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EVOC20 Vocoding

Logic Notes & Techniques By Stephen Bennett
Published March 2007

EVOC20 Vocoding

We attempt to demystify the EVOC20 vocoder plug-in and suggest ways in which it could be useful in all kinds of music, not just as a tool to produce silly noises!

The EVOC20 vocoder is one of Logic's most misunderstood and misused virtual instruments. It's ignored by many who think that vocoders can only produce a certain type of sound, and in some cases (if it's used at all) it's only implemented as a basic synthesizer. I think this is partly because it's not particularly 'plug and play' in its operation, and partly because it actually consists of three plug-ins rolled into one. EVOC20 PS (polyphonic synthesizer) is a combined vocoder and synthesizer virtual instrument, while EVOC20 TO (tracking oscillator) and EVOC20 FB (filter bank) are both effect plug-ins, and all require different approaches to achieve the best results.

Vocoders (derived from the term Voice Encoder) were originally developed by the telecommunications industry in the 1930s, but came into wider musical use in the '70s, with the advent of affordable synthesizers. Its use on numerous novelty records, as well as for the production of 'robot' voices in sci-fi television programmes, has created the impression that the only thing vocoders are useful for is sound effects.

A vocoder allows you to impose the envelope and tonal characteristics of an incoming audio signal (the modulator input) onto a synthesizer sound (the carrier input). Traditionally, the modulator input has been a voice, but you can use any type of audio, including a live microphone feed for real-time vocoding. Both modulator and carrier inputs pass into the vocoder through a bank of band-pass filters, and the more bands you use, the more the characteristic of the input audio is reflected in the output from the vocoder. In the case of EVOC20, you can implement up to 20 filter bands from the GUI. At this point, before we get too far, it's important to mention that Apple use their own terminology for EVOC20 's two input signals: carrier becomes 'synthesis', while modulator is referred to as 'analysis'. We will use Apple's terminology throughout this article to avoid confusion.

Using The EVOC20 PS Vocoder

Setting up the vocoder is a little more involved than using your typical virtual instrument, but Apple have generously supplied several presets to help you start vocoding right away. First of all, you need to plug in a microphone and route its input into Logic. Any microphone will do the trick, but the vocoder does benefit from clearly defined high-frequency information, so a condenser model is probably best. In our example, we'll use input track four, as this is the physical channel I have my microphone connected to on my audio interface. Make sure you have a signal coming into Logic, then select the No Output option in the pull-down menu on the track channel strip; you don't usually want to actually hear your unprocessed voice along with the vocoded signal. Next, insert EVOC20 onto an instrument track and select a side-chain input preset from the plug-in's preset menu; any of them will do at this stage. You'll notice that the signal parameter has been set to 'Voc', as this allows EVOC20 to respond to a side-chain input, which is where the analysis input will enter the EVOC20. The 'Syn' setting is the one to select if you're using EVOC20 as a 'normal' synthesizer. Click on the side-chain button on EVOC20 's GUI and choose the source track on which you have routed the microphone input (in this case, input four again; see the top screen opposite). Now select the instrument track that has EVOC20 inserted, and play the keyboard while speaking or singing into the microphone. You should hear the distinctive vocoder sound. You'll have to both play and speak to get the vocoder to work, and it's worth flicking through the side-chain presets to get a feel for the type of sounds that the instrument is capable of.

Somewhere Under A Menu... Two More EVOC s

Starting this month, we're exploring Logic 's hidden gems, and documenting some of the more exotic tools it has in its kit. To kick off this mini-series, where better to start than by looking at some of the secrets inside EVOC20?

EVOC20 Vocoding As mentioned in the main article, EVOC20 has two companion plug-ins that you'll find in the filter section of Logic 's effects pull-down menu. EVOC20 TO could be considered a more basic version of the EVOC20 PS vocoder; it works directly on recorded monophonic audio and is inserted onto an audio track or used with a side-chain input to provide the analysis input. The synthesizer section is a basic two-oscillator FM type, but there's no filter (though you could use one of Logic 's EQ plug-ins with automation to modulate the frequency and cutoff values, to create filter sweeps). The Pitch Quantise section (below left) is used to keep the synthesis section in tune with the input audio; the plug-in uses this instead of a MIDI input, and to make this work best, you need to select the root and scale of the incoming audio from the drop-down menu. The LFO can be synchronised to Logic 's metronome, making the plug-in particularly useful on rhythmic material. Like the vocoder itself, this plug-in can add interest to tired drum patterns or create wacky vocal effects.

EVOC20 VocodingEVOC20 FB is a plug-in that can use two banks of up to 20 bands of band-pass filters. These are a little more complex than the ones on the vocoder, in that you can not only choose the number of bands you want to use and the slope of the highest and lowest frequency filters, but you can change the level of individual bands by dragging the little arrows. Also, you can draw filter curves directly onto the band display with the mouse. Two filter banks are available, and the boost controls determine the relative level of each bank's effect (above right). As you can see in the screenshot, the left-hand LFO modulates the Formant Shift parameter and the right-hand one controls the fade between the two filter banks. These controls, along with Logic 's automation, allow you to change the filter effects over the duration of a song.

The 'm/s' setting processes the audio in mono, while the 's/s' setting assigns a separate filter bank to each side of the stereo signal, for some spectacular stereo effects. The actual 'sound' of EVOC20 FB is very different to Logic 's other filters as the 20 bands allow for very precise tonal shaping. The Fade LFO makes it easy to create complex filter shifts: its particular forté is emulating those multi-stage rackmount phasers I used to lust after in the '70s. The LFO's sample and hold waveform can be used to generate pseudo-random phasing effects and, along with the overdrive control, allows you to simulate tape-based phasing and flanging.

Both EVOC20 's plug-in effects are quite complex to use, but reward the effort by producing sounds unobtainable elsewhere within Logic.


Quite a lot of skill is needed to vocalise while playing EVOC20 from a MIDI keyboard and it's often easier to record the MIDI part first with one of the plug-in's synthesizer presets, then add the audio input later and swap to a side-chain preset. However, if you persevere, live vocoding can be very rewarding, as the interplay between the vocal input and the keyboard can be used to produce a very expressive performance.

Routing the analysis (modulator) signal into EVOC20 PS from input four on the audio interface. Note that the side-chain on EVOC20 is set to 'input 4', and that there is no output from the audio track in the Environment.Routing the analysis (modulator) signal into EVOC20 PS from input four on the audio interface. Note that the side-chain on EVOC20 is set to 'input 4', and that there is no output from the audio track in the Environment.Of course, you won't want to have to do this manually every time you play back a track, so you'll probably want to record your vocal performance as an audio file and route this back through EVOC20. You'll need to select an audio track and choose the same audio interface input as for the input track. Now, put the audio track into into record, select it, hold down the shift key and also select the vocoder instrument track so both your MIDI and audio inputs will be recorded together.

It's pretty obvious that any audio recording can be used as an input to the vocoder but generally, audio with a good high-harmonic content will produce the most musical results. But remember; it's not the pitch of the recording that's important, as that is derived from the MIDI input into EVOC20, but the dynamics and harmonic characteristics. EVOC20 responds especially well to rhythmic input and routing a hi-hat through it, for example, can produce some unusual and dynamic-sounding patterns. I find that gently scraping or tapping a dynamic microphone or recording your table-top percussion can create some nice effects. You can throw any type of recording at EVOC20, even whole tracks, but the results can be rather unpredictable, so it pays to experiment.

Send Your Mix All Over The House!

EVOC20 Vocoding Like most of you, I suspect, I like to check mixes on as many systems as possible before I'm happy with the final result. In the studio, I have my main monitors, a ghetto-blaster-type device and a pair of iPod headphones, so I can have a listen to tracks as I'm going along. However, most of my serious listening is done on my living room hi-fi and this is the system I know the best. In the past, listening through this system has meant burning a CDR and traipsing down three flights of stairs just to find out that the bassoon is louder than the drums in the mix. Recently, I've been using a combination of Apple's Airport Express and Rogue Amoeba's Airfoil ( to send audio directly from Logic to my stereo. It's not quite 'CD quality' but it's good enough for checking a mix without endless burning of CDRs. You need the Airfoil software, as Apple's own only allows iTunes audio to be sent via wireless, whereas the Rogue Amoeba software enables any audio software on the Mac to use the Express. An added bonus is that the Express acts as a booster for my pathetically underpowered Airport Extreme base station. Two solutions for the price of one!

Better Vocoder Effect Quality

The number of filter bands can be changed by clicking and scrolling up or down in the 'Bands' box, situated to the left of the centre screen. You can alter the high- and low-frequency roll-off points using the slider above the display.The number of filter bands can be changed by clicking and scrolling up or down in the 'Bands' box, situated to the left of the centre screen. You can alter the high- and low-frequency roll-off points using the slider above the display.If you're going for that classic vocalised sound, any extraneous noises or changes in dynamics or tone will affect the quality of the results. You can minimise these effects by using a compressor and a noise gate on the audio track used for the side-chain input. It may also help to insert an EQ or exciter to increase the high-frequency harmonic content of the audio. Obviously, the more bands you use within EVOC20, the more the vocoder effect will resemble the audio used in the side chain input. This may not always be what you are after, so you can try reducing the number of bands using the input box to the left of the Bands display (below). As the vocoder was originally designed to be used in spoken telecommunications, the analysis section of EVOC20 has several features related to vocal processing. When we speak, vowels and consonants are produced in different ways, the former tending to be more tonal in nature and the latter having a higher noise component. Vowels are said to be 'voiced' while consonants are said to be 'unvoiced'. EVOC20 's U/V detector section can determine the voiced and unvoiced composition of the input audio. The voiced part of the signal always passes directly to the synthesis input without any further processing. What happens to any unvoiced components that are detected depends on the setting of the U/V pull-down menu and its associated controls. If this is set to off, all unvoiced sounds are sent to the synthesis input, along with the voiced component. Using the other menu items, you can replace the unvoiced signal with synthetic noise or a blend of the original signal and noise. You can control the level of this noise and the sensitivity of the unvoiced signal detection using the corresponding knobs (below). A high level of unvoiced input reduces the intelligibility of the output signal, so it's a good idea to experiment with these settings to get the effect you are after.

EVOC20's U/V detector section can be set to a variety of modes that will react differently to voiced (vowel) and unvoiced (consonant) sounds. Voiced sounds tend to be tonal and melodic, whereas unvoiced sounds have a much higher amount of noise, and setting the U/V Detection level to 'full' when in Noise mode can create some very interesting results. EVOC20's U/V detector section can be set to a variety of modes that will react differently to voiced (vowel) and unvoiced (consonant) sounds. Voiced sounds tend to be tonal and melodic, whereas unvoiced sounds have a much higher amount of noise, and setting the U/V Detection level to 'full' when in Noise mode can create some very interesting results. Reducing the Sensitivity control lowers the proportion of the unvoiced signal sent to the synthesis input and thus increases the amount of the input audio's character in the final audio output.

Once you have your basic vocoder sound, you can use EVOC20 's synthesizer section to further modify the timbre. It's a fairly conventional variable-polyphony digital synthesizer with filter, LFO, glide and ensemble parameters. There's also a simple FM synthesis section, but it'd probably make more sense to use the EFM1 if you want to use this type of sound as a conventional synthesizer. It's useful to have all the same.

EVOC20 can happily work as a stand-alone polyphonic synthesizer in the usual way, but its tonal characteristics are covered rather better by Logic 's other included instruments. However, don't let that stop you having a listen to the synthesizer presets, as you may find a few gems! As you might expect, EVOC20 also has some more specialised functions for shaping the final audio output. The bar above the Bands display limits the upper and lower frequencies of the analysis and synthesis inputs, and you can also change the filters at either end of the frequency range to high- and low-pass versions. Narrowing the frequency range makes the vocoded effect sound more 'closed in', which is useful for those lo-fi sounds, and you can drag the bar around to define the position of the frequency range over which the band applies. There's a side-chain analysis section where you can adjust the attack and release of the audio coming into the vocoder, which effectively defines how long EVOC20 takes to respond to the analysis input. The Freeze button captures the current side-chain input and can be employed to good effect if you use Logic 's automation to turn it on and off as a song plays.

Beneath the Bands display are three controls you can use to modify the relationship between the analysis and synthesis inputs of the vocoder. The Formant Stretch knob compresses or expands the range of the synthesis band with respect to the analysis band. At low values, the range is squished up, producing a thinner sound. The Formant Shift button moves the synthesis band up and down the frequency range and changes the apparent pitch of the effect. The Resonance control emphasises the mid frequency of each band, making the sound brighter at higher values. It's a different effect to the filter's resonance control, as the sound changes when you adjust the Formant Stretch and Shift knobs. Again, these controls are all extremely useful when you're using automation, as they allow you to make subtle or extreme changes to the vocoded sound contimuously throughout a song. Alternately, you can use the Shift LFO function to modulate the Formant Shift; it's especially good for special effects or spicing up a vocoded drone.


The EVOC20 vocoder might surprise you with its flexibility. By using real audio or live performances to shape the sound, it can add a warm, organic and human feel to a performance, especially when you are using the voice as the analysis input. Like many useful tools, you'll need to put in some work with it before you obtain the best results, but its unique sounds could be just what you need to transform an ordinary recording into something unusual. 

Further Listening

Examples of a vocoder in full flight can be heard on Stanley Kubrick's A Clockwork Orange soundtrack and Wendy Carlos' collaborations with Bob Moog. Laurie Anderson also used one as the centrepiece of her hit single 'O Superman'. Artists including Air, Daft Punk and Muse use the vocoder extensively and ELO's 'Mr Blue Skies' also famously featured the instrument. Apparently, an early vocoder was used to great effect on Sparky's Magic Piano, a series of children's audio stories from the 1940s, but I've yet to meet anyone who's heard it.

Published March 2007

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