I have two audio signals and I want to fade in one signal in just as the other signal fades out and repeat the process backwards and so on... I want to control the circuit using some digital means or voltage control, however the actual circuit should be analog. Any suggestion for an affordable solution?
Or you can use digital pot chips like
AD5242 series dual I2C digital pots, coupled with some op amps.
Here is a reference design for you:
The two pots there is the digital pot chip
AD5242BRUZ10 commanded by the Arduino. You can program the Arduino to handle MIDI interface and plug this into your professional audio system rack.
OPA4134 is a proper audio grade op amp but it is expensive. You can experiment with orders of magnitude cheaper but pin compatible
LM324, or get rid of the two voltage followers and use the cheaper
OPA134 audio op amp that is pin compatible with
TL431 is a 2.5V reference chip used as a virtual ground here.
Updated answer 01/23/2017:
Maybe this is an answer: Digital Optical Volume Control with LED and photoresistor (light to seperate circuits, optical pot)
Using light to control the 'fades'/volume of each channel by using a LED and photoresistor(s). You can simply control the LED with an Arduino. Because of the LED is driven by PWM, you need to change the PWM frequency. See also example at my answer (and the reason why).
I made some drafts/examples at my question, my question is about if there is component that combines the two. There is, called vectrols but they are very expensive. You can make your own, it's cheap! More info and pictures at my question and answer.
Changed and simplified schematic:
More info at my question: Digital Optical Volume Control with LED and photoresistor (light to seperate circuits, optical pot)
Use a digital signal processor (DSP). Convert both inputs from analog to digital, multiply them with values ranging from 0 to 1, sum them together and feed to the DAC.
You can also use digital volume control ICs, such as CS3310 (http://www.cirrus.com/en/products/cs3310.html) but a DSP might even be cheaper in the end. Depends on the number of channels and what kind of ICs you can find.