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Advanced Music Synthesis


Sequencer Anatomy

Fig 1. Analogue sequencer block diagram.

Not so long ago, sequencers came in only one form - analogue sequencers. These were units which allowed the synthesist to be able to have a series of notes repeated as a melodic/rhythmic accompaniment to a piece of music. Analogue sequencers could only hold a limited number of notes (in the region of 8 to 24) and were somewhat laborious to set up, but that didn't prevent them from becoming the trademark of artists such as Tangerine Dream, Klaus Schulze and many others. Their usage became something of a cliche though and as a result it was easy for anyone with a sequencer, a string synth and an echo unit to sound quite impressive - hence all the Euro-synth clones that are so abundant nowadays.

Fig 2. Digital sequencer block diagram.

Modern technology brought us the digital sequencer which proved to be a much more versatile device. Instead of having to tune each note individually to achieve the required sequence as with analogue sequencers, all you had to do was play the notes into the sequencer off the synth's keyboard. They allowed you to record in 'real-time', that is, with every note played back exactly as you played it, warts and all. Alternatively you could program the sequencer in 'step' or pulse-time' which played back the notes with metronomic precision. The other main advantage they had over the analogue variety was that they could store far more in the way of notes (as many as 600 or more) but this facility was not really taken advantage of. Instead, most synth players would program a sequence of say 8 or 16 notes and then use them in much the same way as an analogue sequencer.

Digital sequencers proved to be very useful as a compositional aid as one could simply plonk in a string of notes and have them replayed almost instantly. Notes played at random could then be used as a backing to a piece.

You may have noticed that I have been talking in the past tense about these devices. That's because although they are still readily available today, they have taken a back seat to the new generation of microprocessor based sequencers such as the Bassline and MicroComposers. These devices offer much more in the way of sequencing power as you can program whole pieces of music into them instead of having one simple sequence repeating over and over ad nauseum. As a result, synthesists have had to re-evaluate their approach somewhat as they now have to sit down and think their basslines (or whatever) through far more, rather than using the improvisational approach that analogue and digital sequencers seemed to enforce. Whether or not this is a good thing is debatable and outside the scope of this article.

The main purpose of a sequencer is to provide varying control voltages (CVs) to control pitch automatically and also to provide a gate/trigger pulse to 'fire' the envelope generators. Each device does this in various ways. On an analogue sequencer the CVs are derived from a row of potentiometers which are individually tuned to give the required pitches. A clock in the sequencer runs systematically through the sequencer and is also available on a separate output in order to trigger the envelope generators. Naturally, the speed of this clock is variable and also, in some instances, voltage controllable so that you could, for example, use another sequencer or, if the sequencer has one, use the second bank of controls (channel 2) to vary the rhythmic nature of the sequence.

A digital sequencer, on the other hand, uses digital circuitry to store CV and gate information which can then be used to control the synthesizer. It, too, has a clock but if it's programmed in real-time the gate information is derived from the digital circuitry so that notes of varying length can be programmed. In step-time playback the internal clock triggers the envelope generators for a more staccatto-type sequence.

Fig 3. Microprocessor-based sequencer block diagram.

Microprocessor-based sequencers are a different kettle of fish, however. Whilst they can be programmed off an external keyboard, it is more usual to program them using either a keypad or a self-contained mini-keyboard. The Roland MC4 Micro-Composer uses the keypad type of data entry whilst the Bassline and MC202 have their own mini keyboard. These machines are very powerful compositional tools indeed and music of incredible complexity can be programmed into them. Their note storage is phenomenal (on the MC4, it's in the region of 11,000. while the MC202 can store 2,600 and the Bassline can handle 4,600 notes if the 'track write' facility is employed). The MC4 and MC202 have more than one channel so that more than one synthesizer can be controlled if required. Because of their extra complexity they can be a bit tedious to program but once it's done you're able to record it in one take, note perfect. It's best to write out the music on manuscript before embarking on programming as this makes things easier, though programming on a 'hit-or-miss' basis can yield some interesting results.

But in spite of the wonders of micro-chip technology, there is still a place for the humble analogue and digital sequencer. It is possible to override their internal clocks so that the sequencer can be sync'd to any other device that produces a steady stream of pulses. This can be a number of things; a square wave from a low frequency oscillator, the clock output of a drum machine, a sample and hold unit, a click track off tape or maybe even a suitably boosted audio signal such as a hi-hat. By connecting up one or more sequencers to a master clock, many interesting and complex patterns can be created in this way. By keying in various sequencers manually so that the sequences overlap, it is possible to create an hypnotic style of music à la Steve Reich, so that, although the sequences may only be 8 notes each, the effect will be of long, non-repeating passages.

Fig 4. Using a sequencer to step through and control VCF cut-off frequency manually.

The pulses required by any sequencer are usually only a simple square wave. When provided by a clock they are just a stream of steady square wave pulses which step through the sequence metronomically. As many of you may know, the trigger/gate pulse from a synthesizer keyboard is simply a one-shot square wave that occurs every time a note is played. This pulse is used to trigger the EGs. Now if we override the internal clock with the keyboard gate/trigger pulse the sequencer will only advance to the next step upon the arrival of a new pulse (or, in other words, when another note is played). If we program a series of four different CVs into the sequencer and then connect the sequencer CV output to the control input of the VCF, every time a note is played the sequencer will advance one step and open and close the filter cutoff frequency. In this way we get a different tone colour for each note. Using this technique it is possible to create the comic singing voice so beloved of Tomita in addition to many other synth and vocal effects.

Putting a lag time integrator between the sequencer and the filter control input will smooth out the jumps in voltages. There is no reason why the sequencer output couldn't be applied to any voltage controlled unit such as a VCA, flanger, phase shifter or whatever.

Fig 5. Using a sequencer to provide a harmony.

We can also connect the sequencer output to another synthesizer. By programming a series of harmony notes into the sequencer, every time you play a note on Synth A, Synth B will play the harmony note. This technique is more suited to digital sequencers with their greater note storage capacity. The results can be usefully employed when recording if you only have a limited number of tracks to play with, as you can put the two synth lines down (with individual EQ, effects, etc.) on one track without having to employ both your hands playing two keyboards.

Another use for sequencers is as a low frequency oscillator (LFO). By programming in a series of CVs and putting them through a lag time integrator you can create an LFO whose waveforms are totally variable. This can be useful when trying to synthesize acoustic instruments, as the vibrato supplied by an erratic human musician is never as symmetrically perfect as the sine or triangle wave outputs of an LFO. A sequencer can therefore be usefully be employed to introduce a more irregular modulation effect.

Fig 6a. Sequencer output being used as an LFO.
Fig 6b. 'One-shot' sequencer output used as a multiple-stage envelope generator. (Dotted line represents smoothed output via a lag time integrator.)

Fig 7. Patch for using sequencer as a multiple-stage envelope generator. Sequencer must be switched to give 'one-shot' play of sequence.

Some sequencers can also be used as multiple envelope generators as they often have the facility to start the sequence remotely with a trigger pulse. This is normally used with a footswitch but you could use the gate/trigger output from your synth to start the sequence every time you play a note. By setting the sequencer clock rate fairly high, switching the sequencer so that it only goes through its cycle once instead of continuously looping and then applying the CV output of the sequencer to the VCA and/or VCF (again via a lag time integrator), you can obtain some very intricate multiple stage envelopes. Again, this can be very useful when imitating acoustic sounds.

These ideas should give you some food for thought on using your sequencer and getting more from it than just trundling sequences. The synthesizer is the only instrument that allows this flexibility and the sequencer can be an invaluable tool to the synthesist. But it's up to you to make the most of them.

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Nov 1983

Donated & scanned by: Stewart Lawler

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