Understanding Audio Signal Flow: Keyboards and Synthesizers

( this is the 2nd Part of the "Understanding Signal Flow" series. You can find the 1st Part at Understanding Audio Signal Flow: Guitar & Bass )

By Peter D.KayContributing Author

Article photo - Understanding Audio Signal Flow: Keyboards and Synthesizers

The source

Article photo - Understanding Audio Signal Flow: Keyboards and Synthesizers Depending on your instrument the source will either be one (usually an assortment of) oscillator(s) or a sampler. Sample based instruments are easy to comprehend since the sample engine basically plays a pre recorded sound every time you press a key. Oscillators on the other hand produce their sound by oscillating. Think of them like a guitar string. They go up and down really fast. The faster they move the higher the pitch produced. Oscillators sport different waveforms like sine waves , sawtooth and square waves. Say an oscillator completes 440 repetitions of its movement per second. That frequency (440Hz) will produce the pitch of A. If we select any waveform other than the sine wave , harmonic tones will be added to our result. They are multiplications of the root tone (440hz in this example) and contribute to the timbre of the sound.

[Sidenote: Harmonic content is present in the real world and is used by our brains to distinguish say a guitar from a ukelele]


Amplitude Envelope

Our sound wave already produces a note but we can add more attributes to it. Using an amplitude envelope we can turn our wave into a pluck like sound or make it fade in like a string section. The amplitude envelope controls the current volume of the played note. It has controls for attack, decay, sustain and release.( Hence the name of A.D.S.R. envelope)

[Sidenote: In modular systems (not exclusively) ADSR envelopes can be assigned to control all sorts of parameters like filters (more on the next section) or effect sends and even waveform selection.]


There is always a filter

Next up the signal passes through a frequency filter. Fancy wordplay for a simplified eq section. We have selection over filter type, cutoff frequency and resonance. Low pass filters let the frequencies below the cutoff frequency through and reduce everything above. A high pass filter will do exactly the opposite and a band pass filter will let through a group of frequencies around the cutoff. Everytime we try to affect the frequency spectrum in any way the nature of our electronics produces a counter action. If we low pass our spectrum at the cutoff frequency of 440hz (that A note again) the circuit will counteract and create a small boost at 440hz then filter out everything above. That boosted counteraction is controlled by the resonance knob. If we boost the resonance high enough we will hear the note A ringing out.


On to the effect section

Article photo - Understanding Audio Signal Flow: Keyboards and Synthesizers Like a guitarist , keyboardists do use effects in the form of rack gear, pedal boxes or even software plug ins. Delays and reverbs fit really well with synths but everything goes as long as it does the job.

[Sidenote: Keep in mind that the filter section of your synth is basically an eq so if you cut all your bass frequencies from the filter section it would be futile to try and boost the bass through an eq stompbox or an equalizer in the mix.] 


On to the mixing desk

Out of your pedal board and into the mixing desk but your signal is currently unbalanced. Meaning that it's susceptible to interference from power lines or phone antennae resulting in added noise. To prevent that, we use D.I.(direct injection) boxes. These relatively cheap (depending on what you reach for) Article photo - Understanding Audio Signal Flow: Keyboards and Synthesizersboxes will turn our signal into a balanced version. Now our signal can travel long distances and will be completely untouched by any interference.The mixing desk can be substituted by an audio interface and these pieces of gear can be equipped with high-z inputs. You can use high-z inputs instead of a DI box but you should take note of the cable run going into that input. Your signal is unbalanced until it gets into that input!

[Sidenote: If there is no alternative and you are forced to carry your unbalanced signal over a long distance you can at least eliminate the low hum noise by making sure that your audio cable never travels parallel to a power cable. They have to cross each other at a 90° angle!]



Don't forget less is more. Depending on your arrangement your sound will be a part of a whole so take note of the other elements in the mix prior to committing a recording.

Consider a cleaner approach. If your effects are complementing your sound and are not a core part of it,  you should prefer to deliver a dry and wet section to the recording. Sending two signals to the mixing desk , one being your dry signal and one being your effects chain signal. This way you have control over the mix of the two during the mix.

About Peter D.Kay

Peter D.kay is a music producer operating in the blooming underground scene of Thessaloniki (Greece). He has worked with numerous artists (Baby shower, Recycle bitch, Seyez Clan) to produce and contribute to their dreams of successful releases. He has been active as a session guitarist in the past and also as a recording engineer. He is currently staying busy producing local artists and diving deeper into the audio visual realm of show making. His aspiration is the creation of a collaborative project including live elements of audio and visual aspects resulting in a truly unique experience.

Contact Peter D.Kay at peter.d.kay@musicngear.com

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