In last month's thrilling first installment of Sequencers we learnt — without the aid of James Burke or Dr Jonathan Miller — exactly what digital and analogue sequencers can do and how they can be used. For those of you who missed last month's issue, I shall very quickly recap; so everyone else should skip the next paragraph. No, not the whole article!

A sequencer provides voltages and trigger pulses to a synthesiser, such that a melody or rhythmic line can be played without using the keyboard. The voltage (or voltages) is normally used to control the pitching of the synthesiser and the trigger pulse generated at the start of each new note activates the instrument's envelopes. This series of notes is usually repeated over and over and often used for a backing track. The differences between the two types of sequencers are as follows: analogue models have rows of knobs that are used to program the required sequence, and hence are usually limited to a 12- or 16-note capability; digital models, however, operate more like a tape recorder with the sequence programmed from the keyboard of a synthesiser. The digital sequencer stores the voltage and trigger information from the keyboard and then plays it back into the synthesiser, allowing more complex and longer sequencers to be used.

This month we will take a look at four of the most popular sequencers available at the moment. Two analogue machines — the Korg SQ10 and the ARP sequencer, and two digital instruments — the Oberheim DS-2A and Sequential Circuits Model 800. These four machines have become the major forces to be reckoned with, and though there are several other models currently available, I consider these to be the most noteworthy.

The Korg Analogue Sequencer SQ-10

The SQ-10 is a 12-step, 3-channel, 6-mode sequencer, with a sequence length of 24 steps. This becomes clearer by looking at fig 1. The three rows of 12 knobs on the machine are the three channels, ie for each of the 12 steps there are three control voltages available. Each of these three channels has an associated range switch, so that the outputs can be set over ±1v or ±5v; this enables more accurate sequences to be set up.

So for each of the three channels there is a jack socket giving control voltages out - Channel A CV out, Channel B CV out, and Channel C CV out.

The triggering system used is very flexible. There is a multiple trigger output that is connected to the synthesiser (this puts out a trigger signal for each step in a sequence). A duty cycle control knob allows the width of the trigger pulse to be adjusted so the pulse can last from around a tenth of the length of the step to 9/10 of the step. This function is extremely useful as it facilitates staccato sequences.

There are two white momentary buttons in the top left hand corner of the instrument that are used for starting/stopping the sequence, or stepping through the sequence. Associated with these switches is the sequence mode selector switch. This six-position rotary control selects the type of sequence: stepped, continuous, or one-time for 12-or 24-step sequences. To get the 24-step sequence channels A and B are run concurrently so that the step 1 channel C voltage will be used on step 1 and step 13.

Of course, it is not obligatory to have only a 12- or 24-note sequence. For each step there is a 'trigger out' jack socket which can be patched into the 'reset trigger in' socket which causes the sequence to go back to position 1. In this way any length of sequence up to 24 can be obtained.

The speed of the sequence is controlled by a voltage controlled clock generator. By patching channel C's control voltage into one of the clock speed inputs the length of each note can be varied giving opportunities for far more interesting sequences.

These are the basic features of the SQ-10 analogue sequencer, and because it has so many 'access' jack sockets it must be one of the most versatile units available. It is primarily designed to link up with the Korg MS range of instruments and is styled to match. In fact, this is my major criticism of the unit: it looks horrible, housed in a bent anodised aluminium case with black end cheeks, its appearance just doesn't match its excellent performance; a shame really.

The ARP Analogue Sequencer

ARP were, I believe, the first manufacturers to market a sequencer, this made up part of the large 2500 modular synthesiser. A lot of interest was shown in the sequencer and as a result ARP set to work designing a free standing version which, as it happened, turned out to be quite different to its predecessor. This model is still being made today, with only a few changes made mostly to the casework.

The ARP Sequencer looks quite different to the SQ-10, and although overall it is less versatile than the Korg, it is easier to use and does have one or two little 'extras'.

A 16-step single channel, or 8-step two channel sequence are the basic capabilities of this neat unit. Two banks of sliders are used to program the sequence, the control voltages derived from these sliders are quantised, that is to say they give semitone steps when connected to the oscillators of a synthesiser; this makes the setting up of a sequence so much easier. Associated with each slider is a slide switch that can be used to reset the sequence, provide a trigger pulse, or skip that step.

The sequence can be started by an external signal, a footswitch, or by the push button on the front panel. LEDs indicate which step is currently in control; these not only look pretty, they are vital. Other features of the ARP Sequencer include multiple gate outputs for diverse triggering functions, pulse width modulation (similar to the duty cycle control in the SQ-10) and several cross and summing patch facilities.

Inevitably, the ARP Sequencer has become a little bit obsolete over the years, mainly because it is not as versatile as its more recent competitors. However, it is probably the easiest model to operate, and I would recommend it to anyone worried about getting bogged down in hardware and who requires just basic synthesiser effects.

The Oberheim DS-2A

The DS-2A is a digital sequencer with a capacity of 144 notes. As previously mentioned, digital sequencers are programmed from an external source, usually the keyboard of a synthesiser, so it is necessary to connect four leads between the sequencer and the synthesiser (a control voltage and trigger send from the synthesiser to the sequencer, and voltage and trigger returns to the synthesiser).

If a Minimoog is being used as the synthesiser in question, a modification is necessary so that the control voltage derived from the keyboard can be sent to the sequencer. All that is required is the fitting of a CV out jack socket to the output panel of the Minimoog and a minor change to the oscillator circuit card. Also when using a Minimoog, it is only necessary to connect the S-trig to the trigger input of the sequencer — no return lead.

The DS-2A is compatible with all major synthesiser makes providing both mini-jack (ARP type) and standard jack sockets.

Having linked sequencer to synthesiser, the rest is easy. The DS-2A's three sequences (of up to 48 notes each) can be run concurrently giving a possible 144-note sequence. When programming, the sequence any length of note up to seven seconds can be recorded with the clock rate set at the 12 o'clock position. To obtain longer notes it is necessary to slow down the clock rate which results in very fast notes being missed out.

To record a sequence simply press the 'load 1' switch, the numeric display will read 1-1 (bank 1, note 1); the sequence is then played into the sequencer. At the end of the sequence, press the terminate button, the sequence will then play back, as programmed, repeatedly until the terminate button is pressed a second time.

The sequence, once programmed, can be played back at any speed by varying the clock speed. The sequence can also be transposed into any key and there are three controls that add preset voltages to the entire sequence. Editing a sequence is also possible by stepping through a sequence and re-recording one or more notes.

The Oberheim DS-2A is housed in a white case, and as can be seen from the photo, has an uncluttered, straight forward control panel. An expensive item, but then quality equipment usually is.

The Sequential Circuits Model 800

The Model 800 is somewhat more complicated than the DS-2A, even though it is considerably smaller in size. It comes in a strong rectangular case and at first glance looks impossible to use, having a rather compact (though some might prefer to say cramped) front panel.

The 800's memory is divided into 16 banks each capable of storing up to 16 notes. Each bank may be activated or deactivated in either record or play mode, by means of the row of front panel switches. Activated banks will play consecutively in left or right order so that either several separate sequences or one continuous sequence of up to 256 notes may be stored.

The 800 is linked to a sequencer in a similar way to that of the DS-2A; however, instead of different sockets for different makes of synth, Sequential Circuits have adopted a switching system and used just one set of ¼in jack sockets.

Pressing the green 'Start Record' button puts the 800 into record mode and selects position 0 on the left most active bank (indicated by LED). The 800 will begin to record when an incoming trigger pulse is received. The restrictions as to the length of note are similar to those of the DS-2A.

When the sequence has been programmed, pressing the red 'stop Record' button instantly switches the 800 to playback mode.

The 800 has one or two extra facilities not normally found on digital sequencers including provision for use of an external oscillator for the sequencer clock in either record or playback mode. This is useful when the 800 needs to be in sync with a click track from a tape machine. Several 800s can also be linked together in parallel in order to record multi-voice sequences in real counterpoint. Comprehensive editing facilities are also included such that both the control voltage and timing, for a desired note, can be changed.

One drawback that both digital sequencers have over their analogue cousins is the inability to remember sequences when the power is removed. This is a particular disadvantage with the 800 as it has a much larger memory capacity.

So, over the past two months we have looked in some depth at sequencers, and I hope I have cleared up one or two mysteries surrounding them. They are useful devices if used sparingly; however, by virtue of their mechanics, they are somewhat expressionless. So fear not all you musicians, technicians haven't taken over YET. Next month I have a rather interesting new instrument to look at so stay tuned... to look at so stay tuned... to look at so stay tuned... to look at so.

Dave Crombie is an experienced electronic design engineer specialising in keyboard instruments.