Kendall Wrightson opens the lid on electronic music
Major electronic musical instrument manufacturers such as Yamaha, Roland and Sequential Circuits have not been slow in incorporating the latest technology into their machines. The rate at which synthesisers, sequencers and drum machines appear is almost as staggering as the facilities they offer.
Most home computers have sound facilities. The better ones, musically speaking, like the CBM-64 can produce reasonable results if you have the patience and aptitude to write your own programs.
Dedicated synths involve a lot of clever hardware as well as software and the more professional musical home computer packages either include extra hardware such as voice cards to transform the micro into a synth, or use the micro to control a dedicated synth as we shall discover.
Synthesisers utilise many techniques to create imitative sounds. The most common method, known as subtractive synthesis, involves control of the frequency of one or more oscillators from a music keyboard, guitar or computer.
The oscillators produce wave forms of complex harmonic structure, like triangle, sawtooth and square waves. These wave forms are then modified by a sort of special tone control called a filter.
Varying the filter’s cut-off frequency removes harmonics of the complex wave form – hence the term subtractive. However harmonics may also be greatly emphasised by increasing the filter’s resonance or Q.
The filtered tone then enters an amplifier, but will sound uninteresting to the human ear, because it is static. This is overcome by generating modulation signals which can be routed to the filter, the amplifier and the oscillators. These modulating signals may be generated by low frequency oscillators or LFO’s, velocity and pressure sensors fitted to the keyboard, and by performance controls.
One special kind of modifying signal is an envelope – so called because of the way it shapes sound. The envelope’s parameters, attack, decay, sustain and release – ADSR – are set up by the user. When applied to the amplifier, the envelope shapes the volume of the sound applied to its input in the following way – having played a note, the attack rate controls the time taken for the sound to reach a maximum level. Decay is the time taken for the sound to reach a sustain level. The sound will remain at this sustain level until the key is released whereupon the sound will die away at a rate dependent upon the release setting – see figure 1.
Figure 1. ADSR envelope.
A collection of control settings is called a patch. Before the advent of the microprocessor and cheap memory, a change of patch involved twiddling all the control knobs to their appropriate positions – a somewhat time consuming activity, particularly during a live performance!
These days life is made easier because patches are stored in memory and may be instantly recalled. Modern synths also allow patches to be saved to and loaded from tape.
Other methods of synthesis include frequency modulation techniques or FM, additive synthesis and more recently, the sampling of real sounds. However, the terminology described above may be applied to any method of synthesis.
Simulating drum sounds electronically is extremely difficult. For this reason contemporary drum machines use samples of real drum sounds. The sounds are digitised through an analogue to digital converter – ADC – and stored in Eprom. So when you hit the trigger pad or press the button, the Eprom’s contents are clocked out through a digital to analogue converter, or DAC, under microprocessor control.
Real time recording on a drum machine means hitting pads or buttons to an accompanying metronome click. The data provided from this activity is then stored in memory. Thankfully, timing errors can be automatically corrected if desired.
Individual rhythm patterns are each assigned a number so that they may be chained together to form a “song”.
For those of us with no intention of hitting buttons in time with a metronome, some machines offer step time programming. In step time, a number of beats to the bar is chosen. Then decisions are made regarding which drums should sound on which beats.
A home computer could make step time programming easier by displaying information on a monitor. Software is becoming available for the Apple IIe, which links via RS-232 to the Drumulator machine, to perform this very task.
MPC Electronics went one step further by incorporating a ZX-81 as an integral part of the percussion computer. The ZX-81 is connected via an interface unit which also contains the software it runs. The addition of the Sinclair 16K RAM-pack increases the machine’s memory, while the software allows the loading and saving of patterns, as well acting as a visual aid to composing.
Sequencers, like drum machines, record data with respect to time. The difference is that sequencer data represents keyboard depressions and the length of time that keys are held down, so why not make a conventional analogue recording?
Well, first, a sequencer can play back at different tempos, without affecting pitch. Secondly, the sequence may be played back using a different pitch to the one it was programmed with. Thirdly, sequencers offer the non-musician the chance to compose music through step time note entry, where pitch and timing information is entered separately.
Again, like drum machines, sequences can be chained together to form “songs”. The sequence order can be changed if it is unsatisfactory.
Originally, synthesisers used to generate a control voltage or CV – proportional to the pitch played, and a gate signal – in proportion to the length of time a key was held down. It was these signals, after analogue to digital conversion, that sequencers used to record. This was fine for synths which were capable of playing only one note – monophonic – since it would require only four cables to connect a synth to a sequencer for record and playback.
Modern integration developments have led us to eight and 16 note polyphonic synths. The problems encountered in connecting up 32 cables from an eight-note polysynth to sequencer do not encourage musical spontaneity! The situation is further aggravated when you discover that different synths use different CV and gate voltages!
However these, and other problems have recently found a solution in the development of a data protocol for electronic musical instruments called MIDI or Musical Instrument Digital Interface.
MIDI is a new word to add to your vocabulary of computer-speak. It came about, like MSX, out of a need to standardise a very non-standard world. Things begin to get very exciting when synths, drum machines and sequencers operate together as an integrated unit. However, before MIDI was agreed by the major synth manufacturers, it was difficult to get excited about such things due to the inherent non-compatibility of products.
Each drum machine had its own way of telling the outside world it had started, then there’s the sequencer problem mentioned earlier.
MIDI cures such problems, as well as allowing the transfer of much more specific messages.
Physically, MIDI appears as two or three 5-pin 180 deg. DIN sockets on a synth, drum machine or sequencer – MIDI In, Out and Through. The MIDI Through socket outputs a direct copy of data entering the MIDI In socket. A manufacturer does not have to fit a MIDI Through facility. Only two of the five DIN pins are used, so MIDI transmits and receives data serially.
This particular facet of MIDI was heavily debated because it was felt by some manufacturers that a serial link would be too slow. In practice there have been complaints of noticeable delays when transmitting keyboard data to more than three synths at once. However, the convenience of using single 5-pin DIN leads rather than multicore cables must have tipped the balance for the supporters of serial transmission.
MIDI runs at 31.25 Kbaud asynchronous. The word format is shown in figure 2. So, to MIDI-fy your micro, wire up an asynchronous communications interface adaptor or ACIA, like the Motorola 6850 to the expansion orifice of your micro. Address the ACIA nicely and tell it to transmit and receive as in figure 2, i.e., one stop bit, one start bit and no parity.
Figure 2. MIDI serial
Wire the ACIA transmit output and input to appropriate 5-pin DIN sockets.
Don’t forget to opto-isolate the MIDI In input otherwise nasty earth loops could develop. Now write some brilliant software and make lots of money! Seriously though, for anyone considering designing their own interface, the MIDI hard and software specification is available from the MIDI Users Group, 8426 Vine Valley D.R. Sun Valley, CA91352, U.S.A.
The MIDI data format is divided into two categories – channel commands and system commands. The channel command format allows for 16 unique channels for communication between instruments.
One of the most fundamental tasks MIDI must allow is for one synthesiser to play another. The channel command structure gives three ways or modes of performing this task.
In omni mode, all synthesisers connected together will transmit and receive on all channels. In poly mode each synth is set by the user to receive on only one channel. The synth will therefore ignore any incoming data which is not on its assigned channel. Figure 3 shows a typical poly mode set up. Note that Synth A. is used as the MIDI transmitter!
Mono, the third possible mode, allows the allocation of different MIDI channels to individual voices within one synth. This opens up the exciting opportunity of one synth playing different patches on each voice. However, at the time of writing, the only reasonably priced synth capable of mono mode operation is the Sixtracks made by Sequential Circuits.
Let’s take a meaningful example – suppose a middle C is played on a synth. In the MIDI scheme of things this is called a note on event. Three bytes will be transmitted from the synth’s MIDI Out socket to represent this:
First Byte – 1001 nnnn
Where 1001 means note on event and nnnn is the MIDI channel number (0 to 15)
Second Byte – 0kkk kkkk
Where kkk kkkk is the key number – 0 to 127 semitones
Third Byte – 0vvv vvvv
Where vvv vvvv is the velocity at which the note was played (0-127 levels). So, if you delicately stroked the key, you would generate a velocity byte equal to 1. If, however, you hit the key with a large mallet, you would generate a velocity of 127 – this practise is not advised.
Synths which do not have velocity sensitive keyboards transmit a velocity byte of 64 (decimal) as a de-fault value. So, if a middle C was played on a non-velocity sensitive synth set to MIDI channel 1, the data transmitted would be:
10010000, 00111100, 01000000 (Binary)
Other channel commands include note off event – 3 bytes – and patch change request – 2 bytes.
The second category of commands, systems commands, is divided into three types: system common; system exclusive and system real time.
System common commands are those intended for all devices in the system. An example is asking synths to tune their oscillators, a tune request – 1 byte.
System exclusive commands are those applicable between instruments of the same internal design. The system exclusive command, 240 (decimal) is therefore followed by a number representing the manufacturer – Sequential Circuits’ number is 01.
The number of bytes which follow is dependent on the nature of the data to be transmitted. An end of system exclusive is flagged by transmitting 247. Examples of system exclusive information are patch dumps and specific control knob changes.
The third category of system commands, system real time, are those messages concerned with synchronisation. They can be transmitted at any time by sequencers or drum machines. Examples are, Start, Stop, Reset and Timing Clock. The timing clock pulses are sent at a rate of 24 clocks per quarter note. Most MIDI drum machines and sequencers have Trigger, Clock or Sync outputs like their non-MIDI counterparts so as not to alienate customers with pre-MIDI equipment.
A micro fitted with suitable MIDI interface could perform wondrous tasks as part of a MIDI set up. Here are some examples:
- Patch data dump to take on disc.
- Patch data display.
- Music transcription.
- Intelligent arpeggioator.
- Educational software.
The following is a survey of commercially available interfaces and software for home micros. The list is not exhaustive, the criteria being to cover as many micros as possible.
Sequential Circuity, the pioneer of MIDI, has taken the CBM-64 under their wing and come up with a 400C note real time sequencer called the Model 64.
The Model 64 allows overdubbing, auto time correction and transposition. Its six tracks can be chained together and both sequences and songs may be dumped to tape or disc. A drum machine sync input is provided, although sequences may be recorded without a drum machine connected.
Passport Designs – designer of the Soundchaser software – will shortly be launching a MIDI card for the CBM-64. The card includes MIDI In, Out and drum sync connections. The MIDI/4 software provides 16 real time tracks, each of which can be assigned its own MIDI channel and instrument name.
The MIDI/4 software can also run on an Apple II or IIe using Passport’s Apple interface card. This card is also used for music transcription software called Polywriter. The software is a four note polyphonic version of the Notewriter monophonic transcriber for the Soundchaser system. Polywriter allows printouts in eight different formats, ranging from single, treble and bass clef parts, to large orchestral stores.
The ZX Spectrum is popular among the small entrepreneurs, like Upstream whose software consists of a six track, 3500 note real/step time sequencer. The interface, which is included in the overall price of £179, boasts a trigger output along with MIDI In, Out and Through connections. Optional extras include editing facilities and a dot and stave graphics display.
XRI Systems is asking £108 for its MicronMidi interface and software for the Spectrum. Micron is an 8,000 note real time sequencer with MIDI In, Out and Through connections as well as a trigger output. The Micron can also handle step time note entry in eight tracks, each of which can hold 3,000 notes. Tracks may then be merged or “bounced’’ onto on track to make space available for further recording.
If you are considering changing your micro, then Yamaha’s MSX computer, the CX5 may well be worth the wait, it is expected in November.
The CX5 is actually going to be marketed as a musical instrument as well as a home computer in this country, due to the fact that it comes fitted with MIDI interface and an FM voice module as standard.
The Yamaha CX5 is not the same as the Yamaha Y1S503 MSX computer reviewed by the British computer press recently.
Having typed Call Music, the CX5 becomes an eight note polytechnic, 48 patch synth. There is also a rhythm box which unfortunately is rather weak.
The CX5 also allows 48 of your own FM synth patches which are used in the CX5’s built-in real time sequencer. The CX5 is expected to retail for about £560, fair dos for its synth facilities alone.
The BBC Model B gets the MIDI treatment from Electro-Music Research (EMR). Its Miditrack software is step time only onto six tracks, however dynamics can be programmed. The Interface which connects to the Beeb’s 1MHz Bus, provides MIDI In, Out and drum machine synchronisation facilities. The interface and software is expected to go for about £120.
- MIDI, though still very young, has definitely caught the imagination of both manufacturers and public. Its now almost impossible to sell any electronic musical instrument that doesn’t feature MIDI in its specification.
- Both the Soundchaser and PDSG systems are likely to incorporate MIDI shortly.
- At present, the cheapest polyphonic synthesiser with MIDI is about £650; the cheapest MIDI drum machine is £950. So, assuming you use a MIDI home micro as the sequencer, a professional set up is going to cost about £1,750. However, the prices of synths and drum machines has been falling sharply over the past five years and this is a trend that is sure to continue.
- It’s clear that there are plenty of ways of getting extremely musical with your micro, even if your micro has not expressed a musical bent in the past. Why not take the plunge? It could prove to be a very rewarding experience.
Casio – ZX Spectrum
Casio has been producing electronic keyboards by the million in the last few years. Indeed a recent survey asking young people to list their favourite toys, had portable keyboards up at the top of the list along with micros and BMX bikes.
It is good to know that you can now link a Casio MT-200 portable keyboard to a ZX Spectrum – and most other popular micros – via the Casio PA1 interface. The software, listed in the MT-200 manual turns the Spectrum into a sequencer with editing facilities.
Alternatively the software is available on cassette from Micro Musical Limited, which is also working on a system called Microlink 2. This will allow the linking of two existing Casio models – the MT800 and PT-80 – to the Spectrum to provide a sequencer which can also turn the Casio auto-rhythms on and off.
Soundchaser for the Apple
The Soundchaser turns an Apple II or IIe micro into a dedicated synth, through the insertion of three cards into the Apple’s magic slots. Also provided is a four octave music keyboard.
Passport Design’s Four Track Performance Software makes the Apple act like an eight voice polyphonic synth. There are two soft oscillators per voice, each with independent ADSR’s and one LFO which can independently frequency modulate either oscillator.
Because the oscillators are soft, you can edit existing waveforms or waves on the VDU, using a joystick or create your own. You can also build up a wave by controlling the amplitudes of a table of 16 harmonics. This is additive synthesis mentioned earlier.
All waves displayed may be printed, as can voice parameters.
The Soundchaser also provides a filter, and although you cannot control it dynamically with the ADSR envelopes, it’s possible to type in the cut off frequency then see as well as hear the result. It is this combination of both additive and subtractive synthesis which makes the Soundchaser produce such a wide variety of sounds.
Also included in the software is a fourtrack real time polyphonic sequencer, although step time editing software is available. The sequencer allows four different patches to play up to eight notes simultaneously. All voice and sequencer information can be stored on disc as wave or track files.
The Hardware and Four Track Performance Software comes to £1,369, which may seem a lot, but comparable dedicated systems cost between £3,000 and £8,000.
PDSG for the BBC
A similar system to the Soundchaser is being developed for the BBC Micro by Clef Products. The Programmable Digital Sound Generator or PDSG, allows 8-32 note polyphony from a five-octave music keyboard which is included in the provisional retail price of £400. Software being developed includes waveform creation and sequencing. It will be interesting to see how the PDSG compares to the Soundchaser when it is formally launched.
First published in Your Computer magazine, September 1984.