How to Make Your Sound Sing with Vocoders
Pages: 1, 2, 3, 4, 5

Strike Up the Band

A bandpass filter is a device that, when fed a signal, allows only the frequencies within a narrow band to pass. The incoming signal may have partials at both higher and lower frequencies, but those partials are filtered out. For instance, if the center frequency of the filter is 1,000Hz and its bandwidth is 200Hz, it would allow partials between 900Hz and 1,100Hz to pass, while filtering out partials below 900Hz and above 1,100Hz.

That is a slight oversimplification, because the upper and lower boundaries of the pass-band are not rigid (especially with analog filters). To continue the previous example, if the lower boundary of the pass-band is 900Hz, it's not the case that a partial at 901Hz will pass through while one at 899Hz will be stopped dead in its tracks. Rather, the amplitudes of partials that lie beyond the upper and lower boundaries will be reduced more depending on how far beyond the boundaries they lie, as shown in Figure 2. But this spillage is not especially significant to the design of a vocoder. For practical purposes, we can talk about the bandpass filter as if it were a frequency window with sharp edges, even though the edges are in fact fuzzy.

Fig. 2. This bandpass filter, constructed in Izotope Ozone, is centered at 2kHz. Because it's in "analog" mode, the sides of the curve drop off gradually.

Fig. 3. A simple drum loop before and after being processed with the bandpass filter in Fig. 2. Click to hear the sound (100KB MOV).

The bandpass filters in a vocoder split the incoming speech signal into a number of separate signals, each of which contains only the sound energy within the narrow pass-band of that particular filter. For instance, each pass-band might be an octave wide. In this case, the filters would have something like the following lower and upper boundaries:

As you can see, that's a 10-band design. If the vocoder has 16 bands, each will be correspondingly narrower. Typical vocoder designs have 8, 16, 24, or 32 bands. With fewer than 8 bands, the speech input won't be detected accurately enough for us to understand the output. Conversely, using too many bands can reduce the personality of a vocoder by glossing over its characteristic distortion.

Follow the Bouncing Ball

An envelope follower is a device that senses the amplitude of an audio signal and outputs a control signal (also known as an envelope) whose level corresponds to the input's amplitude. In other words, if the incoming signal is loud, the control signal output has a high level. If the incoming audio is soft, the control signal has a low level. When there's no incoming audio, the control signal drops to zero.

Envelope followers are used for various effects. A compressor uses a type of envelope follower, as does an auto-wah effect. Most envelope followers have attack and release controls. The attack parameter governs how quickly the envelope rises when the incoming audio increases in amplitude. The release parameter (sometimes called decay) governs how quickly the envelope falls back toward zero when the amplitude of the incoming audio drops.

In a compressor, a long release time may give better-sounding results. In a vocoder, however, both the attack and the release are normally kept fairly short. That ensures that the vocoder will be able to track the incoming speech signal accurately. In the Reason examples, you can try experimenting with the Vocoder module's Attack and Decay knobs. Increasing the Decay causes the sound to smear, while setting it too short makes the sound rather grainy.

In a vocoder (again, refer to Figure 1), each bandpass filter in the speech path feeds through a dedicated envelope follower. The result is this: when the speech signal has partials within a given frequency band, the output of that particular envelope follower rises. When there are weak partials or none at all within that band, the output of that particular envelope generator falls.

If we were to look at the outputs of the vocoder's envelope followers over time, perhaps displaying them as squiggles on the computer screen, we'd have a fairly accurate picture of the frequency content of the speech signal over the course of time. (Indeed, vocoders were initially developed to compress speech for telephony.)

The vocoder in Propellerhead Reason offers up to 512 bands, but for many applications, using fewer bands sounds better.