Automatic Gain Control (AGC): help me understand something

[Darryl In Canada]

Is the following correct about Automatic Gain Control (AGC)?

1. Automatic gain control (AGC) was a feature added to VCRs.
2. The goal was to adjust the brightness of the video image to keep it as close to the middle as possible (similar to how some televisions have a feature to reduce the sound automatically if there is a very loud scene).
3. The anti-copying feature of Macrovision soon followed in VCRs through an ingenious/devious use of the AGC feature, by fooling the recording VCR to auto adjust the video image to be too bright or door dark, thus ruining the copy.
4. Many (all?) external frame TBCs have the ability to ignore Macrovision's ingenious/devious use of the AGC feature.
5. Some analog-to-digital capture devices have the AGC feature, but many older capture devices from ATI and others do not.

Question: Is the AGC feature in many modern capture devices meant to improve video capture by adjusting brightness on the fly? Or is AGC forced into capture devices to comply to anti-piracy rules?

Question: Is AGC a good thing for capture hobbyists or a bad thing?

[dellsam34]

I wouldn't put it that way, AGC neither good or bad, it is a tool. If you got some MV tapes just look for a better copy on DVD or blu-ray, if not look for an old capture card that ignores it. Ignoring MV does not mean lacking AGC, The device knows what scan lines have MV and therefore ignore those lines while still using AGC.

[betwixt]

1. correct
2. correct
3. correct
4. depends on the design. Timebase correction is really to align the scan lines to avoiding slight timing errors. In itself it has no knowledge of or effect on Macrovision, however TBC often replace the part of the video that carries the protection with an internally generated one, hence rendering it useless.
5. correct.

AGC is to 'even out' the effects of poor lighting or excessive lighting. It is intended to improve shoots where conditions were less than ideal, it has nothing whatsoever to do with enabling protection. Macrovision found a way of utilizing it to make recordings useless.

Whether it is good or bad depends on your shooting technique, if you can avoid using it all the better but for most people, spending time adjusting exposure scene by scene isn't an option, they just want to "point and shoot".

Going back to Macrovision, I will try to explain how it works as simply as possible: A TV picture (or any composite video format) has two component parts, the video itself and synchronizing pulses. Only the video part is visible on your screen or capture, the synchronizing is used behind the scenes to make sure the parts of the image you capture match the image reaching the camera's sensor (CCD or whatever). Without sync, the top of the image might be half way down the screen or the scan lines starting with different left margins. So you don't see the synchronizing pulses, the TV blanks them off, you might think of them as being hidden off the edges of the picture. Normally AGC is simply a measurement of the whole video content, including the picture and sync signals so it can try to 'normalize' the levels to be about right. What Macrovision does is add a varying level of bright pulse inside the blanked off picture edge. The simple AGC circuits in analog recorders can't tell it isn't where it would be seen so they take it into account as part of the picture content and adjust the whole picture level accordingly. The brighter the Macrovision signal, the brighter the AGC thinks the whole picture is so it turns the levels down and vice versa.

Brian.