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I have a breadbox Commodore 64 with a Commodore 1702 monitor.

My monitor has three RCA-style inputs on the back:

  • Audio (White)
  • Luma (Yellow)
  • Chroma (Red)

I purchased a cable which claims to be an original Commodore 64 monitor cable, but I believe it must have been made for a later model of the monitor (or perhaps for a different Commodore computer altogether) because it has four leads: White, Black, Red, and Blue. It's possible that the white may have originally been yellow, because all of the colors are quite faded. The cable sheath is light grey (but may have originally been white).

Through trial and error, I've discovered the red lead carries the Luma signal and the blue lead carries the Chroma signal.

Are the black and white cables both audio? Can I use an RCA Y-splitter to merge them? Can I connect them directly into a standalone speaker set (with the standard white-red/left-right RCA-style female connectors)?

  • The SID pinout I found — ist.uwaterloo.ca/~schepers/MJK/pics/6581.gif — shows only one 'audio out'. So I'm confident the machine is fully internally mono, making two audio outputs unlikely. – Tommy Sep 26 '17 at 18:47
  • @Tommy is it possible that the c128 or perhaps the pet or amiga used sound chips with two channels? – Woodrow Barlow Sep 26 '17 at 18:51
  • i just found these pictures of a 1084ST monitor -- it looks like they have two audio inputs. – Woodrow Barlow Sep 26 '17 at 18:53
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    Various pinouts suggest the extra cable is either composite video out, or audio in. – snips-n-snails Sep 26 '17 at 18:54
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    @WoodrowBarlow definitely possible. I've gone the traal route in producing an answer based on the pinout, but he's right that it could be audio in, as the SID is designed to be able to work in series with other audio. I think composite out is more likely though, as per the quantity of Apple ][ monitors. – Tommy Sep 26 '17 at 18:56
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[Do over answer]

Based on your clue that an Atari monitor has matching coloured inputs I looked up the Atari 8-bit video connector, compared to the C64:

enter image description here

It is likely you have an Atari lead rather than a Commodore one. So your four signals are intended to be (1) luminance; (2) audio; (3) composite video; (4) chroma where available.

Plugging that cable into a C64 will give you luminance, audio out and composite video on the same outputs as they would have been on an Atari. But the connection that's offered for chroma output will actually function as an audio in.

Composite video should be usable as luminance because that was a key concern affecting its original design — it's the same issue as backwards compatibility with black and white sets.

Therefore of your two spare ends, one is audio out and one is audio in.

  • hello tommy, i get a black screen if i put the black or white cables into luma. if i put the blue cable into luma, i get a grayscale picture. – Woodrow Barlow Sep 26 '17 at 18:59
  • i just found these pictures of a 1084ST monitor and it looks like they accept audio left/right channels. and the colors seem to match up with my cable. i'm confused, though, because (as you said) the SID only has one audio output. – Woodrow Barlow Sep 26 '17 at 19:00
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    @Tommy Did you mean to make this community wiki? – wizzwizz4 Sep 26 '17 at 19:19
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    @wizzwizz4 yes, since Woodrow ended up doing a lot of the work himself in spotting the Atari monitor inputs. So it was a team effort. – Tommy Sep 26 '17 at 19:20
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    Perhaps the cable was built to support the Commodore's audio input, though I have no idea whether anything actually used that. It might have been helpful for some audio expansion devices to allow their output to be combined with the C64's main audio, though I have no idea how well that would work. – supercat Sep 26 '17 at 22:16

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