Joe’s Broadcast Legalizer uses a different strategy than Final Cut Pro’s Broadcast Safe filter. Joe’s Broadcast Legalizer performs a slight full-range correction to the entire image while FCP’s Broadcast Safe filter rolls off out of range values. There are advantages and disadvantages to both strategies and both can be used to bring video within NTSC broadcast standard levels.
All corrections are based on the on the maximum reported value range of 0-116 IRE. Output values have been calibrated against Final Cut Pro’s Video Scopes. Please check your output levels on external equipment whenever possible.
Joe’s Broadcast Legalizer also works with Final Cut Express and all versions of Final Cut Pro since 1.2.5. Even though these do not have video scopes, the calculations should be consistant with those in Final Cut Pro 3.
Luma Adjustment
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| Waveform and Histogram for Original Image |
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| Joe’s Broadcast Legalizer |
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| Corrected Waveform and Histogram with Joe’s Broadcast Legalizer |
| Luma Ceiling: | 100 IRE |
| Luma Floor: | 7.5 IRE |
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| Final Cut Pro’s Broadcast Safe |
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| Corrected Waveform and Histogram with Final Cut Pro’s Broadcast Safe |
| Mode: | Conservative [115] |
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Some things to note about these examples:
- Joe’s Broadcast Legalizer scales image values so changes are distributed evenly across the entire range.
- FCP’s Broadcast Safe affects only out of range values compressing them at the top of the range but leaving other values unchanged. Out of range values are rolled in by Broadcast Safe.
- Joe’s Broadcast Legalizer is also elevating the Luma floor to 7.5 IRE. FCP’s Broadcast Safe filter only corrects Luma ceilings with it’s default settings.
Chroma Adjustment
The Chroma Amplitude, Phase and Offset controls can be used to correct Chroma problems in video clips. Amplitude is a simple Chroma Limiter, which reduces the saturation of a clip’s color information without affecting it’s overall hue.
Phase and offset work by shifting the chroma relative to the view in the Vectorscope. This can be used to quickly affect the overall color balance of a clip towards a specific hue.
Combining Amplitude limiting with large offsets creates unique color effects.
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| Amplitude: | -50 |
| Phase: | 105° |
| Offset: | 40 |
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Y/C Correction
The Y/C Correction controls shift the Chroma or Luma components of a video image relative to one another. A common use of Y/C correction is to compensate for poor image alignment in analog dubs but it’s also quite useful for shifting DV chroma sampling against right-side color drift.
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| Original Image (200% enlargement) |
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This detail shows a slip in color of the pepper against the blue background resulting from NTSC DV’s 4:1:1 chroma sampling. All the color in the image is drifting to the right, resulting in a blue cast on the left edge of the pepper and a trailing reddish cast on the right edge. Additionally, the green stem has a yellowish left edge and a green shadow.
Using the Y/C Correction controls, the color information can be shifted to the left, helping to better align the sampled color point against the Luma information. This still isn’t perfect, partly because of DV’s oversized and blocky Chroma sampling, but centering the color on the pepper helps to improve the quality of the image.
When viewed on a video monitor, this image showed a significant reduction in dot-crawl along the pepper’s horizontal edges .
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| Horizontal offset: | 2 px |
| Joe’s Channel Blur: | Chroma only, 2px, aspect: 4 |
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When combined with Joe’s Channel Blur to smoother out the Chroma, the resulting image appears smoother and less blocky. This process can be used to help improve the appearance and success of color keys pulled from DV material.
Adam Wilt’s DV Sampling page has much more information and examples about different methods of video color sampling.
Controls
Luma Adjustment
- Ceiling (70…110 IRE)
- Sets the absolute maximum value to output.
- Floor (0…25 IRE)
- Sets the absolute minimum value to output.
Chroma Adjustment
- Amplitude (-100…0)
- Sets the intensity of the Chroma (color) signal. -100 removes all color information.
- Phase (-360…360°)
- Determines the direction to use with the Offset value.
- Offset (0…112)
- Shifts all color information towards the direction set in Phase.
Y/C Correction
- Target (Both, Luma, Chroma)
- Chooses which channels to offset. When ‘Both’ is selected, both channels are offset half the total distance relative to each other.
- Horizontal, Vertical (-10…10)
- Sets how far to offset the selected channels.
Acknowledgements
Thank you to Michael at Pixelmover for suggesting the Y/C correction tools.
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