u/tommytwothousand

I'm making an audio video switch for a collection of retro game consoles. This circuit would be replicated for multiple inputs and is configurable for use with composite and component video. Note that logic for controlling the mux and the power supply circuit are excluded as they are still a work in progress.

J1 and R1 are the input RCA connector and terminating resistor.

C1 is a DC blocking capacitor to AC couple the signal. Seems like most similar equipment does AC coupling instead of DC from researching best practices.

R2 re-establishes a known DC offset at the midpoint between 5V and ground. This is to ensure the signal doesn't float and stays positive so it passes cleanly through the mux. Note that the mux only accepts signals in the 0-5V range with a single supply. I'm open to adding dual supply if its worth risking the noise and complexity for my use case.

Between the mux (ADG708) and buffer (THS7314) there is a resistor and a capacitor that set the mode on the buffer. Capacitor only for AC Sync Tip Clamp mode and capacitor plus resistor for AC-Bias mode. This is the configurable part I mentioned. I would stack four boards together for the full design, each taking a single channel of a signal. Composite and Component Y are configured for sync tip clamp mode while Component Pb and Pr are configured for AS biasing. All of this comes straight out of the datasheet.

After that the signal is sent out with an AC coupling and impedance matching with the resistor and capacitor.

I really hope that wasn't gibberish as I am still learning all this.

I'm mainly looking for confirmation that this looks right before I start prototyping but I'm open to any and all feedback!

I'd like to request a review on my design before starting the PCB layout. This circuit is for switching analog video (composite and component specifically) in a larger AV switch box project I am working on. It will be used with retro game consoles released between 1985 and 2005.

Requirements for the project:

1. Must have 16 inputs accepting composite and component video options
2. Must have 2 outputs that are independently selectable (i.e. two TVs side by side with different or same inputs on each)

This schematic is for a single board switching a single channel. There would be 4 boards in total. One for composite video and three for component video. Each board is controlled by another circuit I have yet to design, but will essentially be a breakout board for an arduino or similar microcontroller.

The signal follows from input connectors, through a multiplexor, and then to an output buffer.

On each input connector I have a 22 ohm resistor for ESD protection and a 75 ohm resistor to ground for terminating the signal. After that the signal is AC coupled with the 47µF cap and then a pull up resistor is used to add a DC bias. The idea of the DC bias is to ensure the signal passes cleanly through the mux chips.

Four 8:1 mux chips are used, two for output A and two for output B. They all have a single positive voltage supply as I wanted to avoid noise and complexity from a negative rail. The absolute maximum rating of the chip seems to go low enough for composite and the Y signal of component, but I would rather stay in the recommended operating range (hence the DC bias after the AC coupling). ADG708 was chosen specifically for its low on resistance but I am open to suggestions if there are better or cheaper options.

The output circuit uses a THS7314 buffer chip which has three channels. I am only using two (one per output). the 0.1µF cap and the 2MΩ resistor are used to set the mode of the buffer. For syncing signals (composite and component Y) the resistor is unpopulated which sets the mode to "AC Sync Tip Clamp Mode" and then when the resistor is populated it gets set to "AC-Bias Input Mode" (for Pb and Pr component signals). I believe this is the correct usage based on the datasheet.

The logic section should be pretty self explanatory, again I'll be using this with an arduino over I2C. The 4 pin connector is so I can daisy chain a single harness.

Lastly the power supply section has multiple circuits. The "Filter and Bulk Capacitor" circuit adds a filtered 5V rail that I will use to supply the mux and buffer chips. The "Signal Bias Voltage" circuit is just a voltage divider creating a mid rail supply for adding the DC bias before the mux. The rest should be self explainatory.

Background on myself: I am a mechanical engineer by profession with a strong interest in electrical engineering but minimal formal training. I've been using Claude AI to help fill in the gaps in my knowledge and learn more. I didn't ask it to make the circuit for me, instead I used it to direct my own learning. Long story short, I have learned a lot in the last few weeks but I don't have any practical experience in analog signals yet. Any and all help is appreciated!