Car stereo architectures ride DSPs into the software era

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As MP3 audio compression and CD technologies grow more popular, consumers increasingly want the convenience and functionality they enjoy at home to be just as available in mobile venues, particularly in their cars. To meet this demand, automobile manufacturers are well on their way to replacing analog solutions with digital media processing technologies to make the driving experience more pleasant and productive.

So much functionality must be added and so much flexibility is required, however, that software-defined radio solutions are emerging as a dominant trend. The challenge facing semiconductor companies is to design and manufacture chips capable of executing complex software of software-defined radios quickly and efficiently. Semiconductors are still the key to the improved automobile stereo and multimedia. Digital signal processing, in particular, will continue to be instrumental in enabling digital media to express all its modalities on car radios.

Digital signal processors (DSPs) have already helped transform car radio from a simple audio processor into a sophisticated hub for high-tech information and entertainment by using the sidebands and intermediate frequency (IF) signals of an automotive entertainment system.

WHY DIGITAL SIGNAL PROCESSING?

As the car radio industry transitions from analog signal processing to digital signal processing, radio manufacturers will be able to provide improved radio performance, enhanced audio quality, much more flexibility, faster design cycles, speedier time to market, production simplicity and environmental stability. DSPs used in cars today offer much more functionality in a single chip compared to the first DSPs for automobiles that were developed during the 1990s.

A trend indicative of the growing importance for automotive DSPs is the fact that radio signals are being converted into a digital format at a much earlier stage in the signal-conversion process. Analog-to-digital conversion is moving up the signal-processing stream from the baseband end, which is just before output, to be closer to the antenna supplying the radio frequency signal. Complex interacting analog filters have been replaced by digital signal-processing circuits. In addition to simplifying radio design, digital processing delivers other advantages, including improved radio performance, better audio quality enabled by more sensitive control, and complete audio processing supported by a DSP architecture. DSPs deliver fully unlimited linearity as well, and because they can handle complex algorithm mathematics, they make it possible to achieve improved radio reception through phase diversity.

Digital reception will increasingly become an important feature. In the future, it is likely that RF signals will be digitized directly from the airwaves. By performing analog signal processing in the digital domain, performance quality will increase significantly, although it will still be limited by the analog performance of the transmitting system. To increase performance in various types of situations, several digital transmission schemes are being developed. Digital radio systems can optimize the reception of information that is transmitted as analog signals. By performing the signal processing in the digital domain, much higher performance can be realized compared to today's analog receivers.

DSPS SPARK SYSTEM-LEVEL INNOVATION

DSPs whose functionality is largely defined by software will provide a simple solution that enables car radio manufacturers to add new features and greater differentiation at a fraction of the time and cost than would normally be required to re-design an entire hardware-based radio IC from scratch.

For instance, car radio manufacturers today are pushing for greater improvement in multipath performance and antenna diversity. They are also bombarded with a myriad of new radio features and broadcast standards.

These consumer demands can all be satisfied fairly easily using software-based architectures, which includes a software-defined radio, an audio software library or IF concepts. Manufacturers could then improve the radio's functionality with new features, incorporate product differentiation and enhance radio performance through simple software upgrades.

SOFTWARE-BASED ARCHITECTURES

Implementing a software radio using a DSP can deliver a variety of audio enhancements, including music elevation, adaptive ultrabass 2, LifeVibes PureStudio, SRS Circle Surround II and more. Sound optimization for each individual car function can be implemented using an equalizing function. Radio and antenna enhancements provided for a single tuner enhancement can include superior adjacent-channel suppression and enhanced multipath suppression. A double tuner can be added for phase diversity to provide the most appropriate way to minimize multipath effects where a software radio algorithm can control incoming signals from both tuners.

Software-based architectures, however, require high processing power. DSPs specially designed for automotive audio within a standard platform that can be easily modified are well suited for these architectures. A real-world example of a software radio based on CarDSPs and peripherals will be presented later in this article.

There are several aspects to software radio innovation. DSPs will provide more value and flexibility as high-definition (HD) radio, Digital Radio Mondiale (DRM) and satellite radio emerge as digital transmission and reception systems.

HD Radio (formerly known as in-and on-channel or IBOC) transmits terrestrial AM/FM broadcast information digitally within the sidebands of the existing spectrum. As such, it provides broadcasters with a simple upgrade path to digital transmission. The same holds true for radio receivers equipped with an IF DSP, which can be upgraded easily with the addition of an HD co-processor.
Digital Radio Mondiale (DRM) was developed as a universal, non-proprietary digital audio and data broadcast system operating on the LW, MW and SW bands with near-FM audio quality, which is available to markets worldwide. Endorsed by the ITU, IEC and ETSI in 2003, Digital Radio Mondiale (DRM) uses existing AM broadcast frequency bands, unlike other digital systems that require new frequency allocation.

The initial focus of the DRM Consortium was to create a broadcast standard that operated below 30 MHz. Following a recent agreement, it is planned to extend the operating frequency up to 108 MHz. Besides providing near-FM quality audio, Digital Radio Mondiale has the capacity to integrate data and text, which can be displayed on compatible receivers to enhance the listening experience.

Satellite radio signals are transmitted on the 2.3 GHz S band by providers like XM and Sirius who can reach a global audience with little to no variation in signal quality. Satellite trans-mission is similar to HD radio in that it enables on-demand features such as the ability to receive signals that are specific to a car's location. A driver, for example, can get information about upcoming weather and traffic conditions. Philips is a leader in the digital radio technology and working closely with digital broadcasting supplier to develop relevant solutions in satellite radio as well.

PLATFORM DESIGN SUPPLEMENTS ASICS

As more and more functionality is added to car audio systems, semiconductor companies must take steps to ensure that their chip designs and the systems they implement are robust, reliable and manufacturable.

The growth of software functionality, therefore, is driving higher levels of silicon integration, reduction in hardware blocks and the development of highly flexible hardware platforms that require very little hardware redesign as the software evolves. Automotive DSPs such as Philips' SAF7730 software radio DSP, for example, combines multiple DSP cores into a single chip. Signal processing is conducted completely using software.

Most recently, intermediate frequency (IF)-based automotive DSPs have been developed that can fully process from the IF level and software blocks within the modules can be used for major tasks. These chips integrate an RF front end, amplifier, MP3 and CD applications. They provide high-performance radio reception and a downsized module with a lower BOM.

Since a larger software component delivers greater flexibility, car radio vendors can use the same basic platform design to add value with unique features. A platform design approach not only improves product yields, but delivers greater reliability. Such a chip will satisfy the market demand for CD and MP3 features, which are expected to be “in the overwhelming majority of car audio systems by the end of the decade.

It is expected that future generations of DSPs will further advance integration and perform key car audio tasks. Compression audio processing and a 32-bit microcontroller will be embedded on the support system and macro control.

Most features can be modified by internal coefficient settings or by different ROM code, while the hardware application does not change. By concentrating on a single platform design, a system designer avoids the effort required to solve quality control issues mainly originating from the hardware. There are still hardware limitations due to processing power restrictions, but new specific functions can be easily adapted by ROM code development.

REAL-WORLD IMPLEMENTATIONS

Designing a hardware platform of this complexity requires a thorough knowledge of system requirements as well as future functionality requirements.

As mentioned previously, Philips has embedded five DSPs in its SAF7730, a CarDSP that is shown in Figure 2. To meet the intense number of processing requirements of software-defined radio, the SAF7730 provides 650 MIPS. But that is only the beginning. For the dual-tuner support mentioned earlier, the SAF7730 manages phase diversity and RDS background scanning in addition to managing digital radio and audio functions.

The level of integration extends to not just the DSP cores but also to analog and digital blocks (mixed signal) as well. This includes analog IF input, digital radio reception and audio processing, sample rate converters and digital and analog audio output.

Signal processing is executed completely in software. This high level of integration is also unique in the digital IF car DSP market and extends a cost-effective digital IF solution realised in a high-density CMOS process.

Software radio DSP offers enormous flexibility that must be supported by libraries. Philips, for example, provides customers a software library for advanced audio and radio-processing features, which can be embedded on the SAF7730. In addition, the software radio system's architecture enables customers to integrate their own software intellectual property for greater product differentiation.

Philips SAF7730 offers adaptive Ultrabass II, music elevation, multipath cancellation and antenna diversity performance capabilities that result in superb radio reception and audio quality. Software radio DSPs greatly improve the sound quality of today's conventional radio broadcasts and offer radio manufacturers the headroom to adapt existing radio designs for future digital radio capabilities. Using an input/output signals such as those established by the Fraunhofer IIS forum, HD radio or DRM applications can be delivered.

HD RADIO SOLUTION

To address the demand for automotive HD radio sets, Philips SAF3550 HD radio processor adds high-quality digital AM and FM functionality. A block diagram showing the relationship between the SAA7730 and the SAF3550 is illustrated in Figure 3.

The SAF3550 incorporates HD radio decoding and is optimized in hardware and software to deliver a dedicated single-chip solution, while remaining flexible for future HD radio features. The chip supports future functionalities including a second audio program channel, replay facilities for missed broadcasts and increased data services such as customized station/program content, news and local traffic information.

Compared to existing devices, the SAF3550 provides the highest integration level in the smallest footprint, enabling cost-efficient signal demodulation and processing. Its design also enables manufacturers using Philips' tuner front-ends and CarDSPs to extend their existing designs with minimal effort. Volume production of the first cost-effective HD radio was powered by Philips' decoder chip.

RADIO MONDIALE SOLUTION

Philips' dedicated Digital Radio Mondiale co-processor will become part of a complete system concept. Since the architecture allows it to share the same platform as high-performance AM/FM radio solutions, the introduction of digital radio into the automotive market will be easier. This is because a digital radio decoder can easily be designed into a complete car radio solution. When combined with Philips tuner front-ends and CarDSPs, such as the RF front-end TEF6730 and SAF7730, the Digital Radio Mondiale co-processor adds further dedicated processing power to the complete system.

This offers manufacturers a simple upgrade path from an existing proven radio design to a Dig-ital Radio Mondiale enabled set, delivering the benefits of Digital Radio Mondiale and digital radio reception.

TUNING IN TO THE FUTURE

Although the basic architecture would be the same, the next generation of DSPs supporting compressed audio will have less IF input and more integration of SRC, ADCs and DACs and a microcontroller, making it possible to minimize the CD mechanism design and system control capability. The microcontroller might be integrated with radio and audio signal processing, as well as USB, Bluetooth and Wi-Fi connectivity interfaces. In addition, compressed audio decoding peripherals for system functions will be needed for easier hardware and software design and applications development.

Flash memory is likely to be embedded so that updates can be affected by revising the code. The microcontroller would enable macro control for Philips' CarDSP. While in the past, the CarDSP was a chip covering mainly higher-end models, it has recently penetrated the mainstream market.

While performance is still limited by the analog capabilities of the transmitting system, further advances in IC process geometry and the performance of CarDSPs will realize the digitizing of the RF signal directly from the airwaves. This will provide the ultimate in processing analog radio signals fully within the digital domain.

In the future, DSPs will continue to offer an extended listening range in the car radio, enabling consumers to receive more stations in a wider radius and minimizing the need to continually tune the radio for better reception. DSPs will also make traditional analog AM and FM broadcasts clearer by delivering improved sound quality with less static.