Today, I will be teaching you how to build a glitchy A/V “mixer” that I’ve affectionately called Dorkwave. “Mixer” is in quotations because it’s a rather simple device that directly connects 2 composite video signals and an audio signal. Despite its rudimentary nature, it produces some rather beautiful, distorted effects. It’s great for live performance set-ups or just to mess with in your room.

This project requires a little bit of hardware knowledge, some breadboarding and/or soldering, and a little bit of patience.

Supplies

2x 2 MOhm Resistor
1x 100 kOhm Resistor
1x 220 kOhm Resistor
1x 100 nF Ceramic Capacitor
2x 1 µF Electrolytic Capacitor
1x 10 kOhm Potentiometer
1x 100 Ohm Potentiometer
1x low-noise, general-purpose operational amplifier
Several old RCA cables to cut up and use
Breadboard and Protoboard
2x Toggle Switches (optional)
2x Composite Video Feeds as input
1x Composite Video Display for output
1x Audio Input (either line level or instrument level)

Background

Composite video is an analog video signal that transmits video information over a single channel. The initial idea for this project was to see how an analog audio signal would warp a composite video signal.

Starting out, I connected my guitar directly to a composite signal before running it into an old TV. Playing the guitar seemed to have no effect on the video. I decided to try to use a line-level audio signal (an old keyboard) as opposed to an instrument-level audio signal to see if that was enough to distort the video feed. With the volume turned all the way up, the video feed was warping. You could see some really cool flickering and rippling on the screen that seemed to represent the frequency of the note being played.

I decided to build a non-inverting amplifier circuit to boost the audio signal before combing it with the video signal. I found that most line-level signals did not need to be boosted to distort the video whereas instrument-level signals did need to be boosted. Adding the amplifier circuit with a potentiometer on the output allowed me to fine-tune the audio signal to the desired level of distortion regardless of what was connected to the input.

The next logical progression was to see what would happen if I added a second composite video signal to the mix. Dorkwave: the glitchy A/V “mixer” was born.

Step 1: Breadboarding/Amplifier Circuit Construction

Note: I recommend breadboarding at first to confirm your circuit works and to make the necessary tweaks before soldering components onto your circuit board.

First, construct the amplifier circuit. U1 can be any low-noise, general-purpose op-amp. I used an MCP6002 chip that I had lying around. Add R1, R2, R3, R4, R5, C1, C2, and C3 around U1.

At this point, you should have a working amplifier circuit. To test your circuit, connect a 5V power supply to the power rail and ground rail on your U1 op-amp. Connect a low voltage (< 1V) signal between the input (C1) and GND. Check the output voltage on the wiper of the R5 potentiometer. Confirm that there is a voltage present. Adjust R5 to be sure the output voltage changes (R5 functions as a volume knob). You may need to burden the output of the amplifier by putting a 1 kOhm resistor between the wiper of R5 and GND to pull current through the op-amp to get a sense of how it will work during normal operation.

Note: To adjust your amplifier gain, you can change R3 and R4. You may need to finetune this based on what audio sources or instruments you plan to connect. To calculate Vout or the gain, you can use the following equations:

Vout = Vin * (1 + R4/R3)

A (gain in dB) = 20 * log(1 + R4/R3)

Now that your amplifier circuit is set up and you’ve confirmed it’s working, it’s time to start adding connectors and running some A/V signals through your circuit!

Step 2: Connectors/Final Hardware Set-up

Before soldering, you will need to plan out how you will connect your inputs and output to your A/V mixer. You will need a way to connect 2 RCA inputs, 1 audio input, 1 RCA output, and your power supply. I decided to have 3 RCA female connectors on my Dorkwave. That way, I could use standard male-male RCA cables to go from the external devices to my Dorkwave.

Instead of buying specialized RCA connectors, I went by a thrift store and found a bunch of old RCA cables to cut up. I cut up a couple of female-female RCA cables and stripped away the insulation until I had 3 appropriate-length female RCA connectors. For my audio connection, I had a 1/4" audio jack lying around from building some guitar pedals. I will mostly be using my mixer with my guitar, so a 1/4" mono audio jack works perfectly for me. Once you have found connectors that work for you, connect all 4 of these sockets/jacks to the appropriate place in the circuit.

You will also need a way to supply power to the amplifier circuit. You can use a battery, USB connection, DC power supply, or anything to provide voltage. I used the 5V rail from a Raspberry Pi I was using to supply one of the video feeds (discussed in more detail in the last section).

At this point, you can also add the R6 potentiometer. You can imagine this as an A/B fader slider on a turntable (i.e. it slides between selecting video feed A and video feed B). You can attempt different values for the potentiometer; however, I got the best results with a 100 ohm potentiometer.

You can add switches (SW1 and SW2) if you want to switch on and off the video feeds. I did not add these to my mixer. I figured I could just unplug whichever input I didn’t want. If you want to quickly transition or switch on/off video sources in a live setting, it could be useful to have these switches.

Step 3: Testing/Troubleshooting

Now you should have all the components on your breadboard. It is time to actually start running real video and audio signals through your Dorkwave. Connect two composite video feeds and an audio source to your Dorkwave. Run the composite video output to a TV or display of some sort. Connect your amplifier to power. You should now see something on the screen!

If you don’t see something at first, play around with the potentiometers. Your video fader (R6) could possibly be too far one way or another. Your audio volume adjuster (R5) could potentially be too high or too low. For best operation and too avoid any potential issues, you want to avoid turning R5 all the way up or all the way down. I've found that you should very gradually adjust R5. There seems to be a very tight range of resistance values for the potentiometer that produces the best results. This range varies based on what instrument you are using.

If you still don’t see anything, check your connections. Use a multimeter in continuity mode to make sure there are no open circuits between connection points. Since there are many different connection points for this small circuit, this is the most likely issue.

Step 4: Soldering/Final Packaging

Now it’s time to solder. Find an appropriately sized protoboard and start transferring your parts from the breadboard to the protoboard. Be sure to keep in mind where you will want to put your connectors and potentiometers for routing outside of the enclosure when planning your layout. Begin by soldering the components before moving on to solder the cables or the potentiometers. You can solder pin headers to the board so you can easily swap out connectors or potentiometers. If you are soldering cables directly to the board, you may want to provide some strain relief especially if the cables run outside of the enclosure and will repeatedly be plugged/unplugged. You can do this in a variety of ways, but some of the easiest ways are zip ties or hot glue.

Once you have finished soldering your circuit, test it again. You want to make sure that everything is soldered properly and nothing got lost in translation from breadboard to final protoboard. Once you’ve confirmed full functionality, it is time to package it as a final solution. I had a spare die-cast aluminum enclosure from building guitar pedals I decided to use. Any sort of enclosure will work. Get creative! Try recycling some old junk and using it as an enclosure. You just want to be sure it can protect your circuit and that you can easily mount/route your connectors and potentiometers.

Step 5: Get Mixing!

Your Dorkwave is ready to go! Have fun! Get mixing! Get remixing!

Step 6: Composite Video Sources?

Struggling to find composite video sources? Thrift stores and flea markets are a great place to go to find cheap gear. You can find all sorts of stuff that outputs composite video: VCRs, DVD players, etc.

If none of that strikes your fancy, don’t fret! Raspberry Pi to the rescue! All models of the Raspberry Pi have a composite video output. The composite video output is shared with the audio jack on the standard models, and the zero models have the composite video output on an unpopulated header. Using a Raspberry Pi opens up so many new video possibilities for you to run through your Dorkwave. You can now run digital video files, streaming video, or live webcam footage through it. Just a reminder that you will have to turn on composite video in the Raspberry Pi configuration settings. It is off by default.

If you are interested in going the Raspberry Pi route and want to use the Dorkwave for a live set-up, Adafruit provides a pretty elegant solution that will loop video files on startup. I have used this and it works very well. For more information on installing and running the video looper, you can go to the following link:

https://learn.adafruit.com/raspberry-pi-video-looper/overview