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Chroma/Apple Interface

Playing the Rhodes Chroma with the Apple II Sequencer

Article from Electronics & Music Maker, April 1983

The Chroma Polyphonic synthesiser is one of the new generation instruments that links directly with a microcomputer to increase its user control capabilities dramatically. Its potential appears only to be limited by the software available, once the correct interface has been made between Chroma and computer. The first 'package' has just been introduced and is a comprehensive 16-track polyphonic Sequencer system, complete with interface kit for the Apple II.

The Interface

Once you have a Chroma keyboard synthesiser and an Apple II Plus (or IIe) microcomputer with 3.3 Dos, you have the basic equipment to realise the increased musical control that the Chroma/Apple link offers. On the Apple side, you will also need one or (preferably) two disk drives, the additional 16K RAM language card (from Apple or elsewhere) - the Apple II won't run without this, a supply of 5¼" disks for backup and sequencer/program storage and, of course, a suitable TV monitor.

The hardware for the computer interface kit comprises a long PCB card that inserts into an Apple 'slot' inside the micro, a small metal box that has standard jack sockets for external functions (this fits on to the rear of the Apple chassis by means of a screwed bracket), a 2-metre multiway link cable for connecting one instrument (either the Chroma Synthesiser or the Chroma Expander), a Sequencer Program (master) diskette, a Sequencer Data diskette for storing your sequences (this includes some sample sequences and program groups - the 3 factory sets of 50 sounds each plus other sets), a footswitch, and a detailed user's manual.

Some further optional items include an extra multiway cable for those lucky enough to have the Expander instrument as well, a control pedal for speed changing (this is almost essential in practice, although it could simply be a suitable control pot in a box), a Programmers Manual (for those who want to write their own Apple software), and an interface manual that discusses the interface bus in general.

As detailed in last month's issue, there are plans for interface kits to the new IBM 16-bit micro, the Commodore 64, and the TRS-80. Facilities will be more or less the same as the Apple system, except that the IBM machine will offer increased memory storage - a big plus in its favour.

Basic facilities

With the interface kit parts in position correctly and the micro disks in operation in Disk drives 1 & 2, the Chroma can send and receive live performance information and also programming information (i.e. all the 50 parameter settings for each of the 50 programs).

The Sequencer disk program offers up to 16 track recording with a different program (i.e. preset synthesised sound) for each. Like most computer orientated systems, the number of tracks you'll get to play at any one time depends entirely on your efficient use of synthesiser voices.

The Chroma Expander is an additional multivoice instrument that is virtually the original Chroma Synthesiser without the keyboard controller. The Expander is therefore played from the main synth keyboard or from the Apple link described here. It increases the number of oscillators available to 32.

The Chroma instrument was reviewed in detail in E&MM October 1982, but it's worth noting that the 16 separate analogue synthesiser 'voices' in both the Expander and Synthesiser have the standard VCO, VCF, VCA format (although very comprehensive in terms of control). The voice cards can then be arranged in various monophonic/polyphonic/system blocks (VCF, VCA etc.) using the 'Patch' and 'Keyboard Algorithm' parameters on the instruments. In practice, 99% of the time the configurations will utilise two synthesiser voice cards for making a more accurate sound (by detuning and multiple envelopes). That means, most of the time, this Sequencer program, although offering 16 track recording, will usually end up with around 8 as the oscillators get used up.

So that's why the Expander is needed to realise the full 16 tracks all working together. Of course, you can still operate the 16 instruments on a Synth alone, provided you don't have them all playing at once. A program can also have a linked program, using up 4 oscillators, and so the system gives priority to new tracks, usually dividing equally when possible the remaining oscillators. So two tracks (using just the Chroma) will take 8 oscillators each, three tracks will take 8, then 3, then 1 - a lot will depend on mono or poly voicing, too. If you use the Expander, voice allocation automatically makes use of the extra oscillators. The system is unlikely to crash, but simply asks you whether you require certain tracks to be 'muted' and allows 'optional' tracks to be recorded, provided memory doesn't run out. Another way round is to change to program sounds that use less oscillators.

Don't be too bothered about this detail at present, I've presented it just to let you feel the kind of methodical approach required when using the system - it's far less complicated than using an analogue 16 track recorder in practice and offers full editing at any time.

Other 'utility' programs will soon be available as well as the Sequencer, such as voice program editing and file management (continuous display of all parameter settings). Every effort is made by the Chroma people to encourage you to create your own software and freely copy the diskettes provided. Using the 'Escape' key on the Apple, you can access all the programs that create the system and the manual gives all the monitor 'overlay quotes' (i.e. important routine calls and function addresses) plus enough computer and electronic data for further development.

Setting up

There is no problem in installing the interface hardware as long as you follow the instruction manual carefully. Although there are ways suggested to continue using other cards in remaining Apple slots, these are best removed to conserve current.

The connector box has 2 multiway sockets for Chroma and Expander, and sockets for footswitch (to start/stop recording), a pedal (to adjust speed of recording and playback), 'Click' - a metronome type of audio output, 'Sync' and 'External Clock' that allow another piece of equipment, like a drum machine, to control the system to run in synchronisation with it.

The review set-up employed two disk drives and this lets you save sequence tracks as you go along, as well as special sets of voice programs (50 in one go) required to play the particular piece you've recorded. A Roland TR-808 drum machine was also linked as an external clock source, as the need for synchronisation with drums in particular is likely to be essential for most users. The TR-808 external clock links to the 'External Clock' input and one of its 3 trigger out lines was connected to the 'Sync' input via the E&MM Trigger Interface project (July '82) to ensure TTL levels were not exceeded. The Ext. Clock will take almost any existing pulse clock ranging from 0.7V to 22V p-p. The 'Click' output feeds to an input on your mixer and the synth sounds can come from 4 separate outputs or a mix output on the Chroma - a tremendous advantage because you can allocate sounds to dry, reverb, echo, and effect channels simply through a parameter change. And, above all, every piece of information you need for the correct playback of a Sequence is recorded when you save it on disk. Saving the program set used completes the information.

The most exciting feature for most musicians will be that you just play your tracks (mono or poly as you like) and all the DYNAMICS, including the new Pressure option, are recorded as well. On playback, all the 'emotional' content is still retained - that for me brings me back to the Chroma in preference to non touch-sensitive machines!

The ultimate set-up? — Chroma Expander, Synthesiser on the Fender Rhodes.


The TV monitor displays three 'pages' of main 'menu' selections. The Apple keyboard gives direct access to these and further 'submenus' from one key or one key plus the 'control' key, with or without 'return' key operation. As you proceed through an operation, you are always given a 'second chance' to opt out in case you've changed your mind and, unless you choose things that don't exist (e.g. no Expander actually available), you'll meet few problems. The 'escape' key puts you back to Applesoft BASIC and Goto 160 gets you back to the main menu. All disk saving operations are very quick - some 20 seconds max for a full memory sequence and just a second or so for full 50 program transfer. As you move from page to page menu and selections there may be some disk drive operation occasionally for a few seconds.

The whole system is presented as a realtime functional operation - in other words, if you're a player, you'll go for this, but if your keyboard skill is limited, you won't find it too friendly to record by numbers or typed data input. Of course, there's a comprehensive editing mode, but it's really for tidying up your realtime keyboard playing during record.

Main Menu

Here's a list of the menu selections with some explanation of what each offers. As you learn to use the selections on each page, you can simply type the appropriate key letter and it will immediately be selected. Although the package is called a 'Sequencer', it's more useful for preparing a complete piece or section that would be used for a studio recording or stage performance. By putting down the normal tape click track, it is quite possible to conceive a whole LP of instrumental music on disks that would be overlaid in the studio situation in some 3-6 minute chunks. Because the Chroma has a noise generator, snare drum, 'syndrum', tom-toms and bass drum percussion tracks can be synthesised (but no cymbals, it appears, although ring mod control should give you something).

The screen menu selections allow three main tasks to be carried out - PLAYING SEQUENCES, RECORDING SEQUENCES and EDITING SEQUENCES.

Playing Sequences

GET SEQUENCE. Sample sequences or your own sequences are displayed in disk file format and loaded into the system ready for playback. Once this is done, the main menu conveniently indicates the percentage of memory used (maximum 99%).

PLAY and PLAY ALONG. Playback can be a straight 'play' of the sequence or you can 'play along' from a selected program on the keyboard with the sequence on playback. If you're using an external clock, the display indicates 'Waiting for Sync' and won't start until you've hit your drum machine foot-switch. If you change tracks on the Chroma during playback, you might lose track 1, so it's best to stick with one keyboard program only.

The sequence is stopped by typing a 'space' or pressing the footswitch. The main menu reappears with an extra indication of the 'Last Event Time' - useful in editing to find a wrong note.

LOOPING. A looping function can be set by typing 'L', although this did cause the biggest headache to get right! As you'll see, there are several ways to record - with or without a Click Track, using internal or external control pulses. The problem is that precise stop points have to be made in each track of music, especially for chords at the last point in the final measure, otherwise an additional bar is inserted. With the click track in operation there's not too much problem, but the extreme is an external clock source which requires a lot of 'time' changes in Edit mode to get right and is too time consuming.

So Tangerine Dream stuff is fine provided you don't forget the click track and don't want external sync.

ENDPOINTS. If you use a 'click track' to record your sequence, each bar, or measure as it's called, is numbered and therefore allows a new start and endpoint to be specified.

The Apple II complete with Interface Kit in place.

I did find that looping was still a little awkward here as well, and what the software doesn't offer at present is a means to repeat certain parts of a piece - this is the most important way of saving memory space, which becomes the constant factor you have to keep an eye on all the time you're recording tracks.

Recording Sequences

CLEAR SEQUENCE. Before recording a new sequence, this selection must be used to get '0% memory used' on the menu. An initialising process takes place ready for you to begin.

CLICK TRACK. This is a hardware/software pulsed audio output that enables measures to be counted, successful looping, and special beginning and endpoints to be located. Interestingly, it can have an emphasised first beat with consequent beats made softer by using dual pulses 28 microseconds apart. The click track requires you to state a normal time signature (it does in fact ignore the 'bottom' figure), and beats per minute between 39 and 234 can be specified, or heard and adjusted up or down until you've got the working tempo.

AUTO SAVE. When this function is set ON (as shown on main menu), you'll save tracks so far recorded at the end of each run. You can get out if you wish, although it can be useful for saving different versions of a sequence as you build it track by track. As you become familiar with the program, it's only used at essential points to save time.

RECORDING. Recording the first track is known as recording from a 'clean slate' and you're asked to name the sequence and the voice in use on the Chroma. Both can have default tags if you press the 'Return' key and the voice can be changed again later. Pressing any key or the footswitch will begin and later end the recording. The actual recording does not start until you play the first keyboard note. If you change Chroma voices as you play, another track will be created.

Further tracks are done in the same way and it's likely you'll soon drop the click track and maybe make some volume adjustments to tracks or even 'mute' them. Certain tracks can be allocated to Chroma or Expander as well.

Recording can be done at slower (or faster) speeds by two methods. The pedal can change the speed but will not change the click track speed. Secondly, a 'Reconfigure' selection lets you set the Times Increment to 1/2X before recording and back to 1X for playback. Other set increment divisions can be chosen: (1/16, ⅛, ¼, and 1, 2, 4 & 8). There's also a Speed Change mode on the menu that can set variations in tempo during recording or playback - quite useful for tidying up your final tempo required before saving on disk.

Pressure can be recorded as well as standard key velocity, but this is an optional selection since it will use up a lot of memory - each note having an individual pressure reading.

Loop recording is another useful function. You can be playing back your sequence which will loop until you press the footswitch. Having 'cued' the system, the next time round you do your recording.


There are 3 groups of editing - various track directory accessing, speed changing, and the Editor mode.

Rear view of the Apple II with connector box and cable links.

TRACK DIRECTORY. This provides a new screen display of each track used in a sequence, including the track number, track name (you make a suitable title like TRUMPET), track port (C for Chroma, X for Expander), track program number (as shown on the instrument display), and track volume (always set initially to maximum 255).

Changing the program is particularly important, because as you build up tracks, some sounds may get lost or be too heavy and so on. You can also set a new volume level for each track, but it will last for the whole sequence. This, however, is hardly a problem with the tremendous dynamic range from the touch sensitive keys.

At present there are a couple of 'bugs' in the software noted in the manual. One is to do with the balancing of linked sounds and the other is the use of lever or pedal modulation for the link program (neither work until you get Rev. 13 software - the disk reviewed was only Rev. 3!).

If you play something wrong, and that can happen frequently, you can ZAP (delete) one or more tracks in a few seconds and try again.

There's also a TRANSPOSE function, for changing the overall pitch of one or all tracks up or down 33 semitones.

The MUTE/UNMUTE selection has its own menu and is useful for listening to selected tracks during the editing process. Muted tracks are displayed on the screen track directory in Inverse Video (black on white).

SPEED CHANGING. Using a swell type pedal or just a pot (value 100k LIN), allows speed changes during recording and playback through a built-in analogue to digital converter on the Interface card. A simple test program is provided to set this up accurately. Two types of pedal speed change can be made - large (twice to one half set speed) or small variations.

A clever effect is heard by looping with slight increased speed change being recorded - making an increasingly fast playback each time.


A unique display is provided in this mode, with editing options conveniently shown at screen top. These allow you to monitor (hear the sounds as they are displayed), change, delete or insert the command statements.

You definitely have to be more careful with this editor than others. If you don't think about what you're doing and make corrections or additions precisely, you can run into trouble on playback.

John Shykun (far right) talks Chroma with (l. to r.) John Hill, Mark Andrews, Peter Vetesse (Jethro Tull) and Vic Emerson (10 C.C.).

The screen display can show one or all of the tracks so far recorded. The process simply starts recording after a recorded time 'event', (depending on how long you pause before keying the first note) and then continues to display an attack or release of the tracks required, followed by the correct time event value in between. If you're just looking at Track 1 out of several, then you still get all the individual time event (interrupts) listed, but only the attack and release info on the one track.

All the necessary time dependent functions are shown and can be changed - these include the measure number, attack time, key number, velocity time, pressure value, volume number, levers 1 & 2 values, pedals 1 & 2 values, sustain and latch up and down, as well as any parameter on the instruments connected. If you enjoy this more exacting side of making music there is obviously a great deal you can do in editing mode to produce playback of continuously changing events.

The 'last event time' mentioned earlier will be the only key to knowing where a wrong note is and the only really annoying part of the editor is that you can only move forward through events - not backward. Using the 'repeat' key on the Apple you can step through your events, but usually run over and have to go back. This is even more of a hassle if your time event count exceeds around 65,500 (quite easy to do with 8X speed increment) because counting starts again. So you have to start at the very beginning, use the find function for time 30,000, then 60,000, then say, 5400 to get to 5500 accurately, (a maximum jump forward of only 30,000 approx can be done in one go). This 'find' function is a real help as it can actually locate any part of a statement such as ATTACK 1-20.

Making changes to time values to get your sequence loops to work is possible but requires calculations, often with large amounts. It's much better to record the problem track again.

Disk functions

The disk drive facility obviously speeds up access to everything you need to know about the Chroma and in a performance situation will let you load in a whole new set of 50 sounds (or 100 with the Expander) in a few seconds between numbers.

The Program File Management section has its own menu to allow Chroma or Expander program sets to be saved on disk, or the other way - from disk to instrument, plus deletion of a program from a disk file. At any time you can check the Disk File catalogue to see what's available as program sets or sequences. The latter can also be deleted.

Two options are available to reduce the amount of memory used up. First, measure numbers can be removed and second, a 'Scrunch' key reduces time counts - unfortunately, neither is really useful. Removing measures takes away the click track facility, and the Scrunch routine is likely to mess up chord timing in places - but just occasionally it will actually tidy up chord attacks for you instead!

RECONFIGURE. This is a sub menu selection that shows the present state of certain operating parameters of the Sequencer. It's used mainly to get fast or slow recording, or external and internal setting up correct. Once this is done, the new configuration can be saved and, if desired, set-up on power-up. Briefly, the options are for Changing Sync (wait for an external TTL type pulse before playing - but unfortunately not for record to keep the drum machine sync'ed after track 1 where it doesn't matter when you begin); Wait For Sync (this allows a single foot pedal to trigger events or inserts a start delay of between 1 and 15 pulses, sometimes necessary with certain external devices); whether Footswitch is being used; Emphasis required for click track or not; Timing Source: Internal, External, Single Step (footswitch or trigger pulse); plus the Sequencer and Instrument software edition (or Rev. No), disk and slot allocation.

Other selections in the menu provide further transfer between Expander/Chroma programs, Reinitialise of the Sequencer and instruments, error code information and access to utility programs that go through test procedures. They also let you write your own routines and access them as part of the set-up and give space for a 115 character comment - this is important for noting the program set in use and the specific external/internal set-up.


It took some two weeks (between other work!) to familiarise with the system. At first it is slightly tedious, but after a short while, with the Apple keyboard conveniently situated to the left or right of the Chroma (on top is a bit risky because of transmitting micro clocking down signal lines etc.), your fingers will make recording and playback extremely easy with, more importantly, little distraction from playing.

Some of the user limitations encountered were as follows: Looping (can corrupt start timing). Records from beginning only - you can't jump in and record a few bars only which would save a lot of time in a long piece. Pedal adjustment of speed change in 'large' variation mode is critical and difficult to precisely set to, say, an external drum without sync. System locks if you carry on playing and you're well out of instruments. On some occasions ext. clock appeared to miss pulses and lose sync (but only very rarely). If you look for an event in Edit mode you must be at a time point prior to the required event. Looping between new endpoints proceeded to mute programs one by one until no sounds were left! (May be a procedure order error on my part that's not stated). No trigger out available to stop a drum machine on playback. No copy/repeat function in edit. No ability to chain sequences.

That might sound a rather awesome list, but it's not really! The potential of the Sequencer package as a whole is tremendous for the serious composer/performer. The fact that you can play a solo keyboard performance track without any limitations makes this a very creative tool that for Chroma users would be in constant use whenever the instrument is played. As always there is never enough memory but I managed to record a 3-minute piece with 8 tracks and a 6-minute piece for solo guitar on 1 track with plenty of lever vibrato and pedal sustain. Already, there are revisions under way and these would be passed freely to Chroma users, and it is fair to say that CBS have taken their own initiative in developing this system over some years. The aim was to produce a player orientated package and this has been done exceedingly well through clear menu displays and a very well written manual. When you start to run your own programs as well (and make a link with MIDI!) the Chroma/computer interface will be a very worthwhile investment for serious stage, studio and film music composing and performance.

The Chroma/Apple Interface kit retails at £301.99 (inc. VAT) and is distributed in the U.K. by CBS Fender Ltd., (Contact Details).

Also featuring gear in this article

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Previous Article in this issue

Sound On Stage

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Naked Eyes

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

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Electronics & Music Maker - Apr 1983

Gear in this article:

Synthesizer > Rhodes > Chroma

Gear Tags:

Analog Synth

Review by Mike Beecher

Previous article in this issue:

> Sound On Stage

Next article in this issue:

> Naked Eyes

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