I'm designing my own custom audio/video switch for my old video game consoles that use RCA jacks. It's got 16 inputs and 2 outputs.
I'm using a bunch of these to do the switching:
Invalid Request
Here's the data sheet:
http://www.ti.com/lit/ds/symlink/sn74ahc4066.pdf
I rigged up a prototype on a breadboard and the video is perfect but I'm getting noise or something on the audio when multiple systems are turned on. It's certainly the other systems because the crackles in the noise sync up with the music of the other game. I don't really know much about this so I would appreciate any suggestions or advice on better ICs or ways to fix this.
I thought it was interference on the grounds of the RCA jacks but have since ruled this out by just passing them through a breadboard with common grounds. The sound was perfect so it's gotta be in the switches right?
Could this be crosstalk? I've got a lot of wires passing over each other. If it were, would I be able to tell that it's syncing up with the other game or would it be more random?
I also tested the switches by hooking the As up to +5 volts and measuring the voltage at each B. I got 4.98 when I closed the switch and 0.026 when it was open. Could the audio be leaking through? The noise isn't significant so it would make sense with that much leaking through. I used different switches in an earlier build and it had more noise with more voltage leaking through so it does correlate. Could something like a pull up or pull down resistor prevent signals from leaking through the open switches?
I'm using a bunch of these to do the switching:
Invalid Request
Here's the data sheet:
http://www.ti.com/lit/ds/symlink/sn74ahc4066.pdf
I rigged up a prototype on a breadboard and the video is perfect but I'm getting noise or something on the audio when multiple systems are turned on. It's certainly the other systems because the crackles in the noise sync up with the music of the other game. I don't really know much about this so I would appreciate any suggestions or advice on better ICs or ways to fix this.
I thought it was interference on the grounds of the RCA jacks but have since ruled this out by just passing them through a breadboard with common grounds. The sound was perfect so it's gotta be in the switches right?
Could this be crosstalk? I've got a lot of wires passing over each other. If it were, would I be able to tell that it's syncing up with the other game or would it be more random?
I also tested the switches by hooking the As up to +5 volts and measuring the voltage at each B. I got 4.98 when I closed the switch and 0.026 when it was open. Could the audio be leaking through? The noise isn't significant so it would make sense with that much leaking through. I used different switches in an earlier build and it had more noise with more voltage leaking through so it does correlate. Could something like a pull up or pull down resistor prevent signals from leaking through the open switches?
The signal is probably crosstalking through the protection diodes. This can happen from simply being too loud and/or asymmetry on the audio signal causing it to trip the protection diodes but only on positive or negative peaks. If you're running single supply the average DC needs to be 1/2 of the power supply.
The HC4066 is not the best choice for the audio because of the low power supply Voltage. You'll have to attenuate the audio before the switch and boost it back up after the switch. The old non HC version (CD4066) can run 15 Volts and will give you more headroom but the control logic has to be 15 Volts also which would also be a complication.
You said the video is OK. Are you using a line driver opamp to restore the video levels into the destination?
There are chips out there tailored for audio switching that are easier to implement in that the logic levels have translators to keep it simple such as the MPC506A from TI.
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The HC4066 is not the best choice for the audio because of the low power supply Voltage. You'll have to attenuate the audio before the switch and boost it back up after the switch. The old non HC version (CD4066) can run 15 Volts and will give you more headroom but the control logic has to be 15 Volts also which would also be a complication.
You said the video is OK. Are you using a line driver opamp to restore the video levels into the destination?
There are chips out there tailored for audio switching that are easier to implement in that the logic levels have translators to keep it simple such as the MPC506A from TI.
G²
I'm not doing anything to the video, it just works. To be honest I don't even know what an op amp is. The only things on the breadboard are 3 switch ICs, 12 RCA jacks and wires.
When you said cross talk in the protection diodes is that within the IC? The asymmetry makes sense too. I tested a sega genesis and a Super Nintendo and the genesis was crystal clear always while the Super Nintendo picked up interference. I guess the genesis has a stronger signal.
Ideally I'd like to control this with an arduino so high control voltages could be an issue but I'll look into ways around it.
One more thing, how would I attenuate the audio like you suggested?
When you said cross talk in the protection diodes is that within the IC? The asymmetry makes sense too. I tested a sega genesis and a Super Nintendo and the genesis was crystal clear always while the Super Nintendo picked up interference. I guess the genesis has a stronger signal.
Ideally I'd like to control this with an arduino so high control voltages could be an issue but I'll look into ways around it.
One more thing, how would I attenuate the audio like you suggested?
The inputs that aren't causing problems don't need the attenuators, only the ones that are interfering. If the video is OK you can leave it as is. Only the audio needs the bias to the center of the range. You'll need one bias source needing a minimum of 2 resistors and some capacitors to prevent crosstalk. Then, each input will require at least one resistor and blocking capacitor. Each input that requires attenuation will need 1 more resistor. You should also have a blocking capacitor on each output to prevent the bias from reaching the destination.
Focus on the audio attenuator and the configuration is the 2 resistor 'L' pad (because it looks like an L).
http://en.wikipedia.org/wiki/Attenuator_(electronics)
I would probably use a pair of 1K resistors for the bias source with a 100uF capacitor in parallel with a 0.1 uF film capacitor. I'd use 10 to 22K resistors for the bias and 1 to 10 uF capacitor for the blocking functions. Attenuators are simply 2 resistors in series with the signal at the junction of the 2 resistors - OR you could use a pot for each attenuator and just adjust to your liking. I tend to think in fixed values so you don't continue to 'fiddle' with it.
G²
Focus on the audio attenuator and the configuration is the 2 resistor 'L' pad (because it looks like an L).
http://en.wikipedia.org/wiki/Attenuator_(electronics)
I would probably use a pair of 1K resistors for the bias source with a 100uF capacitor in parallel with a 0.1 uF film capacitor. I'd use 10 to 22K resistors for the bias and 1 to 10 uF capacitor for the blocking functions. Attenuators are simply 2 resistors in series with the signal at the junction of the 2 resistors - OR you could use a pot for each attenuator and just adjust to your liking. I tend to think in fixed values so you don't continue to 'fiddle' with it.
G²
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