Y/C DUB color-under

Y/C DUB color-under is a separated Y/C video signal format used on U-matic, VHS, Betamax, and other tape formats via DUB connectors. Like standard S-Video (Y/C), the signal carries luminance (Y) and chrominance (C) on separate conductors, but the chrominance subcarrier remains at the color-under frequency used during tape recording rather than being upconverted to the standard 3.58 MHz (NTSC) or 4.43 MHz (PAL) subcarrier.

Overview

During recording, U-matic and VHS-family formats downconvert the chrominance subcarrier to a lower frequency for storage on tape — a process known as color-under recording. On playback, the VTR's internal circuitry normally upconverts the chrominance back to the broadcast standard frequency before outputting composite or S-Video. The DUB connector bypasses this upconversion step, providing the raw color-under signal directly.

This approach preserves signal quality by avoiding an unnecessary decode/re-encode cycle, making it ideal for VTR-to-VTR dubbing and for feeding time base correctors that accept color-under input natively.

Color-Under Frequencies

Format	Chrominance Frequency
U-matic low-band	685.546 kHz
U-matic hi-band / SP	983.803 kHz
VHS / VHS-C	629 kHz
S-VHS / S-VHS-C	629 kHz
Betamax	688 kHz
Video8	732 kHz

Equipment labels may refer to specific frequencies (e.g., "Y/C-688" for U-matic low-band/Betamax frequency) or use general terms like "DUB" to indicate color-under Y/C support.

Connector

The DUB signal is typically carried on a Hirose 7-pin DUB connector on U-matic equipment and some professional VHS decks. Some time base correctors and transcoders (such as the Digital Processing Systems DPS-210) provide both Hirose 7-pin DUB connectors for direct VTR connection and BNC connectors with separate Y and C for flexible signal routing.

Y/C-688

Y/C-688 is a common designation for U-matic color-under signal, referring specifically to the 688 kHz (more precisely 685.546 kHz) chrominance carrier frequency used by U-matic low-band decks. The name derives from the "Y/C" (luminance/chrominance) component separation combined with the nominal carrier frequency.

The term "Y/C-688" was used by some manufacturers and in professional documentation to distinguish U-matic DUB connections from standard S-Video (which uses a 3.58 MHz subcarrier for NTSC). The AMIA Cable Bible notes that Y/C-688 predates S-Video as a separated Y/C format, was first seen on 3/4" U-matic decks, and continued to be used on professional Betacam and VHS/S-VHS decks.

An important characteristic of Y/C-688 connections is that different devices modulated chrominance at different frequencies depending on the tape format—making signals from one Y/C-688-capable deck potentially incompatible with another, or with S-Video devices. This is why the more general term "Y/C DUB color-under" is preferred when describing the signal type, as it encompasses all color-under frequencies without implying a specific modulation standard.

See Also

• S-Video (Y/C) — Standard Y/C with upconverted chrominance
• Hirose 7-pin DUB — Common connector for DUB signals
• Composite (CVBS) — Composite video signal format