Stereo Audio Mod (with RF video) for Atari 2600?

[Andromeda Stardust]

My 4 switcher has a couple of mods already. For starters, I added an LED Power indicator. I also gutted the stock RCA cable and installed a right angle RCA/F connector on the original RCA jack that I found on the motherboard. Inside the console, there's a 1 foot RF cable with terminates at an F-coupler that is glued into a hole I installed in the case plastic. Now you simply plug a real RF cable onto a jack in the back of the Atari and connect it to the TV, just like you would a VCR or cable box. I'm kind of an audio guy, and I read about getting stereo sound out of the current A/V mod. A lot of homebrews use this feature. Basically, I'm hesitant to do the A/V mod because with current versions of the mod, you'll lose the RF output. I have at least one old CRT that doesn't accept composite (it's also the one with the nicest, biggest screen too, and my Atari picture is gorgeous on it). Sometimes I like to hook my NES to the TV and connect the audio out to my Hi-Fi stereo system, but I'm currently unable to do that with the Atari. The NES and the SNES can simultaneously output RF and composite, so I don't see why that isn't possible on the Atari.

 

Basically, what I would like to do with my Atari is to keep the existing RF output for video + mono audio and install a pair of stereo RCA jacks in the back of my Atari to send stereo audio to my hifi stereo, if that's at all possible. Composite/S-video would be nice, but as I've said before I don't want to lose the RF output, and none of the available A/V mods support passing the video signal to the RF unit. I want to add features, not take them away! I'd imagine a resistor bridge could be used to combine the mono audio signal for the RF modulator without losing the stereo effect. Also, homemade audio circuits aren't as easy to screw up as video, due to the fact the bandwidth is much less (<20kHz for audio circuits compared to ~3.5Mhz for video and >50Mhz for RF), and within a fairly wide tolerance, if the signal level is low or high you can simply adjust the volume on the receiver to preference. If necessary, an Op-Amp could be used to isolate and boost the signal if it's too weak, or use a resistor/pot as a voltage divider if it's too strong.

 

I'm pretty skilled when it comes to electronics (my college major is Electrical Engineering technology), and I'm more than capable of building a custom schematic on a solder type breadboard. I designed a digital turbo circuit for an NES arcade controller using off-the-shelf parts (no vintage controllers were harmed) and the arcade controller works quite well.

Edited February 25, 2013 by stardust4ever

[Andromeda Stardust]

Bump. Probably needs to be moved to "hardware."

 

I just want to output stereo audio from my VCS without composite or messing up the RF circuit.

[RevEng]

According to the 2600 circuit diagram, the audio channels are bridged together. My first approach to isolating them would be to lift the pins and re-connect them to the mainboard through diodes. Re-create the audio portion of the Longhorn mod, connecting to the TIA pins (pre diode) to get the stereo signal.

 

The RF will get mono audio, but the diodes will drop the volume some. If the result is too quiet, add a transistor amp between the diodes and the mainboard connection, or just use 2 transistors as buffers instead of the diodes.

[Andromeda Stardust]

So basically you are telling me that there is a pair of audio pins on the TIA, which are bridged together and then fed to the RF circuit. And if I lift those pins up, I can source the audio from there. I am thinking maybe a pair of diodes isn't the best way to separate the audio circuits. Diodes would introduce a voltage drop, and I would imagine that anything under 0.6V would get clipped in the process. A resistor bridge may work better, but too low a value and stereo separation would suffer; too high a value would reduce volume levels to the RF modulator. I was thinking perhaps of using a dual Op-Amp IC chip on a small daughterboard to isolate the signal first, and possibly use a pair of trim pots to set the gain. I could then use a resitor bridge to combine the op-amp output and feed the mono downmix back to the RF unit. Also I can create a RC high-pass filter to the RCA outputs to remove DC bias. I prefer using bigger caps when possible to keep the bass rolloff well below 20Hz. Are the TIA audio lines at a suitable signal level for line output or do I need to pre-amplify them? Also, since the input impedance of the Op-Amp is extremely large, is it necessary to "load" the TIA output with a "dummy" load to make the audio sound properly?

[RevEng]

Sure, your plans sounds fine to me. Pretty sure you don't need to dummy load the TIA output.

 

I haven't measured the TIA pin levels myself, but many mods just pass the pin outputs through a decoupling cap and nothing else, so I'm assuming the levels are fine.

[retromod]

Since 2013 there are stereo/surround mods available for all Atari, Colleco and Intellivision systems (doesn't matter if PAL or NTSC). It is not based on the TIA audio separation trick (which is only supported by a few games). Instead it uses a MCU AD/DA function controlled by it's firmware to split and modify the audio output to the required channels. The sound is great and the whole solution quite cheap. It is currently combined with an AV mod as well for all these systems (but also runs standalone without modding the videou output). Check out the youtube demonstration videos.

Edited January 9, 2014 by retromod

[PFL]

Do you have any links, at all?

[retromod]

Sure search for atari audio mod within youtube to get those links and demos.