In audio processing, we can say there are two main ways of adding effects to a chain in just about any DAW. The first one works by basically interrupting the dry audio signal of an audio or software instrument. You can do this by using the insert slot to add the effect. The dry signal is first fed to the processor. It then feeds the processed result to the next processor and so on. The name of this approach is “Serial processing”. This is because the chain of the effects runs in series from one to the next.
The second approach, called ”Parallel processing”, uses the auxiliary track. In such a way, the original audio signal remains untouched before this second audio feed (via the auxiliary) provides a second audio stream, complete with its effects. Mainly, the auxiliary buses are considered as places where spatial effects such as reverb and delay are added. Producers use them to blend the required amount of effect back in with the dry signal. This ensures a balance between the clarity of the original and the desired effect added at the required level.
To conclude, we can summarize parallel processing as a process where the second audio signal runs alongside with the original audio signal. It enhances or enlarges the outcome.
Reverb on Vocals
Consider first the use of reverb on a vocal track. The right use of reverb might embellish and support the vocal track. This is especially useful for vocals recorded in a highly absorbent room with a close microphone. It is not merely a matter of support. A touch of the just right amount of reverb (sometimes called a “magical dust”) can enable the vocal to soar into the mix, creating an emotional presence for the vocal fighting its way out of a pair of loudspeakers. The distinguishing characteristic for this type of processing is that it is added to the signal. But, it does not replace it. Consequently, we hear the original audio signal plus the output of the reverb.
One more useful way of parallel processing the audio signal is dynamic processing the specific frequency regions. By setting up the dynamic processor, such as a compressor, as a send (aux) effect, it is possible to equalize its return channel to tailor the processed tone. Therefore, where fast attack/fast release compressor working in the parallel configuration would typically emphasize the overall sustain, a low shelving cut to the return signal would direct that sustain more toward the upper-frequency regions. This would be a good way of compensation for the shorter sustain of higher piano notes, to give one example.
Additional Resources & Source Texts