Nokia 9000i, Service Manual

Page: 59 / 434

After Sales
Technical Documentation
RAE/RAK–1N
Baseband
Page 2–33
Original, 08/96
Block description
The codec has three microphone inputs and two earphone outputs. The
microphone inputs from handportable and PHF microphones plus the
external audio input can be therefore connected directly to the codec. The
handportable earphone output and external audio are also connected
directly to the codec using the two earphone output channels. The booster
amplifier for PFH uses the same codec output channel as the external
audio. The codec has internal switches for selecting which input or output
is used. It also has microphone amplifier with programmable gain and
earphone attenuator with programmable attenuation. Input/output
selection and amplification/attenuation setting can be done using codec
registers. The registers are controlled by the MCU.
Handportable microphone (MICN, MICP) and PHF microphone
(PHFMICN, PHFMICP) are connected directly to the codec‘s differential
inputs. There is a bias switch in the AUDIO block, which connects the DC
bias voltage for both microphones (electret type) when a call is activated.
The external microphone signal (EXTMIC) is connected single–ended to
the third microphone input channel. There is 21 dB attenuation in the
external microphone line before the codec to prevent clipping.
The handportable’s earphone (EARN, EARP) is connected directly to one
of the two differential output channels. The output can drive directly a 32
load. The external audio signal (EXTEAR) is connected single–ended to
the other output channel. This output is also connected to the input of the
Personal Hands–Free booster amplifier. The booster is disabled by MCU
when an external audio accessory is used. In PHF mode, the booster is
enabled and its differential output is fed to the PHF speaker (PHFEARN,
PHFEARP). The load impedance of the PHF speaker is 8
.
Inside the codec, the currently selected microphone signal is routed to the
microphone amplifier. After that it is fed to the bandpass filter and then to
the A/D converter. After the conversion the digital speech is sent to the
DSP in PCM format.
Digital downlink signal from the DSP is fed to the D/A converted. After the
converter there is low pass filter and attenuator before the earphone
output. All these are inside the codec. The ASIC generates the 512 kHz
and 8 kHz clocks for the codec and data transmission between the codec
and the DSP.
The audio codec communicates with the DSP (digital speech) through an
SIO (signals: PCMIN, SYNC, CODEC_CLK and PCMOUT).The MCU
controls the audio codec function through a separate serial bus (signals:
PCMCDO, PCMCDI, PCMCLK and XSELPCMC). The PHF power
amplifier can be enabled or disabled using the AMP_PWR signal from the
MCU. The power amplifier is enabled only when needed due to power
consumption reasons.