Waveform comparison and display system using cathode ray tube

In an oscilloscope having a cathode ray tube for visually displaying a pattern, the combination comprising:

• input means for receiving an input signal;
• controlled digital memory means responsive to clock pulses for storing a sequence of digital values representative of an input signal applied to said input means and for reading out an analog signal having values equal to the sequence of stored digital values under control of the clock pulses;
• vertical control means for controlling the vertical dimensions of the cathode ray tube pattern in response to signals applied to said vertical control means;
• switching means connected to said input means and said controlled digital memory means for alternately applying the input signal and the signal read-out from said digital memory means at a frequency substantially higher than a horizontal sweep frequency at which said cathode ray tube is operated to simultaneously display the waveform of the input signal and the waveform of the signal read out from the controlled digital memory means in a chopped mode;
• triggered horizontal sweep control means for controlling the horizontal sweep of the cathode ray tube;
• triggering means connected to said input means for triggering operation of said triggered horizontal sweep control means in response to said input signal;
• and
• clock pulse generating means responsive to operation of said triggered horizontal sweep control means for generating and applying the clock pulses to said controlled digital memory means only during a horizontal sweep period of said triggered horizontal sweep control means to display the waveform of the signal read out from said controlled digital memory means stationary with respect to the displayed input signal waveform.