Ensure your computer is powered off. Connect your audio interface to your computer using the provided USB or Firewire cable. Power the computer on and boot into your operating system.
Download and install your Vocoder software, if necessary. Vocoder has become a popular effect, and a software unit is usually included as standard with most fully featured recording or production applications. Propellerhead Reason, Ableton Live and Apple's Logic Pro Studio all include a Vocoder plug-in as standard. Alternately you can download the Sirlab or Epiphyte free versions from the links below. Most platforms are supported.
Launch your recording software and open the "Preferences" dialog. This will be found at the bottom of your PC's "Edit" menu. If you use a Macintosh computer, click on the application name in the top left corner of the screen, then select "Preferences" from the drop-down menu.
Click the "Audio" or "Devices" tab in the preferences menu, and locate the drop-down box for choosing your main audio device. Select the name of your audio interface from the available items in the drop-down list.
Connect the microphone to Input 1 on the interface, assuming that there is more than one. Create a new audio track in the recording software, and click on the track controls, usually located at the far left of the screen. Click the "Track Input" field, usually located just above the volume fader, and select "Input 1" from the drop-down box.
Click on a free "Insert" field, usually located just above the Track Input field. Select your Vocoder plug-in from the list of available effects. Singing or speaking into the microphone will provide the Formant signal used to modulate the synthesizer sound. Place a recorded audio file or MIDI sequence onto the track, and your vocals should modulate the audio.