handset development
Robert Hatfield, technical marketing engineer, Wolfson Microelectronics, looks at the technical parameters which are shaping the next generation of mobile handset speaker designs
The entire mobile services supply chain has been facing falling ARPU throughout the 2/2.5G era. Their response has been to vigorously encourage subscribers to consume a wider variety of services, in addition to basic telephony, using mobile handsets.
Fortunately, 3G bandwidth now makes many high-value services feasible, allowing service providers to generate revenue from activities such as music filesharing, gaming and streaming video. But consolidating all the necessary capabilities into the most popular handset form factors brings significant challenges in manipulating numerous audio signals, and managing noise and power throughout the device. To allow these new services to become successful also requires handset designers to achieve a quantum leap in audio playback performance.
Boosting audio fidelity
To ensure market acceptance of emerging wireless multimedia services, louder and more powerful speaker drivers are a must, to deliver better speakerphone operation for services such as “video snacking” and mobile TV. Hi-fi audio is also necessary through headphones, for personal music, as well as during voice calls for added value features such as karaoke. However, the small enclosure size of the typical mobile handset does not pre-dispose the device to high fidelity audio performance.
As far as improving speakerphones is concerned, the latest high output speakers deliver much improved audio quality. But their high supply voltage requirements conflict with the generally falling operating voltages elsewhere in the handset. Designers therefore need to ensure a stable supply near battery voltage level, in addition to digital logic and analogue rails.
Manipulating multiple audio streams
Combining the numerous telephony features of a modern handset with advanced music features results in numerous audio streams comprising a variety of digital formats as well as analogue. Digital signals exist in a range of sample rates and word lengths. Arranging for these varied signals to co-exist without interference is demanding enough. But extensive, flexible mixing capabilities are also required, to support advanced entertainment features, such as music playback during voice calls, or various karaoke options.
Multimedia handset design also calls for flexible digital audio sampling and much more powerful processing capabilities to enable hi-fi quality audio recording and playback features. Sophisticated audio processing such as multi-band equalisation or 3D sound, for example, will become necessary to optimise the signal for playback through speakerphone, headphones, and to enable a range of audio effects. For example, a multi-band equaliser can be automatically adjusted to jazz, classical or rock settings pre-calculated to achieve the most authentic sound for a given genre. By tailoring filters to match the response of the human ear, listening through headphones or on speakerphone, for instance, designers can compensate for some of the shortcomings imposed by the small handset enclosure and relatively limited speaker response.
Power management
Despite the additional functionality of multimedia phones, users will not accept shorter battery charge intervals. Since the RF stages will continue to account for most of the power consumed, sophisticated audio can be implemented with little impact on power consumption, especially if a good power management strategy is devised.
Careful design and operation of signal paths, for example, allows tight control over portions of the circuit to be turned off when not required. Voice memo operation for example, since it requires only basic mono audio, can be run with only one ADC active, thereby saving the power consumed by two DACs and the DSP in the stereo audio signal chain. On voice calls, sample rates can be reduced to save power without suffering any noticeable loss of voice quality. More sophisticated power saving techniques may also include the reduction of analogue bias levels to achieve an optimal balance between audio output and power consumption.
Integrated audio hub for mobile multimedia
Handset designers, therefore, must meet the diverse challenges presented by these speaker drive, signal processing, noise performance and power management demands within a small physical footprint and – more testing still – within the tight design cycles characteristic of the mobile market. Greater integration, incorporating sophisticated audio capabilities, is called for.
The integrated baseband chipset approach, comprising a mixed-signal voice CODEC (vocoder) analog front end with a low power DSP to perform the channel coding and compression/decompression algorithms in order to lower the data rate over the air interface, has successfully supported basic voice telephony in 2G/2.5G networks. To deliver the compelling sound that next generation multimedia services will demand, handsets must embody a more audio-centric approach, leveraging true stereo audio CODEC capabilities and more powerful DSP for audio effects, plus flexible speaker outputs.
All of this advocates leveraging high-end audio design expertise and mixed-signal architectures to create an integrated handset audio hub supporting digital features such as enhanced 3D surround sound capabilities, multi-band equaliser, auto level control and bass boost. Wolfson Microelectronic’s WM8983 stereo audio CODEC, for example, implements this feature-set around a 24-bit high-order oversampling ADC and hi-fi DACs as well as implementing additional digital application filtering functions such as wind noise filters and programmable IIR filters as notch filters to eliminate noise at specific frequencies. Implementing the digital processing in dedicated hardware, instead of software, allows handset integrators to reduce the processing demand on the device’s main multimedia processor and thereby benefit from additional cost, memory and power savings.
This application-centric approach enables enhanced support for emerging usage scenarios, such as audio playback during calls to allow the called party to hear playback as well as the caller’s voice, or initiating normal mobile phone functions while continuing to listen to MP3 or FM radio. Other functions now becoming attractive to music phone users include karaoke during voice calls, with full mixing capability for headphone playback as well as recording in I2S format, facilitating sharing of karaoke files via MMS or email. Flexible digital and analogue interfaces implemented by devices such as WM8983 add versatility by allowing synthetic content such as ringtones or midi files to be mixed with audio or voice all at different sample rates.
Integration to save power
The multitude of signal paths necessary to support flexible multimedia features provides opportunities for designers to implement numerous power saving modes by turning off unused circuit elements, and adjusting analogue bias and digital signal levels for minimum power consumption at optimum audio quality. Consolidating CODEC, mixing and signal processing functions into a single, integrated hub device allows designers to invoke power saving modes more quickly and easily – for example by ordinary register operations within software – than would be possible for a discrete solution. Automatically reducing signal levels such as the digital voltage swing in the ringtone signal path, or analogue bias levels during voice calls (as described earlier) can be achieved relatively easily and will deliver valuable power savings at no audible cost in terms of ringtone playback or voice quality. Many of these modes can be made to operate transparently to the user.
Maximising audio quality
The WM8983 implements 3D processing and a 5-band equaliser, which is software adjustable not only for level but also for frequency range. This provides numerous options for designers to optimise the speaker output to playback particular audio effects at high quality. For instance, user selectable settings for particular music types may be provided. Other effects can also be applied by adjusting the equaliser through software controls. This may be transparent to the user if required. Wolfson also provides an accurate simulator tool, which enables designers to quickly optimise response to take into account the properties of the handset enclosure and shape, selected speaker parameters and driver design, and the response of the human ear.
Drives such as a BTL driver for high-output speakers, which can drive up to 1W into an 8W-impedance BTL speaker for high-fidelity sound at high-volume, need to be powered separately from the digital logic and analogue signal voltages. Indeed, the WM8983 provides analogue supply voltages from 2.5V to 3.3V, as well as supply voltage to the digital core as low as 1.71V. If a separate 5V supply is to be provided for the BTL speaker driver, for a high volume audio output, calls for at least three different supply voltages. The WM8983’s mixed signal technology eliminates the need for level shifting circuitry by allowing high analogue signal and supply levels to co-exist with digital supplies and signals on the same silicon.
Noise performance
However, integration of different digital and mixed-signal functions on the same die creates many potential noise and crosstalk problems. Hence the responsibility for protecting the sensitive analogue audio circuitry from noise is transferred from the circuit designer to the chip designer.
In the WM8983, integrated speaker and microphone drivers also enhance noise performance. Applying high-end audio design principles to multimedia handset CODEC design also enables very low noise microphone bias properties for extremely high quality recording. This would be a very unusual feature for a mobile handset, potentially attracting whole new groups of users. Hence existing IP can be leveraged very effectively and at reasonable implementation cost, to support advanced features such as this. Designers can therefore provide unusual and powerful differentiating features with the potential to open new avenues of usage and thereby create additional revenue opportunities for the entire mobile supply chain.
Conclusion
The importance of application-centric design as well as the skills of mixed signal designers to achieve peaceful coexistence of analogue, digital and power circuits on-chip, highlights the extensive interdependencies between audio quality, power consumption and noise performance. The integrated audio architecture which is now emerging should save designers of multimedia handsets from negotiating these alone.
