A vocoder 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 also lacks treble. This is also the case when the synthesis signal has 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 to obtain the best results.
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 tips will help you achieve the best possible results.
The less the level changes, the better the intelligibility of the vocoder. You should therefore compress the analysis signal in most cases.
Due to the way human hearing works, the intelligibility of speech is highly dependent on the presence of high-frequency content. To aid in keeping speech clear, consider using equalization to boost or cut particular frequencies in analysis signals before you process them.
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 important for speech intelligibility.
If the synthesis signal lacks treble energy, it can be generated with a distortion effect.