Dial-up tone spectrogram.
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Previously:
https://www.homemadetools.net/forum/...844#post202345
https://www.homemadetools.net/forum/...812#post183245
https://www.homemadetools.net/forum/...435#post189090
https://www.homemadetools.net/forum/...090#post187756
https://www.homemadetools.net/forum/...420#post181647
