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emerson

intellivision stereo mod

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While I was looking at the intellivision service manual I noticed the sound chip has 3 outputs all tied together along with the external audio. I decided to try and put together a stereo mod to go along with my rgb mod and this is what I came up with. It seems like the three outputs are part of an internal DAC so I took the output of pin 3 and blended it into pins 4 and 38. This seemed to provide the most uniform sound output from either channel. The external audio is blended into both channels and has a 10pf capacitor to ground to eliminate hum. The two signals are then fed into a common base amplifier.

 

Attached at the bottom you can find the audio recordings and other relevant info. Enjoy!

 

Here is the schematic:
[attachment=614331:schematic.png]

 

This is a recording of the intro music from the crimson tower. The mono sample is taken from the solarfox av mod circuit.

[attachment=614304:crimson_tower_signals.png]

 

Here is the intro music to frogger. The mono sample is taken from the solarfox av mod circuit.

[attachment=614314:frogger_signals.png]

 

Here is some audio from pitfall. The sound order is two log hits, jump, vine swing, then game over. Notice how only one channel is prominent in the stereo recording. Adding a jumper wire between the two emitters resulted in dual mono signals, so add either a switch or blend pot to the emitters if you like.

[attachment=614324:pitfall_signals.png]

 

This is the intro from b-17 bomber to demonstrate external audio. It's dual mono regardless of the emitter jumper discussed earlier.

[attachment=614305:b17_signals.png]

 

 

Edit - the 10pf capacitor should be 1nf

Edited by emerson

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It reminds me of the Amiga which has two DMA channels on the left and two channels on the right for a very spatial stereo. It can be pleasing on a speaker system, not quite as nice in headphones. If you would be able to mix the three audio channels plus the overlayed noise in some configuration where you have 75% left and 25% right for one voice and vice versa for the other, it probably would get even better but also a more complex circuit. For simple jingles using one or two unison voices it doesn't matter, for more advanced music using each voice independently it does.

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The description of the 75% and 25% mixing makes me think of the resistor network for digital to analog conversions like the one I'm using in the rgb mod. I'll have to experiment with that. Assuming it works, the circuit wouldn't be much more complex, just a few more resistors. The external audio could be sent through low/high pass filters for a "stereo effect" but I'm not sure how good that would sound. I'm leaning towards keeping it dual mono.

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The external audio could be sent through low/high pass filters for a "stereo effect" but I'm not sure how good that would sound. I'm leaning towards keeping it dual mono.

 

Not the most elegant, but maybe on the external audio you could find a way to cause a slight delay on the signal between the left and right? This would cause a psuedo stereo like effect if you could change the timing ever so slightly but I'm not sure how feasible that actually is.

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Not the most elegant, but maybe on the external audio you could find a way to cause a slight delay on the signal between the left and right? This would cause a psuedo stereo like effect if you could change the timing ever so slightly but I'm not sure how feasible that actually is.

 

I feel an added delay on the external audio (or any channel really) would cause more of an echo effect than a stereo effect. Also, I'm guessing the delay circuit would need more components than I care to deal with. I want this circuit to fit on the same pcb as my rgb mod which will go in place of the rf modulator.

 

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I agree, stereo is about hearing different things in each channel, not hearing the same thing with room delay. Perhaps if you go to a rock concert where all the sounds are amplified and blasted by speakers, you will experience the delay in composite sound more than you experience how different instruments/voices arrive sooner than others.

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Okay...I was specifically talking about the delay in regards to the external audio line since he mentioned prior up in this thread that he would have to mix it in mono on both channels or just choose one channel. That is what my suggestion was for...not for the entire circuit as a whole. Sorry for not clarifying that earlier.

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Okay...I was specifically talking about the delay in regards to the external audio line since he mentioned prior up in this thread that he would have to mix it in mono on both channels or just choose one channel. That is what my suggestion was for...not for the entire circuit as a whole. Sorry for not clarifying that earlier.

 

You were clear about the delay being just for external audio. I was saying that delay in general might not provide the desired effect. I think it was my (or any channel really) remark that took your idea off course, so I apologize for the confusion.

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I experimented with using a DA style resistor network to blend the channels but the results were poor. I decided to use a 50k dual potentiometer with a 5k6 resistor in series with each half of the dual pot. I found the results to be rather pleasing. I chose 50k because that was the only dual pot I had in the bin, but a smaller value may be better if you decide to install it in a console. By adjusting the pot I came up with the following. 12k to 22k will give nicely blended audio, while 33k to 56k will give very spacial audio, similar to the original circuit. If you go much below 12k, you get dual mono. Resistance measurements are measured across one half of the pot and its series 5k6 resistor. Both of which were removed from the circuit when measured.

 

Included are audio samples comparing the old and new circuits. The audio samples were recorded around the 12k setting.

 

[attachment=615959:schematic_v2.png]

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