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With polyphonic synthesisers appearing in the shops more frequently now, it must be natural for a manufacturer's design team to want to make their instrument have more 'orchestra' groups and effects than their competitors. Yamaha, however, in their SK20 have produced a synthesiser that takes the three most important polyphonic sounds and utilises their unique custom chips to give an advanced technology machine for its price. The SK20 functions as a polyphonic synthesiser and a string synthesiser (with the POLYSYNTH section), as well as a harmonic drawbar ORGAN. The keyboard plays up to seven notes over its 5 octave range or fourteen notes in either of the split keyboard modes. In addition, string chorus effects, Leslie-type tremolo, delayed vibrato and detuning are featured.

The Construction

Yamaha's instruments always have a high class finish in terms of physical materials, and the SK20 is certainly no exception. Styled like the CS40/20M and 15D it has rosewood grain end panels, semi-matt black metal panels and wooden base. The weight of the whole package is only 331b which must make it one of the lightest polyphonic instruments for its size (10(W) x 15.8(H) x 40.6(D) cms). No case is provided and the footswitch (FC-4) and footcontroller (FC-3) are optional extras at £13 and £19 respectively. Servicing couldn't be easier with all of the main panel opening up as in Figure 4 and a removable keyboard (see Figure 3).

The Circuitry

The SK20 essentially comprises three separate sound sources — organ, string and polysynth driven by a common keyboard. The machine is built around 4 Yamaha custom large scale chips. The 61 note keyboard speaks to the digital tone generators via a 40-pin chip called the 'Key Assigner YM62100'. There are 13 note lines, 5 upper and 5 lower octave lines on the input, interfacing with the key switches through a diode matrix. Octave and note identities are sent by the Key Assigner and output as 12-bit codes indicating octave, note, keyboard and keying state. These 12-bit codes or parties are output on 4-bit data lines KC1 to KC4. They are organised as 4-bits wide by 3-bits long. Each bit is 1 microsecond duration. The Key Assigner is the same device used in Yamaha's more upmarket organ range and for this reason has the output capability of instructing up to 18 sound channels: 7 upper, 7 lower, 1 pedal and 3 auto arpeggio sounds. In this particular application, only the 7 upper and 7 lower instructions are used.

Tone Generation

Two tone generator LSIs are used for production of the basic waveforms and their associated sound amplitude envelopes. The 2 ICs share the same 4-bit input data lines KC1 to KC4 from the Key Assigner.

The organ wave generator IC YM70200 receives 13 master input frequency instructions C₁-C from an LSI daughter chip, YM62200, which combines with the keyboard data on its KC1 to KC4 terminals. This produces a set of sine wave outputs 16' through to 1' by Yamaha's Pulse Analogue Synthesis System (Pass). These sine signals are low pass filtered and sent through to the volume slider controls of the panel. 4' and 2' pitches are also connected through two VCA envelope control gates for the percussive attack voices. Three preset flute combinations are derived from mixing resistors and their associated FET resistor gates. Normal flute voices are collectively passed through a 'brilliance' filter circuit and on to a 'level' control and 'tremolo/ensemble' selector gating.

The polysynth and string basic sound sources are produced from another (Pass) wave generator LSI chip, YM70400 — this device produces 8' and 16' square waves and 4 sawtooth waves which pass through mixing amplifiers into selector gating which directs these sources to the appropriate treatment (sawtooth 16 and 8' are derived from the square wave mixing with the 4' sawtooth).

Polysynth and sources are routed through a voltage controlled filter, TG00156, giving gate selectable high pass or low pass outputs. The filter provides voltage control for attack time, decay time, sustain level and release time.

Three diode resistors matrices provide preset control for the various parameter voltages (e.g. initial level attack) required to set the particular presets on the panel.

Two string voice sound sources, 16' and 8' sawtooth, are passed through their selector gates and a common LDR to the string level control (which can be the optional Foot Controller FC-3).

Polysynth and string level outputs are mixed and passed through a common 'brilliance' filter and out to the tremolo/ensemble selector gates.

Effects

Tremolo/Ensemble effects are obtained from a common board employing 3 delay paths (BBDs). Another Yamaha custom LSI, YM63300 clock generator, provides 3 outputs of 0.64Hz with 120° phase difference between them and 3 outputs of 6.4Hz, also with 120° phase difference. These 0.64Hz and 6.4Hz output groups are mixed and provide 3 composite control lines for the VCO drives to the BBDs which then have a 120° phase difference. This produces either a complex ensemble effect or by modulating the VCA at its output achieves a rotating speaker/tremolo effect.

A slightly unexpected method of interfacing some of the parameter controls is employed, e.g. organ sustain lengths. Here the sustain slider selects diodes via internal switch connections and outputs a 3-bit code to a parallel to series converter (multiplexer) through to a serial data input on the wave generator instructing it to produce the appropriate sustain lengths.

The two tone generator ICs (organ and poly) are driven from the two keyboard code groups — upper and lower. The polysynth IC is driven by upper key codes and the organ generator IC is driven by lower key codes. By dividing the connections of the upper and lower octave input lines with the common note lines, the tone generator groups are similarly divided, giving the keyboard split.

The two tone generator ICs have separate master clocks which can be frequency modulated by the vibrato drive circuit. Modulation may be applied to either one or both in separate degrees. Pitch control is applied in a similar fashion to the two clock circuit inputs giving individual pitch control on both tone generator groups.

The Controls and their Musical Effects

The controls are well placed on the angled main control panel with microtouch switches (that have LED indicators) in a row conveniently above the keyboard. There are 5 control blocks above the switches from left to right: Output, Pitch, Vibrato, Organ and Polysynth. Sliders make setting up much easier although it takes a while to get used to the organ section — maximum volume on each drawbar is in its lowest position — as if you were pulling a Hammond drawbar towards you.

A connection panel on the rear links a 'sustain' footswitch (for organ and polysynth sections — either or both can be selected from the panel) and a foot controller for 'string' or 'mixed' output volume. There are three standard jack outputs available — polysynth/string, organ and mixed polysynth/string/organ. A low impedance stereo phone socket gives the mixed output and a standard 11 pin connector is provided for use with a Leslie Speaker System (2 channel input type). An adjacent switch allows the speed of the rotating speaker to be switched from the panel to fast/slow and on/off.

The 'Output' block has independent sliders for setting the volume of organ, string and polysynth with a rotary master volume control that can have its output signal (the mixed output) switched on or off. Since the background noise of the instrument is very quiet it's not really needed as a standby switch or noise reducer for a multi-keyboard set-up, but can prove useful when the other two outputs go to effects boxes or different spatial positions. If you get our demonstration tape for this issue you'll hear a very broad stereo image from the SK20 which is due to the mixed output feeding to the centre with slight echo added. The separate organ and poly-synth/string outputs are then panned left and right respectively with a phase box and flanger linked in the polysynth signal line for further effects. Incidentally, the Roland CR-78 gives the drum sounds (adapted for stereo with split instrument outputs), and the CS-30, operating from a Yamaha pedal board and drum trigger pulse, gives a bass line to the opening piece (see page 96 for details of demo tape).

There are two rotary controls in the 'Pitch' block for tuning adjustment to the organ and polysynth sections. Since the string sound is derived from the polyphonic synthesiser section, the polysynth tuning also affects the pitch of the strings. Interesting detuning effects can be obtained by first tuning the organ to your playing 'standard pitch' (e.g. 440Hz=A) and then very slightly moving the polysynth pitch sharper so that slow beating occurs. A small amount of detuning will enrich the overall sound and a large amount will give, for example, organ and strings (without tremolo/ensemble) a fairground organ effect. Pitch is variable from A=435-450Hz.

Vibrato can be applied to organ and polysynth in separate amounts using 'depth' and 'speed' sliders and has an initial delay of up to 3 secs. It's useful on organ for improving the fast tremolo effect and with delay will allow playing with or without vibrato depending on your use of slow or quick notes.

Organ

The organ block contains the main features of a good harmonic drawbar organ with separate sliders for selecting 16', 8', 5¹/₃', 4', 2²/₃' and 1' pitches in varying degrees. Despite the lack of the two top mixtures, it has a full-bodied tone that is particularly clean in the bass. There's a 'brilliance' control to boost the top as well. Two switches select 'sustain' (which gives a decay after release of the keys) or 'decay' (which replaces the normal organ on/off key action), both adjusting from one slider. To complete the section there are 2 percussion sliders that give 4' and 2²/₃' punch to the sound using 'decay' on minimum. Increasing the decay time gives glockenspiel or vibraphone (with vibrato) tones. There is a certain amount of key click present, especially with increased brilliance, for jazz/pop organ sound. A slight drawback is that with 'sustain' on this bright click effect is removed. Four panel switches give 3 presets: 16'8'5¹/₃'; 16'8'4'2'and full, plus 'manual' selection.

Trigger action is worth noting — the polysynth only triggers when all notes are released and a new note or chord is played. The strings always play and the organ plays according to decay/sustain settings. The note limitation is only evident with long 'sustain' set, playing runs up the keyboard when it only sounds 7 notes at any time.

Tremolo/Ensemble

Tremolo is a very realistic rotating speaker effect that is produced electronically. It can be applied to both main synth sections and has two-speed switch selection that speeds up or slows down during changeover for jazz chorus and church organ sounds. Ensemble is best suited to applying phase to the strings although it can be used with organ and polysynth. Unless you are buying an external tone cabinet then you have a compromise here — when ensemble is selected for polysynth/strings, then tremolo is cancelled on organ and also reverts to ensemble! Nevertheless, these two treatments make a big contribution to the final sound.

Polysynth

Within this section are two controls that also affect strings. They are switches for adding 'slow attack' and 'sustain'. Once again the latter is really the release control for the VCA. Two string pitches can be selected, either 16' or 8'.

The Polysynth section can have its oscillator and wave source to be either 16', 8' or 4' sawtooth, 16' o r8' square-wave, or 8' sawtooth in bandpass mode — the others go through a low pass filter. All the useful synthesiser filter controls are provided — cut-off frequency, resonance, EG depth and ADSR — and if you've played any of the CS50, 60, 80 range you'll know the kind of sounds you can get from these. It's a powerful synth sound by any standard with very smooth filter control using resonance that really picks out the harmonics. Three presets for Polysynth give brass sounds — so now perhaps you're realising this instruments potential!

Finally, the keyboard can be split with the organ on the left and the polysynth on the right and vice versa. You can get a walking percussive bass from the organ in your left hand with the right making rich strings swell in from the foot controller whilst polysynth sounds shoot through the harmonics using added 'brilliance'! All very exciting and there's an SK50 two manual on the way shortly so obviously Yamaha have a lot of hopes for this type of instrument which has benefited from their research and development in the lucrative home organist market — this machine would have been quite impractical for manufacturers who use conventional off-the-shelf devices. If you can't afford its price of £870 including VAT, then take a look at the smaller SK10.