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Application Report
SPRA960A − September 2004
Clarity Clear Voice Capture (CVC) Hands-Free Kit
Suite of Application Reports
Texas Instruments
Clarity Technologies
HFK DP Software Applications
ABSTRACT
Clarity Clear Voice Capture Hands-Free Kit (CVC-HFK) is an eXpressDSP-compliant,
acoustic echo and noise suppression algorithm suite. The user is allowed to fully customize
the performance of the system, by adjusting the parameters which control the adaptation of
the algorithms. This document describes the functionality of each parameter and the
system-level design requirements that must be met to integrate the CVC-HFK and achieve
optimal performance levels.
Contents
1
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2
System Signal Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
User Adjustable Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
List of Figures
Figure 1. Generic Hands-Free DSP Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Figure 2. Processing Flow Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
List of Tables
Table 1. CVC-HFK Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1
Overview
The CVC-HFK is available as a TMS320C54x eXpressDSP-compliant library that can be
integrated into hands free applications to achieve near Type I acoustic echo cancellation
performance.
Figure 1 shows a typical DSP circuit configuration that would be used to implement the
hands-free kit. The CVC-HFK requires two audio inputs and provides one audio output, and is
parameterized, which allows the DSP sub-circuit to be configured by an external
microprocessor.
Trademarks are the property of their respective owners.
1
SPRA960A
Microphone
amplifier
CODEC
DSP
Speaker
amplifier
CODEC
Microprocessor
interface
Digital Bluetooth
interface
Figure 1. Generic Hands-Free DSP Circuit
2
Clarity Clear Voice Capture (CVC) Hands-Free Kit Suite of Application Reports
Cell phone
audio interface
SPRA960A
CVC-HFK components Send Out
OMS
voice extraction
Buffer Synchronization
A/D
AEC
Decimation
Microphone
single element
D/A
Speaker
discrete or car
audio system
Receive In
D/A
Buffer Synchronization
Send In
A/D
Control
Bluetooth
module
Figure 2. Processing Flow Diagram
Figure 2 shows the CVC-HFK system audio processing flow with the CVC-HFK suite in a typical
configuration. The hands-free kit requires a framework which includes a codec driver and
parameter control to interface to the codec and external processing functions. The Send In,
Send Out, and Receive In points shown in Figure 2 for the audio path, correspond to the
pointers described in Table 1. The code is shown here:
void HFK_CLARITY_process (IHFK_Handle handle, XDAS_Int16 *input, XDAS_Int16
*reference, XDAS_Int16 *output);
Table 1. CVC-HFK Functions
Parameter
Type
Description
Handle
IHFK_Handle
Handle for instance of algorithm.
input
XDAS_Int16 *
Pointer to 32 sample input buffer (Send In− from MIC)
reference
XDAS_Int16 *
Pointer to 32 sample input buffer (Receive In − from PHONE)
output
XDAS_Int16 *
Pointer to 32 sample output buffer (Send Out − to PHONE)
To achieve acceptable performance, careful attention must be paid to the system gains,
frequency response, and distortion as measured at the Send IN, Send Out, Receive In, and
Receive Out points.
Clarity Clear Voice Capture (CVC) Hands-Free Kit Suite of Application Reports
3
SPRA960A
2
System Signal Requirements
To achieve optimal audio performance the audio gain levels, frequency response and system
distortion must be within tolerable levels. Non-linear signal processing should not be placed in
the audio path after the Receive Out point, or prior to the Send In point.
•
Frequency Response
The frequency response measured at Send In and Receive In should be 0 dB ± 0.5 dB
(referenced to a 1 kHz 80% full scale sine wave at the codec), from 200 Hz to 3.6 kHz (flat
frequency response), and have a minimum of a second-order analog roll-off from 0−200 Hz
before the A/D converter. For optimal voice quality, the microphone signal should be
sampled at 16 kHz and decimated by 2 kHz − 8 kHz to obtain a flat frequency response up
to 3.6 kHz, if the anti-aliasing filters of the chosen A/D converter do not provide a satisfactory
response. The anti-aliasing filter should provide a minimum of 40 dB attenuation at the
sub-sampled Nyquist rate.
•
THD+N
The harmonic distortion measured between Receive In and Send In must be minimized to
achieve optimal performance. The maximum achievable echo return loss is primarily limited
by the distortion created in the system between the Receive In point and the Send In point.
The maximum achievable Echo Return Loss Enhancement is directly proportional to the
distortion introduced in the Receive In to Send In path. Other non linear components such as
fast time constant dynamic gain and equalization, and audio effects such as concert halls,
must not be placed in the audio path after the Receive In point.
•
Electrical Noise Floor
The electrical noise floor of the system should be less than −65 dB referenced to an 80% full
scale sine wave at 1 kHz.
•
Gain Levels For Speaker Reference (Receive In)
Nominal speech should measure at 50% of a full scale 16-bit two’s complement number at
the Receive In point.
•
Gain Levels For Microphone Input (Send In)
Nominal speech should measure at 50% of a full scale 16-bit two’s complement number at
the Send In point.
•
Gain Levels For Microphone Output (Send Out)
Send Out gain levels of the CVC-HFK are set by the configurable parameters according to
the system needs.
3
User Adjustable Parameters
•
Aggressiveness
The CVC-HFK provides a means to increase/decrease Send Out noise cancellation
aggressiveness according to the desired noise suppression and voice quality preferences. The
normal usable range is from 0 to 32767. The more aggressive the value, the greater the noise
suppression. However, voice quality will degrade as the noise suppression level is increased.
4
Clarity Clear Voice Capture (CVC) Hands-Free Kit Suite of Application Reports
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