Audio Line
Hybrid Echo Canceller
for TMS320C25/'C5x
by DSP Software Engineering, Inc.

Software Overview

The Line Echo Canceller (LEC) software cancels electrical echo due to 2-to-4 wire conversion 
hybrids in telephone lines for audio applications. The LEC may be used to cancel echo caused by 
other phenomenon as well. The cancellation window location and length are user configurable and 
determine the number of filter coefficients and processor loading. This algorithm is designed for 
integration with other DSP software, such as the DTMF Encoder/Decoder, to enhance performance.
This implementation works on a sample-by-sample basis and incorporates 32 locations (4 ms) of 
history, double-talk detection, and an adaptive-tapped delay line. The software can handle delays 
of greater than 4 milliseconds with additional memory and processing time, but no additional ROM is 
required.
The Line Echo Canceller was developed for answering machine devices, but can be used for many other 
applications. Any DSP application that has some board echo, or a local hybrid, can be enhanced with 
the addition of this Line Echo Canceller.

Features and Benefits

•	Convergence time < 500 ms (speaker dependent)
•	Adapts on speech, eliminating white-noise blasts during adaptation
•	Greater than 27-dB attenuation of echoes
•	Sample rate independent
•	Double-talker detection (6 dB attenuation of echo)
•	Independent C-callable functions
•	Coarse and fine mode adaptations
•	SPOX compatible

Processor and System Requirements

•	Devices supported: TMS320C5x
•	Algorithm category: Telecommunications
•	Requirements:
All memory requirements are in units of 16-bit words. The MIPs ratings presented require the listed 
amounts of dual-access RAM, zero-wait-state program and data memory, sample by sample processing, 
and a 4-ms cancellation window length.  All measurements were made using an executable 
demonstration built for a third-party PC plug-in board.
Function
MIPS
DARAM
Data
Program
Adapt
2.7* **
76*
0
0.2 k
Cancel
1.4*
76*
0
0.2 k
*  Adaptation occurs as needed, as part of the normal cancellation process.
** The DARAM and MIPs loading vary with configuration.

User Functions

The LEC software consists of two independent, C-callable functions that perform echo cancellation 
and initialization of the echo canceller. The echo canceller interfaces with linear-PCM samples as 
input and output.
Echo Canceller
echo_cancel (...)
Produces an echo-cancelled version of the input and updates the filter
init_echo (...)
Sets the initial state of the echo canceller

Algorithm Verification

•	Call DSPSE for verification details

Availability

•	Source and object code currently available for licensing.

Company Background and Contact Information

DSP Software Engineering, Inc (DSPSE), founded in 1989, is a leading provider of highly-complex 
digital-signal-processing software used in telecommunications and multimedia applications such as 
computer telephony, digital wireless, PSTN, and satellite communications, and videoconferencing. 
DSPSE sells its products to OEMs and directly to manufacturers of telecommunications products. 
DSPSE provides the complete solution to your DSP needs with DSP solution software packages and 
individual DSP software component algorithms and expert service.

Contact:	DSP Software Engineering, Inc
		175 Middlesex Turnpike
		Bedford, MA 01730  USA
		(617) 275-3733
		Fax: (617) 275-4323
		e-mail:  info@dspse.com
		www:  http://www.dspnet.com