And VCRs too. The original Macrovision technique relied on the low tolerance to noise in the old record-mode AGC circuits on first generation VCRs. But when VCRs improved to the point that Macrovision was ineffective, legislation was passed so that VCR manufactures had to include a special circuit to recognize the Macrovision noise bursts, and emulate the old behavior. (Source: my memory of an old article in an electronics magazine, so the above may be somewhat inaccurate -- the article may have been only referring to proposed legislation, or possibly industry self-regulation).