Listen up, audio gets on the move
14 January 2008
Hearing clearly amid the traffic/party/background noise when mobile is vexing audio design engineers.
When digital audio was either hi-fi sound or telephony, life was so much easier for audio engineers. The hi-fi codecs used multi-bit sigma delta technology to improve sound and phone ICs were mono, low data rate (8bit and 8kHz, typically) parts.
How simple life was then. Now, the inter-IC sound, shortened to the clumsy acronym I2S, data format has emerged from its hi-fi origins to compete with AC’97 standard parts, traditionally the codec dedicated to sample rate conversion in software, making it the de facto industry standard for computer audio. Audio engineers have a choice of AC’97 or I2S codecs to connect to an applications processor.
I2S Codecs
Texas Instruments believes it can offer the first I2S codecs that can interface to digital or analogue microphones. They can therefore be used in practically any portable consumer devices where audio is required, from wireless phone handsets to personal navigation devices; from digital still cameras to portable media players.
The trick with such devices is to maximise battery life and enhance performance, as by nature of being portable, they are used in areas where other noise can fight for the listener’s attention. The TLV320AIC34, TLV320AIC33 and TLV320AIC3106 stereo codecs all have low power consumption and noise filtering capabilities. They are the latest addition to the existing family of software-compatible, low-power codecs from Texas Instruments and complement the DSP and analogue company’s portable audio products of low jitter clocks, Class D audio power amplifiers and Direct Path headphone amplifiers.
Digital sound
Digital microphones are gaining ground on their analogue counterparts. Part of the appeal is that they can generate a digital audio signal that is less susceptible to electrical noise than an analogue one. Audio devices are facing electromagnetic interference as well as that from radio frequencies, which can compromise quality wit static noise and ‘drop-outs’ in voice conversations.
TI has worked with Akustica, which produces digital microphones, to jointly present digital microphones with low power consumption for use in portable audio applications. The four-channel stereo codecs can open up design opportunities as they can be placed close to the digital microphone without impairing performance. This proximity has increasingly become an issue as portable applications are providing densely populated circuit boards and so if placing the codec can be unrestricted, there are more options in circuit design.
Performance
The codecs can support 8ksample/sec up to 96ksample/sec rates. They also exhibit high signal to noise ration (SNR), i.e. 102dB for DAC and 92dB for ADC. A wide range of audio clocks can be supported courtesy of the integrated phase locked loops (PLLs). For the listener, the headphone, speaker and playback modes are all low power, enhancing the battery life of portable devices. They also support programmable digital audio effects including 3D sound, bass, treble, EQ and de-emphasis
The pricing is also tailored at the cost-conscious consumer market. The TLV320AIC34 is available in a BGA package, measuring 6mm x 6mm and has a recommended price of $6 in quantities of 1,000. The TLV320AIC33 and TLV320AIC3106 are available in a 5mm x 5mm BGA and 7mm x 7mm QFN package and both sell for just under $4 in the same quantities. An application note, detailing the set-up of the evaluation module and instructions to connect the codec to a digital microphone are available at www.ti.com/audiocodecdigmic-appnote.
Power management
Mixed-signal specialist, Wolfson Microelectronics has introduced a group of products specifically to extend battery life and reduce system cost for portable audio and navigation applications. The UK company announced AudioPlus Power Management with the first product, the WM8350, an integrated, hi-fi audio codec and power management sub-system.
The level of integration means that the chip represents up to 50 per cent board footprint saving and can reduce the bill of materials by up to 25 per cent, claims the company. It offers an alternative in multimedia applications to using separate codec and power management chips, combining as it does an audio codec, on-chip clock generation, six DC/DC converters, four LDO regulators, a battery charger, two white LED drivers and an auxillary ADC. The WM8350 is packaged in a 129pin, 7mm x 7mm BGA.
Performance control
SNR performance is 98dB and there is support for six analogue inputs, two stereo analogue outputs and two mono line outputs to drive headphones directly.
The company claims that the WM8350 has integrated power management and power efficiency that is comparable with discrete solutions available today. It has an integral power management controller to regulate the output voltage settings, start-up sequence and low power modes for all internal DC/DC converters and LDO regulators. Four 2MHz DC/DC buck converters generate supplies for the CPU core, I/O, memory devices and other system peripherals. Each converter provides a programmable output voltage of between 0.85V to 3.4V in 25nV increments. Peak efficiencies are quoted as over 90 per cent, using an external inductor and capacitor. Two 1MHz DC/DC boost converters can be used to backlight displays and generate 5V supplies for USB OTG (universal serial bus on the go). A single cell, lithium battery charger supports programmable target voltage, trickle and fast charge modes as well as safety features for the end application.
Cross Codec Operation
Some processors can operate with I2S and AC97 codecs. An example of this is the C-Media Oxygen HD CMI8788 PCI audio processor (www.cmedia.com.tw). The CMI8788 is a 32bit, multi-channel audio processor that supports up to 12 output channels and eight input ones. The multi-channel DMAs have sampling rates of 192kHz.
The ability to work across all popular codecs increases design options for audio engineers. The I2S, AC-link, two-wire master bus and SPI interfaces are used to transfer audio data and control data between the processor and the codecs. There are four synchronous I2S input and four output pairs. All I/O pairs support 32bit high-definition PCM data transfer and adjustable sample rate up to 192kHz.
Other hardware features support the digital loop back path to switch between optical and RCA connection. A serial bus communicates with the microcontroller unit and there is an EEPROM control interface and nine direct-access GPIO pins.
Surround Sound
Software delivers audio for consumer applications, such as gaming. For example, the DTS Interactive iV real-time 5.1 encoder takes two or more channels and encodes the data into DTS bitstream and a DTS NepPC matrix turns any two channel audio into 7.1 surround sound. The Dolby Pro-Logic IIx surround processor spreads stereo audio into 7.1 channel surround sounds. The user’s voice can be digitised using the C-Media Magic Voice software in online chats.
Software also provides real-time encoding of the bitstream to connect with the receiver. There is software support for most industrial standards of PC 3D sound, used for gaming as well as for digital audio playback of rWinXP64, WinXP, Win200 (DirectX v9.0 and above). A Linux driver is available but without Dolby/DTS. The processor is packaged in 128pin LQFP.
The player gets serious
Now ubiquitous, the MP3 player is also benefiting from advances in silicon to reduce both the power consumption and bill of materials of these portable, consumer devices. The number of mobile handsets with MP3 players is increasing rapidly, with over 260million this year and expected to reach over one billion new unit sales by 2012. Informa carried out research that suggested that by the time of the London Olympics, over 70 per cent of all handsets sold around the world will be music-enabled.
Two UK companies have adopted MP3 development with vigour. Arc International (www.arc.com) has optimised the MP3 decoder for its ARC Sound Subsystem, which operates at less than 7MHz and dissipates less than 0.46W, when decoding a typical MP3 stream at 128kbit/sec/44.1kHz. This is a reduction of around 20 per cent, claims the company on competitive MP3 decoders.
The other UK company, CSR (www.csr.com), concentrates on personal wireless technology and uses its Bluetooth technology, BlueCore, to deliver what it claims is the world’s lowest cost Bluetooth-enable MP3 player. The bill of materials is under $10.00. Now instead of getting tangled up in your MP3 player wires at the gym, the BlueCore Player, based on the company’s BlueCore5-Multimedia silicon means MP3 players can be wireless, or music can be streamed to two headsets simultaneously or MP3 files can be directly streamed to reduce power consumption.
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