Decoding the signal from 1988 Videophone

Nowadays it’s easy to have a video call with almost every part of the world. But it took a long time for the technology to reach this level. I will show what solutions engineers used back in 1988, so you could not only talk but also see your friend or partner during the call.

Once I saw a YouTube review of Sony PCT-15 — this device, made in Japan, was connecting to an ordinary telephone line and allowed not only to talk with another person but also to send and receive images during the call. At that time the high-speed communication was available only for institutions, the military and government, so the telephone line was the only possible way to be connected to the world:

Source © Popular Photography 1988

How does it work? Let’s figure it out.

Decoding

The video review was published on Techmoan’s YouTube channel, which is specialized in retro devices. But the remarkable for me was the fact that the data transmission was audible in the video. I just saved the signal in WAV and decided to analyse it using Python.

Let’s first check the signal waveform:

import scipy.io.wavfile as wav
import scipy.signal as signal
import matplotlib.pyplot as plt
import numpy as np

fs, data = wav.read('videophone.wav')

plt.rcParams["figure.figsize"] = (20,6)

time =…