So you're brand new to streaming, or maybe you've been playing new games and you're wondering how the "retro" players capture footage from the old games. Well, here's a quick visual guide on what hardware you'll need.

The capture device
IO Data GV-USB2
$45 (Amazon)
I-o DATA USB connection video capture GV-USB2 This device allows you to capture RCA (Yellow-Red-White) and S-Video outputs. Nearly every retro console uses this output! The GV-USB2 is the current "best-in-class" for quality capture.
The powered splitter
Radioshack 1-to-4 RCA/S-Video
$35 (Amazon)
RadioShack 1500320 1-Input/4-Output A/V Component Video Sound S-Video Distribution Amplifier Splitter Playing your old games straight from the capture device is okay, but any gamer will immediately notice the input lag. In order to stream at full quality & with less input latency, I recommend getting a powered splitter. There are cheaper, non-powered splitter adapters available, but the result is a weaker signal in your TV and your capture.
RCA Cables
6ft - $5 (Amazon)
12ft - $6 (Amazon)
RCA Cables You'll need 2 of these cables to split the signal to your TV and the capture device.

Once you're all hooked up, it's very easy to add this new video capture device as a source in OBS Studio.

Note: Prices are approximate, and may be different than current prices.
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