This is an adapter attached to a TV set. This machine changes a TV to a Oscilloscope. This photo indicates a sine wave 50mV 120Hz. On the other hand , an amplifier makes 300 times of input signal. And it is shifted to 6V center.
Differencial circuit makes a small pulse on the changing timing of the pulse.
Therefore it is called as an adder. This machine is able to indicates only 60Hz or 120Hz or 180Hz. Therefore it is not so useful. But I think that this circuit is very good sample to learn about OP amp circuit. If you can not understand my explanation.