A Design Scheme for Digital Car Audio System

Abstract: This paper introduces the solution of a new digital multimedia storage intelligent car audio system. The high-efficiency switching power supply is used to achieve low-power operation in power saving mode. The single-chip microcomputer controls the corresponding registers of the radio, MP3 decoding, and sound processing chips through the I2C bus to realize the digital FM stereo radio, audio decoding, and sound processing functions respectively. The system incorporates intelligent self-checking and abnormal error response mechanisms, realizes memory operation and MP3 resume playback functions, and optimizes user operation and audio-visual experience.

With the rapid development of automotive electronics technology, the field of in-car audio is undergoing an unprecedented technological change. Users have put forward requirements for in-car audio with diversified functions and user-friendly operation, mainly including the following three aspects: (1) better radio reception and simpler digital tuning operation; (2) support for a variety of external storage devices, such as large-capacity USB flash drives and SD cards; (3) provide richer sound processing, such as adjustment of treble, bass, equal loudness, balance, etc., as well as providing pop, rock, jazz, classical and other sound processing. Based on these requirements, a digital in-car audio system was designed.

1 I2C Protocol

The I2C bus is a synchronous serial data output bus consisting of a serial data line (SDA) and a serial clock line (SCL). It is a true multi-master bus. If multiple masters initiate data transmission at the same time, conflict detection and arbitration can be used to prevent data from being corrupted. Each device connected to the bus can be addressed by software with a unique address and a simple master/slave relationship that always exists. The master can act as a master transmitter or a master receiver.

2 Hardware Circuit

2.1 Circuit Design

According to the characteristics of the car audio system, the design uses the STC12C5624AD series microcontroller, which is a 51 enhanced microcontroller launched by Hongjing Company. It has the characteristics of low power consumption and fast computing speed. When the audio system is turned off, the microcontroller enters a low-power sleep state and cuts off the power supply of all peripheral modules; when the system needs to work, the microcontroller is awakened by an external interrupt to minimize power consumption. The circuit diagram of the audio system is shown in Figure 1.

Figure 1 Audio system module circuit diagram

2.2 Peripheral Hardware Modules

The system is based on the STC12C5624AD microcontroller and controls the operation of other modules through the I2C bus. The hardware module block diagram is shown in Figure 2.

Figure 2 Block diagram of the vehicle audio system hardware module

2.2.1 Power Module

The power module uses the current output step-down switch integrated voltage regulator circuit LM2576 and LM2575 launched by National Semiconductor Corporation. They contain a fixed frequency oscillator (52 kHz) and a reference voltage regulator (1.23 V), and have a complete protection circuit (current limit and thermal shutdown circuit). The integrated voltage regulator circuit introduces closed-loop control, and only a few peripheral devices are needed to form an efficient voltage regulator circuit, with stable output voltage and small ripple.

The input voltage range is 10 V~40 V. The wide input voltage range enables the system to adapt to both 12 V/24 V power supply vehicle series, and also solves the problem of output voltage changes when the vehicle works in different working conditions.

2.2.2 MP3 Decoding Module

The MP3 decoding module uses AU7842, which integrates a microcontroller, MP3/WMA decoder, USB host controller, SD/MMC card controller, 16-bit audio decoder and an infrared decoder, as shown in Figure 3.

Figure 3 MP3/WMV decoding module circuit

The AU7842 chip decodes music files (MP3 and WMA) stored in digital signal form into playable analog signals. After powering on, the MCU detects the actions of the circuits around the decoding chip in a loop. When it detects that a storage device is connected to the chip, the MCU controls the chip to directly access (DMA) the contents of the memory, reads the data and sends it to the MP3/WMA decoder. The decoded digital signal is converted into an analog signal through a digital-to-analog converter (DAC), and after analog audio amplification and low-pass filtering, the music you hear can be obtained.

2.2.3 Radio Module

The radio chip uses a low intermediate frequency tuner TEF6606 with a PLL tuning system designed by NXP Semiconductors for car radio hosts. In addition to basic features, TEF6606 also provides good weak signal processing functions and a dynamic bandwidth control. The working principle of TEF6606 is shown in Figure 4. Its local oscillator signal is generated by the PLL tuning system. The frequency division coefficient of the programmable divider is tuned through the I2C bus to change the local oscillator frequency output by the voltage-controlled oscillator (VCO), thereby achieving the purpose of digital tuning [4]. The circuit schematic of the radio system is shown in Figure 4.

Figure 4 Schematic diagram of radio system circuit

The TEF6606 chip has excellent weak signal processing capabilities, which improves the radio's reception performance when the car is driving at high speeds and between mountains.

2.2.4 Sound Processing Module

The sound processing module processes the sound from the MP3 decoding chip and the radio chip. The main control chip controls the corresponding registers of the sound chip through the I2C bus, and can set the volume, tone (bass, treble), balance (left, right) and loudness (front, back); select pop, rock, jazz, classical and other sound effects; the chip compensates for hearing by enhancing high-frequency and low-frequency sounds at low volume, and can obtain hearing effects with low distortion, low noise and low DC level drift.

2.2.5 Key system and display system circuit

The key system uses two EC11 encoders and four push button switches, which are convenient for the driver to operate while the car is driving. The LCD uses a segment LCD customized according to the functions of the car audio, which can display the status including radio frequency, audio playback time, USB disk status, SD card status, RPT single repeat playback, RDM random playback, ST stereo status, LOUD loudness, MUTE mute and left and right channel level indication.

3 System Software Design

The overall flow framework of the system software is shown in Figure 5. With the help of software, the system can complete the power-off memory function, that is, when the system is manually shut down or automatically powered off, the system can remember the system's actions and various settings before the power off, and directly call them when it is turned on next time. The radio head circuit can realize manual and automatic station search in steps of 10 kHz, and can store 18 frequently listened frequency bands.

Figure 5 System software flow chart

The power-off memory subroutine is as follows:

The power-off memory subroutine is used. No matter the system is shut down manually or the power is off, the system will automatically save the sound effect values ​​set by the user before the power off, such as volume, balance, loudness, etc., as well as the frequency band value of the radio or the songs being played in USB and SD and the playing time value, and store them in EEPROM through the Write_EEPROM function. The next time the system is turned on, the user does not have to reset it, and the system can directly read the various setting values ​​before the power off from the EEPROM value.

The radio head receiving subroutine is as follows:

The main control chip controls the radio head register through the I2C bus, and controls the working status of the radio head by changing the logic value of RD_DAT. The radio head can be controlled to switch and search channels in three bands by operating the keys on the keyboard.

By rotating the channel search button, you can manually search for channels in steps of 10 kHz.

By controlling the automatic channel search button, the radio head can automatically search for channels in steps of 10 kHz.

The digital car audio system designed in this paper is compatible with radio and MP3 playback functions. It has made a great breakthrough in MP3 audio file storage devices. The new large-capacity storage device (USB/SD) replaces the traditional cassette storage. The system is easy to operate and has good radio and MP3 playback performance. To ensure the stable operation of the system, the experiment simulated the driving conditions of the car and placed the system in -40℃ and +60℃ environments and a four-dimensional vibration test bench. The results show that the system can run for a long time and its functions are stable. The system has been successfully applied to some brand engineering vehicles.