Cirrus Logic CS6422-CS, CDB6422 Datasheet

CS6422
Enhanced Full-Duplex Speakerphone IC

Features

l Single-chip f ull-duplex hands-free operation l Optional Tx Noise Guard l Programmable att enuation during do uble-talk l Optional 34 dB microphone preamplifier l Dual channel AGC’ed volume controls wi th
mute
l Dual integrated 80 dB IDR codecs l Speech-trained Network and Acoustic Echo
Cancellers
l Rx and Tx supplementary echo suppression l Configurable half-duplex training mode l Powerdown mode l Microcontroller Interface
DVDDNC1 NC2 NC3 NC4
16 1
Rx
Suppression
TVol
(Mute, -12 to +30 dB)
17
NI
(0, 6, 9.5, 12 dB)
Network
Sidetone
(none, -24,
-18, -12 dB)
4
NO
9101112
RGain RVol
ADC
NSdt
DAC
+
Σ
+
­(Mute, -12 to +30 dB)
Network
Echo
Canceller
Pre-emphasis
Filter
Suppression
Microcontroller Interface
Tx

General Description

Most modern speakerphones use half-duplex operation, which alternates transmission between the far-end talker and the speakerphone use r. This is done to ens ure sta­bility because the acoustic coupling between the speaker and microphone is much higher in speaker­phones than in handsets where the coupling is mechanically suppressed.
The CS6422 enables full-duplex conversation using echo cancellation and suppression in a single-chip solu­tion. The CS6422 can easily replace existing half-duplex speakerphone ICs with a huge increas e in co nv er sa tio n quality.
The CS6422 consists of tel ephone & audio interfaces, two codecs and an echo-cancelling DSP.
ORDERING INFORMATION
CS6422-CS 20-pin SOIC CDB6422 Evaluation Board
AVDD
3
Pre-emphasis
Filter
Acoustic
Echo
Canceller
-
+
+
Σ
DAC
ASdt
ADC
Acoustic
Sidetone
(none, -24,
-18, -12 dB)
TGain
(0, 6, 9.5, 12 dB )
Voltage
Reference
Clock
Generation
Mic
34 dB
1 k
14 13
20
18
AO
CLKI CLKO
API
APO
8
DATA STROBE
76
DRDY
Preliminary Product Information
P.O. Box 17847, Austin, Texas 78760 (512) 445 7222 FAX: (512) 445 7581 http://www.crystal.com
5
RST DGND AGND MB
This document contains information for a new product. Cirrus Logic reserves the right to modify this product without notice.
15 2 19
Copyright  Cirrus Logic, Inc. 1998
(All Rights Reserved)
AUG ‘98
DS295PP1
1

TABLE OF CONTENTS

ABSOLUTE MAXIMUM RATINGS .....................................................................................4
RECOMMENDED OPERATING CONDITIONS .................................................................4
POWER CONSUMPTION ...................................................................................................4
ANALOG CHARACTERISTICS .........................................................................................4
ANALOG TRANSMISSION CHARACTERISTICS .............................................................5
MICROPHONE AMPLIFIER ...............................................................................................5
DIGITAL CHARACTERISTICS ...........................................................................................5
SWITCHING CHARACTERISTICS ....................................................................................6
Overview ............................................................................................................................8
Functional Description .....................................................................................................8
Analog Interface .......................................................................................................8
Acoustic Interface ..............................................................................................9
Network Interface ............................................................................................10
Microcontroller Interface .........................................................................................10
Description ......................................................................................................10
Register Definitions .......................................................... ...... ..... ...... ..............11
Register 0 ............................................................ ...... ...... .........................12
Mic - Microphone Preamplifier Enable ..............................................12
HDD - Half-Duplex Disable ...............................................................12
GB - Graded Beta .............................................................................13
RVol - Receive Volume Control ........................................................13
TSD - Transmit Suppression Disable ................................................13
ACC - Acoustic Coefficient Control ...................................................13
TSMde - Transmit Suppression Mode ..............................................13
Register 1 ............................................................ ...... ...... .........................15
THDet - Transmit Half-Duplex Detection Threshold .........................15
Taps - AEC/NEC Tap Allocation .......................................................15
TVol - Transmit Volume Control ......................... ...... ..... ...... ..... ...... ...16
RSD - Receive Suppression Disable ................................................16
NCC - Network Coefficient Control ...................................................16
AuNECD - Auto re-engage NEC Disable ..........................................16
Register 2 ............................................................ ...... ...... .........................17
RHDet - Receive Half-Duplex Detection Threshold ..........................17
RSThd - Receive Suppression Threshold .........................................17
NseRmp - Noise estimator Ramp rate ..............................................18
HDly - Half-Duplex Holdover Delay ...................................................18
HHold - Hold in Half-Duplex on Howl ............................ ...... ..... ...... ...18
TDSRmp - Tx Double-talk Suppression Ramp rate ..........................18
RDSRmp - Rx Double-talk Suppression Ramp rate ......................... 18
IdlTx - half-duplex Idle return-to-Transmit .........................................18
Register 3 ............................................................ ...... ...... .........................19
TSAtt - Transmit Suppression Attenuation ........................................19
PCSen- Path Change Sensitivity ......................................................19
TDbtS - Tx Double-talk Suppression attenuation ..............................20
RDbtS - Rx Double-talk Suppression attenuation .............................20
TSThd - Transmit Suppression Threshold ........................................20
TSBias - Transmit Suppression Bias ................................................20
Register 4 ............................................................ ...... ...... .........................21
AErle - AEC Erle threshold ...............................................................21
AFNse - AEC Full-duplex Noise threshold ........................................21
NErle - NEC Erle threshold ...............................................................21
NFNse - NEC Full-duplex Noise threshold .......................................22
RGain - Receive Analog Gain ...........................................................22
TGain - Transmit Analog Gain ..........................................................22
Register 5 ............................................................ ...... ...... .........................23
HwlD - Howl detector Disable ...........................................................23
TD - Tone detector Disable ...............................................................23
CS6422
2 DS295PP1
APCD - Acoustic Path Change detector Disable ..............................24
NPCD - Network Path Change detector Disable ..............................24
APFD/NPFD - Acoustic Pre-emphasis Filter Disable/Network
Pre-emphasis Filter Disable .............................................................24
AECD - Acoustic Echo Canceller Disable ........................................24
NECD - Network Echo Canceller Disable .........................................24
ASdt - Acoustic Sidetone level .........................................................24
NSdt - Network Sidetone level ..........................................................24
Reset ............................................................................................................... 25
Cold Reset ...............................................................................................25
Warm Reset ............................................................................................. 25
Reset Timer .............................................................................................25
Clocking .................................................................................................................25
Power Supply .........................................................................................................25
Power Down Mode ..........................................................................................26
Noise and Grounding ......................................................................................26
Design Considerations ............................. ..... .................................................................28
Algorithmic Considerations ....................................................................................28
Full-Duplex Mode ............................................................................................28
Theory of Operation ................................................................................28
Adaptive Filter ..........................................................................................29
Pre-Emphasis ................................................................................... 29
Graded Beta ........................................... ..... .....................................30
Update Control .........................................................................................30
Speech Detection ....................................................................................30
Half-Duplex Mode ...........................................................................................31
Idle Return to Transmit ............................................................................31
AGC ................................................................................................................31
Suppression .................................................................................................... 32
Transmit Suppression ..............................................................................32
Receive Suppression ...............................................................................33
Double-talk Attenuation ....................................................... ....................33
Noise Guard .............................................................................................33
Circuit Design .........................................................................................................34
Interface Considerations .................................................................................34
Analog Interface .......................................................................................34
Microcontroller Interface ..........................................................................34
Grounding Consideration s .............................. ................................................34
Layout Considerations ................................................................ ...... ..............35
System Design .................................................. ..... ...... ..... ...... ...............................35
Gain Structure .................................................................................................35
Testing Issues .................................................................................................36
ERLE .......................................................................................................36
Convergence Time ..................................................................................36
Half-Duplex Switching ..............................................................................37
Pin Descriptions ..............................................................................................................38
Analog Interface ..............................................................................................38
Microcontroller Interface .................................................................................39
Clock ............................................................................................................... 39
Power Supply ..................................................................................................39
Miscellaneous .................................................................................................40
Glossary ...........................................................................................................................41
Package Dimensions ......................................................................................................44
CS6422
DS295PP1 3
CS6422

ABSOLUTE MAXIMUM RATINGS

Parameter Symbol Min Max Units
DC Supply (AVDD, DVDD) -0.3 6.0 V Input Current (Except supply pins) I
Input Voltage Analog
Digital
Ambient Operating Temperature T Storage Temperature T
in
V
ina
V
ind
A
stg
WARNING: Operation beyond these limits may result in permanent damage to the device.
Normal operation is not guaranteed at these extremes.

RECOMMENDED OPERATING CONDITIONS

Parameter Symbol Min Typ Max Units
DC Supply (AVDD, DVDD) 4.5 5.0 5.5 V Ambient Operating Temperature T
AOp
-10 +10 mA
-0.3
-0.3
AVDD+0.3
DVDD+0.3
V
-40 85 °C
-65 150 °C
0257C

POWER CONSUMPTION

(TA = 25°C, DVDD = AVDD = 5 V, f
= 20.480 MHz) (Note 1)
XTAL
Parameter Symbol Min Typ Max Units
Power Supply Current, Analog (RST Power Supply Current, Analog (RST Power Supply Current, Digital (RST Power Supply Current, Digital (RST
=0)
=1) =0) =1)
P
P
P
P
DA0
DA
DD0
DD
10 20 mA
50 60 mA
1mA
1mA
Notes: 1. AO and NO outputs are not loaded.

ANALOG CHARACTERISTICS (T

= 25°C, DVDD = AVDD = 5 V, f
A
= 20.480 MHz)
XTAL
Parameter Symbol Min Typ Max Units
Input Offset Voltage (APO, NI) 2.12 V Output Offset Voltage (AO, NO) 2.12 V Transmit Group Delay (Note 2) 6 ms Receive Group Delay (Note 2) 6 ms Input Impedance (APO, NI) Z Load Impedance (AO, NO) Z
in
load
10 k
1.5 M
Power Supply Rejection (1 kHz) 40 dB
Notes: 2. These parameters are guaranteed by design or by characterization.
4 DS295PP1
CS6422

ANALOG TRANSMISSION CHARACTERISTICS (T

= 25°C, DVDD = AVDD = 5 V, f
A
XTAL
20.480 MHz, RVol=TVol=RGain=TGain= 0 dB, HDD=TSD=RSD=1, analog inputs and outputs loaded with resistors and capacitors as shown in the typical connection diagram, Figure 1)
Parameter Symbol Min Typ Max Units
Idle Channel Noise C-Message weighted (0-4 kHz) (Inputs grounded C-Message weighted (0-4 kHz) through a capacitor) Psophometrically weighted (0-4 kHz)
Signal-to-Noise Ratio C-Message weighted (0-4 kHz) (1.0 V
1 kHz sine wave input)
rms,
Total Harmonic Distortion C-Message weighted (0-4 kHz) (1.0 V
1 kHz sine wave input)
rms,
Programmable Analog Gain RGain/TGain = 00
RGain/TGain = 01 RGain/TGain = 10
RGain/TGain = 11
SNR 75 80 dB
THD 0.030 0.1 %
-80 11
-78
0 6
9.5 12
-75 dBV dBrnC0
dBm0p
dB
Volume Control Stepsize (TVol/RVol) 3 dB ADC Full-scale Voltage Input 0.9 1.0 V DAC Full-scale Voltage Output 1.0 1.1 V
rms rms
ADC Noise Floor C-Message weighted (0-4 kHz) -83 dBV DAC Noise Floor, DAC muted C-Message weighted (0-4 kHz) -83 dBV

MICROPHONE AMPLIFIER (T

= 25°C, DVDD = AVDD = 5 V, f
A
= 20.480 MHz)
XTAL
Parameter Symbol Min Typ Max Units
Gain (Z
= 50Ω)A
source
mic
34 dB Signal-to-Noise Ratio C-Message weighted (0-4 kHz) SNRm 70 dB Input Impedance Z Input Offset Voltage V
inm
offm
8k
2.12 V
=

DIGITAL CHARACTERISTICS (T

= 25°C, DVDD = AVDD = 5 V,f
A
= 20.480 MHz)
XTAL
Parameter Symbol Min Typ Max Units
High-Level Input Voltage V Low-Level Input Voltage V Input Leakage Current I Input Capacitance C
IH
IL
leak
IN
DVDD-1.0 V
1.0 V 10
µ
5pF
A
DS295PP1 5

SWITCHING CHARACTERISTICS

Parameter Symbol Min Typ Max Units
Digital input rise time t
low time
RST CLKI frequency f CLKI duty cycle t CLKI high or low time t
Min DRDY STROBE high or low time t
DRDY DATA valid to STROBE rising setup time t STROBE rising to DATA valid hold time t
STROBE rising to DRDY Min RST Max RST
falling to DRDY falling (CLKI = 20.480 MHz)
HLSTROBE
falling to STROBE rising setup time
rising hold time
rising to 4th extra STROBE pulse (cold reset)
rising to 4th extra STROBE pulse(warm reset)
rise
t
RSTL
CLKI
LCLKI
HLCLKI
t
DRDY
t
sDRDY
sDATA hDATA
t
hDRDY
t
cRST
t
wRST
CS6422
1.0 µs
1.0 µs
20.480 MHz
40 50 60 %
19.5 ns 125 µs
55 ns 30 ns
30 ns 30 ns 30 ns
110 ms 100 ms
RST
STROBE
DRDY
DRDY
STROBE
DATA
t
RSTL
t
sDATA
1
f
CLKI
Bit15
Bit14 Bit2 Bit1
t
sDRDY
t
hDATA
Bit15
Bit14
HLCKI
t
HLCKI
t
CLKI Timing
t
cRST
t
wRST
Reset Timing
t
DRDY
Bit0
Bit0DATA
t
hDRDY
four extra strobe pulses
1234
t
HLSTROBE
Bit15
Bit14

Microcontroller Interface Timing

6 DS295PP1
Network Line Out
Network Line In
3300 pF
0.47 µF
10 µF
3300 pF
+
6.04 k
0.1 µF
12.1 k
CS6422
ferrite bead
+5V Analo g
0.1 µF
16
DVDD
15
DGND
4
NO
17
NI
CS6422
AVDD
AGND
MB
APO
API
1
2
19
18
20
0.1 µF
0.022 µF
0.47 µF
+
10 µF
10
+
µ
F
Mic Bias
Network Line Out
Network
Line In
From
Microcontroller
8 7
6 5
DATA
STROBE DRDY
RST
NC1NC2NC3NC4
20.480 MHz
22pF
12.1 k
3
AO
3300pF
CLKOCLKI
13149101112
22pF

Figure 1. Typical Connection Diagram (Microphone Preamplifier Enabled)

ferrite be a d
+5V Analog
API
APO
MB
1
2
20
18
19
0.1 µF
0.1 µF
+
0.47 µF
6.04 k
3300 pF
10 µF
0.47 µF
10 µF
+
External Mic
3300 pF
0.47 µF
10 µF
3300 pF
+
6.04 k
0.1 µF
12.1 k
16
DVDD
15
DGND
CS6422
4
NO
17
NI
AVDD
AGND
Speaker
Driver
Preamp
8
DATA
7
From
Microprocessor
6 5
STROBE
DRDY
RST
AO
NC1NC2NC3NC4
22pF
20.480 MHz
CLKOCLKI
13149101112
22pF
12.1 k 3
3300pF
Speaker
Driver

Figure 2. Typical Connection Diagram (Microphone Preamplifier Disabled)

DS295PP1 7
CS6422

OVERVIEW

The CS6422 is a full-duplex speakerphone chip for use in hands-free communications with telephony quality audio. Common applications include speakerphones, inexpensive video-conferencing, and hands-free cellular phone car kits. The CS6422 requires very few external components and allows system control through a microcontroller interface.
Hands-free communication through a microphone and speaker typically results in acoustic feedback or howling because the loop gain of the system ex­ceeds unity by the time audio amplitudes are ad­justed to a reasonable level. The solution to the howling problem has typically been half-duplex, where either the transmit or the receive channel is active, never both at the same time. This prevents instability, but diminishes the overall communica­tion quality by clipping words and forcing each talker to speak in turn.
Full-duplex conversation, where both transmit and receive channels are active simultaneously, is the conversation quality we enjoy when using hand­sets. Full-duplex for hands-free communications is achieved in the CS6422 using a digital signal pro-
cessing technique called “Echo Cancellation.” The end result is a more natural conversation than half­duplex, with no awkward breaks and pauses, allow­ing both parties to speak simultaneously.
Echo Cancellation reduces overall loop gain and the acoustic coupling between speaker and micro­phone. This coupling reduction prevents the annoy­ing effect of hearing one’s own delayed speech, which is worsened when there is delay in the sys­tem, such as vocoder delay in digital cellular phones.
The CS6422 is a complete system implementation of a Digital Signal Processor with RAM and pro­gram ROM, running Echo Cancellation algorithms developed at Crystal Semiconductor using custom­er input, integrated with two delta-sigma codecs. The CS6422 is intended to provide a full-duplex
speakerphone solution with a minimum of design effort while displacing existing half-duplex speak­erphone chips.

FUNCTIONAL DESCRIPTION

The CS6422 is divided into four external interface blocks. The analog interfaces connect the device to the transmit and receive paths. Control functions are accessible through the microcontroller inter­face. Two pins accommodate either a crysta l or an externally applied digital clock signal. Analog and digital power and ground are provided through four pins.

Analog Interface

In a speakerphone application, one input of the CS6422 connects to the signal from the micro­phone, called the near-end or transmit input, and one output connects to the speaker. The output that leads to the speaker is called the near-end or re­ceive output. Together, the input and output that connect to the microphone and speaker form the Acoustic Interface.
The signal received at the near-e nd input is pa ssed to the far-end or transmit output after acoustic echo cancellation. This signal is sent to the telephone line. The signal from the telephone line is received at the far-end input, also called the receive input, and this signal is passed to the receive output after network echo cancellation. The far-end input and output form the Network Interface.
The analog interfaces are physically i mplemented using delta sigma converters running at an output word rate of 8 kHz, resulting in a passband from DC to 4 kHz. Because the inputs are analog to dig­ital converters (ADCs), anti-aliasing and full-scale input voltage must be kept in mind. The ADCs ex­pect a single-pole RC filter with a corner at 8 kHz, and they are post-compensated internally to pre­vent any resulting passband droop. The ADCs also expect a maximum of 0.9 V puts (which are biased around 2.12 VDC). A signal
(2.5 Vpp) at their in-
rms
8 DS295PP1
Receive Path
CS6422
NI
FAR-END
(0,6,9.5,12 dB)
NO
17
4
RGain
ADC
D S P
DAC
DAC
ADC
CS6422
Transmit Path
Figure 3. Analog Interface
of higher amplitude will clip the ADC input and will result in poor echo canceller performance. See the Design Considerati ons section for more details.
The outputs are delta-sigma digital to analog con­verters (DACs) and have similar requirements to the ADCs. The DACs are pre-compensated to ex­pect a single-pole RC filter with a corner frequency at 4 kHz. The full scale voltage output from a DAC is 1.1 V
(3.1 Vpp) maximum, 1 V
rms
typical, bi-
rms
ased around 2.12 VDC.

Acoustic Interface

The pins API (pin 20), APO (pin 18), AO (pin 3), and MB (pin 19) form the Acoustic Interface. A block diagram of the Acoustic Interface is shown in Figure 3.
API and APO are, respectively, the input and out­put of the built-in microphone pre-amplifier. The pre-amplifier is an inverting amplifier with a fixed gain of 34 dB biased around an input offset voltage of 2.12 V. APO is the output of the pre-amplifier
AO
3
NEAR-END
API
TGain
(0,6,9.5,12 dB)
Mic
1k
Voltage
Reference
34 dB
20
1918
MBAPO
after a 1 k (typical) resistor. The circuitry con­nected to the amplifier input must present low source impedance (<100Ω) to the API pin or the gain will be reduced. When using the internal mic preamp, a 0.022 µF capacitor should be placed be­tween APO and ground to provide the anti-aliasing filter required by the ADC, as shown in Figure 1. The pre-amplifier may be bypassed by clearing the
‘Mic’ bit (Register 0, bit 15) using th e Microcontrol­ler Interface (see Microcontroller Interface section). If the intern al mic pream p is no t used, a 0.022 µF ca­pacitor should be tied between API and ground, and APO should be driven directly. In this case, the sig­nal into APO must be low-pass filtered by a single­pole RC filter with a corner frequency at 8 kHz (see Figure 2).
Following the pre-amplifier is a programmable an­alog gain stage, called TGain, which is controlled through the Microcontroller Interface. This gain stage allows gains of 0 dB, 6 dB, 9.5 dB, and 12 dB
DS295PP1 9
CS6422
to be added prior to the ADC input. The default gain stage setting is 0 dB.
The signal at APO should not exceed 2.5 Vpp at the 0 dB gain stage setting. If a different gain setting is used, then the full-scale signal at APO must al so change. Table 1 shows full-scale voltages as mea­sured at APO for the given programmable gain:
Gain Setting Full-scale Voltage
0 dB 6 dB
9.5 dB 12 dB
Table 1. Full scale voltages for each gain stage.
2.5 V
1.25 V
0.84 V
0.63 V
pp
pp pp pp
MB serves to provide decoupling for the internal voltage reference, and must have a 0.1 µF and a 10
µF capacitor to ground for bypass. Noise on MB will strongly influence the overall analog perfor­mance of the CS6422.
The acoustic output, AO, should connect to a sin­gle-pole low-pass RC network with a corner fre­quency of 4 kHz, which will filter out-of-band components. The full-scale voltage swing at AO is
3.1 Vpp maximum, 1 V
typical. AO is capable of
rms
driving a load of 10 k or more.

Network Interface

The pins NI (pin 17) and NO (pin 4) form the Net­work Interface. The details of the Network Inter­face are shown in Figure 3.
NI is the input from the telephone network into the CS6422. The signal into NI must be low pass fil­tered by a single-pole RC filter with a corner fre­quency of 8 kHz.
RGain, a programmable analog gain stage accessi­ble through the Microcontroller Interface, ampli­fies signals received at NI. This gain st age allows a gain of 0 dB, 6 dB, 9.5 dB, or 12 dB to be added pri-
or to the ADC input. The default gain stage setting for the network side is 0 dB.
The signal at NI should not exceed 2.5 Vpp at the 0 dB gain stage setting. If another gain setting is se­lected, then the full-scale signal at NI will change. Table 1 shows full-scale voltages as measured at NI for the given programmable gain.
The output to the telephone network side, NO, should connect to a single pole RC network with a corner frequency at 4 kHz, which will filter out-of­band components. The maximum swing NO is ca­pable of producing is 3.1 Vpp maximum, 1 V
rms
typical. NO is capable of driving a load of 10 k or more.

Microcontroller Interface

The registers and control functions of the CS6422 are accessible through the Microcontroller Inter­face, which consists of three pins: DATA (pin 8), STROBE (pin 7), and DRDY (pin 6). These inputs can connect to the outputs of a microcontroller to allow write-only access to the 16-bit Microcontrol­ler Control Register (MCR).

Description

The Microcontroller Interface is implemented by a serial shift register that is clocked by STROBE and gated by DRDY. The microcontroller begins the transaction by setting DRDY low while STROBE is low. The most significant bit (MSB), Bit 15, of the 16-bit data word should be presented to the DATA pin and then STROBE should be brought high to shift the data bit into the CS6422. STROBE should be brought low again so it is ready to shift the next bit into the shift register. The next data bit should then be presented to the DATA pin ready to be latched by the rising edge of STROBE. This pro­cedure repeats for all sixteen bits as shown in Fig­ure 4. After the last bit (Bit 0) has been shifted in, DRDY should be brought high to indicate the con­clusion of the transfer, and four or more extra
10 DS295PP1
CS6422
STROBE pulses must be applied to latch the data into the CS6422.
Since the MCR is a shift register, the STROBE can be run arbitrarily slowly with a duty cycle limited only by the minimum high and low time specified in the Switching Characterstics table. The Micro-
controller Interface is polled at 125 µs intervals, so register writes must be spaced at least 125 µs apart or the register contents may be overwrit­ten.
STROBE

Register Definitions

The six control registers accessible through the MCR are described in detail in the following tables. These registers are addressed by bits b3-0 of the MCR. Bit ‘b0’ must always be ‘0’. Table 2 shows the register map with the default settings. Tables 3 through 8 show the control registers in more detail.
The Register Map at the top of each register de­scription shows the names of all th e bit s, with th eir reset values below the bitfield name. The reset val­ue can also be found in the Word column of the bit-
field summary as indicated by an ‘*’.
four extr a strobe pulses
12
3
4
Bit15
Bit14 Bit13 Bit12
DRDY
# b15 b14 b13 b12 b11 b10 b9 b8 b7 b6 b5 b4 b3-0
Mic1HDD
0
1
RHDet
2
3
4
HwlD0TD0APCD0NPCD0APFD0NPFD0AECD0NECD
5
0
THDet
00
00
TSAtt
00
AErle
00
GB
10
Taps
10
RSThd
00
PCSen
0
AFNse
00
Bit11 Bit10 Bit9 Bit8
Figure 4. Microcontroller Interface
NseRmp
00
TDbtS
000
NErle
00
Table 2. MCR Control Register Mapping
RVol
0100
TVol
1010
Bit7 Bit6
HDly
00
RDbtS
00
NFNse
00
Bit5 Bit4 Bit3
TSD
RSD
HHold0TDSRmp0RDSRmp0IdlTx
0
0
0
TSThd
RGain
00
00
ASdt
00
Bit2
Bit1
ACC
00
NCC
00
Bit0DATA
TSMde
AuNECD
TSBias
00
TGain
00
NSdt
00
0
0
0
0000
0010
0100
0110
1000
1010
DS295PP1 11
CS6422
Register 0
b15 b14 b13 b12 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b1 b0
Mic HDD GB RVol TSD ACC TSMde
1 0 10 0100 0 00 0
A4 0 0
Bits Name Function Word Operation
15 Mic Microphone preamplifier enable
14 HDD Half-Duplex Disable
13-12 GB Graded Beta
11-8 RVol Rx Volume control
7 TSD Tx Suppression Disable
6-5 ACC AEC Coefficient Contro l
4 TSMde Tx Suppression Mode
0
1* 0*
1
00 01
10*
11 0000 0001
---
0100*
---
1010
1011
--­1101 1110
1111
0*
1
00*
01 10
11 0*
1
0000
disable mic preamp
enable mic preamp
enable half-duplex disable half-duplex
0.00 dB/ms
0.75 dB/ms
0.38 dB/ms
0.19 dB/ms +30 dB +27 dB
---
+18 dB
---
+0 dB
-3 dB
---
-9 dB
-12 dB mute
enable Tx suppression
disable Tx suppression
Normal
Clear
Freeze
reserved
enable noise guard
disable noise guard
* Denotes reset value
Table 3. Register 0 Bit Definitions
Mic - Microphone Preamplifier Enable
The microphone preamplifier described in the Acoustic Interface section is enabled by default, but
may be disabled by setting Mic to ‘0’. Refer to the Acoustic Interface section for more details on using the Microphone Preamplifier.
enough loop gain reduction to prevent howling. This half-duplex mode is active at power-up while the adaptive filter begins to train. Half-duplex mode prevents howling and also masks the conver­gence process.
In some cases, such as when measuring conver­gence speed (see Testing Issues), the half-duplex
HDD - Half-Duplex Disable
In normal operation, the CS6422 will be in a half-
mode is undesirable. By default, the half-duplex mode is enabled.
duplex mode if the echo canceller is not providing
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CS6422
GB - Graded Beta
The room-size adjustment scheme called “graded
beta,” provided for the acoustic echo canceller in the CS6422, is controlled by GB. The network echo canceller does not support graded beta.
Graded beta is an architectural enhancement to the CS6422 which takes advantage of the fact that acoustic echoes tend to decay exponentially with time. The CS6422 can increase the beta, or update gain, for the coefficients of the adaptive filter which occur earlier in time and decrease it for thos e that occur later in time, which increases conver­gence speed while maintaining stability. In order to make this improvement, there is an implicit assump­tion that the decay rate of the echo is known. The graded beta control allows the system designer to ad­just this. For very acoustically live rooms, use either no decay (00) or slight decay (11). Cars and acousti­cally dead rooms can benefit from the most rapid de­cay (01).
RVol - Receive Volume Control
Volume in the receive path is set by RVol. The vol­ume control in the receive direction is implemented by a peak-limiting automatic gain control (AGC) and digital attenuation at the near-end output DAC.
The AGC is discussed in detail in the Design Con- siderations section. See the sub-section on AGC for a full explanation of how it functions.
When the reference level is set to +0 dB, the AGC is disabled. Volume control is implemented by dig­ital attenuation in 3 dB steps from this point on down. The maximum gain is +30 dB and the mini­mum is -12 dB. The lowest gain setting (1111) mutes the receive path.
The default setting for RVol is +18 dB.
TSD - Transmit Suppression Disable
The Transmit Supplementary Echo Suppression function is a non-linear echo control mechanism. Transmit Suppression introduces TSAtt (see Regis-
ter 3) of attenuation into the transmit path when it is engaged. When TSMde = ‘1’, the transmit sup­pressor engages when there is spee ch detected in the receive path and no near-end speech is present. When TSMde = ‘0’, the default case, the transmit suppressor engages when there is no near-end speech present. When near-end speech is present, the suppression attenuation is removed. By default, the transmit suppression function is enabled.
ACC - Acoustic Coefficient Control
The coefficients of the AEC adaptive filters in the CS6422 are controlled by ACC. The default posi­tion (00) yields normal operation, which means the coefficients are free to adjust themselves to the echo path in order to cancel echo. When set to the clear position (01), the adaptive f ilter coefficients are all held at zero, so the echo canc eller is effec­tively disabled. Note that unless the half-duplex mode is disabled, this will force the CS6422 into half-duplex mode. The freeze position (10) causes the coefficients to retain their current values and not change.
TSMde - Transmit Suppression Mode
TSMde enables the Noise Guard feature of the CS6422. Noise Guard is a noise squelch feature that operates in the transmit path (from the near­end microphone to the far-end speaker). In tradi­tional hands-free systems where the near-end talker is located in a noisy environment, the near-end sys­tem will remain in transmit mode and send that noise to the far-end listener. This creates a real problem if the listener is using a traditional half-du­plex speakerphone because the far-end phone will stay in receive mode, thus preventing the far-end talker from being heard. Noise Guard eliminates this problem by squelching the transmit channel at the near-end unless near-end speech is detected, permitting the far-end speakerphone to switch nor­mally during the conversation.
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Noise Guard is also useful in cellular hands-free car applications because it prevents car noise from reaching the far-end while the near-end talker is si­lent.
Noise Guard is usually disabled when “half-duplex Idle return-to- Transmi t” is ena bled. See t he Regis­ter 2 description for more information. Noise Guard is enabled by default.
CS6422
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