Y/C-688 is a color-under Y/C video signal format used on U-matic VTRs and some professional VHS equipment 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 of approximately 688 kHz (NTSC low-band) 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 |
Although "688" specifically refers to the approximate U-matic low-band and Betamax frequency, the term Y/C-688 is commonly used as a general label for color-under DUB connections on professional equipment, sometimes encompassing other color-under frequencies as well.
Connector
The DUB signal is typically carried on a Hirose 7-pin DUB connector on U-matic equipment, or on a Mini-DIN-4 connector on some professional VHS decks. Some time base correctors and transcoders (such as the Digital Processing Systems DPS-210) accept DUB input via BNC connectors with separate Y and C connections.
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
