GB2214695A - Electronic musical instrument - Google Patents

Abstract

An electronic musical instrument stores and replays music in a plurality of voices simultaneously. A memory 3 holds both instructions and the music data in digital form. The music data are read out and allocated to the voice channels, and are converted by a D/A converter 4 and a demultiplexer 5, followed by a sample-and-hold circuit for each voice channel, all this being under the control of a CPU 2. A WAIT control 15 allows for instructions which require an extended time for their execution. The memory 3 may comprise ROM and RAM, the ROM being paged with the RAM. <IMAGE>

Description

Electronic Musical Instrument The invention relates to electronic musical instruments, and in particular to electronic instruments arranged to store and replay musical sounds in a plurality of voices sounding simultaneously, the sounds being held in a memory in digital form.

An object of the invention is to provide an instrument of this kind which is versatile in performance and relatively inexpensive to construct.

The invention is defined in the claims appended hereto, and one example of it will now be described with reference to the accompanying drawings, in which Figure 1 is a simplified block diagram showing the main components of the instrument, and Figure 2 is a memory map showing the organisation of the memory.

SYSTEM OUTLINE Referring first to Figure 1, the main components of the instrument include a system clock 1 operating at a frequency of 6 Mhz, a CPU 2 in the form of a Z80B microprocessor chip, a memory 3 which stores both the data constituting the recorded music and also programming for the instrument itself, a D/A converter 4 for converting the digital data representing the music into a musical waveform, and a demultiplexer 5 which distributes the waveforms over the separate voice channels, of which there are four in the present example. The memory 3 includes RAM which is non-volatile so that it retains the recorded data from one session to another, and also ROM holding programmes, and optionally also, frequently used data such as standard drum rhythm patterns. The ROM may be paged with a section of the RAM, as will be described below.The instrument also includes various other components which will be described in due course.

The system requires only a single micro-processor which addresses directly both the data and programs stored in the memory and the output converter and demultiplexer. This renders it relatively inexpensive, and this open architecture, without specialised hardware, makes it resemble a programmable computer rather than a dedicated electronic musical instrument. This enables new functions to be introduced by software, and makes the instrument very versatile.

By way of illustration, the simplest mode of operation of the system will first be described, and this consists of the playing out of a piece of music previously stored in memory.

The music to be played out is in four voices, which might, for example, be independent instrumental parts, or two pairs of stereo channels, etc. At the time of recording the four waveforms will have been repeatedly sampled in turn, digitised, and the digital values stored in the memory 3.

The CPU sends the addresses of the locations in which the music is stored over an address bus 6 to an address decoder 7, which causes the memory 3 to retrieve the stored data and place it on a data bus 8. The data is held in a latch 9 from which it is read into the D/A converter 4. The analogue voltage output from this is then applied to the demultiplexer 5, which allocates it to one of the four output channels, each of which is provided with a sample-and-hold circuit. A low-pass filter is included in each output channel. Production of the separate outputs in this way is more efficient in terms of hardware than using a separate converter for each voice, and produces higher quality output than digitally summing the samples before conversion. This arrangement has application in systems other than that of the present invention.

Correct allocation of the signals to their respective output channels is maintained as follows: the four least significant digits of the address standing on the address bus 6 are recognised by a decoder/demultiplexer 12 and two of these digits specify the voice output channel to which the data relate, as will be more fully explained below. This decoder/demultiplexer provides the signals which, inter alia, synchronise the demultiplexer 5 and the latch circuit 9.

Addressing of the memory to obtain each word of music data as described above requires only a short time. However, the memory also holds programme instructions requiring considerably longer for their execution, for example when input is to be taken from a keyboard, or input or output is to be sent to or from tape or via an interface to some other device, or data are to be displayed, and provision is made for this by means of a WAIT control 15.

This comprises a counter controlled by signals from the demultiplexer 12 and decoder 7 which recognise the addresses of such instructions and provides a WAIT signal to the CPU 2. The WAIT control and its operation will be described in more detail below.

Input to the instrument may be from a keyboard 20, which is timed by signals from the decoder/demultiplexer 12, and the instrument is provided with a display 21, which may be of the liquid crystal type. External triggering devices, such as an electronic drum pad 22, may also be provided. A MIDI interface 23 allows the instrument to control or be controlled by other MIDI equipment such as synthesizers, sequencers or computers.

The peripheral devices are controlled by a control unit 24 which also receives control signals from the decoder/demultiplexer 12.

OUTPUT CONVERSION AND DEMULTIPLEXING The particular example of the instrument being described can play up to four sound samples from memory simultaneously, each souiid having its own scparate audio output channel. In order to achieve ttiis the single D/A convertor is "tinie-stiared" by each channel in turn, and its analogue output simultaneously routed to the relevant audio channel by tile de-multiplex circuitry.

When the system is replaying sounds, the CPU reads one byte of data from each of tile four samples being played, at the rate of 31250 Hz. Each of these four bytes is then written by ttie CPU to tile I/O address corresponding to the relevant audio channel.

Which of tile four channels the output of the DAC is sent to is determined by bits 2 arid 3 of tiie address, giving the for-ln xxxxccll where cc is tlio channel:- OO channel A 01 cliarinel B 10 channel C 11 channel D This is ttie sequence of events performed by tile CPU: : head next byte of 1st sample Output to channel A Read next byte of 2nd sample Output to channel B nead next byte of 3rd sample Output to ciianriel C Iead next byte of 4th sample Output to channel D Thins happens 31250 tiles per second when the system is playing sounds, in addition to tti other tasks tile CPU is also perforlllirlg. siicli as reading the keyboard MIDI, synchronisation, time correction, and reading data to and from memory.

'WAIT' CONTROL The WAIT control comprises e binary counter arranged to count a suitable number of clock pulses, for example 12. The two least significant digits of the address on tile address bus 6 indicate whether the ROM or an interface are to be addressed. These digits are decoded by the decoder/demultiplexer 12, which triggers the counter, and this in turn provides a WAIT signal to the CPU until the count is completed.

INPUTJOUTPUT As mentioned above, the two least significant digits of the address are decoded by the decoder/demultiplexer 12, and are used to select one of four port select lines. These are: Al AO no. Direction Name Function . 0 0 0 read/write PORT o MIDI and LCD o 1 1 read KB KEYBOARD READ 1 0 2 read PAD EXTERNAL PAD READ 1 0 write CTRL INTERNAL CONTROL 1 1 3 write E DAC WRITE PORT 0 triggers tlie CPU WAIT control, and is used with further address decoding by 141D1 and. tlie LCD iliterface KB provides the enable pulse ts latch . tlie output of the keyboard onto the data bus Address xxxxx.'.10 (binary) on input (i.e read) reads the external drum PAD (8 lines) Address xxxxx):lO on output (write) enables the control unit 24 which latches the output from the data bus.

This provides control lines tor memory paging, tape output, and GOUND enable.

Address xxxxxall is used for writing data to the D/A convertor.

MEMORY ORGANISATION The memory. organisation is'shown in Figure 2.

The ROM is mapped to the bottom'l6k of the Z80 memory map, and contains the system firmware. This comprises USER INTERFACE FILING 6Y6TEM INPUT/OUTPUT CONTROL REAL-TIME ROUTINES plus various housekeeping and calculation programmes.

16k of RAM is mapped onto the next 16k of the memory map, (H4000 to H7FFF), and is used for system variables, editing buffers, and filming space for programmed drum patterns.

The remaining 64k of RAM is arranged as a continuous block (H000 to HFFFF), all of which is used for audio sample storage except the top 4k which is used for the machine stack and real-time routines.

The bottom 32k of the memory map is pageable, that is, when accessing addresses H000 to H7FFF the processor can decide to look at either 16k ROM + 16k RAM or the bottom 32k of the sample RAM block. (6ee diagram ).

On power-up the ROM side is paged in, but when real-time operation is required for sample playback the RAM block is paged, giving 64k of continuous RAM. The real-time routines from the ROM are loaded into the top 4k of RAM in real-time operation.

Claims (7)

1. An electronic musical instrument arranged to store and replay musical sounds in a plurality of voices sounding simultaneously, the instrument comprising a central processor, a memory storing both system control commands and digital data representing waveforms to be output simultaneously in analogue form on a plurality of output channels, and means for converting and de multiplexing the signals on to output channels, the central processor being arranged to address the memory to obtain each data signal, and to address the converting and demultiplexing means to allocate the signal to its proper channel.

2. An electronic musical instrument according to claim 1 in which the memory is a random access memory and the instrument is provided with input means whereby data representing sounds can be input and stored in the memory under the control of the central processor.

3. An electronic musical instrument according to claim 2 wherein the input means include at least an input port for receiving sound signals in digital form, and a keyboard.

4. An electronic musical instrument according to any preceding claim including both a random access memory and a read-only memory containing system control programs.

5. An electronic musical instrument according to claim 4 in which the read-only memory can be paged with the random access memory, the system control programs being loaded from the read only memory into the random access memory.

6. An electronic musical instrument according to any preceding claim including a clock circuit operating at a constant frequency and controlling the central processor and a WAIT control, the AWAIT control being arranged to respond to the address of an instruction requiring an extended time for its execution and thereupon to cause the addressing of the random access memory to be delayed for a predetermined number of clock cycles.

7. An electronic musical instrument according to any preceding claim in which the means for converting and de-multiplexing the signals comprises a digital-to-analogue converter arranged to convert each byte of digital data to a corresponding voltage, followed by a demultiplexer cdmprising switching means for distributing the analogue signals to their respective channels and a sample-and-hold circuit for each channel, the digital-to analogue converter and demultiplexer being under the control of a decoder decoding the address to which the byte of data was written.