Analog signals
An analog signal is a continuous function of
a continuous variable. Referring to Figure 1, this corresponds
to that both the 1st AND the 2nd axis is continuous. The 1st axis
will in general correspond to the variable
 signal range  the possible amplitude values the signal can take
 signal axis  the time interval for which the signal exists
Time discrete signals
A time discrete signal is a continuous signal of a discrete variable.
Referring to Figure 1, we have the 1st axis discrete while the 2nd axis is continuous.
Often we assign the values of the 1st axis to a variable
Digital signals
Let the signal be a discrete function of a discrete variable, e.g. 1st and 2nd axis discrete, then the signal will be digital. Examples of digital signals are a binary sequence. Digital signals often arise from sampling analog signals and the samples being assigned to a discrete value.
Periodic vs non periodic signals
All the signals mentioned above can be periodic. For time discrete and digital
signals one has to be extra cautious when "declaring" periodicity as we
will see in Frequency definitions & periodicity.
Figure 3 shows a periodic signal with period
