From the discussions here on hooking up a DVD player to a VCR that in turn is hooked up to a TV it appears that on some setups the DVD is totally unwatchable due to the macrovision. On other
peoples systems it appears to be watchable as long as they aren’t trying to record it on the VCR as they watch the DVD. If the VCR record button is pushed the DVD becomes unwatchable on the screen.
On this second type what accounts for the difference in picture quality once the record button is pushed, as opposed to the picture always showing the effects of macrovision?
Macrovision™ copy protection works by fooling the VCRs AGC (Automatic Gain Control) circuit. The AGC circuit ensures that the video record level will be set properly for a clean readback bu monitoring the voltage level of a portion of the vertical synch and adjusting the record level apporopriately. What Macrovision does is intermittently replace this section with a much higher-than-normal voltage level, alternated with a normal level. Thus, the image on the recording alternates between dark and light, making it unwatchable. The reason for alternating the Macrovision signal is so it’s not possible to compensate for it by merely darkening the picture brightness control.
Now, to address your question. In the second type of VCR you mention, the AGC circuit is only activated when the unit is recording, so the picture looks fine until the REC button is pressed.