Vocoders are an instantly recognizable synthesizer sound, having been used in popular music since the 1960s. They allow you to ‘talk like a robot’, which while fun, is often not musically useful. However, used properly within a song they can work well, and add a (now rather retro) techno-effect to a piece of music. So how does it all work?
A vocoder needs two inputs to function properly. A ‘carrier’ wave, and a ‘modulator’ input. The carrier is the sound you want to vocode through, and the modulator is your voice. The modulator takes your voice, finds the fundamental frequencies (important bits) of it, and converts them into levels of amplitude on a series of band pass filters (this is why some vocoders have different numbers of bands) – in general, the more bands available the more understandable your speech will be. These band pass filter signals are then passed onto the carrier wave where your final sound is created.
It’s important to remember that anything can be used as the carrier and modulator in a vocoder – it’s just that most people want to use speech as the modulator to achieve the classic robotic vocoder effect. For creative results, try running other synthesizers or even other instruments through the modulator input and see what results you get. The carrier wave should usually be a nice big synthesizer sound, such as a long saw wave pad or string sound, but anything with considerable depth and length will achieve useful results.
Like sequencers and onboard effects, vocoders have now become standard features of computer-based hardware and software synthesizers, and inevitably some will say that nothing sounds quite like a dedicated analog vocoder. Due to the analog voltage-controlled circuitry from the ‘golden age’ of vocoders this is true, but like most things in life you get what you pay for. The legendary Roland VP-330 will set you back at least a grand or so for one in good condition, while the equally-as-old Korg VC-10 is much cheaper and much naffer too. For a modern alternative, look into the dedicated Roland VP-550 or the Korg R3, or even something as small and affordable as the MicroKorg – it come with a microphone for a reason.
Remember that not all ‘robot voice’ sounds come from a vocoder – a talk box is often used (in ‘Robot Rock’ by Daft Punk for example), and recent years have seen the use of speech synthesis (anybody remember Microsoft Sam?) and auto-tune software becoming increasingly popular.