Getting the Best Results with the EVOC 20 PolySynth

Achieving a great “classic” vocoder effect requires both the analysis and synthesis signals to be of excellent quality, and it also requires care to be taken with the vocoder parameters. These sections outline a number of tips that will help you achieve the best possible results.

Editing the EVOC 20 PolySynth Analysis and Synthesis Signals

Avoiding Sonic Artifacts in the EVOC 20 PolySynth

FM Mode EVOC 20 PolySynth Oscillator Parameters

Editing the EVOC 20 PolySynth Analysis and Synthesis Signals

The following section outlines how you can edit the analysis and synthesis signals to achieve better speech intelligibility.

Compressing the EVOC 20 PolySynth Analysis Signal

The less the level changes, the better the intelligibility of the vocoder. You should therefore compress the analysis signal in most cases.

Enhancing High Frequency Energy

The vocoder, in a way, always generates the intersection point of the analysis and synthesis signals. If there’s no treble portion in the analysis signal, the resulting vocoder output will also lack treble. This is also the case when the synthesis signal features a lot of high frequency content. Because this is true of each frequency band, the vocoder demands a stable level in all frequency bands from both input signals in order to obtain the best results.

Due to the way human beings hear, the intelligibility of speech is highly dependent on the presence of high frequency content. To aid in keeping speech clear, it may be worthwhile using equalization to boost or cut particular frequencies in analysis signals before you process them with the vocoders.

If the analysis signal consists of vocals or speech, a simple shelving filter should be sufficient to boost the high-mid and treble range, which is so important for speech intelligibility.

If the synthesis signal lacks treble energy, it can be generated with a distortion effect. The Logic Studio Overdrive effect is perfect for this purpose.

Avoiding Sonic Artifacts in the EVOC 20 PolySynth

A common problem with vocoder sounds involves sudden signal interruptions (ripping, breaking sounds) and rapidly triggered noises during speech pauses.

Effective Use of the Envelope Parameters in the Analysis Section

The Release parameter defines the time it takes for a given synthesis frequency band to decrease in level if the signal level of the respective analysis band decreases abruptly. The sound is smoother when band levels decrease slowly. To achieve this smoother character, use higher Release values in the analysis section of the interface. Don’t go too far with this, however, as overly long release times result in a less distinct, washy sound.

Short Attack values are no problem. They may, in fact, even be desirable when a fast reaction to incoming signals is required.

Gating Background Noises in the Analysis Signal

If the analysis signal is compressed, as recommended, the level of breath, rumble, and background noise will rise. These unwanted signals can cause the vocoder bands to open unintentionally. To eliminate these artifacts, you can employ a noise gate before using compression and boosting the treble frequencies. If the analysis signal is gated appropriately, you may be able to reduce the (Analysis) Release value.

When you are gating speech and vocals with the Logic Studio Noise Gate plug-in, use Threshold to define the level above which the gate will open, and use Hysteresis to define a lower Threshold level, below which the gate will close. The Hysteresis value is relative to the Threshold level.

Figure. Noise Gate window.

The figure above shows a Threshold setting that is well-suited for speech compression. Unwanted triggering by low or high frequency noise is avoided by the dedicated sidechain filters of the Noise Gate plug-in. The Hold, Release, and Hysteresis values are suitable for most vocal and speech signals.

Enhancing Speech Intelligibility in the EVOC 20 PolySynth

Keep these points in mind when trying to achieve the best possible speech intelligibility:

The spectra of the analysis and synthesis signals should almost completely overlap. Coupling low male voices with synthesis signals in the treble range doesn’t work well.

The synthesis signal must be constantly sustained, without breaks. The incoming side chain signal should be played or sung legato, as breaks in the synthesis signal will stop the vocoder’s output. Alternatively, the Release parameter of the synthesis signal—not the Release time of the analysis section—can be set to a longer time. You can also achieve nice effects by using a reverberation signal as a synthesis signal. Note that the two latter methods can lead to harmonic overlaps.

Do not overdrive the vocoder. This can happen easily, and distortion will occur.

Enunciate your speech clearly if the recording is to be used as an analysis signal. Spoken words, with a relatively low pitch, work better than sung vocals—even if the creation of vocoder choirs is your goal! Pronounce consonants well, as exemplified in the rolled “R” of “We are the Robots,” by Kraftwerk, a classic vocoder track. This exaggerated pronunciation was specifically made to cater to the vocoder.

Feel free to do as you wish when setting the Formant parameters. Shifting, stretching, or compressing the formants has a surprisingly minimal effect on the intelligibility of speech. Even the number of frequency bands has a minimal impact on the quality of speech intelligibility.

The reason for this is our ability to intuitively differentiate the voices of children, women, and men, whose skulls and throats vary vastly by nature. Such physical differences cause variations in the formants that make up their voices. Our perception, or recognition, of speech is based on an analysis of the relationships between these formants. In the EVOC 20 plug-ins, these relationships are maintained even when extreme formant settings are used.