(1+r)