WM8978 Wolfson Microelectronics Ltd., WM8978 Datasheet - Page 25
Manufacturer Part Number
The WM8978 is a low power, high quality stereo codec designed for portable applications such as Digital still camera or Digital Camcorde
Wolfson Microelectronics Ltd.
Preliminary Technical Data
The WM8978 is a low power audio CODEC combining a high quality stereo audio DAC and ADC,
with flexible line and microphone input and output processing. Applications for this device include
multimedia phones, stereo digital camcorders, and digital still cameras with either mono or stereo
record and playback capability.
The chip offers great flexibility in use, and so can support many different modes of operation as
Two pairs of stereo microphone inputs are provided, allowing a pair of stereo microphones to be
pseudo-differentially connected, with user defined gain using internal resistors. The provision of the
common mode input pin for each stereo input allows for rejection of common mode noise on the
microphone inputs (level depends on gain setting chosen). A microphone bias is output from the chip
which can be used to bias both 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.
LINE INPUTS (AUXL, AUXR)
The inputs, AUXL and AUXR, can be used as a stereo line input or as an input for warning tones (or
‘beeps’) etc. These inputs can be summed into the record paths, along with the microphone preamp
outputs, so allowing for mixing of audio with ‘backing music’ etc as required.
The stereo ADC uses a 24-bit delta sigma oversampling architecture to deliver optimum performance
with low power consumption.
The hi-fi DAC provides high quality audio playback suitable for all portable audio hi-fi type
applications, including MP3 players and portable disc players of all types.
Flexible mixing is provided on the outputs of the device. A stereo mixer is provided for the stereo
headphone or line outputs, LOUT1/ROUT1, and additional summers on the OUT3/OUT4 outputs
allow for an optional differential or stereo line output on these pins. Gain adjustment PGAs are
provided for the LOUT1/ROUT1 and LOUT2/ROUT2 outputs, and signal switching is provided to
allow for all possible signal combinations. The output buffers can be configured in several ways,
allowing support of up to three sets of external transducers; ie stereo headphone, BTL speaker, and
BTL earpiece may be connected simultaneously. Thermal implications should be considered before
simultaneous full power operation of all outputs is attempted.
Alternatively, if a speaker output is not required, the LOUT2 and ROUT2 pins might be used as a
stereo headphone driver, (disable output invert buffer on ROUT2). In that case two sets of
headphones might be driven, or the LOUT2 and ROUT2 pins used as a line output driver.
PTD Rev 2.6 November 2005