Comment by tshaddox

HDCP is how modern digital displays (and digital display recorders) do it.

You might be thinking of Macrovision, which was integrated in a lot of DVD players and would embed pulses into the vertical blanking interval of the analogy video output. These pulses could be detected by compliant DVD recorders and used to refuse recording. The pulses would also cause playback defects in some older VCRs and TVs.

I remember connecting my first DVD player to an old TV via a VCR (effectively using the VCR as an RF modulator) and being plagued with the image brightness constantly lowering and rising. At the time, I fixed this by switching to a dedicated RF modulator. I now suspect Macrovision is what caused this.

https://en.wikipedia.org/wiki/Analog_Protection_System