RU2734572C1 - Method for determining pitch in electric musical instruments of heterodyne type - Google Patents

Abstract

FIELD: electric musical instruments.SUBSTANCE: invention relates to electric musical instruments, in which an audio signal is obtained by heterodyning - formation of a tone of difference frequency (beats) during interaction of signals from two high-frequency generators. Method of determining pitch height of heteromedical electric musical instruments is to measure frequencies of signals supplied to differential frequency generator, or one frequency at known value of the other, and frequency of the resulting audio signal is calculated based on their difference.EFFECT: technical result is possibility to determine current tone height with high accuracy, high speed and wide range, including fact of falling into so-called negative beat region.3 cl, 4 dwg

Description

The technical field to which the invention relates

The invention relates to electromusical instruments, in which the sound signal is obtained using heterodyning - the formation of a tone of the difference frequency (beats) when signals from two high-frequency generators interact. Determination of the pitch (the frequency of the sound signal) is required for visual indication, automatic sound mute upon transition to the "negative beat area" and dynamic change of sound characteristics (timbre, special effects, etc.) depending on the current pitch.

The problematic characteristics in determining the pitch are accuracy (resolution) and speed.

State of the art

For electro-musical instruments of heterodyne type that are awaiting entry into the game (in particular, theremins), there is a known method for determining the pitch "by ear" using a small-sized telephone capsule inserted into the performer's ear, to which a sound signal is constantly supplied (for example, from the output of "Phones "With an appropriate configuration of the theremin" Etherwave Plus "by Moog Music Inc., USA). This method is an alternative to the visual pitch indicator and allows the musician to quickly and with minimal miss-play after a long pause. The disadvantage of this method is the performer's auditory disorientation and the need to have perfect pitch when introducing without musical accompaniment.

A known method for determining the pitch by measuring the frequency of the audio signal, taken from any point in the sound path. The measurement is carried out, as a rule, by a digital method, and the result is converted into a form convenient for subsequent processing or direct perception by the performer. For example, the practice of connecting industrial electronic tuners to the “Tuner” jack of the “Etherwave PRO” theremin from Moog Music Inc., USA, or to a specially organized connection point (manual “Understanding, Customizing, and Hot-rodding Your Etherwave Theremin” - URL: http://www.suonoelettronico.com/downloads/HotRodEtherwav.pdf, date accessed: 01/25/2020). The disadvantage of this method is the impossibility to provide the required accuracy and speed when determining very low frequencies. Also, this method fundamentally does not allow to establish whether the instrument operates in the so-called "negative beat region" (ie, whether the frequency of the tunable generator exceeds the frequency of the reference one or not), which is necessary to implement the automatic mute function.

There is a known method for determining the pitch by converting the frequency of the audio signal into voltage (for example, for subsequent indication in the "Theremin Pitch Visualizer" L. Korolev - Radio, 2005, No. 10-11, p. 48, or for controlling external devices in the theremin “Theremax” by PAiA Electronics Inc., USA), but it also works unsatisfactorily in the low frequency region and does not allow identifying the “negative beat region”.

There is a known method for determining the "area of negative beats" in the theremin "Skywave H1" (URL: http://www.thereminworld.com/files/Pages/28/files/SkywaveHi.pdf, access date: 01/25/2020), which consists in comparing periods of high-frequency signals fed to the input of the difference frequency driver (mixer). This method does not determine the pitch.

A known method of obtaining a sound signal in digital contactless instruments that imitate playing the theremin (for example, in the project "The Digital Theremin" - URL: http://eet.etec.wwu.edu/simonsk3/project.html, date of access: 25.01. 2020). In this method, the current frequency of the high-frequency tunable generator is measured, and then, based on the data obtained, a digital synthesis of the audio signal is carried out. The task of determining the pitch of the generated tone is not worth it, since the initial data specifying the frequency of the sound signal are already known and can be immediately used for processing and display. Such synthesizers are not heterodyne type instruments.

The closest analogue to the invention is a method for determining the current operating frequency, used in some radio transmitters (transceivers). For such transmitters (for example, for the Desna shortwave transceiver - URL: http://www.cqham.ru/desna.htm, date of access: 01/25/2020), the emitted signal is generated using a mixer, the input of which is formed on some intermediate frequency, the original signal and the signal from the tunable generator ("smooth range generator"). The signal frequency after such conversion and subsequent filtering is equal to the sum or difference of the frequencies of the original signals. To determine the frequency of the emitted signal, the signal frequency is measured at the mixer input (i.e., the frequency of the tunable oscillator), and the desired value is calculated by adding or subtracting some constant from the result.

Disclosure of the essence of the invention

The technical result of the proposed invention is the ability to determine with high accuracy, high speed and in a wide range, the current pitch, including the fact of falling into the "region of negative beats".

The specified technical result is achieved by measuring the frequencies of the signals arriving at the difference frequency generator, with the subsequent calculation of the frequency of the sound signal (pitch) based on their difference. The sign of the difference determines the achievement of the "area of negative beats".

Pitch from a physical point of view is the frequency of a sound signal, expressed by the octave number, the name of the note and the amount of deviation from the note in fractions of a semitone or in cents. A direct measurement of audio frequency, as noted earlier, may not give the required accuracy and / or speed. Thus, the common method of measuring the number of signal periods in a fixed period of time is potentially inapplicable due to poor performance characteristics. For example, with a measurement time of 0.1 s and a resolution of a quarter of a semitone (minimum requirements for visual indication), the range of measured frequencies is limited from below to 688 Hz (this is the 2 nd octave "F" note area).

Measurement of the duration of the signal period with further conversion of the results into frequency, although it provides the necessary resolution (at a sufficiently high frequency of counting pulses), the measurement time is limited by the duration of one signal period. So, to ensure a measurement time of 0.1 s, the determined frequency should not fall below 10 Hz. This is quite enough for a visual indication of the pitch, however, the search for the "zero beat point" (in order to quickly tune the theremin) involves reaching lower frequencies.

An additional reason why measuring the frequency of an audio signal may not give the required accuracy is the need to convert the audio signal to a rectangular shape (so as not to depend on its shape and amplitude). The remaining high-frequency components present in the audio signal after heterodyning will affect the position of the edges of the resulting square-wave signal and thereby introduce additional error.

In electro-musical instruments of the heterodyne type, it is possible to measure the frequency not of an audio signal, but of the original signals supplied to the difference frequency driver. In this case, the frequency of the audio signal can be calculated as the difference in the frequencies of the original signals. The transition from measuring low (sound) frequencies to high (radio frequencies) allows you to reduce the duration of the measurement, since the period of the latter is rather short. The best results are obtained by a method (which became possible only with the development of digital technology), in which, instead of measuring the duration of one period with further conversion of the results into frequency, the duration (T) of the interval is measured, which includes several (N) periods of the measured signal, followed by the calculation of the frequency using the formula :

ƒ = N / T

The value of N can be set rigidly (and then the measurement time will directly depend on the input frequency), or it can be determined dynamically depending on the period of the input signal (then the measurement time can be made almost constant).

Described in AN2283. Measuring Frequency "by Cypress MicroSystems, Inc., USA (URL: https://www.cypress.com/file/67281/download, date accessed: 01/25/2020)," hybrid method "gives a relative error approaching 1 / 2 ⁿ (where n is the bit width of the binary counter used to measure the duration of the interval of interest using counting pulses). The number of counting pulses at a repetition rate of 24 MHz (typical for microcontrollers of this company) for a measurement interval of 0.1 s will be 0.1 / 24000000 ^-1 = 2400000, which will give n ≈ 21 and a relative error of 4.8⋅10 ^-5 % ... When measuring frequencies in the 100 kHz region (which is typical for generators used in theremins), we get an absolute error of 0.048 Hz, which, when calculating the difference frequency, will double to 0.095 Hz. If the frequency of the second oscillator is known (for example, stabilized by a quartz resonator), there will be no doubling. Thus, the determination of the difference frequency will occur with an accuracy of fractions of a Hertz at a speed that is comfortable for the musician. And since during heterodyning the frequency of the generators changes within insignificant limits, this accuracy will be ensured throughout the entire range of audio frequencies, including infrasonic beats.

Brief Description of Drawings

Figure 1 shows a method for determining the pitch by measuring the frequencies of signals supplied to the difference frequency driver.

Figure 2 shows an embodiment of the invention in which only one frequency is measured and the second is considered known.

Figure 3 shows an embodiment of the invention in which the signal frequencies are measured at intermediate acquisition points (before the frequency dividers).

Figure 4 shows an embodiment of the invention in which signal period durations are measured.

Implementation of the invention

The proposed method can be implemented as shown in figure 1. Reference oscillator 1, tunable oscillator 2 and the difference frequency driver 3 form the basis of an electro-musical instrument of a heterodyne type, for example, a theremin. Node 4, which measures frequencies, calculates their difference, processing (bringing the result to the required form) and displaying, is connected to the inputs of the difference frequency generator and can be performed on the basis of a microcontroller or a programmable logic chip.

In another embodiment, shown in Figure 2, only the tunable oscillator frequency ( ₂ ) is measured, and the frequency of the reference oscillator ( ₁ ) is considered known and can be stored in non-volatile memory as a constant.

The third implementation option (figure 3) reflects the presence in the signal path of additional units 5 that produce frequency conversion (in this case, division). In this embodiment, the frequency is measured not at the input of the shaper, but at intermediate points of its receipt, which is taken into account when calculating the difference frequency.

In a fourth embodiment (FIG. 4), the frequencies of signals ₁ and ₂ are obtained based on measuring the durations of their periods T ₁ and T ₂ (as the reciprocal of ¹ / T ₁ and ¹ / T ₂ ). To increase the resolution, the duration of several such periods can be used instead of the duration of one period. All measurements and calculations can also be performed by a microcontroller or programmable logic chip.

Claims (3)

1. A method for determining the pitch of electric musical instruments of heterodyne type, characterized in that the frequencies of signals supplied to the difference frequency former, or one frequency with a known value of the other, are measured, and the frequency of the resulting sound signal is calculated based on their difference. 2. The method according to claim 1, characterized in that the frequency is measured at an intermediate point of receiving at least one of the signals, taking this into account in the calculations. 3. The method according to claim 1 or 2, characterized in that the measurement of frequencies is performed by an indirect method based on measurements of signal periods or time intervals determined by these periods.

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