Rockwell SoniCrafter BT8960 - Manual (Page 33)

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2.0 Functional Description
2.2 Receive Section
Bt8960
Single-Chip 2B1Q Transceiver
N8960DSB
2.2.4 Echo Canceler
The EC removes images of the transmitted symbols from the received signal and
consists of two blocks: a linear and nonlinear echo canceler. The organization of
the blocks is displayed in Figure 2-3.
2.2.4.1 Linear Echo
Canceler (LEC)
The Linear Echo Canceler (LEC) is a conventional LMS Finite Impulse Response
(FIR) filter, which removes linear images of the transmitted symbols from the
received signal. It consists of a 60-tap FIR filter with 32-bit linear adapted coeffi-
cients.
When enabled, the last data tap of the echo canceler is treated specially. This
serves to cancel any DC offset that may be present.
A freeze coefficient mode may be specified via the microcomputer interface.
This mode disables the coefficient updates only. A special mode exists to zero all
of the coefficients; it is also enabled through the microcomputer interface.
An additional mode exists to zero the output of the FIR with no effect on the
coefficients. It is also enabled through the microcomputer interface. Individual
EC coefficients can be read and written through the microcomputer interface.
Adaptation should be frozen prior to reading or writing coefficients.
2.2.4.2 Nonlinear Echo
Canceler (NEC)
The Nonlinear Echo Canceler (NEC) reduces the residual echo power in the echo
canceler output caused by nonlinear effects in the transmitter DAC, receiver
ADC, analog hybrid circuitry, or line cables.
The delay of the transmit-symbol input to the NEC can be specified via the
microcomputer interface: Nonlinear Echo Canceler Mode Register
[nonlinear_ec_modes; 0x09]. This allows the NEC to operate on the peak of the
echo regardless of differing delays in the echo path.
A freeze coefficient mode may be specified via the microcomputer interface.
This mode disables the coefficient updates only. A special mode exists to zero all
of the coefficients; it is also enabled through the microcomputer interface.
An additional mode exists to zero the output of the look-up table with no
effect on the coefficients. It is also enabled through the microcomputer interface.
The 64, 14-bit, individual NEC coefficients can be read and written through the
microcomputer interface. Adaptation should be frozen prior to reading or writing
coefficients.
2.2.5 Equalizer
Four LMS filters are used in the equalizer to process the echo canceler output so
that received symbols can be reliably recovered. The filters are a digital automatic
gain controller, a feed forward equalizer, an error predictor, and a decision feed-
back equalizer. Their interconnections are shown in Figure 2-3.
2.2.5.1 Digital
Automatic Gain Control
The DAGC scales the echo-free signal to the optimum magnitude for subsequent
processing. Its structure is that of an LMS filter, but it is a degenerate case since
there is only one tap.
A freeze coefficient mode may be specified via the microcomputer interface.
This mode disables the coefficient update only.
The DAGC gain coefficient can be read or written through the microcomputer
interface. Adaptation should be frozen prior to reading or writing the coefficient.