In today's mobile phone market, higher audio quality and higher power speakers not only require the system to have more advanced data converters, but also bring huge challenges to the design of lithium batteries as power sources, which requires Wolfson put forward the design idea of ​​using "audio hub" to help mobile phone designers solve these problems, and reduce costs and simplify system design.
Opportunities and challenges of high-quality audio
In a traditional mobile phone design, only a mono speaker is needed to play the ringtone, which may only need to be played for a few seconds every few hours. However, the latest models of multimedia mobile phones may support movie playback, mobile TV, games and other multimedia functions, and begin to use stereo speakers in the design of the system, so the design of the system must not only consider audio converters, but also require low power consumption Technology combined with it.
However, stereo speakers require twice as much power as mono speakers, and need to remain in operation for a long period of time. A 10-minute movie clip consumes 120 times as much battery power on a stereo speaker as a 10-second monophonic ringtone. Today's consumers are also expecting higher volume, and speakers with 1W output power are now quite typical requirements, and this places higher demands on battery energy.
Adding functions to mobile phones usually requires adding circuits. Today, mobile phones are shrinking, which means that the space left for batteries is smaller than ever. Due to the requirement to add more very power-hungry audio features to the mobile phone while using the smallest possible battery, the mobile phone designer has to carefully review each power consumption reason and inefficiency problem on the mobile phone in order to Save battery energy where possible. This requirement for longer battery life is driving the trend towards Class D power amplifier technology, which can eliminate the largest source of inefficiency in audio circuits.
The shrinking overall size is also driving the integration of mixed-signal audio functions, but this integration also brings new challenges because it is necessary to improve audio quality without increasing power consumption or requiring additional external components (such as voltage regulation Device or passive component). The new generation of 'audio center' devices are gradually solving this complex design problem, while solving the problem of improving audio quality, minimizing power consumption in working and standby modes, and reducing PCB area and component count.
The audio center came into being
Portable multimedia devices such as mobile phones usually contain some analog and digital sound sources in different data formats. These audio streams must be converted and properly converted before being output to the real world through different converters (such as low-power speakers, high-power speakers and headphones). the mix of. In order to save space, cut costs and reduce design complexity, it is certainly beneficial to concentrate all of these audio processing functions on a single device (ie, 'audio center').
Digital data sources can also exist in different data formats, word lengths, and sample rates. Since the telephone usage mode usually only requires the audio center to process mono 8kHz PCM format data, the integrated digital music playback function requires the audio center device to process different sample rates, word lengths and data formats (such as stereo 16-bit 44.1kHz I2S data). A flexible digital audio interface and clock solution on the audio center, coupled with Hi-Fi quality data converters, makes it possible to implement digital music playback on mobile phones without the need for additional mixed-signal components.
The audio center must be able to connect analog signals with different amplitudes, source impedances, DC offsets, and bandwidths, such as FM receivers, microphones, sending / receiving voice data, ringtones, or Hi-Fi line inputs. Flexible input configuration can support these different signal characteristics in different system architectures, and at the same time minimize the number of pins, save space and reduce costs.
Mixing in the analog domain of the audio center can eliminate the difficulty of sample rate conversion, and the flexible mixing channel can facilitate the emergence of new application features. Devices like WM8983 and WM8985 allow arbitrary mixing of microphone input, digital music, FM receiver and received voice data, and provide the ability to re-digitize this mixed audio, which can facilitate the implementation of functions such as karaoke recording .
Energy-saving power supply design for audio center
The power requirements for each audio function in the above signal chain are the most different in audio center devices. Generally, there are 3 to 4 independent power domains, and each domain has its own unique voltage / current requirements and noise characteristics. Audio center devices need to be designed very carefully to work under the different limitations of these power supplies. Minimizing power consumption without sacrificing audio signal quality is the key to providing Hi-Fi quality music for portable designs while reasonably reducing battery life. Each power domain must use different power-saving technologies.
Since reducing the power supply voltage of the digital part will not affect the audio quality, the digital core will use the lowest possible voltage to save power. The use of these low-voltage DC / DC converters can greatly improve the power conversion efficiency compared to linear regulators. The power supply ripple caused by the high-frequency switching of DC / DC converters can be more easily suppressed by digital circuits, and An analog block requires a stable supply voltage to keep the noise level as low as possible.
In a similar way, using a low-voltage digital I / O buffer power supply will consume less power and the audio quality will not be affected, although for some practical reasons this power supply voltage is sometimes higher than the power supply of the digital core Voltage (for example, to maintain the same level of signaling voltage between devices that communicate with each other).
Unlike digital functions, analog signal processing components (such as ADCs, DACs, mixers, amplifiers, and microphone interfaces) are very sensitive to noise. The signal-to-noise ratio can be improved by increasing the analog supply voltage, but at the cost of increased power consumption. System designers must make reasonable compromises based on their own audio quality and power consumption goals.
Maintaining a stable and clean analog power supply is also important to prevent power supply noise from degrading audio quality. Although good design and differential techniques can improve the power supply rejection ratio (PSRR), a high PSRR linear regulator is usually used to power the analog circuits in the audio center. It is also important to have a sufficient margin between the output voltage of the regulator and the minimum input voltage, which can maintain a high PSRR level when the battery is discharged. Analog voltages between 2.7V and 3.0V are quite typical in portable audio applications.
The most effective strategy for power saving of analog circuits in the audio center is to provide flexible and granular power management control, so that circuits that are not necessary for a given application scenario can be turned off. For example, most audio centers have at least 2 ADCs and 2 DACs on-chip, but the voice recording function requires only one ADC, PCM voice calls require one ADC and one DAC, and MP3 playback requires two DACs.
As long as there is a trade-off between power consumption and audio quality in a particular circuit, the low-power mode can be used, so that when the quality requirements are reduced (such as in voice communication), the performance can be appropriately reduced to save power. As the complexity of audio center devices continues to increase to match the increasing performance of mobile phones, the number of possible device configurations also grows, and low-level control of different blocks becomes necessary to avoid wasting power.
Combining high-quality data converters with Class D drivers
Traditional Class AB speaker drivers usually consume more power than they deliver to the speaker, which reduces battery life and may cause the device itself to overheat. Class D speaker drivers-The technology used by speaker drivers is by far the biggest factor affecting overall efficiency.
For example, a stereo Class AB speaker driver that provides 1W of power per channel with 40% efficiency needs to consume at least 5W of power from the battery, where 3W is converted into heat and dissipated on the device. In some applications, all other audio-related power consumption is two orders of magnitude lower, which makes the speaker driver a major source of low efficiency and unnecessary battery power consumption. Class D speaker drivers are increasingly used to improve efficiency, extend battery life, and simplify thermal management issues. Excessive heat can limit device functionality and increase costs.
In portable applications that support movie playback, games, or other multimedia functions, the speaker needs to remain in operation for a long period of time, and Class D technology is very effective in extending battery life. Even mobile phones that have been relatively short until the recent working hours of the speaker (such as when playing ringtones) now support speakers and multimedia streaming functions, which require longer use of the speaker driver. Therefore, Class D amplifiers are increasingly replacing Class AB amplifiers in mobile phone design.
Crackle and Tick Suppression-The audible crackle and tick sounds generated during the audio circuit setup will reduce the user experience, and often expend great effort to eliminate these noises during system development. An audio center device that integrates pop and tick suppression circuits can further shorten development time and improve the perceived audio quality. Interestingly, high-quality audio that is free from crackles, ticks, and other unpleasant noises also has the effect of improving video image quality.
to sum up
The combination of high-quality audio codec and Class D amplifier technology is very effective in extending the battery life of portable multimedia devices, which allows new features (such as games and TV streaming) to work longer. The high level of silicon integration provides a significant benefit of smaller PCB area and component count, but as Hi-Fi audio becomes more important, prioritization was not very high for mobile phones designed only for voice communication The other challenges must now be resolved.
Well-designed audio center devices can integrate functions such as data converters and power management to provide users with Hi-Fi audio and more mobile phone functions with a smaller form factor and longer battery life. For mobile phone designers, the additional benefits of ease of use, reduced component count and flexible power management performance are the main reasons for adopting these devices.
Higher levels of integration can provide real benefits to end users, but they must always maintain a focus on audio quality. Mixed-signal designers must maintain audio quality along the entire signal chain and keep in mind the limitations of mobile phone batteries. With a good design, a better audio experience does not necessarily result in additional power consumption or the cost of many additional components.
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