Audio Line
Hybrid Echo Canceller
for TMS320C3x/'C4x
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.
The Line Echo Canceller operates in training and cancelling modes. Software-generated Gaussian 
noise is used to train the canceller on a given channel. Once the LEC is trained, it is operated in 
the cancel mode and no longer tracks changes in the channel.

Features and Benefits

•	User-selectable processing frame size
•	Object-based interface
•	Sample rate independent
•	Independent C-callable functions
•	Configurable window location, window length, and convergence speed
•	SPOX compatible

Processor and System Requirements

•	Devices supported: TMS320C3x, TMS320C4x
•	Algorithm category: Telecommunications
•	Requirements:
All memory requirements are in units of 32-bit words. The MIPS ratings presented require the listed 
amounts of on-chip, zero-wait-state program and data memory, and a processing frame size of 256 
samples 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
Chip RAM
Data
Program
Noise Generate & Train
7.5*
0.4 k
1.4 k
1.2 k
Cancel
1.4
0.4 k
1.4 k
1.2 k
* Noise-generator MIPs do not vary with configuration (4.2 of the 7.5 MIPs). 
** The chip RAM, data memory and MIPs loading vary with configuration.

User Functions

The LEC software consists of two independent, C-callable functions that perform echo cancellation 
and noise generation operations. The echo-cancelling function performs training and cancellation 
and the noise-generation function provides Gaussian white noise for channel training. There are 
also functions that reflect the implementation's object-based interface. The echo canceller 
interfaces with arrays of linear PCM samples and returns status.
Echo Canceller and Noise Generator Functions
LEC_cancel (...)
Produces an echo-cancelled version of the input and provides status
NSE_gauss (...)
Creates a frame of Gaussian white noise samples
Interface Functions (general form)
XXX*_create (...)
Dynamically allocates and initializes data memory for the software object
XXX*_delete (...)
Frees object's usage of dynamically-allocated data memory
* The XXX is the mnemonic for each module (LEC or NSE).

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. 
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