Figure 0 - This black box allows routing effects through your phone system
This black box allows routing effects through your phone system

This project converts an old desktop telephone into a versatile audio mixing station that lets you route your telephone calls through an effects processor in order to create a state of the art voice changer. By using a real time computer voice filter or a professional quality effect box, you can change your voice in ways that will make you sound like a completely different person. You can make a man sound like a woman, or a girl sound like a man, or a man sound like an elderly lady, or any possible combination imaginable with results that will fool anyone. Unlike those "spy toy" voice changers that make you sound like a funny cartoon, a real vocal effects unit or computer vocal filter will alter a voice in a perfectly convincing manner, allowing fine control over both the formant (gender) and the pitch of your voice. Sure, you can have a lot of fun with evil and chipmunk voices as well, but if you really want to mask your voice identity in a convincing manner then this useful device will allow you to connect any microphone compatible audio processing unit into your phone so you can alter your voice in real time.

There are many extremely powerful audio processing programs available for a computer that allow a person to alter his or her voice by talking into a microphone. Many of them are inexpensive or even free. Music stores also offer digital effect boxes that are designed for vocal processing, and these have the same functionality as the computer programs, but do their processing in a dedicated DSP (digital signal processor). I will be using both the computer software voice changer as well as the "effect box" version of the vocal processor to show how each one can be connected to the phone system using this project.

Figure 1 - This basic desktop phone will be converted into an effects mixer
Figure 1 - This basic desktop phone will be converted into an effects mixer

To alter your voice in real time, you must have a microphone connected to the input of a digital signal processor that can break your voice down into a digital signal, apply a complex algorithm to it, and then feed it back to an audio output without any noticeable delay. These audio processors are widely available in the form of computer programs or musical effect boxes, and all of them will allow you to speak into a microphone and hear an altered voice as an output. Ok, that's great, but how do we get this altered voice into the phone system so that you can have a conversation with your newly disguised voice? Well, we will make a high quality telephone audio input/output mixer using a cheap desktop phone, that's how!

Any hard wired desktop phone will work for this project, and the less features the phone has, the easier it will be to hack. The cheap desktop phone shown in Figure 1 is a prime candidate for hacking as it has no built in answering machine, and will be easy to take apart. A cordless phone will not work for this project - you need a phone that plugs directly into the wall. When you are sourcing a phone to hack, look for one that has as few extra features as possible, and preferably one that does not require an external power supply. Just a plain old desktop phone that still functions is all you need.

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