PCD6003 datasheet(61/96 Pages) PHILIPS

61 / 96 page

2001 Apr 17

Philips Semiconductors

Product speciﬁcation

Digital telephone answering machine chip

PCD6003

13 THE CODECs

13.1

Deﬁnitions

In the description of the CODECs, amplitude units in dB

are used. The following definitions apply:

• dBm: used for absolute analog signal power levels.

0 dBm equals 1 mW power dissipation in 600

Ω. A

single sinewave signal with a power level of 0 dBm

corresponds to an RMS voltage value of 774.6 mV.

• dBmp: used for absolute analog signal power levels

with psophometric weighting according to

“CCITT

Recommendation G.223”. This unit is used to express

analog noise power levels.

• dBm0: used for relative digital signal power levels.

0 dBm0 is defined in

“CCITT Recommendation G.711

(Section 4, Table 5)”. It follows that the maximum digital

signal power level is 3.14 dBm0 (A-law). Thus

3.14 dBm0 is the RMS value of a sinewave signal whose

peaks just reach the full-scale of the digital code. For the

(internal) bitstream signal (output of ARS and DNS) the

positive full-scale value is a continuous stream of ‘ones’,

whereas the negative full-scale value is a continuous

stream of ‘zeroes’. For the (internal) digital 14 or 16-bit

words, represented in 2s complement (MSB first) the

positive full-scale value is a ‘zero’ followed by 13 or 15

‘ones’, whereas the negative full-scale value is a ‘one’

followed by 13 or 15 ‘zeroes’.

• dBm0p: used for relative digital signal power levels with

psophometric weighting according to

“CCITT

Recommendation G.223”.

• dB: is used for the signal level gain between any two

nodes within the speech path. As different signal

representations are used within the speech path, the

gain value depends on the used signal definitions.

• dBp: is used for the signal level gain between any two

nodes within the speech path with psophometric

weighting according to

“CCITT Recommendation

G.223”.

• The uniform PCM reference point is the (virtual) signal

node in the DSP at the input of the PCM encoder for the

analog-to-digital speech path and the output of the PCM

decoder for the digital-to-analog speech path.

13.2

CODEC architecture

The PCD6003 is provided with two CODECs that perform

the analog-to-digital and digital-to-analog conversion of

speech signals. In Fig.29, the CODECs are the interface

between the external analog peripherals and the DSP.

CODEC1 is used for the line interface and CODEC2 is

used for the loudspeaker and the microphone.

The DTCON register bit DTCON.4 selects the input to

CODEC1 (LIFMIN1 or LIFMIN2).

The main CODEC functions are (refer to Fig.29):

• AMP - Pre-amplifier

• ARS - Analog Receive Sigma delta ADC

• DDF - Digital Decimation Filter

• DNS - Digital Noise Shaper

• ATD - Analog Transmit DAC.

For CODEC1 the balanced line interface input is fed to the

ARS block that performs analog-to-digital conversion, the

gain of the input can be set to the amplification steps:

7, 23 and 35 dB (see Section 17.5 for typical/maximum

gain specifications). This programmable range is used by

the microcontroller on command of the DSP to perform

limit or automatic gain control. The analog data is

converted by ARS to a bit stream. The basic sampling

frequency (fs) is 8 kHz. The DDF decimates the bit stream

down to 16-bit linear PCM data. The DF has a gain of

3.14 dB (which has to be added to the programmable ARS

gain) to achieve a uniform reference point at the DSP input

for linear PCM data. Finally, the DSP will decimate this

data to 16-bit linear PCM data at a rate of 8 kHz.

The reverse operation is performed in the transmit path.

The DSP produces 16-bit linear PCM to the DNS. The ATD

which is a DAC converts the bit stream into an analog

signal. The converter has a programmable amplification

range of 18 dB. This programmability is

−12, −6, +0 and

+6 dB.

CODEC2 is built-up in a similar manner as CODEC1, the

only difference being the microphone amplifier before the

ADC. This will amplify the balanced analog (microphone)

signal in the receive path with a fixed +15 dB (see

Section 17.5 for exact gain specifications). For direct

connectivity of an external microphone, a software on/off

switchable supply voltage is available.

Several registers are available for the CODECS control:

• DTCON: for selecting the input to CODEC1

(DTCON.4 = 0 means LIFMIN1 is selected,

DTCON.4 = 1 means LIFMIN2 is selected) and for

alternative gain settings (see Section 13.2.2)

• CDVC1: the volume control register for CODEC1

• CDVC2: the volume control register for CODEC2

• CDTRx: test mode control registers for both CODECS

• PMTRx: test mode control registers for both CODECS.

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