Nowadays, application-centric design in smartphones has become the mainstream trend, and peripheral functions such as various modems in such designs have been offloaded to various independent chipsets. There are many reasons for this result, partly due to fierce market competition, including the kind of hope to achieve differentiation through leading performance, as well as the rapid market pressure faced by mobile phone manufacturers, as well as multimedia technology and wireless Different technological development ideas in the field of chipsets are intertwined.
For the innovation of the "function-driven" of various application processors, the trend is often to deviate from the development path of "standard-driven" and to the development trend of wireless communication chipsets. The core capabilities required in these areas are different: Bluetooth, cellular or Wi-Fi chipsets need to meet their respective specifications, while requiring the lowest cost and competitive power consumption; in addition, in order to respond to competition, applications Processors need to achieve a large degree of differentiation in their digital functions and performance. Increasing competition has pushed the capabilities of single-chip solution providers to the limit, and it is no longer practical to maintain their leading edge in all aspects of device functionality. Today, cost and time-to-market pressures are driving the industry to develop solutions with a more efficient division of labor. In the long run, this is beneficial to both chip designers and consumers because it allows chips Group providers focus their efforts on what they do best and eliminate many costly duplicate functions in the system.
In the past, these peripheral functions were usually integrated in some application processors or modems, the purpose of which was to add value through integration. Due to time and cost pressures, their implementation solutions are often unsatisfactory, especially where professional skills are required. Audio has been a victim of over-integration for a long time. The implementation of audio functions is not only repeated but of poor quality, because the implementation of audio relies on two or more chipsets, plus one A large number of "patch" devices, thus supporting the mixing, switching, popping and click noise suppression and power amplification required by their end products.
The audio-centric codec WM8994 launched by Wolfson Microelectronics takes full advantage of the latest application-centric modem architecture, which integrates all mixed-signal audio functions to ensure the selection of a low-cost processor to successfully implement the system, and Ensure the consistency of the audio signal path characteristics of various data sources (Figure 1).
Figure 1: WM8994 uses an application-centric modem architecture.
WM8994 integrates a variety of analog and digital I / O, the purpose is to drive all types of audio transducers in mobile phones, but also to achieve the interface between various processors, such as used to deal with Bluetooth transceivers , FM radio, cellular modem and application processor. Such a high level of audio integration, while bringing flexibility, consistency, and high performance to the mobile phone, requires only a small amount of planning in the design stage to maximize the advantages of the system architecture.
For most mixed-signal systems, attention should be paid to the good control of the clock scheme and system noise, coupled with the use of a good power supply, grounding scheme and a low noise reference, so that the best system performance can be achieved. However, there are many other challenges for complex audio systems. The following design tips will tell design engineers how to make full use of WM8994's advantages in a highly challenging smartphone environment.
Ten tips for designing smartphones with WM8994
1. Plan audio application scenarios in advance. For each application, we must first figure out which chipsets will be started, and then we must figure out where the signal comes from and where to go? At the same time, what is the most efficient transmission path? In the process of audio streaming Some unexpected events must be taken into consideration. For example, when a specific audio channel is activated, if an event occurs, what will be the result? Should the channel be terminated? Attenuated? Mixed with some other sound? Or re-find another transmission path? Actual How many such small apps will run at the same time?
2. To outline audio clock schemes for key application scenarios, and then to formulate clock schemes for those corner applications. From the perspective of "convention", today's edge applications may be the "must-have" function of tomorrow. Make sure that each phase-locked loop (FLL), clock distributor, and audio interface are properly configured so that the memory buffer in the processor is not overfull or empty due to non-optimal configuration. Because WM8994 has enough powerful clock programmability, it can cover most scenarios; but the question worth considering is where should the FLL reference clock come from each application? Which port should be configured as the operating master Mode? Connect to the always-awakened 32KHz clock through one of the multiple clock input pins to achieve power savings in many cases, especially in standby mode that requires a clock to debounce the GPIO input, or in Simple low-power music playback mode. By using the audio interface bit clock and frame clock as the reference input for FLL, no additional high-frequency main clock input is needed, thereby realizing power saving.
The WM8994's sample rate converter allows the device to work in two completely independent clock domains, supporting audio mixing and transmission (routing) across these clock boundaries. Since the sampling rate converter of the device works on the largest of the two full-duplex channels, there is almost no limit when the sampling rate converter is connected to audio interface 2 and / or audio interface 3, while audio Interfaces 2 and 3 are usually interfaces to wireless chipsets that rarely require more than one or two concurrent channels.
3. Don't worry about the use of D-type horn amplifier. The power saving of this mode of amplifier is very much, the power saving can usually reach tens or even hundreds of milliwatts. As more mobile phones with stereo speakers appear on the market, the power consumption savings will naturally double. Imagine that if the saved power budget can be used for other functions in the system, for those engineers who are not familiar with the class D amplification technology, the data sheet gives specific suggestions on various aspects, including electromagnetic compatibility ( EMC) design, how to choose speakers, how to improve efficiency and how to achieve the best circuit board design. In the early days of mobile phone design, people usually paid more attention to EMC; nowadays, design engineers tend to pay attention to thermal design, because in fact, in some cases, heat dissipation has a greater constraint on the performance of mobile phones. So design engineers began to pay more attention to the power utilization efficiency. Class D amplifiers have higher efficiency, thus reducing the current surge on the power supply used by the speaker amplifier. This surge will cause the battery voltage to drop and the system to stop working earlier, and shorten the battery life. When the surge occurs with other current surges in the system, the above problem will be more serious. The reduction in battery voltage drop will help reduce distortion at high signal levels, and in some cases will provide more room to increase the maximum output signal of the speaker.
4. Wherever possible, the analog connection to the modem should be replaced with a digital connection. Although WM8994 can also support analog voice data channels, this analog scheme will consume more system-level power consumption, making it more susceptible to crosstalk violations. This will cause PCB rework in many cases and will increase the number of signals on the signal channel. The additional cost of the source components and the board area overhead (Figure 2).
Figure 2: Using WM8994 will greatly improve audio performance.
For example, when a pulse code modulation (PCM) link is used between the codec and the modem to transmit a voice call with a wired headset, if it is connected to a digital data stream, it will be supported at the headset output to activate Class W mode, which It will save the power consumption of the codec; also by turning off the built-in DAC, which usually consumes more power, the modem can also save power consumption. In a handset call or hands-free mode, the use of digital connection also allows the speaker tuning to use integrated parameter equalization (EQ), and the control of EQ settings only needs to be achieved through a device (usually an application processor), not Rely on two or three processors to repeat the same tuning function.
5. Where possible, the benefits of digital microphones should be considered at the system level rather than at the device level. Abandoning excessively long analog microphone connections can help reduce costs and save space. Nowadays, in order to reduce noise, microphone arrays are being widely adopted. With the increase in the number of mobile phone microphones, in the wiring of many microphone signal channels, there is a high probability that one of the channels is close to the noise source. Although the noise immunity can be improved by differential connection and careful wiring, the demand for high-quality capacitors in the signal path is doubled. Using digital connections will improve noise immunity. For this reason, the WM8994 provides up to 4 digital microphone channels. It must be noted that in order to obtain the best performance, when using a microphone array, it is essential to select a microphone with the best matching characteristics.
6. Make sure to connect the serial interface of the application processor to the WM8994 so that it can support the highest sampling rate required for all kinds of audio applications. In addition, the number of TDM channels needed to support large processing volumes such as microphone array processing or multiple concurrent audio digital stream processing needs to be checked.
7. An effective power management architecture should be selected. In fact, WM8994 only needs two external power supply voltages. Usually, one is directly connected to the battery, and the other is connected to the 1.8V power supply. Since the 1.8V power supply powers the charge pump and then the headphone amplifier whose reference level is ground, it is best to use a high-efficiency switching power supply regulator for this power supply. For example, a low dropout voltage can be used. Linear regulator (LDO) power supply to ensure that the low power consumption performance of the headset is not affected.
8. When setting up voice calls for earpieces and wired earphones, use digital sidetone characteristics where possible; compared to analog sidetones, the digital channel ’s wind noise filtering and low delay can provide a more natural Surround side sounds. In the analog side sound, the low-frequency mechanical components and wind noise components that are easily distracting are included, or the software is used to implement the side sound in the baseband processor, which will introduce some delay and cause some changes in the sound.
9. Please use analog volume control in the headphone output channel, or at least use the analog volume control in the range of the first few decibels below full scale. For the improvement of noise performance, this may bring an additional 4 decibels, which is invaluable for today's high-sensitivity headsets. Since the acoustic efficiency of headphones has been greatly improved in recent years, the requirements for the signal-to-noise ratio performance of digital-to-analog converters (DAC) and the noise performance of headphone amplifiers have become more demanding; if the signal-to-noise ratio performance in the DAC is insufficient, When using a headset, it will cause an audible tone or "hiss" sound. If analog volume control is used, the WM8994 can provide a signal-to-noise ratio of up to 100dB for the load, which will reduce the noise on the headphones by an additional 4dB. Typically, compared to many codecs on the market, the noise performance improvement provided by this solution can be as high as 10dB. For users, the difference that can be clearly felt through this improvement is the huge difference between being able to directly hear the noise and not hearing the noise at all.
10. Careful selection of series resistors for headphone output is required. In many countries, there are clear regulations on the maximum output power of headphones, the purpose is to avoid damage to hearing, so design engineers must keep this in mind when choosing resistance values. The resistance value should be small enough so that the output voltage attenuation at the line load (such as Hi-Fi input) can support a voltage of 1Vrms at full scale, which is usually expected; on the other hand, the resistance value must also be sufficient Large, so that for a small load, after moderate attenuation, it can also meet the regulatory requirements of those countries on the power of headphones. For the WM8994, due to its high output power rating, even if a series resistance with a relatively large resistance value is selected, the headphone output can drive the headphones with a very low distortion level. The output of 1Vrms can be achieved even when the total load is as low as 32Ω due to demand.
Future-proof audio performance
The Audio Hub (Hub) solution developed in recent years can easily integrate high-performance application processors and more cost-effective modem systems by integrating all the required audio function sets on a single chip . This solution eliminates duplication in the system, minimizes component interdependence, and provides a stable platform for future product development.
Based on the rich signal channels and available mixing functions on the WM8994, it provides a simple solution for various new audio applications in the future without the need for heavy software development work. The device supports numerous digital and analog channels, which will enable mobile phone design engineers to implement further design innovations without adding new audio components to the system. If you can follow the above basic guidelines for beneficial system design, it is entirely possible to achieve excellent audio performance in a wide range of applications.
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