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BV512 VOCODER

210

Introduction

The BV512 is an advanced vocoder device with a variable number of filter

bands. It also has a unique 1024-point FFT vocoding mode (equivalent of 512-

band vocoding) for very precise and high quality vocoded speech. By connect-

ing the BV512 to two instrument devices, you can produce anything from vo-

coded speech, singing or drums to weird special effects.

Even if you have worked with a vocoder before, please read the following sec-

tion. Knowing the basic terms and processes will make it much easier to get

started with the BV512!

How does a vocoder work?

Carrier and modulator

A vocoder accepts two different input signals, a “carrier” and a “modulator”. It

analyzes the modulator signal, applies its frequency characteristics to the carrier

signal and outputs the resulting “modulated” carrier signal.

In the most typical case, the carrier signal is a string or pad sound and the mod-

ulator signal is speech or vocals - the result will be a talking or singing synth

sound. The modulator could also be drums or percussion (for rhythmically mod-

ulated sounds and effects) or any sound with changing frequency content.

Filter bands

Technically, a vocoder works in the following way: The modulator signal is di-

vided into a number of frequency bands by means of bandpass filters (called the

“modulator filters” or “analyzing filters”). The signal in each of these bands is

sent to a separate envelope follower (which continuously analyzes the level of

the signal). The carrier signal is sent through the same number of bandpass fil-

ters (the “carrier filters”), with the same frequency ranges as the filters for the

modulator signal. The gain of each bandpass filter is controlled by the level from

the corresponding envelope follower, and the filtered signals are combined and

sent to the vocoder’s output.

In this way, the carrier is filtered to have roughly the same frequency characteris-

tics as the modulator. If the modulator signal has a lot of energy in one of the fre-

quency bands, the gain of the corresponding filter band for the carrier signal will

be high as well, emphasizing those frequencies in the output signal. If there is no

signal at all within a frequency band in the modulator signal, the corresponding

band in the output signal will be silent (as the gain will be zero for that filter).

There are several factors determining the quality of the vocoder sound, but the

most important is the number of filter bands. The larger the number of filter

bands, the closer will the output signal follow the modulator’s frequency charac-

teristics. The BV512 offers 4, 8, 16 or 32-band vocoding.

✪ Even if a high number of bands will make the sound more precise

and intelligible, this isn’t always what’s desired! Vocoding with a

lower number of bands can give results that sound different, fit bet-

ter in a musical context, etc.

FFT vocoding

The BV512 has an additional FFT mode, in which the vocoding process isn’t

based on bandpass filters as described above. Instead, FFT (Fast Fourier Trans-

form) analysis and processing is used. This equals 512 “conventional” fre-

quency bands and results in a very precise and detailed vocoder sound. Note:

• The FFT mode is best suited for vocoding speech or vocals, giving crystal

clear and highly intelligible results. It is not so well suited for vocoding drums

and percussion, since the FFT process is inherently “slower” than the regu-

lar filtering and doesn’t respond as quickly to transients, and also there will

be a slight delay added to the signal (in the region of 20ms). A workaround

solution to this would be to move the modulator signal slightly ahead to com-

pensate for the delay.

• Where the conventional filter bands are distributed logarithmically (i.e. the

same number of filter bands per octave), the 512 bands in the FFT mode are

distributed linearly. This means a lot of the bands will be in the high fre-

quency range - this is one of the reasons for the clear sound but it is also

something to keep in mind when making settings for the vocoder in FFT

mode.

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Propellerhead Reason Spécifications Page 212

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