Si3220/25
attenuation for signals above 3.4 kHz are part of the 3.25.4. TXEQ/RXEQ Equalizer Blocks
combined decimation filter characteristic of the A/D
The TXEQ and RXEQ blocks (see Figure 11 on page
converter. One more digital filter, THPF, is available in
the transmit path. THPF implements the high-pass
attenuation requirements for signals below 65 Hz. An
equalizer block then equalizes the transmit signal path
24) represent 4-tap filters that can be used to equalize
the transmit and receive paths, respectively. The
transmit path equalizer is controlled by the TXEQCO0-
TXEQCO3 RAM locations, and the receive path
equalizer is controlled by the RXEQCO0-RXEQCO3
RAM locations. The Si322x Coefficient Generator
software uses these filters in calculating the ac
impedance coefficients for optimal ac performance.
Refer to “AN63: Si322x Coefficient Generator User’s
Guide” for detailed information regarding the calculation
of ac impedance coefficients.
to compensate for series protection resistance, R
,
PROT
outside of the ac-sensing inputs. The linear PCM data
stream output from the equalizer block is amplified by
the transmit-path programmable gain amplifier, TPGA,
which can be programmed from –∞ to 6 dB. The DTMF
decoder receives the linear PCM data stream and
performs the digit extraction if enabled by the user. The
final step in the transmit path signal processing is the
A-law or µ-law compression, which can reduce the data
stream word width to 8 bits. Depending on the PCM
mode select register selection, every 8-bit compressed
serial data word occupies one time slot on the PCM
highway, or every 16-bit uncompressed serial data
word occupies two time slots on the PCM highway.
TPGA or RPGA
PCM
In
PCM
Out
X
3.25.2. Receive Path
M
In the receive path, the optionally-compressed 8-bit
data is first expanded to 16-bit words. The PCMF
register bit can bypass the expansion process so that
two 8-bit words are assembled into one 16-bit word.
RPGA is the receive path programmable gain amplifier,
which can be programmed from –∞ dB to 6 dB. An
8 kHz, 16-bit signal is then provided to a D/A converter.
An analog mute function is provided directly after the
D/A converter. When not muted, the resulting analog
signal is applied at the input of the transconductance
amplifier, Gm, which drives the off-chip current buffer,
where M = {0, 1/16384, 2/16384,...32767/16384}
Figure 39. TPGA and RPGA structure
3.25.5. Audio Characteristics
The dominant source of distortion and noise in both the
transmit and receive paths is the quantization noise
introduced by the µ-law or the A-law compression
process. Figure 5 on page 20 specifies the minimum
Signal-to-Noise and Distortion Ratio for either path for a
sine wave input of 200 Hz to 3400 Hz.
I
.
BUF
Both the µ-law and the A-law speech encoding allow the
audio codec to transfer and process audio signals larger
than 0 dBm0 without clipping. The maximum PCM code
is generated for a µ-law encoded sine wave of
3.17 dBm0 or an A-law encoded sine wave of
3.14 dBm0. The device overload clipping limits are
driven by the PCM encoding process. Figure 6 on page
21 shows the acceptable limits for the analog-to-analog
fundamental power transfer-function, which bounds the
behavior of the device.
3.25.3. TPGA/RPGA Gain/Attenuation Blocks
The TPGA and RPGA blocks are essentially linear
multipliers with the structure illustrated in Figure 39.
Both blocks can be independently programmed from –∞
to +6 dB (0 to 2 linear scale). The TXGAIN and RXGAIN
RAM locations are used to program each block. A
setting of 0000h mutes all audio signals; a setting of
4000h passes the audio signal with no gain or
attenuation (0 dB), and a setting of 7FFFh provides the
maximum 6 dB of gain to the incoming audio signal. The
device signal scaling assumes that dBm is always
referenced to 600 Ω. To compensate for this, the correct
RXGAIN and TXGAIN settings are given in the
coefficient generator software. The DTXMUTE and
DRXMUTE bits in the DIGCON register are also
available to allow muting of the transmit and receive
paths without requiring modifications to the TXGAIN or
RXGAIN settings.
The transmit path gain distortion versus frequency is
shown in Figure 7 on page 21. The same figure also
presents the minimum required attenuation for out-of-
band analog signals applied on the line. The presence
of a high-pass filter transfer function ensures at least
30 dB of attenuation for signals below 65 Hz. The low-
pass filter transfer function attenuates signals above
3.4 kHz. It is implemented as part of the A-to-D
converter.
70
Rev. 1.2
SILICON [ SILICON ]
