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 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.

[Darryl In Canada]

Thanks for confirming this. I am not interested in Macrovision as I only capture home movies. But I've read posts about, for the instance, the Diamond VC500 USB capture device that suggests its AGC feature is a negative because it annoyingly adjusts a video brightness/contrast automatically in the middle of a scene for no obvious reason.

Bottom line: if capturing commercial VHS tapes, avoid capture cards with AGC or invest in an external frame TBC for the workflow. If capturing home videos, then the presense of AGC in the capture device if fine, provided it was not implemented poorly. Is that about right?

[betwixt]

Almost. Don't assume a TBC will remove macrovision although inmost cases they would. TBCs try to compensate for slight horizontal displacement of the scan lines so vertical parts of the scene line up with each other. Perhaps the easiest analogy would be to imagine the picture is printed on the edge of a stack of papers. If the stack has each sheet exactly on top of the one below it, the picture looks good. Now take all the sheets off and then re-stack them, it would be difficult to get them lined up perfectly and there would inevitably be slight, sheet by sheet, displacement that makes edges look fuzzy. The sheets in the analogy are like the scan lines that the TV picture is built from. In practice the displacement between lines is caused by imperfect tape motion, record and playback head rotation speed and noise on the signal that confuses the circuits that detect the synchronizing pulse edges.
TBCs work in one of two ways:
1. some digitize the line or frame into memory using the timing derived from the video source then play it back using a more accurate locally derived timing.
2. some use delay lines, the signal is fed through a series of short delay circuits, each only maybe 1% of the length of a single scan line, then the electronics works out which 'tap' along the delay chain is selected to correct the delay so all lines end up with the same timing.

Nothing intentionally removes macrovision but method 1 usually preserves the visible part of the picture but regenerates the syncs and blanking signals during playback, hence chopping out the part where the macrovision is hidden.

Brian.

[dellsam34]

All capture cards work the same way, The Diamond VC500 bug is a design flaw in the driver, I don't believe it is an AGC issue or at least it has nothing to do with the AGC being good or bad, Again the whole framing of the issue here is upside down, Just find a good capture card and worry less about AGC.