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 C64s had a 5-pin Video port that are missing the pins 6-8. It means there’s not separate chroma signal available there. In other words: no S-Video, only composite available via the video port.

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 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.

Talking about SCART, please not that SCART is not a video signal standard, it is a connector standard. A SCART lead can carry video in different formats plus audio – just like the modern equivalent HDMI. So when talking about SCART cable, it should be mentioned which signal type is being used, only referring to SCART alone does not necessarily mean anything. According to the standard, a SCART connection can carry composite, S-Video and RGB video signal. A C64 outputs both composite and S-Video, but not RGB.

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, it could well be 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. The picture quality will somewhat suffer of course.

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, SCART pins 17 and 4.

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

The plugs before closing them. The cable shielding is connected to the 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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17 Responses to C64 A/V cable – theory and practice

  1. MiaM says:

    AFAIK the picture quality improved a lot on later C64′s, perhaps the circuit boards got redrawn in a better way.

    Notice that you get a darker shadow on the right of color 6 (on Samsung S-video). That is because the C64 doesn’t really output a normal S-video signal. S-video were not commonly used when the C64 was new, so therefore there were no real standard for such signal in consumer electronics. The output from C64 has a different luma-chroma-timing than standard S-video.

    If you have an old CRT TV that has for example TDA4546 as delay line (combined with CTI) then it’s possible to select different delays and find one that suits C64′s output better.

    I’ve modified my old CRT TV in this way when I added a S-video input. It has a thumb wheel with 8 active positions that selects the delay used in s-video mode. :)

    • ilesj says:

      Hey, thanks for the informative comment!

      Yes, generally newer C64s have better picture than older ones. The VIC-II chip had several different versions and revisions, and it probably got better over time. Especially the early VIC-II chips are said to make rather poor video signal. It’s no wonder if the VIC-II got better with later revisons, and especially the HMOS version used in late cost reduced C64E boards produce much sharper picture.

      Also the RF-modulator plays a big role in the picture quality of C64. It is part of the video circuitry, and the final Y/C and composite signals are generated in the modulator. Also the modulators have been revisioned over time. I guess some models are better than others.

      You are absolutely right in that the C64 does not technically output S-Video, for the reasons you mention, but rather separate (well they aren’t completely separate in reality but..) luma and chroma in a C64 specific mix.

      I recently read a post by Bil Herd where he mentions this signal delay. He also mentions that the Commodore monitors were tweaked for the C64′s signal, and thus, “CBM signals look best on a properly matched era CBM monitor”. So it sounds like your TV mod is very clever one :) I guess the delay could be also tweaked at the C64 side for modern TV compatibility…

  2. SCART? Why… ;”’-(

  3. LeventY says:

    Is there connection pin5 to pin2 ? yes, handwrite.

    • ilesj says:

      Yes, there is connection between pins 2 and 5 in the DIN plug. This simple jump wire connects AV port’s audio in pin to ground. The result is reduced background noise.

  4. abo says:

    Works great, thank you.
    But I want to use a converter from Scart to HDMI. The converter works just fine with my PS2 and my GameCube. The C64 cable shows from time to time a picture. The duration without image and also the duration of image are random.

  5. Joe says:

    SCART = horror

    • ilesj says:

      Again, SCART is just one option for the TV/monitor side connection. If you are building a cable for yourself, you can choose whatever connector types suit you the best.

  6. [...] TV / Monitor to C64 – Theory: http://ilesj.wordpress.com/2012/03/30/c64-av-cable-theory-and-practice/ – Buying: http://www.amibay.com/showthread.php?t=30409 – [...]

  7. Walter says:

    Stupid question… For the stereo setup, don’t understand exactly how the pin 3 + pin 7 (which in theory is NC) works… Is this supposed to be just to simplify soldering or something instead of soldering 2 cables to pin 3 or what? (And if so, isn’t the pink cable that is mentioned still a second cable soldered to pin 3?).

    • ilesj says:

      Hi Walter,

      The C64 only has a mono audio. But in practice all televisions (or audio amplifiers) have stereo input. That’s why we want to connect both the left and right audio channels. Otherwise we would hear the C64 sound only from the left or right speaker.

      So in the cable, both the left and right audio wires are connected to the DIN plug pin 3, audio out. We can call this ‘dual mono’ – same sound from both left and right speaker.

      And correct, the C64 Video port pin 7 is not connected.

      However, it is possible to modify the C64 to have a second SID chip.

      Normally there is no output for second audio channel in the C64. So the audio from the second SID chip can be carried over the pin 7, which is otherwise unused.

      I hope this clarifies!

  8. Anonymous says:

    Hello,

    first I’d like to thank You for that very nicely written article. It helped me alot while I was crafting my own video cable. I made two cables – one composite and the other one s-video.

    From my experience:

    At start I was sitting on “antena” cable (good colors – but bad quality of picture). Then i made an composite cable – good colors – better picture quality but still not much different then antena cable. After that i made an s-video version – very nice picture quality, clear pixels but colors are messed up :( I can clearly see some red and green in places where it shouldnt be. Im afraid im experiencing “…TVs can amplify the colour signal way too much resulting in complete colour chaos…” its not that bad overall, but you can clearly see the difference for example when looking at pure white text on antena cable and then switching to s-video cable you can clearly see the text is not pure white anymore is more like white with some red and green in it. I tried with and without 300 ohm resistor (cant see any difference).

    I really like the picture quality that s-video cable provides. Is there anything i can do, besides swapping my TV for something else? Btw im using an old Sony KV-14M1K CRT TV. Thanks in advance for Your time!

    • ilesj says:

      Hi! Did you make the S-Video cable using a SCART connector? I’m replying now assuming you did.

      Check that your TV is really receiving the S-Video signal. Many TVs have two SCART sockets, and only one of them is compatible with S-Video signal. And often TVs won’t auto detect S-Video over SCART and default to composite. So you may have to manually choose between S-Video or composite over that AV input with some input setting or selection.

      What you describe sounds very much like S-Video signal fed into composite input; the picture is sharper than composite, but colors are way off and messy. Reason for that behavior is that composite and S-Video signals are carried over using the same pins on SCART connector. S-Video luminance signal that includes some leaked and weak color signal is then treated like a composite signal by the TV.

  9. Anonymous says:

    Ha, ha, ha :D. Funny How many answers this got (in this day and age). I too made a cable (cause back in the days I used only the coaxial cable (RF port) and the picture was bad and it influenced sound – especially when using yellow letters – I remember – noisy as hell :D). Yesterday I got around to try to make a cable – just for laughs. Previously I bought DIN, 6-wire cable with shielding and SCART to construct it, but then I figured – I’m not gonna use SCART – I have a camera cable with three part audio jack and separate composite and audio plugs on the other side – that’s far more practical. Cable constructed – so now I have DIN on one side and two audio jacks on the other – one for AV and the other for Audio-IN – which of course begged a question – would I burn the chip if I plugged in microphone or line (from MP3 player for example) – actually what can we plug in the AUDIO-IN – what is it meant for originally, what is it used for?
    I found this post because I was kinda wondering if I made my cable correctly and if there is the best way … and now I see that this falsely named “Luminance” output gives out far sharper picture :D , darn it. …Maybe I’ll switch it. Or – can both be on the same wire?

    • ilesj says:

      To anyone asking about the AUDIO IN on the AV port, my advice is: Unless you’re absolutely sure what you’re doing – DON’T DO IT!

      Would you burn the chip? If you are not 100% sure about the accepted voltage levels etc, it’s better not to try your luck. You just might burn the chip, yes. Many have. That is a valid concern.

      Luminance signal is just what it says. Except that in case of C64, the color encoding signal tends to leak into it. I’m not sure what you’re asking exactly, but if you’re planning to use this luminance signal for composite input or jack, you would probably get a sharper picture, but with bad colors. Connecting the different signal options together doesn’t sound like a good plan.

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