Atari 7800 Video Mod Theory

How It Works

Here's the background on how the Atari 7800 Video Mod does what it does.

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The Electronics Behind the Atari 7800 Video Mod

I'll break this thing down into bits, then tell you what the bits do. If you find this mod at all daunting, hopefully this will put your fears to rest.

I also try to answer the question "Why do this at all?" near the end.

Synchronization and Luma, the Brightness Signal

The most complex part of the video mod is involved with the sync and luma signals. This is largely because these signals need an amplifier, and the luma portion needs to be converted from a digital signal to an analog one.

The Sync/Luma Part of the Circuit


The sync signal coming in is the part of the video signal that tells the TV or monitor how to frame up the video image--where it starts and ends, basically. Without this signal the image on screen will roll and flutter, looking like you've played with the Horizontal and Vertical controls on an old TV set.

This signal is included as part of the Luma (Luminence, or brightness) signal in S-Video signals. The diode here lets the signal do its thing, and the diode itself provides sufficient isolation, so we don't need a resistor on this line (also because the signal doesn not need attenuating.) The signal is run past the brightness control potentiometer since we don't want to have the signal be too weak if we have to turn this potentiometer up to a moderately high resistance level.


The four Luma signals are digital, we have a four-bit luma signal, providing sixteen possible levels of brightness for each pixel. The resistor ladder here--the four resistors that the signals are each coming in through--is a simple digital to analog convertor.

Each bit is either on or off. How much each bit's electrical signal affects the total luminance value is based on how much that signal is attenuated (decreased) by the resistor it goes through. The high-order bit, bit 3, gets attenuated half as much as the next most significan bit, bit 2. This means that bit 3 affects the brightness of the pixel twice as much as bit 2. And bit two is twice as important as bit 1, and bit one lords it over lowly bit 0, which will change the brightness by only a paltry amount.

You'll notice the resistor values are roughly doubled with each lower-order bit. You could get really persnickety about the values of the resistors, and get resistors that are precisely the right ratio of values, but there really won't be any noticable improvement in the image over just using whatever is pretty close value-wise and easily obtainable.

Once we've done the digital-to-analog conversion, there's a potentiometer to provide an overall brightness adjustment. I've tried building this circuit without a pot here, but the variability of the components used, and the display you'll be using, require some adjustment be available.


The sync and luma signals are combined after the brightness control. The signals here are not capable of driving a display directly, unfortunately. So we need an amplifier.

The next part of the circuit is the amplifier, a rather simple one with only a single transistor that is perfectly adequate for a wide range of tasks. If you are an amplifier fanatic you may want to insert your own favorite amplifier circuit here, but once you get the signal to your monitor you'll wonder why you bothered. This one is perfectly adequate for what we need.

The resistors R9 and R10 balance the amplifier to drive an impedance of about 75 ohms, which is about what video systems typically use. I could go into a whole bunch of technical detail here that would leave your head spinning unless you're the sort who deals in such things. In either case there's no real need to go into impedance matching and all here.

The 100uF cap gives us a little power storage and decoupling for the circuit. Its value is not critical, but I'll go into that in the construction section, where I discuss component substitutions.

Chroma (the Color Signal)

The Chroma signals are generated at two different sources depending on whether you are running a 7800 game (which uses the Maria chip), or a 2600 game (which still uses the old TIA chip for its video.) We need to take these two signal lines and bring them together for a single chroma output, one of the signals of S-Video.

This part of the circuit is pretty simple. Each signal is attenuated a bit, and in the case of the 2600 chroma, we are isolating the interference it produces when it's not being used from our glorious 7800 video signal. If you were to jumper out this resistor and start up a 7800 game, you would see some diagonal lines running up and down through your 7800 video image.

Since this is exactly the sort of thing we're working to get rid of here, I included the resistor in the circuit. I'd hate to have you spend a couple of your valuable evenings putting this together only to get a clearer version of a rotten video image.


The audio part of the circuit takes the two parts of the 7800's audio signal and mixes them together. The "AUDIO" signal is the audio as generated by the 7800 itself. This signal is also present in the "EAUDIO" signal. So why do we need both?

In the 7800 the EAUDIO signal (or External Audio signal) allows a cartridge to generate its own sound and add this to the sound generated by the 7800 itself. For example, if a game manufacturer decided they wanted to include a Computalker chipset on their cartridge to enhance their software, they could do so and use the EAUDIO line on the cartridge port to bring the output out of your regular TV speaker along with the audio generated in the 7800 itself. The same would go for putting a POKEY or SID chip in the cartridge.

One approach to getting audio out of the 7800 would then be to simply pick up the EAUDIO signal and ignoring the AUDIO signal entirely. The thing is that the EAUDIO signal is seldom used by any cartridges (there are probably a couple out there, I don't know what they are, personally), and the EAUDIO signal would need another amplifier since it's not enough to drive the audio input of a normal TV or monitor by itself (unless you want to turn the volume way up and get blasted through the far wall when you switch to another input on the TV.)

So I take a compromise approach and mix the AUDIO signal, which is more than strong enough to drive an audio input on a typical device, with the EAUDIO signal. This is also the approach used by Jay Tilton earlier, and after going back and forth on whether to just pick up the EAUDIO and amplify it, I finally decided to just do it this way.

If you want to do the amplifier thing, refer to the 2600/7800 FAQ for a method which works fine.

Why Do All This At All?

One could make a case that this circuitry already exists, pretty much, inside the 7800 itself, so why go to the trouble of duplicating it all?

The idea is that we could just find the right point to pick these signals off inside the 7800 and put them out through a connector, rather than building our own circuits for D-to-A conversion, amplification, and mixing of signals.

The problem is that I've tried this, and here's why I ended up building things as I have:

  • The composite video and luma signals inside the 7800 are not capable of driving an external display without removing the RF converter to remove its load from the 7800's internal signal sources. One of the things I wanted to do was leave what is already inside the 7800 intact. So I would have to build an amplifier for the luma signal regardless (or for a composite signal if I decided I didn't want to go with a full S-Video output.)

  • The 7800 has a resistor ladder D-to-A convertor for Luma inside, similar to the one in my circuit. It uses higher resistance values, so the signal on the amplifier side is weaker than the signal that I get out of my resistor ladder. I would have had to increase the gain of the amplifier, and I decided I'd rather keep the gain low and be more conservative with the design of the amp to avoid stability issues. So I built my own resistor ladder to suit the amp I was going to have to build no matter how I did this. Even though it adds several components to the mod, I didn't see this as a big deal since they are only a diode, resistors, and a pot. I'd probably need a pot to make this a general-purpose circuit anyway--different transistors of the same type will often vary enough in their characteristics that I'd need the pot to make up the difference (unless I change the amp design.)

  • Likewise for the Chroma and Audio. I have a choice of taking weaker signals and building more amps (three total), which would not make things simpler, or modifying the 7800's base hardware to remove loads so as to free up signal for my new outputs. I didn't want to make this sort of mod to my system, as I mentioned before. So it was either duplicate some circuitry with resistors and caps, or build amplifiers.

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