什么是 Periodic signal, period？

Example 8.5 (Mains and concert pitch)

The mains voltage oscillates at $f = 50\,\mathrm{Hz}$: its period is $T = 1/f = 0.020\,\mathrm{s} = 20\,\mathrm{ms}$. An orchestra’s concert A is a pressure signal at $440\,\mathrm{Hz}$: one cycle lasts $T = 1/440 = 2.3\,\mathrm{ms}$. Higher frequency, shorter cycle: $f$ and $T$ are inverses of each other.

Example 8.17 (The heart as a signal)

Each heartbeat sends a small voltage pulse across the chest; electrodes record it as the electrocardiogram (ECG), a nearly periodic signal. If the tall spikes are $T = 0.75\,\mathrm{s}$ apart, the heart beats at $f = 1/0.75 = 1.33\,\mathrm{Hz}$, i.e. $1.33 \times 60 = 80$ beats per minute: reading $T$ off a trace is how a cardiologist takes your pulse.