1. Field of the Invention
The present invention relates to a tone waveform generation apparatus capable of efficiently mixing a plurality of tone generation processing modes.
2. Description of the Related Art
Tones of acoustic instruments have complicated harmonic structures. For example, the frequencies and amplitudes of harmonics always vary, and in some instruments, harmonics include those of fractional orders. In addition, tones have noise components unique to respective instruments (e.g., shock noise upon attacking of a piano). Such harmonics and noise components characterize tone colors of instruments.
In order to faithfully reproduce such tones by an electronic musical instrument, or in order to create tones of an unprecedented feeling, various tone generation processing methods are employed in the electronic musical instrument.
These processing methods include many methods, such as a PCM method, a frequency modulation (FM) method, a phase modulation method, a harmonic addition method, and the like. When these methods are appropriately combined, the above-mentioned object can be accomplished to some extent. For example, since the harmonic structure is complicatedly varied in an attack portion, the PCM method is employed. For the following sustain portion (steady portion), the PCM method may be switched to another method since a memory having a large storage capacity is required if the PCM method is employed.
For example, in Published Unexamined Japanese Patent Application No. 58-102296, a tone waveform for an attack portion is generated by the PCM method, and a tone waveform for the following portion is generated by the FM method, thus obtaining tones.
This method will be described below.
When the depression of a given key is detected, numerical value data corresponding to a pitch of the ON key is repetitively accumulated to obtain an accumulated value whose value changes at a speed corresponding to the pitch. Thereafter, waveform data of an attack portion is read out from a PCM waveform memory using the accumulated value as an address, and an envelope of the attack portion shown in FIG. 1A is given to the readout waveform data.
On the other hand, the accumulated value, and another constant are input to an FM tone generator, thus obtaining a tone waveform. Thereafter, an envelope shown in FIG. 1B is given to the tone waveform. The tone waveform obtained by the PCM method and the tone waveform obtained by the FM method are added to each other to generate a total waveform signal of a tone having an attack portion, a sustain (steady) portion, and a release (decay) portion of a tone, as shown in FIG. 1C.
As the first prior art of an FM method which can be utilized in the above-mentioned method using a combination of the PCM method and the FM method, an FM method described in, e.g., U.S. Pat. No. 4,018,121, or the like is known. This method basically defines a waveform output e given by the following equation as a tone waveform: EQU e=A.multidot.sin {.omega..sub.c t+I(t) sin .omega..sub.m t}(1)
With this method, a carrier frequency .omega..sub.c and a modulation frequency .omega..sub.m are selected at a proper ratio, a modulation index (modulation depth function) I(t) which can be changed over time is set, and an amplitude coefficient A which can be similarly changed over time is set, thereby generating a tone which has complicated harmonic characteristics, and whose harmonic characteristics can be changed over time. Therefore, an electronic musical instrument using this method can synthesize a tone similar to that of an acoustic instrument, and can obtain a very unique synthesized tone.
As the second prior art of the FM method, a method described in Published Examined Japanese Patent Application No. 61-12279 is known. This method adopts a triangular wave arithmetic operation in place of the sine arithmetic operation of equation (1), and defines a waveform output e given by the following equation as a tone waveform: EQU e=A.multidot.T{.alpha.+I(t)T(.theta.)} (2)
where T(.theta.) is the triangular wave function generated by a modulation wave phase angle .theta.. With this method, a carrier wave phase angle .alpha. and the modulation wave phase angle .theta. are appropriately advanced, and a modulation index I(t) and an amplitude coefficient A are appropriately set, thus generating a tone waveform.
A tone of an acoustic instrument such as a piano includes a fundamental wave component based on a pitch frequency, and harmonic components at frequencies as integer multiples of the pitch frequency, and a harmonic component of a considerably higher order may be present. Furthermore, the tone often includes harmonic components of fractional orders. Tones having good sound quality are generated by these harmonic components. In a decay process of a tone of an acoustic instrument, the amplitudes of harmonic components are decreased in the order of higher orders, and finally, a single sine wave component corresponding to a pitch frequency remains. Some tones include only single sine wave components. Furthermore, when a key of, e.g., a piano is strongly depressed, a tone including many harmonic components of high orders can be generated. Contrary to this, when a key is very weakly depressed, a tone including only a single sine wave component can be generated.
Since the first prior art of the FM method is based on modulation using a sine wave, when a value of the modulation index I(t) in equation (1) is changed to be closer to 0 over time, a process for decaying a tone to only a single sine wave component, or generation of a tone consisting of only a single sine wave component can be realized, as in a tone of an acoustic instrument. However, frequency components of a tone generated by equation (1) are concentrated on harmonic components of low orders (having low frequencies). Thus, even when deep modulation is executed with a large modulation index I(t), harmonic components of high orders (having high frequencies) cannot satisfactorily appear. Therefore, the first prior art cannot generate a tone having good sound quality, and sound quality of a tone which can be generated is limited. For example, even when the modulation index I(t) can be controlled to have a large value, the levels of harmonic components of high orders which can be generated are limited, and a tone including many harmonics of high orders cannot be generated. As a result, if the first prior art of the FM method is applied to the method of synthesizing a tone while switching the PCM method and the FM method, a desired tone cannot be obtained.
In contrast to this, since the second prior art of the FM method is based on modulation using a triangular wave which originally includes many harmonics, a tone in which harmonic components of high orders are clearly present as frequency components can be easily generated. However, since equation (2) does not include a term of a single sine wave component, a process for decaying a tone to only a single sine wave component, or generation of a tone consisting of only a single sine wave component like in, e.g., a decay portion after an ON event of a high-note key of a piano cannot be realized. Therefore, even when the modulation index I(t) can be controlled to have a small value (e.g., 0), no control for generating only a single sine wave component can be performed, and hence, a soft tone cannot be generated. As a result, even when the second prior art of the FM method is applied to the method of synthesizing a tone while switching the PCM method and the FM method, a desired tone cannot be obtained.
On the other hand, a polyphonic type electronic musical instrument which can use a plurality of tone synthesis methods is known. In a conventional electronic musical instrument of this type, tone generation channels are predetermined in units of tone synthesis methods, and a plurality of tone generators perform time-divisional operations to generate tones of the respective channels. For this reason, each tone generator cannot generate a tone from a channel other than those assigned in advance. For example in a 16-tone polyphonic electronic musical instrument, assume that the PCM method is assigned to the first to eighth channels, and the FM method is assigned to the ninth to 16th channels. In this case, for example, the sixth channel cannot produce a tone generated by the FM method. Even when the FM method is not used at all, the PCM method can only be used in eight channels, resulting in poor efficiency.
When different tone generation methods are merely mixed in an electronic musical instrument, a circuit arrangement is complicated, thus inevitably increasing cost. For example, in each of the above-mentioned prior arts, a portion common to the two methods includes only a circuit for obtaining numerical value data corresponding to a pitch of an ON key, and a circuit for accumulating the numerical value data. Thus, other circuits, i.e., an FM tone generator, a PCM waveform memory, two envelope generators for giving envelopes to the corresponding tone waveforms, multipliers, and the like constitute an uncommon circuit arrangement.
Furthermore, in an electronic musical instrument in which a plurality of different tone synthesis methods are mixed, the above-mentioned method as a combination of the PCM method and the FM method of the first or second prior art may be assigned to different tone generation channels. In this case, however, the FM method of the first prior art cannot generate a tone including many harmonics of high orders, and the FM method of the second prior art cannot generate a soft tone. As a result, a desired tone cannot be obtained.