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170 Chapter 14 EVOC 20 PolySynth
Avoiding Sonic Artifacts
A common problem with vocoder sounds are sudden signal interruptions (ripping,
breaking sounds) and rapidly triggered noises, during speech pauses.
Release Parameter in the Analysis Section
The Release parameter defines the speed that a given synthesis frequency band can
decrease in level, if the signal level of the respective analysis band decreases abruptly.
The sound is smoother when the band levels decrease slowly. To achieve this smoother
character, use higher Release values in the analysis section of the interface. Longer
release times result in a washy sound.
Short Attack values are no problem. They may, in fact, even be desirable when a fast
reaction to impulse signals by the vocoder is required.
Gating Background Noises in the Analysis Signal
If the analysis signal is compressed, as recommended, the level of breath, rumble, and
background noises will rise. These background noises can cause the vocoder bands to
open, but this is normally not intended. In order to eliminate these noises, it’s therefore
a good idea to employ a noise gate before compression and treble boosting. If the
analysis signal is gated appropriately, you may find that you want to reduce the
Analysis Release value.
When gating speech and vocals, the Hysteresis parameter is important. Threshold
defines the level, above which the gate will open. Hysteresis defines a lower Threshold
level, under which the gate will close. The value is relative to the Threshold level.
The graphic above shows a Threshold setting, which is well-adapted to compressed
speech. Unwanted triggering by low or high frequency noise is avoided by the Noise
Gates’ dedicated sidechain filters. The Hold, Release, and Hysteresis values shown are
typical level envelopes, suitable for most vocal and speech signals.