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Signal Processing With Sequencers


You can use your computer sequencer for lots more than sequencing — how about augmenting your effects processors with some unique sequenced effects? Craig Anderton shows you how.

In today's multitimbral synth world, it seems there are never quite enough synth outputs — let alone signal processors — for the average mix. However, your sequencer can provide many signal processing effects by tickling your sound generators in particular ways. There are a few trade-offs; some of the following techniques involve synergistic operation between sequencer and synthesizer, and some techniques use up more voices than non-processed parts. But there are advantages as well: sequencers can give effects you can't get with outboard units, so let's start with some of those.

SYNCHRO-SONIC TREMOLO



'Synchro-sonic' is a term that describes the process of imparting rhythmic characteristics to a sound that normally doesn't have such characteristics. A simple example is using a noise gate triggered by a kick drum to gate a bass sound on and off, thus 'syncing' it to the kick. Tremolo has fallen out of favor in recent years compared to the halcyon days of tremoloed surf music and Bo Diddley beats, but this effect can help make a synth sound less static, especially if the tremolo effect is synchronised to a tune's rhythm. Unlike conventional tremolo boxes, sequencer-generated tremolo is easy to synchronise to the beat. To add tremolo, simply 'draw in' the desired modulation waveform and apply it as a continuous controller that affects a signal's master volume (Figure 1, measure 1). Usually controller 7 affects this, although you may want to dedicate a different controller to tremolo (such as controller 92, standardised as 'tremolo depth') and route it to a DCA that affects the signal path so that you can continue to use controller 7 for overall volume.

Figure 1.


If you come up with any particularly useful waveforms, save them in a separate sequence that serves as a 'waveform library.' When you need that waveform again, just copy it and merge the desired waveform into the track to be processed. Ramps, for example, work very well. A few other tips: to fade in or fade out tremolo effects, use the sequencer's 'change smoothly by percentage' command (Figure 1 — measure 2 fades into measure 3).

Define the fade-in start point (zero volume) and end point, then smooth from 1% to 100%. Do the reverse for fade-outs. To offset the tremolo waveform, add a constant to all values (Figure 1, measure 4, shows the same waveform as measure 1 with 40 subtracted). Finally, note that you can often thin the controller stream to an almost absurd degree (see Figure 1, measure 5) without noticing much difference — the human ear is far more sensitive to pitch variations than level variations. So, tremolo effects need not take up too much sequence memory.

SYNCHRO-SONIC VIBRATO



You can just use the mod wheel to add vibrato, but it probably won't sync to the music. However, you can use pitch bend messages to control vibrato in a similar way to how we added tremolo. Since this signal will be centered around zero, changing smoothly by a percentage brings in the vibrato effect without offsetting the pitch centre (Figure 2 shows an 'eighth-note' waveform fading in from measure 2, beat 1 to measure 4, beat 1; the fade-out starts at measure 5). Vibrato with a period of an eighth-note, eighth-note triplet, or 16th note seems to work the best. Waveform libraries are also handy for vibrato, but find a MIDI guitarist who can give you some good finger vibrato patterns: They're just what the doctor ordered for synthesized lead parts. Note that you will not be able to thin the data as much as tremolo without the vibrato sounding 'grainy.'

Figure 2.


DRUM PANNING



Here's a tip for those who like to use samplers as drum sound generators. Assign the same sound to two different notes, but set different pan positions — for example, centre and left of centre. At the sequencer, use a 'change filter' or 'logical edit' filter to cut, for example, just the 2nd and 4th beats of every measure driving one of the drum sounds. Paste this into another track, then transpose it to the pitch of the other drum sound.

As the part plays, the percussion sound will bounce over to one side periodically. Musically, I find this works best on short percussion sounds, such as claves, tambourine hits, claps, etc. Moving primary drums like the kick, snare, or toms, can be disorientating.

CHORUSING



If a keyboard has an on-board chorus, that's convenient — but sequencer-created chorusing allows interesting rhythmic possibilities. There are several ways to create chorusing. First, decide whether to use pitch bend messages, or a continuous controller to modulate pitch. If you modulate with pitch bend, you will not be able to do pitch bending and add chorusing. Using continuous controllers, which I recommend, keeps pitch bending independent from the chorus effect. Sequencer-driven chorusing works best with two-oscillator-per-voice synths. For the most obvious chorus effect, program the two oscillators for the same waveform and pitch. Set one oscillator to respond negatively to the controller, and the other oscillator to respond positively (in other words, upon receiving a controller message the two oscillators shift pitch in different directions). This maintains a proper pitch centre, instead of having one oscillator simply go flat or sharp compared to concert pitch. Figure 3 shows one of my favorite chorusing curves. It creates a sudden thickening at the beginning of a measure that fades out over the measure, with the next measure devoid of chorusing. Figure 4 is a pattern that works well for dance music. This turns chorusing on and off rapidly, four times per measure. Note that the 'low' chorusing value is set to a non-zero number. When set to zero, there is no pitch difference between the oscillators and they sound static. Even a low controller value, like 3 or 4, adds a bit of animation. The amount of modulation need not be large. Since it's easiest to draw sequenced curves with fairly large values, I usually trim modulation depth way back at the synth oscillators themselves.

Figure 3.


Figure 4.


PANNING/DELAY EFFECTS



The following technique uses delay to spread a signal across the stereo field, and is particularly effective for thickening brass and string pad parts. Copy the track to be thickened, and assign it to a different MIDI channel compared to the primary track. Shift the copied track 20-40 ms (that's approximately 20-40 clocks at 120 BPM, if the sequencer resolution is 480 ppqn). Assign the same synth program to both channels, with one panned left and the other panned right. The overall sound will be much fuller.

NASTY NOISES



Figure 5 looks a little strange - it shows a controller signal where on one clock, the signal is full on, and on the next clock, it's full off. Feed this to an oscillator, and repeat the pattern for as long as you want the nasty noises. This started as an attempt to get ring modulation, but what actually happens depends on a variety of factors (the synth itself, MIDI bandwidth, etc.). Usually it produces an effect that sounds as if the synth has some intermittent hardware problem. Who says digital has to sound perfect? The truth of the matter is that if properly abused, digital can produce ugliness that analogue circuits can only dream about.

Figure 5.


And that concludes our foray into signal processing via sequencer. The main advantage is the ability to add rhythmic effects; this makes your overall mix bigger and more interesting, even with a limited number of sounds.

DRUM FLANGING

I've never been a big fan of the 'randomise/humanise' functions found on sequencers, at least for their intended application. But randomisation can work great for flanging drum parts, even though this uses up twice as many drum voices. Copy the drum part (or just the drums you want flanged) to a separate track, then randomise the copy track within about a 15ms time window. Assign both tracks to the same channel, and voila — instant flanged drums. This works for keyboard parts too.


MIDI ECHO VARIATIONS

Most people are familiar with the procedure of copying a track, pasting it to another track, lowering the velocity of the copied track, and shifting its timing to provide echo effects. However, there are some ways to improve this basic approach.

Program the synth so that lower velocities close down the filter somewhat. This prevents the echo from 'stepping on' the straight signal.

Unless you run out of tracks, never merge the echoed track with the original track. If you decide you don't like the echo effect, you'll have a hard time removing or editing the notes.

Transposing the echo track (octaves work well) is a great effect. This is particularly useful when creating octave-jumping, eurodisco-style bass parts.

Try changing the duration of the echoed notes by a percentage, such as 50% or 150% of the existing value. This gives an entirely different feel.

Although it takes more work, polyrhythmic echoes can really liven up a tune. Try having one track provide an echo one beat after the straight sound, with a second track adding an echo one eighth note after the first echo. Throw in triplets every now and then, too — the variations provide an effect you'll never hear from a standard digital delay.

Stereo echo effects are lots of fun, and also create a more wide open mix compared to having the straight and echoed sounds located in the same place in the stereo field. Some synthesizers make this easy by letting you create a 'combi' patch where the same program can be assigned to different MIDI channels and have different panning. With other synthesizers, you'll need to copy the program being echoed to another program location, pan each program individually, and assign these to two different MIDI channels in a combi setup. Three echoed parts — centre, right, and left — is even cooler. Synths where the programs need to be assigned to different channels can eat up MIDI channels pretty fast. I sometimes offload the echo parts to a keyboard's on-board sequencer, synced to the main (computer) sequencer.


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Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Nov 1993

Feature by Craig Anderton

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