[0001] The present invention relates to electronic musical instruments and in particular
to a digital-to-analog converter circuit by means of which samples of a complex audio
waveform produced by digital circuitry of the instrument are converted into the respective
audio waveform.
[0002] Modern electronic musical instruments, as electronic organs, comprise memories for
one or several wave forms from which amplitude samples are read under the control
of signals produced by the player via at least one manual, stops, tabs, and switches.
The output of the digital part of the circuitry is a data stream comprised of pulses
having equal durations, the magnitude of the pulses being representative of the sample
amplitude, and the repetition frequency of the pulses being representative of the
audio frequency to be produced.
[0003] If the pulse magnitude is simply proportional to the sample amplitude, the respective
D-A-converter is usually a simple RC lowpass. If, however, the pulses each represent
the amplitude difference of the respective sample relative to the preceding sample,
the D-A-converter comprises an integrator circuit including an operational amplifier.
[0004] Usually, the audio signal is passed via filters and amplifiers to a loudspeaker system
so that, for example, the character of the sound may be affected at will. Frequently,
the "brilliance" of the sound is to be modified. "Brilliance" means that in a sound
comprising treble and bass compo nent, the treble components are emphasized.
[0005] It is the object of the present invention to provide a D-A-converter for instruments
wherein the pulse magnitudes represent the amplitude differences of the respective
sample amplitudes which convert, moreover, provides an "automatic brilliance control",
i.e. the converter provides an attenuation of the bass frequencies only if and when
there are also treble components in the sound. In other words, the player once selects
a particular bass/treble balance, and this is automatically adapted during play with
a tendency to more "brilliance".
[0006] The converter circuit according to the invention is defined in patent claim 1. Surprising
enough, the complex effects outlined above require only a simple RC network provided
that its time constant be substantially longer than the duration of each pulse. It
is preferred to have this time constant in the order of magnitude of one period of
the highest audio frequency to be produced. With such figures, the magnitude of the
analog signal on the capacitor is sufficiently high to provide a satisfactory signal
to noise ration.
[0007] The attached drawing is a simplified circuit diagram of the converter circuit according
to the invention.
[0008] Block 10 symbolizes the digital circuitry of an electronic organ adapted to produce
a pulse sequence wherein all pulses are of identical duration (or "width"), the repetition
frequency is commensurate with the audio frequency to be generated, and the pulse
magnitude is representative of the amplitude difference of successive wave form samples.
[0009] The pulse duration, in the embodiment, is 300 nanoseconds for each pulse.
[0010] Block 12 symbolizes a multiplex switch, e.g. of the CMOS type 4051. Its purpose is
to decouple block 10 from the converter during interpulse intervals so to make the
pulse feeding circuitry have a high output impedance.
[0011] The very converter comprises resistor R having a value of 3.9 KiloOhm and a shunt
capacitor C of 22 nanoFarad in the present embodiment. One terminal of the capacitor
is connected to ground as reference potential. An amplifier 14 having a high input
impedance is connected to the R-C-junction.
1. - Feeding circuit means having a high output impedance and supplying pulses of
equal duration, the magnitude of sa id
pulses representing samples of an audio wave form such that each pulse magnitude is
the difference of the amplitude of the represented sample relative to the preceding
sample (Delta modulation), the repetition frequency of said pulses being representative
of the audio frequency of said audio wave,
- a resistor connected with its terminals respectively to said high impedance output
of said feeding circuit means and to a first terminal of a capacitor whose other terminal
is connected to a reference potential,
wherein said resistor has a value R and said capacitor has a value C such that the
product R.C is substantially greater than said pulse duration.
2. A converter as defined in claim 1 wherein said product R.C is substantially equal
to one period of the highest audio frequency to be generated.
