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Wolfson Introduces Class D Technology for Audio Power Amplifiers

Edinburgh, UK, 12 January 2004 – Wolfson Microelectronics plc (LSE: WLF.L) today introduced the WM8608, the first of a family of feature-rich class D devices. The WM8608 is capable of providing up to 7.1 channels of pulse-width modulation (PWM) output from four stereo PCM inputs. This product marks a new entry for Wolfson in the Class D technology field and is the result of a significant development project.

The WM8608 comprises a high-performance multi-channel PWM digital power amplifier controller. Six identical full audio bandwidth channels, plus a reduced bandwidth sub channel are provided as PWM outputs. PWM outputs are selectable as either CMOS or LVDS, the latter being advantageous in helping reduce EMI and potential radio interference and allowing greater flexibility in PCB layout, as tracking can be longer. By simply adding appropriate power output stages a multi-channel power amplifier may be built.

Benefits of Class D
Class D, or switch-mode amplifiers, are rapidly replacing linear amplifiers in audio systems, especially in portable, power-sensitive CD and DVD players, providing an acceptable balance between power usage and distortion. Class D devices replace multiple digital to audio converters, increase power efficiency in battery-powered devices and offer reduced costs to final products.

“Although the WM8608 PWM device is primarily digital, Wolfson has been able to build on its skills and expertise in mixed signal audio conversion products to enter this new technology field” said Jim Reid Technical Director of Wolfson. “Class D enables a pure digital path for today’s digital sound sources right through to the final output stages and transducers. As no intermediate D-A conversion is required, the design of a digital chip with DSP functions is possible.”

Supporting up to 7.1 audio input channels in PCM format, these may be mixed down to 6.1 or 5.1 outputs, by mixing rear channel data into centre rear or surround speakers if required. If 5.1 channel data is input, the surround data may be processed internally to create a pseudo 6.1 signal with rear channel output. In the digital domain a 4-band Graphic Equaliser function is provided, selectable high frequency equalisation to suit different speaker types and independent volume control for each channel (+24dB to -103.5dB in 0.5dB steps).

The DSP architecture utilises a dynamic peak compressor for each channel which prevents digital clipping when gains of greater than 0db are applied and to ensure adequate signal amplitude to allow headroom for graphic equalisation. System clocking in simplified by utilising the onboard PLL which not only enables the device to run off MPEG and audio clocks but enhances performance within the system.

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The WM8608 may be used with integrated power stages from vendors such as TI or ST or with a suitable pre driver and discrete FETs (e.g. Zetex, Vishay, Fairchild). Typical performance using an integrated power stage has given 100dB SNR, 0.1% THD @ 30W.

The WM8608 is available in a TQFP-48 operating on 2.7V to 3.6V supplies. It is available now along with evaluation platforms for a 6.1 channel system.

About Wolfson Microelectronics
Wolfson Microelectronics plc (LSE: WLF.L) is an award winning, fabless semiconductor company based in Edinburgh, UK. Wolfson develops and markets high performance mixed signal semiconductors for multimedia and communications applications worldwide. The company currently has sales offices in the USA, Japan, China and Taiwan, with a further office planned for Korea early in 2004. For further information please contact:

For more information contact:
Wolfson Microelectronics plc, Bernard Terrace, Edinburgh, EH8 9NX, United Kingdom, Tel: +44 (0) 131 272 7000, Fax: +44 (0) 131 272 7001

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