A secondhand capture card I picked up had no breakout cable for its custom 9-pin din connector. This card is based on a bt848 and some other vendors of bt848 capture cards also have the same type of connector, I believe. Using a multimeter I measured the resistance to ground of various pins with the card detached from the system. Then I measured the DC component of the voltage of certain pins with the card inserted into a powered up system. These are the values given (very approximately) in the below figure. If you use this information, you may want to measure yourself before trying, in case your card is different.
A linux driver by Gunther Mayer mentions a 9-pin connector on a "bt832 CMOS Camera". This could be the same as well. His notes mention that the pins are used for "4-wire SDATA, 2-wire i2c, SCLK, VCC, GND." Well, as far as I can tell there are two grounds on this connector, and as far as I know, "SCLK" would be one of the two i2c (a.k.a. GPIO, roughly) wires. So maybe that was a typo.
Anyway, the S-Video specification says that the Y and C inputs use an impedence around 75 Ohms, and various i2c documents mention that 7 KOhms is a common value used for pull-down resistors. The pin marked 5V showed a low resistance to power bar pins on the PCI connector, so that would be VCC. Sorry I do not know which of the GPIO pins is SCLK and which is DATA, since I had no use for them.
I am also probably not really connecting the Y and C grounds -- at first I thought the S-video grounds were just reference voltages, but after some reading I discover that one of the S-video channels is not biased DC neutral. So obviously those 250MOhm connects might not be intended to be the Y and C ground :-) Why S-video even needs separate ground pins is still a bit of a mystery to me, given the low overall impedence/high (for a signal) current probably does not leave it vulnerable to RF. Anyway, I didn't even ground the shields (both systems grounded together on their mains) and it's working fine -- I'm leaving on the diagram where I have the Y and C grounds connected because there have been no problems with it connected that way.
Here is a diagram, looking at the female connector on the card:
Now, I bet you are really glad I posted this diagram. If you are like me, you have tried in the past to figure it out, and spent a lot of time searching on the web and found nothing. I have no doubt this information used to be posted on the web, back when bt848s were very popular. However, since then most of the web has started to become invisible to global search engines. Take a short time to read this and please do your part to keep the power of the Internet intact.
Oh, and if you want more information that does not show up in search engines about TV tuner cards, you may still be able to go here, at least you should be able to until the link breaks, which it will eventually. Sigh.