WM8974 datasheet(13/63 Pages) WOLFSON

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WM8974

PP Rev 1.1 September 2004

DEVICE DESCRIPTION

INTRODUCTION

The WM8974 is a low power audio codec combining a high quality mono audio DAC and ADC, with

flexible line and microphone input and output processing. Applications for this device are anticipated

to include digital still cameras with mono audio, record and playback capability and also voice

recorders.

FEATURES

The chip offers great flexibility in use, and so can support many different modes of operation as

follows:

MICROPHONE INPUTS

Two microphone inputs are provided, allowing for either a differential microphone input or a single

ended microphone to be connected. These inputs have a user programmable gain range of -12dB

to +35.25dB using internal resistors. After the input PGA stage comes a boost stage which can add

a further 20dB of gain. A microphone bias is output from the chip which can be used to bias the

microphones. The signal routing can be configured to allow manual adjustment of mic levels, or to

allow the ALC loop to control the level of mic signal that is transmitted.

Total gain through the microphone paths of up to +55.25dB can be selected.

PGA AND ALC OPERATION

A programmable gain amplifier is provided in the input path to the ADC. This may be used manually

or in conjunction with a mixed analogue/digital automatic level control (ALC) which keeps the

recording volume constant.

AUX INPUT

The device includes a mono input, AUX, that can be used as an input for warning tones (beep) etc.

The output from this circuit can be summed into the mono output and/or the speaker output paths,

so allowing for mixing of audio with ‘backing music’ etc as required. This path can also be summed

into the input in a flexible fashion, either to the input PGA as a second microphone input or as a line

input. The configuration of this circuit, with integrated on-chip resistors allows several analogue

signals to be summed into the single AUX input if required.

ADC

The mono ADC uses a multi-bit high-order oversampling architecture to deliver optimum

performance with low power consumption. Various sample rates are supported, from the 8ks/s rate

typically used in voice dictation, up to the 48ks/s rate used in high quality audio applications.

Hi-Fi DAC

The hi-fi DAC provides high quality audio playback suitable for all portable mono audio type

applications.

DIGITAL FILTERING

Advanced Sigma Delta Converters are used along with digital decimation and interpolation filters to

give high quality audio at sample rates from 8ks/s to 48ks/s.

Application specific digital filters are also available which help to reduce the effect of specific noise

sources such as ‘wind noise’.

The filters include a programmable ADC high pass filter, a

programmable ADC notch filter and a 5-band equaliser that can be applied to either the ADC or the

DAC path in order to improve the overall audio sound from the device.

OUTPUT MIXING AND VOLUME ADJUST

Flexible mixing is provided on the outputs of the device; a mixer is provided for the speaker outputs,

and an additional mono summer for the mono output. These mixers allow the output of the DAC, the

output of the ADC volume control and the Auxilliary input to be combined. The output volume can

be adjusted using the integrated digital volume control and there is additional analogue gain

adjustment capability on the speaker output.

AUDIO INTERFACES

The WM8974 has a standard audio interface, to support the transmission of audio data to and from

the chip. This interface is a 4 wire standard audio interface which supports a number of audio data

formats including I

2S, DSP Mode, MSB-First, left justified and MSB-First, right justified, and can

operate in master or slave modes.

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