There is a hard-to-find connector used for interconnecting older professional videotape machines which confusingly is used by multiple (partially incompatible) video standards.
This "dub connector" as it is commonly referred to began as a forerunner of today's component video. It was intended to be used for direct machine-to-machine connection to improve the quality of videotape duplication. Usage was pioneered by the U-Matic family of videotape recorders and later found its way into subsequent generations of professional analog VTR families including Betacam/Beta SP and VHS/S-VHS. (If anyone knows whether the M/MII format included this connector I'd like to know.)
Originally, the connector was used to keep the luminance (luma) and chrominance (chroma) signals separated, but as true component video came into professional use a variant pinout was developed to take advantage of the existing hardware design. In some cases, like Betacam, the dub connector matched the U-Matic format's output to ease the transition between formats. In other cases, such as S-VHS, the same cable and pinout was used but the chroma signal was modulated at a different frequency. In fact, depending on the product family and variant there are several possible frequencies in use: 629 kHz, 688 kHz, 3.58 MHz, 4.43 MHz. And that's just for NTSC! PAL has its own set of frequencies. So although the video quality obtainable from the dub connector is far superior to composite video, the adaptive reuse of the connector type can make using it for digitizing archive video a daunting challenge! In many cases some form of conversion may be necessary for use with a capture card and in most instances a custom cable will be required.
In all cases though, these machines use what is called a Locking 7-Pin DIN connection, so named because it has a ring that locks the connector in place much like a BNC connector does for composite video. This connector is currently made only by Hirose Electric. Here is a diagram and pinout for the two versions of the standard:
|Locking 7-Pin DIN|
|Part # RM12BPG-7S||Part # RM12BPG7-PH|
|1||Y Signal||Y Signal|
|2||Y Ground||Y Ground|
|3||Head Switcing Pulse (VHS)||R-Y Signal|
|4||NOT USED||R-Y Ground|
|5||C Signal||B-Y Signal|
|6||C Ground||B-Y Ground|
I hope this helps someone else down the line as there is a huge amount of misinformation out there regarding this, especially for people with first generation JVC S-VHS units with this connector. As Y/C interconnection became standardized with the 4-Pin Min-DIN S-Video cable, there is a tendency to assume that all locking 7-Pin connectors use the U-Matic dub signal standard. The fact that the connector may have a nonspecific label ("Dub" by itself isn't terribly helpful to a newcomer) which does not specify the signal format just makes it worse. But the good news is, every variant I've researched is pinout-consistent so the most likely outcome of connecting two incompatible components with a properly wired cable will be monochrome video, or at worst, simply no video at all.