Addressing Next- Generation Sound Quality and Efficiency Challenges Whitepaper

Car Audio: Addressing next- generation sound quality and efficiency challenges Whitepaper

Car Audio: Addressing next-generation sound quality and efficiency challenges

The automotive world is undergoing huge changes as vehicle makers develop new technologies to meet the dual needs of autonomous vehicles as well as electric vehicles (EV) and hybrid electric vehicles (HEV). However, as consumer expectations rise on all fronts, almost all areas of modern vehicles are experiencing technological change.

The infotainment system is becoming central to the driver experience. Now much more than the humble sound system that provided basic audio entertainment several years ago, modern infotainment systems guide us to our destination, listen and respond to our voice commands, make and receive phone calls and much more – and, of course, they continue to fill the vehicle cabin with high quality audio.

With higher consumer expectations, greater sophistication in vehicles and far more electronic content than ever before, designing high-end in-vehicle audio systems is becoming ever more challenging. In this technical white paper, Toshiba Electronics Europe will look at how modern requirements are evolving and how these impact the challenges designers are facing. They will then look at some of the modern amplifier topologies and discover how they are being implemented in the latest devices to reach the market.

An ever-increasing quantity of electronics in the vehicle Once an almost purely mechanical system, with a few electrical items, the modern vehicle is becoming more and more reliant on electronics as new functions are added and existing mechanical functions migrate to electrical equivalents to save weight and increase efficiency. In fact, according to Statista, the electronic content represents around one-third of the cost of a modern vehicle, and this is expected to increase to one-half by the end of the next decade.

Figure 1: The electronics content of vehicles is rapidly increasing [Source: Statista]

While there is barely an area of the vehicle that has not been touched by electronics, the central infotainment system contains much of the functionality and is now a significant in-vehicle resource for the driver. The importance is highlighted in a recent survey by Accenture that shows that the vast majority of drivers either already do, or want to surf the Internet, read and dictate emails, operate a smartphone safely and stream music while driving.

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Figure 2: An Accenture survey shows the importance of infotainment, especially for audio streaming [Source: Accenture]

In fact, the Accenture survey goes on to show that, these days, more people base their vehicle buying decision on the quality and functionality of the in-vehicle technology than the basic performance of the vehicle – a significant change in consumer opinions. As a result, designing and implementing high-end infotainment, including high-performance audio, is becoming a competitive differentiator for vehicle manufacturers.

Requirements for in-vehicle audio amplification The head unit is the ‘brains’ of the system and integrates an increasing number of inputs from multiple devices and removable media, often incorporating wireless technologies such as Bluetooth. Increasingly, along with the important audio output, they now incorporate touch screen technology.

Figure 3: Modern infotainment systems combine multiple audio inputs

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Speakers are one key area where there is a significant difference. Whereas most home audio systems have different speakers for each of the major frequency ranges (subwoofer, woofer, mid-range and tweeter), the requirement to save space and weight in a vehicle requires a different approach.

The most basic speaker in a vehicle is known as ‘full range’ as it is designed to reproduce the full frequency range of human hearing, while taking up as little space as possible. While this is possible, the reproduction accuracy is far from ideal.

Some vehicle audio systems combine a woofer and tweeter into a single coaxial speaker, allowing the small size of the tweeter to deliver the higher frequencies, while the larger woofer produces the lower frequencies that require greater volumes of air to be moved.

The amplifier is one of the key element that defines the audio quality with the primary consideration being a linear output that faithfully reproduces the quality of the input source.

Audio amplifier devices are also able to detect system faults. For example, degradation of input capacitors over time can cause a DC offset voltage to appear on the output of the amplifier. This DC voltage can destroy speakers and, for this reason, increasingly many modern audio amplifiers include a full-time DC output offset detector.

The Bill-of-Materials (BoM) is another important consideration. Not only is the modern automotive business extremely cost-competitive but a large BoM also implies increased space requirements, neither of which is desirable in a modern vehicle.

If this were not challenge enough, the vehicle itself is becoming a more challenging environment for electronics and, especially, any system that is trying to faithfully reproduce an analog signal. The huge on-board electronics content, as well as devices such as mobile phones, can be a significant source of electro-magnetic interference (EMI). In EV/HEV switched high voltages and large currents power the motors that drive the wheels, thereby producing another potential source of EMI. In an effort to improve fuel efficiency, modern vehicles regularly turn off the engine when stopped thereby causing a variable battery voltage that affects the supply to the amplifier.

Types of amplifiers available Over the years, designers have dreamed up many amplifier configurations to improve the faithfulness of reproduction of the audio signal, increase power, improve efficiency – or a combination of all three. Along this path of evolution, analogue amplifiers have been supplanted by digital versions in some applications. While each of the available configurations has different strengths and weaknesses, the three most commonly used in audio amplifiers are Class AB, high-efficiency Class AB, and Class D.

The Class AB amplifier is an evolution and combination of Class A and Class B amplifiers and is one of the most commonly used types of audio power amplifier for low-end infotainment systems.

Figure 4: The Class AB amplifier is one of the most common audio power amplifiers

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Class B amplifiers used a push-pull design to overcome the significant efficiency limitations of the Class A amplifiers. However, as they do not work purely in the linear region of the transistor, the overall linearity of the design was poor. In a Class AB design, the switching devices are able to conduct simultaneously at the waveform’s crossover point, thereby eliminating the crossover distortion problems with Class B designs.

Class D amplifiers incorporate some digital elements into what is essentially still an analogue design. A pulse-width- modulated (PWM) train of pulses is generated to represent the amplitude variations of the input signal. This digital signal drives an output stage that, as the switching components do not operate in their linear region, consumes very little power.

Figure 5: Class D amplifiers use pulses to drive the output stage, significantly reducing power consumption

A simple LC low-pass filter passes the low frequency audio signal to the speaker, filtering out the high-frequency elements of the digital pulses.

The main advantage of Class D amplifiers is their efficiency, they often achieve greater than 90% efficiency at loads greater than 25% and achieve 50% efficiency even at very light loads.

Introducing Class TB amplification While the progress in amplifier configurations has yielded significant improvements, the demands of modern vehicle makers mean that constant progress is required. Addressing this challenge, the new Class TB (Tied BTL) amplifier from Toshiba offers a number of significant benefits over previous configurations.

In terms of power efficiency, Class TB matches the high efficiency levels of Class D at low power levels, while delivering a staggering 80% reduction in power consumption when compared to a Class AB amplifier. This increase in efficiency allows vehicle audio systems to run much cooler.

In terms of BoM cost, Class TB is similar to high-efficiency Class AB yet contains less than half the components required to construct a more complex Class D amplifier design. No low pass LC filter is required on the output to smooth digital pulses and remove high frequency components. With its ability to reproduce linear sounds in excess of 120 kHz, Class TB amplifiers are able to realize high quality sound, free of the limitations imposed on the Class D amplifier that rolls off at around 20 kHz due to the low pass filter.

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The TCB701FNG Class TB audio power amplifier Implementing the considerable benefits of Toshiba’s Class TB amplifier configuration (for which a patent is in process), the TCB701FNG is a four-channel, 49 W audio power amplifier IC that offers highly efficient operation and high-quality audio (THD=0.02%) alongside an extensive set of built-in diagnostic functions.

The inherent robustness against EMI reduces the number of external components required. Coupling this with the lack of a LC low pass filter and the small device size of just 15.9 mm x 11.0 mm x 3.41 mm allows for an extremely compact audio amplifier solution with a low BoM cost.

Among the wide range of inbuilt protection and detection circuits, is a full time DC offset detection that with the support of the external MCU prevents a speaker burn that can occur with a significant DC offset voltage on the output. The protection/detection circuits are complemented by an output cross connection detection & protection as well as output clip detection. The device also includes basic power-related protections such as thermal shut down, over voltage, short to VCC, short to GND and output-to-output short.

Figure 6: Block diagram of the TCB701FNG audio amplifier

A built-in I2C bus provides extensive functionality including the ability to adjust many operating parameters. Through the I2C bus functions such as muting, entering standby mode and changing the front / rear gain can all be performed. It is also possible to change the output offset or clip detection as well as to setup and initiate the self-diagnosis function. Finally, the wide supply voltage (VCC) range of 6-18 V DC allows the amplifier to deliver high quality audio without interruption, even in vehicles with engine idle reduction.

To ease the design task, an evaluation board is available for use in conjunction with a Raspberry Pi system.

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Summary With the increased sophistication in modern vehicles, the infotainment system is central to the driver experience and is rapidly becoming a key differentiator upon which purchasing decisions are made. A key element of a successful infotainment experience is the ability to efficiently deliver high quality audio without EMI interference from the vehicles other electronic systems.

Through implementing a Class TB amplifier configuration, the new TCB701FNG from Toshiba achieves these goals and delivers a high level of functionality and configurability in a small package that requires very few external components to realize a sophisticated amplifier solution.

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