This all started from trying to inspect some Macrovision tapes on my waveform monitor and other devices. I've gotten increasingly confused by inconsistencies, and measuring MV-free pre-recorded Hollywood movie tapes hasn't clarified anything yet.
I've fallen back to one of the most basic VCR models I own, a JVC HR-J693U, because I'm hoping to eliminate the variable of possible signal tampering by the VCR. The tape used for this post was the US release of The Fifth Element.
The attached capture was done with a SAA713x card on Windows XP with Vertical Sync recovery mode set to Free Running, Automatic Gain Control disabled, manual gain set to 0% ("-3dB") and Brightness boosted to make the Vertical Blanking Interval more visible. I encoded the Huffyuv capture to 4:2:2 x264 for posting.
The other attachment is a smartphone recording showing the Leader 5870 waveform monitor. It's a different playback of the tape than the capture, but I trimmed them to match. The first flash takes place on frame 414 (WFM) and field 414 (bottom of capture).
Note: I haven't bought any 75 ohm terminators for the WFM. Instead I'm feeding the loopthrough output into the input of a DMR-ES15. This should work just as well, no?
Waveform monitor display
The WFM recording starts with the VCR's blue back, showing that -40 IRE and 0 IRE are where they should be on this generated screen. On the WFM, the first two flashes (at 0:08 and 0:10) cause it to lose sync. There's another blip at the 0:40, but it's much more mild. The COUNT overlay seems constant, at just under 90 IRE (except it's slightly lower on blue back vs tape playback).
Sync level is always clipped, so there's no way to see its actual value.
At the first flash on the capture:
- Blanking level drops, both horizontal & vertical.
- COUNT overlay drops.
[Attachment 50956 - Click to enlarge]
[Attachment 50957 - Click to enlarge]
Over the next several fields, the overall level of the flash itself, the COUNT overlay, and vertical blanking all gradually rise. Here is field 420, where they're nearly back to the level of 414. Horizontal blanking is still clipped, and it doesn't return to normal until it jumps up at field 430.
[Attachment 50958 - Click to enlarge]
At the end of the next flash (fields 591 & 592) there's a corresponding effect in reverse. When the bright field flashes immediately back to dark:
- Blanking level abruptly rises (H & V).
- COUNT overlay abruptly rises.
[Attachment 50959 - Click to enlarge]
[Attachment 50960 - Click to enlarge]
Followed by a gradual drop toward normal. Here is field 599.
[Attachment 50961 - Click to enlarge]
Other flashes later in the video produce similar AGC effects.
The Avisynth script for the screenshots is just:
Questions:Code:[source filter] AssumeTFF().SeparateFields() VideoScope("both",false,"Y","Y","Y")
- What's so special about those two or three flashes that hiccup the WFM, while the rest of them don't?
- Is this expected behavior for a waveform monitor fed unstabilized VHS?
- Why is the capture card showing an AGC effect when it's in "manual gain" mode?
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Last edited by vaporeon800; 26th Nov 2019 at 05:23.
@vaporeon800: what's the goal of your post ?