Music – Instruments – Electrical musical tone generation
Reexamination Certificate
1999-05-19
2001-09-04
Martin, David (Department: 2837)
Music
Instruments
Electrical musical tone generation
C084S622000, C084S659000, C084SDIG009
Reexamination Certificate
active
06284965
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to musical tone synthesis techniques. More particularly, the present invention relates to what is known as “physical-modeling synthesis” in which tones are synthesized in accordance with the mechanisms which occur in natural musical instruments. Such synthesis techniques are particularly useful for simulation of wind instruments and string instruments. By accurately simulating the physical phenomena of sound production in a natural musical instrument, an electronic musical instrument is capable of providing high quality tones.
BACKGROUND OF THE INVENTION
In the case of a wind instrument, the structure for synthesizing tones typically includes a filtered delay loop, i.e., a closed loop which includes a delay having a length corresponding to one period of the tone to be generated, and a filter which may attenuate and/or further delay signals circulating in the loop.
When the acoustic bore being imitated is not a uniform cylinder or cone, a plurality of closed loops are typically employed. Each closed loop corresponds to a single section of uniform cylindrical or conical acoustic tube, and the sections are coupled together by means of so-called scattering junctions. In the case of two adjacent acoustic tube sections having different diameters or different conical taper angles at the point of contact, the corresponding scattering junction typically receives an input signal from each section and provides an output signal to each section. The output to each section typically includes a filtering of the input signal from that section, called the reflected signal, and it typically also includes a filtering of the input signal from the adjacent section, called the transmitted signal from the adjacent section.
To excite a filtered delay loop with an input signal, a filtered delay loop may be coupled to a nonlinear junction which receives control inputs such as a signals corresponding to blowing pressure and embouchure, and which nonlinearly interacts with the signal received from the filtered delay loop so as to produce a sustained oscillation in the loop. The nonlinear junction typically corresponds to the reed in a woodwind instrument, the air-jet in a flute-like instrument, or the lips in a brass instrument.
Excitation signals introduced into a closed loop circulate in the loop. A signal may be extracted from the loop to form a tone signal. The signal will decay in accordance with the filtering characteristics in the loop. Additionally, the resonance frequencies of the loop are also affected by the filtering characteristics. The filter models frequency-dependent losses and delays within the bore of the instrument. A portion of these losses correspond to sound propagating out of the the bell or toneholes of the instrument. Other losses may correspond to the conversion of acoustic wave energy to heat. A portion of the frequency-dependent delay may correspond to the different effective tube length at the end of an open acoustic tube. More generally, acoustical waves experience partial reflection and dispersion when they encounter changes in the flare angle of the acoustic tube in which they propagate. In order to accurately model a natural musical instrument, therefore, it has been necessary to provide one or more filters in the filtered delay loop which give the desired losses, dispersion, and reflections found in a natural acoustic wind instrument.
A block diagram for a tone synthesis system as described above is illustrated in
FIG. 1B
for the case of a brass-type instrument shown in FIG.
1
A. The instrument in this example consists of a mouthpiece
2
driven by the player's lips
1
, a first conical bore section
3
, a cylindrical bore section
4
, a second conical bore section
5
, and a final flaring bell segment
6
. Since cylindrical and conical bores support traveling waves, the sound within them can be expressed in terms of left-going and right-going traveling-wave components. The right-going traveling-wave components in the cylindrical section, for example, correspond to the contents of delay element
16
, and the left-going traveling-wave components in the cylindrical section correspond to the contents of delay element
17
.
The cylindrical section
4
can be modeled well as simply a pair of delay elements. In reality there are small losses associated with acoustic propagation from one end of the cylinder to the other. However, for greater processing efficiency, such losses are typically applied elsewhere, such as at the junctions
7
and/or
8
between the cylinder and the adjacent acoustic tube sections.
As is known in the art, the conical sections
3
and
5
can also be modeled as a pair of delay elements. However, in this case, pressure waves traveling to the right are attenuated by 1/x, where x is the distance of the wave from the extrapolated apex of the cone containing the conical section.
When a right-going traveling wave encounters the bell
6
, a portion of it transmits through the bell into the outside air (which can be heard), and a portion of it reflects back into the bore
5
to help sustain the oscillation. This reflected signal is obtained by applying lowpass filter
13
to the right-going traveling-wave-component signal
30
to obtain the corresponding reflected wave
31
which then becomes the left-going traveling-wave-component signal entering the conical section
5
on its right side. The signal transmitted from the bell may be obtained by applying a transmission filter
61
, which typically resembles a highpass filter, to the incident right-going wave
30
to obtain the final output tone signal. A simple implementation of the the output highpass filter (transmission filter) is formed by simply adding the pressure-wave signals
30
and
31
, corresponding to a lossless bell model, with a microphone placement just outside the bell. (For pressure waves, the reflection filter
13
inverts at low frequencies.) In simplified tone generators, any traveling-wave component, such as
30
, can be taken as the output signal.
In a manner analogous to the bell reflection just described, when a right-going traveling-wave
40
in the cylindrical section encounters a change
7
in the taper angle of the bore, it splits into a reflected signal
41
and transmitted signal
42
. The reflected signal
41
is obtained by applying the reflection filter
18
to the incident wave
40
, and the transmitted signal
42
is obtained by applying the transmission filter
43
to the incident wave
40
. It is well known in the art that the ideal reflection filter
18
has the transfer function
R
(
s
)
=
-
c
c
+
2
xs
,
where c is the speed of sound, x is the distance from the apex of the conical segment
5
(which must be extrapolated far to the left in
FIG. 1A
) to the junction
7
, and s is the Laplace-transform variable. For very large wavelengths, the imaginary part of s is close to zero, and the reflection filter becomes close to a simple inverting reflection R(0)=−1.
It is known in the art that the reflection filter
14
, for waves
32
traveling in from the other side of the junction, has the same transfer function R(s). Finally, it is also known that the transmission is filters
43
and
44
both have the transfer function
T
(
s
)=1+
R
(
s
).
A filtered delay loop
10
is formed by delay elements
11
and
12
, lowpass filter
13
, and reflection filter
14
. Input signals are introduced in to this filtered delay loop by means of adder
15
. The delay elements
11
and
12
are normally combined into a single delay element for greater efficiency, as is known.
A second filtered delay loop is formed by delay elements
16
and
17
, reflection filter
18
, and reflection filter
23
. Input signals are introduced in to this filtered delay loop by means of adder
25
and adder
45
. The delay elements
16
and
17
can also be combined into a single delay element for greater efficiency.
A third filtered delay loop is comprised of delay elements
51
and
52
, reflection filter
21
, and
Smith, III Julius O.
Van Walstijn Maarten
Fletcher Marlon
Lumen Intelluctual Property Services
Martin David
Staccato Systems Inc.
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