What is the frequency response of the North American Public Switched Telephone Network?

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The conventional North American Public Switched Telephone Network, or PSTN, has a frequency response range of 300 Hz to 3,400 Hz. The normal hearing range of humans is typically 30 Hz to 20,000 Hz. So the conventional telephone transmission system is unable to carry bright, high-frequency and deep, low-frequency tones.

But, somewhat surprisingly, because our ears are so used to hearing poor-quality audio over the telephone, our brains actually “fill in” the missing frequencies. As an example, the crisp “s” sound in the word “Christmas”. So in effect, the telephone audio often sounds better than it actually is to us.

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