This is a slide from my lecture notes: enter image description here

My professor used the following words " We denote digital frequencies with capital letters and analogue frequencies with lower case letters"

The problem I have is $$\Omega=\omega T_s$$

Wherever I am looking it is actually the other way round i.e.

$$\omega = \Omega T_S$$

For example here

Is my proffesor wrong?

  • $\begingroup$ I think it can be however you want to define it; as long as it is clearly defined it is not wrong. $\endgroup$ Commented Jan 13, 2019 at 20:09

1 Answer 1


Again there is no wrong or right here. In the Alan Oppenheim's Discrete-Time Signal Processing book, the notation is as follows:

  • when there are only continuous-time signals we use $\omega$ for radians per second frequency.
  • when there are only discrete-time signals we use $\omega$ for radians per sample frequency
  • when both types of signals are present, (as in sampling), we use $\Omega$ for the continuous-time radians per second frequency, and $\omega$ for the discrete-time radians per sample frequency.

Furthermore the relationship between the two frequencies because of sampling normalization is:

$$ \Omega = \frac{\omega}{T_s} $$

or equivalently

$$ \omega = \Omega ~ T_s $$

However, your instructor seems to prefer the opposite notation for frequencies. That's why most literature seems to be reciprocal of your instructor's.


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