Color lock is a process for reconstructing the color signal of primarily PAL-encoded U-matic tapes (Low and High Band) on playback. If the signal frequency is off, “the colour subcarrier will shift phase for the duration of one line, which means that colours will be reproduced incorrectly.”1 The result is quite noticeable:

A colour gradient stretches from the left-hand edge to the right-hand edge of the image, with a narrow achromatic area down its vertical center axis. More precisely, all scan lines start at the left-hand edge of the image with a colour shift specific to each hue. Colour saturation drops to zero towards the centre of the image, then rises to reach maximum saturation in the complementary colour hue at the right-hand edge of the image.2

Black-and-white recordings and NTSC recordings are not susceptible to loss of color lock. However, if an NTSC recording “is played back on a multi-standard player with a colour lock control set in the wrong position, colour disappears completely and a characteristic diagonal herringbone pattern occurs.”3

Can it be fixed?

Adjust the color lock control on a PAL playback device until the color is rendered correctly. If these adjustments do not resolve the artifact, try the same adjustments while playing a test tape in the same device. If the test tape’s colors can be rendered correctly, then the tape recording has uncorrectable defects; however, if the issue persists while playing the test tape, then the device needs servicing.

References

1. Johannes Gfeller, Agathe Jarczyk, and Joanna Phillips, “Loss of colour lock” in Compendium of Image Errors in Analogue Video (edited by Swiss Institute for Art Research, Zürich: Scheidegger & Spiess, 2012), p.56, videos 5 & 6.
2. Gfeller et al, “Loss of colour lock.”
3. Gfeller et al, “Loss of colour lock.”

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