G.728
Low-Delay CELP Speech Coder
by Atlanta Signal Processors, Inc.

Software Overview

The International Telecommunications Union (ITU) G.728 standard (1992) is a 16-kbps algorithm for 
coding telephone-bandwidth speech for universal applications using low-delay code-excited linear 
prediction. The ASPI G.728 coder is a fully compliant and interoperable implementation of the ITU 
G.728 recommendation. This coder is well-suited to a wide range of applications, including both 
voice storage and voice communications. It is ideally suited for telecommunications networks in 
which toll-quality speech is a requirement and total communications link delay is an important 
factor.
The G.728 coding algorithm is based on a standard analysis-by-synthesis CELP coding technique. 
However, several modifications are incorporated to meet the needs of low-delay high-quality speech 
coding. G.728 uses short excitation vectors (five samples, or 0.625 ms) and backward-adaptive 
linear predictors. The algorithmic delay of the resulting coder is 0.625 ms, resulting in an 
achievable end-to-end delay of less than 2 ms.
The G.728 standard was designed to provide speech quality equivalent to or better than that of the 
G.721 32-kbps ADPCM international standard, even after three tandem connections. The G.728 coder 
was also designed to behave well in the presence of multiple speakers and background noise, and to 
be capable of handling non-speech signals such as DTMF tones and voice-band modem signals at rates 
of up to 2400 bps (if perceptual weighting and postfiltering are disabled). Techniques such as 
bandwidth expansion of the LPC filter coefficients and codebook structuring were incorporated into 
the standard to improve resistance to moderate channel error conditions. The G.728 coder achieved a 
MOS score of 4.0.

Features and Benefits

•	Complies fully with every aspect of ITU G.728
•	Callable in TI TMS320 floating-point C
•	Provides toll-quality speech at 16 kbps
•	14.4-kbps and 12.8-kbps data rates also available
•	One-way coder/decoder delay 2 ms or 
8 ms

Processor and System Specifications

•	The ASPI G.728 Encoder/Decoder runs full duplex on a 40-MHz TMS320C3x with 9 kwords of memory or 
on a 50-MHz `C3x with 4 kwords.
•	Ready to run on the Elf DSP Platform from Atlanta Signal Processors
•	Portable to any TMS320C3x target platform


Usage Limitations or Performance Considerations

•	None

Algorithm Verification

•	The ASPI G.728 algorithm has been tested against the ITU Recommendation G.728 test vectors using 
the ITU G.728 test procedures and is fully compliant in all aspects.

Availability

•	Available now through Atlanta Signal Processors.
•	Demonstration available from Atlanta Signal Processors

Company Background and Contact Information

Atlanta Signal Processors was founded in 1981 by Ronald W. Schafer, Thomas P. Barnwell III, and 
Russell M. Mersereau, three Professors of Electrical Engineering at the Georgia Institute of 
Technology with over 80 years of experience in the field. The company has focused its efforts on 
developing tools for professional signal-processing design engineers and now offers digital 
signal-processing hardware and software products to DSP designers worldwide.

Contact:	Atlanta Signal Processors, Inc.
		1375 Peachtree St. NE, Suite 690
		Atlanta, Georgia  30309-3115
		(404) 892-7265
		Fax: (404) 892-2512
		e-mail: aspi@aspi.com