CSP1027 Datasheet, PDF (13/64 Pages) Agere Systems – CSP1027 Voice Band Codec for Cellular Handset and Modem Applications

Data Sheet
December 1999
CSP1027 Voice Band Codec for
Cellular Handset and Modem Applications
4 Architectural Information (continued)
4.2.7 Digital-to-Analog Path
Starting at the bottom right of Figure 3 on page 5, the
∆-Σ D/A conversion process begins with a 16-bit two's
complement PCM signal read from the DI serial input.
The PCM is interpolated up to 1 MHz in two stages and
low-pass filtered at each stage to attenuate 8 kHz
images.
The PCM input is latched into the cdx(D/A) register at
a nominal word rate of 8 kHz. The signal is then option-
ally high-pass filtered. This filter has the same transfer
function as the A/D high-pass filter.
A digital sample-and-hold increases the word rate by a
factor of 5 from 8 kHz to 40 kHz. The seventh-order IIR
digital low-pass filter then removes the spectral images
between 4 kHz and 20 kHz and predistorts the pass-
band to compensate for the filtering done during the
interpolation up to the 1 MHz word rate. The transfer
function of this low-pass filter is the same as the one
employed in the A/D converter.
The output of the low-pass filter feeds a programmable
gain adjustment block that serves as a volume control.
The gain can be changed in 3 dB increments from
0 dB to –45 dB. The attenuation level is set by writing
the OGSEL field in the cioc0 register (see Table 7 on
page 26).
The digital modulator block further increases the word
rate by a factor of 25 from 40 kHz to 1 MHz. Through
quantization and noise shaping, the digital ∆-Σ modula-
tor creates 1-bit output words at 1 MHz.
The modulator 1-bit output drives a structure combining
a 1-bit D/A converter and a second-order switched-
capacitor filter having a cutoff frequency of 8 kHz
(based on a 1 MHz clock). This is all shown as the D/A
block in Figure 3 on page 5.
This is followed by a second-order active Chebychev fil-
ter having a cutoff frequency of 35 kHz.
The passband ripple of the analog filters is small
enough such that they have virtually no effect on the
passband response.
The output amplifier buffers the analog filter output.
The frequency responses of the A/D and D/A paths are
essentially the same. See Figures 6 through 15 for the
magnitude and delay responses versus frequency.
4.3 Programmable Features
4.3.1 Active/Inactive Modes
The CSP1027 has active and inactive modes of opera-
tion which are selected by the ACTIVE field in the
cioc0 register (see Table 7 on page 26). The default
value upon reset and powerup is ACTIVE = 0 (i.e.,
inactive). In the inactive mode, the codec clocks are
disabled, data transfers by the codec are disabled, and
analog bias currents are shut off. This state is useful in
battery-powered applications when prolonged periods
of inactivity are expected. It takes approximately
600 ms for the codec to reach full steady-state perfor-
mance in going from inactive to active. This is primarily
due to the charging of the large external capacitors,
CREF and CREG. However, the codec is functionally
useful after 100 ms.
4.3.2 Input Select
When the A/D preamplifier is selected (EIGS = 0), the
INSEL field of cioc0 (see Table 7 on page 26) switches
the preamp input between the MICIN and AUXIN
inputs. When external gain select is used (EIGS = 1),
the INSEL field has no effect.
4.3.3 A/D Input Ranges
When the preamplifier is used (EIGS = 0), the IRSEL
field of the cioc0 register (see Table 7 on page 26)
selects the 500 mVp range when IRSEL = 0 and the
160 mVp range when IRSEL = 1. IRSEL has no effect
when the external gain select mode is used (EIGS = 1).
When EIGS = 1, the inverting amplifier of Figure 5 on
page 7 replaces the preamplifier. The input range in
this mode is the following:
VFULL-SCALE
=
-^R----ⁱ--ⁿ--
Rfb
× 1.578
Vp
4.3.4 Output Mute Function
The D/A converter output can be selectively muted with
the MUTE field in the cioc0 register (see Table 7 on
page 26). The default value upon reset is muted
(MUTE = 0). The mute function is implemented (Figure
3 on page 5) internally by a MUX following the D/A
input. Placing the mute function here causes the signal
at the analog output to gradually decay/rise over
approximately 1 ms upon muting/unmuting. This effect
is due to the impulse response and group delay of the
digital filters. This implementation will reduce any
potentially undesirable transient effects such as pops,
when the D/A is muted.
Lucent Technologies Inc.
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