Design and implementation of multi-channel digital analog hybrid sound simulation system

    Abstract: Using digital and analog hybrid control technology and a combination of multiple sound sources, the simulation of aircraft engine sound and environmental sound is realized.

    Microcontroller integrated service network ISDN dial-in phone number (010) 62770940

    Microcontroller integrated service network ISDN dial-in phone number (010) 62770940

    The flight simulator not only provides the pilot with realistic vision, force and movement during the entire flight, but also has realistic sound effects, making the pilot feel immersed in the scene and helping the pilot to correctly judge the flight of the aircraft from the auditory sense. state. There are two main types of sound effect simulation: analog and digital. Analog sound simulation is suitable for real-time control, while digital sound simulation has high fidelity. Based on the spectral characteristics of aircraft engines, this article designs an analog circuit that can realize single and dual rotor noise and jet noise; based on the characteristics of environmental sound, a multi-channel digital sound effect playback card that can realize various environmental sounds is designed. This sound simulation system can be used to simulate the sound of propeller engines and single- and double-rotor jet engines. It can simulate various in-machine control effects, external friction, lightning and other effects. It can also be developed and used Sound effect simulation for ships and land mobile equipment.

1 Spectrum analysis of engine sound

    In order to control the simulated sound of the engine in real time, the sound spectrum of the engine must first be analyzed. For example, after on-site recording and collection, the noise spectrum of the J-6 aircraft engine is shown in Figure 1.

    The noise spectrum of the J-6 aircraft engine is analyzed through a spectrum analyzer. It is composed of three sounds, namely turbine sound, jet sound and air intake sound. Turbine sound is the scream produced when the air is compressed during the rotation of the engine turbine rotor. It is composed of two single-frequency waveforms. The frequency changes from 0 to 46 Hz, and its frequency and amplitude are both functions of the engine speed. When the engine runs from 0 to 5500r/min, the amplitude of the turbine sound continues to increase; from 5500r/min to the rated speed, the amplitude continues to decrease. After 8500r/min, the human ear basically cannot feel the turbine sound. The frequency of jet sound is basically between 200 and 500 Hz. Its frequency and amplitude are also functions of engine speed. The human ear feels it is a roar. The air intake sound is negligible.

2 Hardware design

    Based on the noise spectrum characteristics of the aircraft engine and the relationship between sound and engine speed, the hardware circuit is designed. The system consists of a sound control computer, a multi-function sound effect generation and playback card, a multi-channel power amplifier and a multi-channel speaker. The core part is a self-developed multi-function sound effect generation and playback card, which consists of sound effect generation and playback cards. Multi-channel digital sound effects playback consists of two parts. Figure 2 shows the schematic diagram of the coach eight aircraft simulation sound effect generation and playback card structure.

2.1 Sound effect generation circuit

    By utilizing the linear proportional relationship between the oscillation frequency of the voltage-controlled oscillator 4046 and the numerically controlled voltage, a high- and low-voltage rotor circuit that generates turbine sound is designed. By inputting the corresponding speed digital signal, turbine sound signals of different frequencies can be generated (see Figure 3). By setting the initial frequency and changing the amplitude in real time through the high and low voltage rotor amplitude control circuit, the corresponding high and low voltage rotor signals can be generated.

    The jet noise of the jet engine is generated by using the white noise (thermal noise, shot noise) of the crystal diode PN junction. After selection and experimentation, the jet noise generation circuit is shown in Figure 4. It generates white noise of different frequencies according to the input throttle position and engine power. The jet noise generated by controlling the amplitude in real time through the jet noise amplitude control circuit is detected and generated. The noise waveform is very similar to the actual jet noise waveform.

2.2 Playback of multi-channel digital sound effects

    The digital audio signal stored in the computer can be restored to the corresponding analog audio signal amplification, and after switching and superimposing multiple sounds, the audio signal can be output for playback (as shown in Figure 5). This digital channel can simulate engine starting sounds, engine stopping sounds, and environmental effect sounds (sounds of retracting and retracting landing gear, retracting and retracting flaps, retracting and retracting airbrakes, and landing friction sounds). The original sound sources of these sounds are obtained from the field Obtained from recording collection or video editing.

2.3 Fitting and switching between digital audio and analog audio

    In order to avoid conflicts with the existing hardware of the computer, a corresponding jumper structure is designed, and its corresponding address can be changed at any time through the jumper. Below is an actual selection of a set of addresses.

    390H high pressure rotor turbine sound frequency

    391H High pressure rotor turbine sound amplitude

    392H low pressure rotor turbine sound frequency

    393H Low pressure rotor turbine sound amplitude

    394H jet noise frequency

    395H Jet noise amplitude

    396H - 39BH Direct channels for multiple digital audio sources

    In order to produce the sound of the aircraft engine realistically, pure digital sound is used when the engine is starting and stopping; in other working states of the engine, analog sound is used to achieve real-time control, so that the engine can be controlled according to the throttle position, rotor speed and engine power. Produce realistic real-time sound. The main flight program is required to switch between digital audio and analog audio.

3 Software design

    The mathematical model of the audio simulation system is as follows.

    Turbine sound: B=c

    F＝ａ＋ｋ（Ｎ－ｎ）

    Jet noise: B=a+k (Ym-0.21)

    F=a+k(Ym-0.21)

    Among them, B is the amplitude, F is the frequency, N is the rotation speed, Ym is the throttle position, a is the reference value, c is the set value, k is the proportion factor, and 0.21 is the throttle reference value.

    The mathematical model of the coach eight aircraft audio simulation system is given below.

    High pressure rotor turbine sound

    B=c

    F＝152.00+0.0124351（N1-9060.0）

    Low pressure rotor turbine sound

    B=c

    F=116.00+0.0129005 (N2-6500.0)

    Jet noise:

    B=156.0+120.8 (Ym-0.21)

    F=18.0+12.8（Ym-0.21）

    Among them, N1 is the high-pressure rotor speed and N2 is the low-pressure rotor speed.

    The software program of the sound simulation system is written in Visual C++. The program block diagram is shown in Figure 6. Drieect x Sound is used to insert the environmental effect sound. For example, the sound effect is simulated when the left wheel of an airplane touches the ground. The background is engine sound (jet sound and turbine sound), which is broadcast by the main channel of the sound system. Because the pilot made a left bank when landing, the left wheel of the aircraft touched the ground first, so the left channel of the audio system broadcast the friction sound of the wheel touching the ground in real time. At this time, in addition to visual inspection through the visual system, the pilot can also use hearing to determine whether the left wheel of the aircraft touched down first.