Today, with the rapid development of automotive electronics technology, audio systems have become an indispensable part of every car. Consumers' requirements for audio systems are: better radio reception; support for a wider variety of additional audio sources, such as DVD, SD card, Bluetooth, digital broadcasting, etc.; provide richer audio processing, such as SRS, Dolby 5.1, etc. Usually the effect of home audio is provided. At the same time, component manufacturers and design companies hope to shorten the development cycle to meet the challenges of accelerating the replacement of audio products.
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Figure 1 is a general architectural diagram of a car audio system. In the entire system architecture, the processing of the radio signal and audio signal is done by the core module (IF/MPX Processor and Audio Processor), and its selection will determine the final sound and available functions of the system. Currently used on the market are analog audio processors (ASP) and DSP core-based audio processors (hereinafter referred to as DSP). NXP Semiconductors' DSP chipset combines radio and audio processing with built-in ADC, DAC and audio selection circuitry for better performance and reliability than ASP.
The following is a description of the response to market challenges from hardware design and software design.
hardware design
According to market demand, DSP solutions should continue to work on four aspects of hardware design.
(1) Provide greater processing power
As end-user demand increases, the signal processing power required in car audio systems is rising dramatically. NXP's range of DSP chips has an increasing processing power. First, the on-chip programmable DSP core EPICS7A/B is optimized for audio applications using the dual Harvard system. In addition, while improving the speed of a single DSP core, a multi-core architecture is introduced inside a chip to further enhance the processing power of the chip.
The following operations are often encountered in audio processing:
y = mx + b
Here, m and b are called coefficients, and x and y are called values. Since the coefficients and values ​​usually have different requirements for accuracy, the design of the EPICS7A/B selects the double Harvard architecture. The data memory is divided into two parts. The 24-bit wide XRAM is mainly used to store values, and the 12-bit wide YRAM is mainly used. Used to store coefficients, the instruction memory PRAM is 32 bits wide.
The SAF7730 is equipped with five DSP cores. The audio processing can support 6-channel mode, which is used to simultaneously process the 3-way I2S signal output from the DVD player. This allows the driver to enjoy the wonderful enjoyment of Dolby 5.1 channel.
(2) Better coordination between DSP and front-end tuner
For the radio processing, the front-end tuner outputs the received radio signal to the back-end DSP, and further performs processing such as demodulation, decoding, and noise removal in the DSP. Depending on the type of signal output by the tuner (MPX/IF/Low IF), NXP has introduced different combinations of DSP and tuner products for users to choose from.
Through the cooperation of DSP and corresponding front-end tuner, the whole scheme can achieve interference removal well. In a mobile environment such as a car, adjacent channel interference and multipath interference have always been a problem that affects the quality of received radio signals. NXP eliminates the former through the "Precise Channel Rejection" (PACS) technology, and the latter is achieved by the dual antenna/dual tuner application architecture and the embedded Antenna/Phase Diversity software algorithm in the DSP.
In addition, through the cooperation of DSP and corresponding front-end tuner, the whole scheme can achieve extremely high efficiency in completing RDS processing. For example, the AF switch is an important application of RDS. If NXP's DSP/tuner overall scheme is adopted, since the tuner chip has dedicated pins and DSP communication, the switching process can be performed without the MCU. The process can be controlled at the millisecond level, and the user does not feel the smooth switching of the radio frequency at all.
(3) Higher integration and lower system cost
NXP's recently introduced SAF7780 integrates four DSP cores and one ARM7 processor to perform radio, audio processing, MP3/WMA decoding and host control functions in a single chip. It will also support USB and Bluetooth connectivity in the future. It can be used to achieve most of the processing work required by the audio system.
At the same time, NXP continues to improve the technology to reduce the cost of existing chips, such as from SAF7730 to SAF7741, from SAF3550 to SAF3555, all of which achieve cost reduction while maintaining the same functional performance, maximizing customer price competition. Product of force.
(4) Support for scalability of new applications
Due to the changing market, the car audio system must provide sufficient extension support for future applications. NXP's DSP chips provide a rich set of analog and digital interfaces, providing different amounts of audio A/D, D/A, I2S input, SPDIF, Host I2S input/output to facilitate new audio source access systems.
In addition, when the processing power of the DSP chip itself is insufficient, or the customer has special requirements (such as special equalization processing), it is convenient to connect the coprocessor through the external interface of the DSP chip, thereby expanding the processing capability. Considering the future application of digital broadcasting, the recently introduced SAF7730 and SAF7780 support digital broadcasting applications.
On the SAF7730 evaluation board, the same hardware interface supports different digital broadcast applications by plugging in different processor modules (such as the decoder SAF355X for HD Radio and the PNX952X for DRM).
software design
In the DSP-based solution, the software used to implement the algorithm has been solidified inside the chip, and what the customer needs to do is to develop the control program running in the MCU. With the modular design of the control software, customers can easily implement feature reduction, reuse and expansion. On the software side, NXP's DSP solution has two features.
(1) From register setting to hierarchical structure API programming
From the perspective of software design, the current application of audio processors is almost always using register operations. With the increasing functionality of the chip, hundreds or even more registers and complex read and write processes allow application developers to spend a lot of time learning to digest. In future solutions, the user should be provided with an easy-to-use function interface, not just a set of register lists and literal definitions.
Take SAF7780 as an example, SAF7780 is ARM+DSP architecture. For the customer to develop the control program, it can be operated to the register level. The chip itself provides the SDK, which can be programmed directly by calling the interface function of the API, thus making the software Development has become easier. For the customer, it is only necessary to complete the programming development of the application layer according to the requirements of the system, that is, the software control of the entire application can be realized.
(2) Open architecture to embed third-party and custom components
As car audio systems become more diverse and personalized, software must also provide a flexible and open architecture to incorporate the specific features customers need.
NXP first worked with third-party audio software vendors, such as Dedekind, SRS, to embed a variety of popular sound modules and corresponding activation keys in the DSP chip to provide more convenience for customers in need. Customers without this need do not have to bear the corresponding costs; in addition, for large customers, NXP's DSP chips can be customized, that is, software components embedded in customer-specific requirements to maximize the customer's actual needs.
Conclusion
The automotive audio field is undergoing an unprecedented technological revolution, with digital broadcasting, rear-seat entertainment, and wired and wireless connectivity all becoming existing or potential needs in the market. Traditional analog audio systems mainly implement functions through hardware, and it has become increasingly difficult to meet future markets under the challenge of new applications. As mentioned above, DSP-based solutions will show more and more technical advantages.
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