C64 A/V cable - theory and practice

A while ago I made a proper SCART cable for Amiga, so why not for C64 as well? So what I did was a C64 SCART cable with selectable composite/S-Video mode, including a ‘chroma fix’ and audio noise reduction ‘mod’ plus a preparation for possible stereo audio use. But let’s start with the theory.

Cable making principle

Building an A/V cable for C64 is not very difficult. In the simplest form, what is needed are plug for the C64 Video port, a cable with enough wires for the desired operation and the output connector(s).

C64 Video port

The C64 Video port is an 8-pin DIN connector, which is a standard multi purpose connector. It means that the required DIN plugs are easy to find and cheap. However, there are two different versions of the 8-pin DIN plug. The two versions differ in the shape of the arc that the pins form. Best shown as a picture:

The two pins marked in grey in the wrong type DIN plug will prevent the plug from being inserted into C64 A/V jack. But not to worry in case you’re holding the wrong type of plug in your hand, these two pins can be cut off or pulled out from the connector plug. In any ordinary A/V cable they are not needed anyway. Here is the Video port pinout:

Some early C64′s had 5-pin Video port that are missing the pins 6-8. It means there’s not separate chroma signal available there.

Make note on the pin 8. It is either directly wired to the +5V DC line from the PSU (after the power switch) or not connected. According to my first hand research, the +5V DC line is present in the C64′s Video port in all but the earliest two models. The +5V line is present starting from model C64B, or motherboard ASSY-NO. 250425. That means machines manufactured starting from 1984.

If we are looking at the numbers in this fascinating research of the number of manufactured Commodore 64s, we can assume that roughly 70% of Commodore 64s have the power line at Video port. And those are the latter 70%, meaning they are likely in the majority of C64s still existing and in operating condition.

This is noteworthy as many sources on the internet claims that the +5V line is present only in the later C64 C models and in C128. The motherboard version C64B / 250425 that introduced this feature is used in old style ‘breadbin’ models in great numbers. All C64 C models have motherboard model C64B or later.

To sum up;

C64 model	Motherboard ASSY-NO	+5V DC on pin 8 of Video port
C64	326298	no*
C64A	250407	no
C64B	250425	yes
C64B3	250466	yes
C64E	250469	yes

I have C64s of different models, including C64A, C64B, C64B3 and C64E. The above is based on measurements and examination of the motherboards I have. The earliest model ASSY-NO 326298 I do not have, thus I’m not able to verify the case with it – *it’s an assumption.

And in fact, the first models probably all have the 5-pin video port, and thus, missing the pin 8.

I think that’s enough for the A/V port itself.

A/V Connectors, number of wires etc.

The signals from the C64 Video port are then carried to suitable connectors that plugs into TV or monitor. The options for number and type of connectors are many. The A/V port gives composite video and separate luma and chroma, which is essentially S-Video (well, kind of, see MiaM’s post on the comments). For composite video, only a single male RCA plug is needed. For separate luma and chroma, two RCA plugs are needed. This kind of connection can be used with e.g. Commodore video monitors that have these inputs for best possible picture quality. Many televisions still support the separated luma and chroma in the form of S-Video, and the commonly used connector for that is a 4-pin mini-DIN plug.

The C64 outputs only mono audio (unless modified with second SID chip), so one male RCA plug for the mono audio is enough. However, many TVs don’t understand mono audio input as such, and you would be hearing the audio only from left (or right) speaker if connected with only one plug. Therefore it might be useful to double the audio output for both left and right channels.

Alternatively all the signals can be wired to a SCART connector. That’s what I did.

Finally between the 8-pin DIN plug at the C64 end and the chosen connectors at the TV/monitor end there should be adequate length of good quality signal cable with enough wires. Just count the number of signals desired, and add one for ground.

For a composite video + mono audio cable, two signal wires + ground wire is enough. For the cable I made, I used a six wire shielded signal cable. That’s luma, chroma, composite, two wires reserved for audio and a ground wire.

Building up the cable

This is how I made my cable. I’m not going to make a step by step instructions, the information given and the diagram below should be more than enough for a nifty DIY person!

Update: I made another post with step-by-step instructions – and this time without using the SCART plug.

These are the ingredients. Six wire shielded cable, a switch, 330 ohm resistor, 8-pin DIN plug, a SCART plug.

And this is the diagram of the cable I made for myself, provided as post-production!

The SCART connection can carry several different video signals, but composite video and S-Video share the pin 20. Therefore a switch is needed to select between them. If only S-Video is desired, the switch and connection to DIN plug pin 4 can be omitted. In case of composite only, the connection to DIN plug pins 1 and 6 and the switch can be omitted.

The resistor in chroma line is an attempt to improve picture on modern TVs. On some TVs there’s a distinctive checkerboard pattern visible when using an S-Video cable with the C64. Apparently this is because of too strong chroma signal. Or that’s what teh internets told me, I suspect it’s something else. Other possible symptom is the colours going mad. On both of our household TVs I get the checkerboard pattern, so I installed a 330 ohm resistor to the chroma line. Some fellow Commodorists have reported getting rid of the pattern that way. Sadly, the resistor didn’t make great difference with our TVs. Seems I have to try out different values, or some other means. Anyway, the resistor does not do any harm, and if it improves the picture on your setup – good for you!

The noise reduction mod, or feature of cable, is the simplest modification ever. I simply joined the DIN plug pins 2 and 5 with a bent piece of wire. This connects the audio input to ground at the Video port, reducing noise that the audio input picks up. This is because the audio in line is left floating on the C64, and that way it works like an antenna picking up all kinds of noisy mess from surrounding electrical fuzz.

Connecting the audio in -pin to ground is perfectly fine (unlike grounding the audio in pin of the actual SID chip!), the only thing you loose is the possibility to feed external audio to SID. I would not recommend that in any case anyway – it’s an easy way to bust the SID chip. The best use for the audio input is to use it for noise reduction.

But don’t get too excited. The noise reduction mod won’t do miracles alone and make the C64 free from background noise. Especially the VIC-II and video circuitry will still make noticeable noise that you can hear from the audio output. You might have noticed that the level of the characteristic audio background buzz is related to what happens on the screen. At best this mod reduces the level of that noise significantly. Getting rid of all noise calls for some other means, as it’s even possible to playback audio without using SID chip at all! One part in Vicious SID plays audio using the video circuitry. I’ve tried it with SID chip removed – and yes, the chords in that part could be heard. That’s how much there is interference in the different signals inside the C64.

Both the S-Video and the noise reduction mod among many others are described in detail at C64 AlphA MOD page.

By the way, an S-Video SCART cable can actually work with receiver that supports only composite video in the SCART input. This is because the composite video is ‘backwards compatible’ with black and white video signal. If the luma signal is fed to composite input, the result should be black and white picture, and that’s how it’s supposed to be. The interesting thing with C64 is that the picture will most likely not be black and white, but a colour picture! This happens because the rather poor video circuitry leaks the color information to the luminance signal. The TV picks up the more or less weak colour signal, and thinks it’s composite video after all. What happens then is up to the TV/monitor. Some TVs can amplify the colour signal way too much resulting in complete colour chaos. Other TVs may show perfectly fine colour picture.

So in a way, the C64 S-Video SCART cable can be “auto sensing” and work in composite TV set anyway, as seen pictures further down.

Building tips

Here you can see how I mounted the switch. The red wire will go to SCART pin 20. Composite and S-Video signals will be connected to the switch terminals. The wire inside SCART plug connects video and audio grounds.

Stripped wires prepared. Note the plug covers on the cable. They better be sneaked on the cable before soldering the connectors into the cable.

The plugs before closing them. The cable shielding is connected to plug bodies, ground line runs separately. The resistor resides inside a piece of clear heat shrinking tube to prevent any shorts. The cable becomes a stereo cable simply by detaching the pink wire from the DIN plug pin 3, and connecting it to pin 7. Then, also a switch for stereo/mono could be mounted to the SCART plug.

Picture time!

Some examples how picture look using the cable, with both composite and S-Video signal. Once again, big pictures unmodified from camera.

Panasonic plasma, composite left, S-Video right:

Samsung LCD

Philips CM8833 CRT. On left: composite. On right: luminance only! As you see, the picture is not black and white. Chroma leak is real, man!

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