An instantaneous system is characterized by which of the following?

An instantaneous system is characterized by which of the following?

• A. Output depends on past input values.
• B. Output depends on input at the present time only.
• C. Output depends on future input values.
• D. Output is independent of input.

Answer: (B)

Instantaneous systems are memoryless: the output at time t depends only on the input value at that same time t, not on any past or future input values. This means y(t) is determined solely by x(t). That’s why the correct statement is that the output depends on the input at the present time only. If the output relied on past values, the system would have memory. If it relied on future values, it would be noncausal. And a result that doesn’t depend on the input at all isn’t an instantaneous (memoryless) system in the usual sense. An example is y(t) = 3 x(t), which uses only the current input.