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Rhodes Chroma

This month’s Industry Profile visits an audio/video recording and retail outlet in the heart of Essex.



The Rhodes Chroma is the first of a new family of advanced musical instruments and has an interesting background to its eventual arrival in the UK. It was originally designed by the ARP company who were recently bought up by CBS and represents the only instrument product to be continued. ARP never actually marketed the Chroma and it is possible that it contributed largely to their insolvency. Working prototypes have been developed over the last two years, although further changes will be seen in the next three months, particularly the supply of a pressure sensitive 'after-touch' option.

The delayed launch of the Chroma has first been to ensure that all the debugging necessary for its micro software was complete — space has even been made on the boards for linking and testing with an external computer system at the end of the production line. And second, more importantly, to decide on the location for its manufacture. ARP products were made at Woburn, Massachusetts and, although it was suggested it was put together at Fullerton, California along with all the Fender products, it is now being made at Hooperstone, Illinois. This is one of the main CBS plants where the Gulbransen organs are made. Meanwhile, the ARP facility at Woburn has been turned over solely to R&D, with some 200 people currently employed at this Rogers/Rhodes division.

John Shykun, who will be well known to synthesists for his fine demonstrating of keyboards, contributed to the Chroma's specification and the instrument is now reaching main dealer shops. Vic Emmerson of Sad Cafe/10cc and a 4-piece band will be doing a promotional tour in the UK (and Benelux).

The Chroma is a polyphonic fully programmable synthesiser, a hybrid of analogue/digital technology and that's what makes it very flexible. With every Chroma is a cassette with 3 program sets containing 150 voices in all. One set contains the presets that come stored in part of the memory (3K of the 7K RAM available) on delivery, powered by means of a battery back-up system. It is therefore possible to commence playing the instrument immediately by simply selecting one of the 50 Program Select 'pressure sensitive' pads. All the voice presets have 50 parameters already programmed into them, including synth routing, keyboard dynamics and oscillator assignment. It was demonstrated at the NAMM show in conjunction with the Apple II and an independent program exists to develop the software (not currently available).

The Chroma can therefore be regarded as a 'dual' instrument: 1) a versatile performance instrument with the ability to make changes to the selected voice (or paired voices) as you play, or 2) as a comprehensive programmable instrument that allows you to synthesise your own library of sounds in batches of 50 that can then be stored on a separate low cost mono cassette recorder.

One of its major features is undoubtedly its velocity pressure sensitive dynamic keyboard that employs 14" weighted wood keys that utilise an internal microprocessor to measure key speeds to within a 1000th of a second. In fact, two processors are employed, with a second generating all the control signals for the Chroma.

Two very good manuals are supplied, one for performance and a comprehensive one for programming — obviously a sensible approach that helps you get started quickly.

Rear connection panel.


Playing the Chroma



Setting up is straightforward, once the instrument is lifted out of its sturdy flight case. This also holds a large piano style dual foot switch unit and other accessories, including a single foot-switch, foot volume pedal, cassette (with 150 programs) and interface card for a cassette recorder (the only item not supplied — the Radio Shack TR-80 is recommended, although a mono recorder with remote auto stop, pause and tape counter should do, e.g. Lloydtron from Comet discount shops).

On the rear panel of the instrument, various connecting options to your mixer or amplifier give studio mono balanced/unbalanced cannon line out, high/low level mono standard jack outputs or choice of 4 separate outputs. These are linked as you wish, along with dual footswitch and a lock/unlock switch is maintained in the 'lock' position to avoid loss of programs.

When the power cord is plugged in and the Chroma switched on, all 16 LEDs in the left section of the main panel blink 16 times over 6 seconds. This indicates the Chroma is going through an initialisation and tuning program. When complete, the two LED displays on the main panel are activated, with 'Program No.' indicating the last program selected (prior to switch-off). If the smaller 'Data Readout' shows 'Err' followed by numbers, then this indicates a fault which can then be diagnosed from the number indicated. Normally, everything will be in order and the five left hand sliders can be adjusted for Bass, Middle, Treble EQ and Volume, plus Tune which sets the overall pitch to match other instruments.

In the right section of the main panel are 50 numbered switches that each have several uses during operation, determined by 4 'Panel Mode' switches with indicator LEDs. In normal use these are 'Program Select' — each numbered switch selects one of 50 different programmed instrument sounds, 'Parameter Select' — above each number is a synthesiser parameter that can be selected and modified. The individual functions are logically divided into numbered groups: Control, Glide, Sweep, Envelope 1, Envelope 2, Pitch, Waveshape, Cutoff and Volume; 'Copy from A' and 'Copy from B' — parameter copying from one program to another. In addition, temporary Panel Modes are set by pressing two of these switches together (shown by flashing LEDs).

Each program contains all the information necessary to process a sound, plus performance control extras. Besides the 50 programs stored in memory, there's a further 'current program' that acts as a workspace for inserting a particular program number (shown on the large 2 digit display). This can then be edited during performance and also stored at any time during programming as a new program in the memory.

There are 16 complete synthesiser channels, each with an oscillator, waveshaper, filter and amplifier, that are split into 8 pairs, enabling detuning parameters to be set up as you play up to 8 notes at once (but this can be reassigned to other mono/poly formats).

Another important option is the ability to link two programs together, with the additional program called LINK as opposed to the initial MAIN program. The LINK channel selected may be stored as a parameter and is shown in the Data Readout display (prefixed by 'L'). Four switch buttons with indicator LEDs set link modes: LINK LOWER, LINK UPPER, LINK UNISON and NO LINK. The first two place the MAIN and LINK sounds above and below the specified 'Keyboard Split' point, whilst unison puts them together over the whole keyboard.

Left hand part of control panel.


Other Performance Controls



Set Split The 64-note keyboard (E to G) in 'default' mode (i.e. without any parameter set) will give the split at D#/E (approx, 3/2 octaves at lower/upper). Using the 'Set Split' switch followed by the note you require the split at, the Data Readout will indicate the new split point (over -32 to 31 range), e.g. SPL 0 = Middle C.

Since balance between the two programs chosen is critical in performance, this is easily modified by a 'Parameter Control'slider over -14 to +14dB in 2dB steps, indicated on the Data Readout.

It is worth noting that the link mode, program number and keyboard split are stored parameters in each program.

Transpose — a section of 4 switches along with LED indicators for putting Main and Link up or down 1 octave.

Auto tune One problem that did occur during editing was some drifting of oscillator pitches. Like most advanced polyphonics, some facility must be provided to easily compensate for drift during use over a period. This is catered for by the Auto Tune switch which goes through a tuning routine similar to switch-on. Not only do the LEDs flash, but soft, high pitch random notes also accompany the process!

Rather than have a joystick controller, the Chroma has two levers at the left of the keyboard. These are spring-loaded, moving backwards or forwards to return to centre positions. The levers, designated 1 and 2, can be programmed parameters, although surprisingly only one program ('Harmonica' in Set 3) uses them for bends and muting.

Two pedal inputs enable connection of volume type footpedals (100k to gnd). Assigned Pedal 1 and 2, these can be programmed for volume and effects. Also placed underneath the instrument is the dual footswitch plate (two piano type brass pedals) and a simple footswitch. Normally the dual pedal is for latching and holding a chord (left) and for sustain (with the right). The simple footswitch is actually for sequencing through programs, done via a stored 'pointer' number in each program, so that jumps from one to another can be specified for moving several around without losing any.

Finally, an optional pressure sensor modification kit will soon be available that gives extra 'after-touch' control from any key. So the player can assign initial touch sensitivity (dependant on the velocity at which a key is struck) and then by pressing further onto the depressed key, the aftertouch is brought in. Both these keyboard controllers can be assigned to change dynamics, tone or pitch in each program.

Main panel opened up.


Programming the Chroma



The Chroma's 16 synthesiser channels are grouped into eight pairs so that they may be reconfigured to provide a wide variety of sounds. The main processor controls the oscillators, filters and amplifiers directly. It also digitally generates 32 envelopes (2 per channel) and 16 low frequency Sweep signals. Signals for the levers, pedals, control panel and keyboard are all encoded digitally prior to the main processor. Because of this all 50 parameters for a sound are able to be stored digitally in the Chroma's memory. The information may also be saved and loaded from cassette, sent to an external computer via the rear panel 25-way socket, or even to control another Chroma.

Chroma's logical structure has four audio synthesis groups: Oscillator, Waveshaper, Filter and Amplifier (controlled by parameters in the bottom row of the 25 numbered pads); four main control signal generators: Glide, Sweep and 2 Envelopes (controlled by the top row of 25 pads); and six performance controls: 2 levers, 2 pedals, keyboard velocity and pressure.

When a sound is chosen by means of one of the 50 Program Select buttons, it is then possible to edit it. Once a change is made to one of the sound's parameters, a small dot appears below the program number in the display. This 'Modified Flag' reminds you to store the current program if you want to retain the new control settings. To do this, you first set the Lock/Unlock switch to Unlock, and then press the Store switch followed by the chosen program number pad. The system also provides retrieval of a previous program in case you decide to start again, as well as 'unselect' and 'unstore' operations — wise precautions with 50 parameters at stake!

Creating a Sound



First, the workspace must be cleared so that you can start from scratch. Unfortunately, no single 'magic button' does this and the user has to go through a routine of first selecting both Edit A and B buttons, then holding the Parameter Select switch down whilst pressing each of the 50 numbered switches in turn. Once completed, the existing program is conveniently set to a raw sawtooth sound, rather than a silent 'all functions shut-down' situation, for one channel per note playing. Continuing in the Parameter Select programming mode, any of the 50 switches on the right hand panel can be activated so that a particular parameter can be setup. All the changes are noted on the 8-digit Data Readout which provides constant monitoring of parameters as they are selected and then changed by the Parameter Control slider. The effect can of course be immediately monitored aurally by playing notes on the keyboard.

Parameters like Decay, Tune and Modulation are variable and have their range scaled appropriately, e.g. for Decay this is 0 to 31, and for Tune it is 0 to 63 whilst Modulation Depth gives bipolar control: -64 to +63 (0 = no modulation). Others are switched continuously by the slider up to a maximum of 16 positions.

The Parameter Control slider works remarkably well with adequate resolution up to 128 settings on critical sound adjustments. It soon becomes the focal point of the left hand when doing program changes. Meanwhile, the right hand selects parameters and monitors sounds from the keyboard as they are changed. In some respects, the location of the slider might have been better placed away from the other switch functions to its left as they are easily switched on by accident as the hand leans on the panel.

Shaping the System



The 'tapper'.

The first parameter that needs attention in creating a sound is PATCH. Incidentally, even though confirmation of the Parameter Select number appears in the 2-digit LED display, the Chroma has an unusual feature — a solenoid-operated plunger attached internally to the main circuit board to cause a mechanical thump to be clearly heard (and possibly felt!). Every switch change including those made with the Parameter Control slider is registered and is presumably an aid to blind players. (I wonder how the 50 program switches are taken in without keeping an eye on the display?) I prefer to program out the effect which becomes slightly irritating after a while.

Back to the Patch! Here's an exclusive feature that enables 16 modes of osc./filter/amp. routing covering one 16 channel/16-note playing, and fifteen 8-note configurations of 4 independent 8-note modes, 4 parallel filter modes, 4 series filter modes and 3 variable mix filter modes — each employing no cross modulation/sync/ring mod/ and (except for the final group of modes) filter FM. Here's the reason for including 2 edit buttons A and B, as 15 modes require separate modification for each A and B pair of 8 channel banks.

This kind of control technology will leave most of us behind — especially if as you read this you're trying to conceive the end results! (Needless to say, if this parameter function has you thinking, 'Is it all necessary?' then a listen to E&MM Demo Cassette No. 8 and be convinced!) The creative possibilities with 2 glides, 2 LFOs, 4 envelopes, 2 oscillators, 2 waveshapers, filters and amplifiers on an 8-note polyphonic that can program and store the lot is exciting, to say the least. Other special sounds possible are dual 2/4 pole low pass, high pass, band and notch filter effects, bells, phase shifter and vocal effects, percussion, flanging, complex harmonics, ring modulation and dual timbre effects.

Another important group of parameters during initial editing is the General Modulation Selections (GMS). These enable you to choose 3 modulation sources for the oscillator, one for the waveshaper, and 3 for the filter. 16 sources are available for each (coded 0-15): Keyboard Glide A, Sweep A, Env 1A, Env 2A, Keyboard Glide B, Sweep B, Env 1B, Env 2B, Lever 1, Lever 2, Pedal 1, Pedal 2, Velocity, Threshold Velocity, Pressure and Threshold Pressure.

In addition, 3 Modulation Select parameters can be chosen from Env 1A or B, or Env 2A or B. Modulation depth for all these is then set by appropriate individual Parameter Select pads.

Right hand part of control panel.


Before coming to the main synthesis sections, there are four other control functions besides 'Patch': FOOTSWITCH mode, which sets 8 combinations of sustain and latch for the dual pedal assembly, either enabling, disabling, using gate or inverted note gate. The latter two are useful for making Main and Link programs come on during playing by correct use of the pedal.

KEYBOARD ALGORITHM. Besides the unique channel assignment modes already discussed, this parameter offers monophonic and polyphonic note playing variations. The mono modes are best selected for lead or bass line (using only one pair of channels so that the remaining 14 channels can be utilised in a polyphonic section of the keyboard (set by Key Split).

Some very useful algorithms are given, including Polyphonic: least recently used note/same note — same channel/pitch ordered, for moving from one chord to another in the same note order/storage of chords for recall on left foot-pedal/sharing of notes by all synth channels.

Monophonic: full synth channel playing on one note/last note, multiple or single triggered/first note of a group (for inner playing with a polyphonic cluster)/bottom note (ideal for bass line)/top note (for lead). There are also 6 mono arpeggio modes played at a speed set by Sweep A 'Rate' control: up, down, up/down, down/up; sequence (played in order from the keyboard while remembering key velocity); random notes playing from your notes played.

DETUNE makes fine pitch adjustment of the B channel against the A channel in 1/32 of a semitone steps (from 0 to 31).

OUTPUT SELECT not only routes a channel to one of 4 audio outputs, but also acts as a send/receive function by using a correctly wired stereo jack in the rear sockets. If a Link program is added, stereo output is then obtained.

Synthesiser Process Controls

Internal view of paired synth channels.

Here we have Glide (for portamento or glissando at different rates); Sweep which is really an LFO oscillator par excellence with 16 different wave shapes available, as well as amplitude modulation selection and delayed sweep. It can run independently, or be synchronised by key depressions. Each note then may have its own sweep generator. The Sweep Rate can be itself modulated in 16 ways, from the keyboard, envelope, pedals and levers. The wave shapes offer advanced synthesis possibilities with sine, cosine, offset sine, half sine, and stepped patterns, as well as the more usual triangle, sawtooth, square and random shapes. Amplitude modulation sets 15 different control effects of the Sweep Depth, available from the same sources as the Rate, plus 4 delay times up to 5.1 secs.

ENVELOPE. There are 2 envelope generators per channel with AR and ADR shapes, although more complex EGs can be used by combining envelopes and by using modulation inputs to Cutoff, Pitch or Amplitude functions. Variable parameters are Amplified Touch (based on Key Velocity), Attack time with modulation control, Decay time with modulation control, Release time and Delay (Env2 only). Maximum times prior to full sustain each measured 10 secs, but modulation controls no doubt extend these further.

As the modulation controls of the instrument become more familiar, the potential of Chroma's keyboard system is more fully realised — not just with pressure and velocity control, but also by means of specific threshold settings (for soft and hard strikes) with individual fingers for exclusive note/control execution. This should inspire experienced players to extend their performance techniques!

Pitch Parameters

Oscillator tuning is set with the TUNE parameter in semitones from one octave below to 4¼ octaves above, whilst modulation can come from 3 sources at once. There are 16 sources to choose from, (the 'general modulation selections' GMS mentioned earlier). What is unusual is the allocation of different depth gain increments, from 1/16 semitone, ¼ semitone to semitones (maximum depth for the latter over -64 to +63 range).

Main processor board with 7K RAM and 16K ROM.


Waveshape Parameters

Further control of the oscillator waveshape is provided here, with 'saws' — a combination of pulse and sawtooth simulating the sound of two sawteeth, and pulsewidth and sawshape adjustable from 0 to 100%. Modulation with the GMS is also possible. Two noise generator sources can be selected here instead of the oscillators — white or pink.

Cutoff and Amplitude Parameters

The filter can be high or low pass with variable resonance, tune, modulation from GMS (up to 3 at once), and depth. It does however not track the keyboard. Two Amplitude Modulation select sources are given, with at least one needed to be set to operate the amplifier. One of four envelopes can be chosen for each. There is also post modulation control of the VCA from performance controls, as well as tremolo and volume compensation over the keyboard.

Hidden Functions

These are useful extras that put another control to the numbered switches and they are accessed by pressing the Set Split switch followed by the correct number. Included are temporary stereo setups, cassette functions and subroutine in/out, diagnostic checks, channel muting, linking to another Chroma, and envelope threshold and fast/slow release parameters.

Cassette Storage



Four switches operate an external cassette recorder for the purpose of storing programs and a special interface via a Din socket will turn on and off the recorder automatically. You can SAVE ONE or SAVE ALL programs with or without spaces on the tape and the LED will show any program SAVE errors clearly. Once saved on cassette, your programs can be reloaded with LOAD ALL or LOAD ONE. The latter is useful as it puts it in the workspace for you to allocate to any program number.

Obviously, once a new set of 50 programs is loaded you lose the previous set unless you've already got it on tape. No problems were encountered with the tape interface. During execution, LEDs flash between programs as they load, with a buzz in the output amp you're using as well.


Chroma Sounds



On E&MM Demo Tape No. 8 we've put a selection of sounds from the three sets of 50 program sounds supplied with the instrument on cassette. Since the output voltage ranges over ±10V, the Chroma really punches out a large signal and quality is good enough for most studios. The range of sounds is vast and often defies description. Most orchestral and band instruments are provided, with plenty of percussive sounds and modern synth effects. You even get the sea, seagulls and ship's bell on one program!

The modulation facilities really do make a huge difference and the overall clarity combined with the full pitch range and the complex synthesis that takes place — often just by the host of performance controls — makes it a challenging and exacting instrument to play.

Considering it can be interfaced to the Apple II, Tandy TRS-80 and possibly other micros (surely not the Spectrum!) it's future looks very bright indeed.

You'll have to pay around £3,850 including VAT for the Chroma and its extras mentioned, but the design seems to have taken in all the requirements that a keyboard player could ask for especially as it offers instant change of any parameter. The touch pads take a bit of getting use to and the instrument runs hot at the rear. I enjoyed the weighted keyboard immensely and could only begin to explore the Chroma's possibilities in the time I had with it.

These higher priced instruments usually offer special sequencing facilities but here we only have arpeggiation. I would expect the external computer interfacing to overcome this, and we'll be looking at the possibilities as soon as the information becomes available.

Large scale computer operated instruments such as the Chroma require plenty of back-up for users, and CBS assure me this will be provided in the U.K. So if you want to have full creativity and dynamic performance control, my advice must be to check out the Chroma as soon as you can.

The Rhodes Chroma is distributed in the U.K. by CBS, (Contact Details).


Also featuring gear in this article



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

 

Electronics & Music Maker - Oct 1982

Gear in this article:

Synthesizer > Rhodes > Chroma


Gear Tags:

Analog Synth
Polysynth

Review by Mike Beecher

Previous article in this issue:

> Industry Profile

Next article in this issue:

> Cassette Review


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