Toll-quality voice is now a measurable specification

The final
judgment always rests on human opinion, Steve Voran said.


Toll-quality voice is a big selling point for voice-over-IP and other computer
telephony products. But who decides what toll quality means?


The Commerce Department’s Institute for Telecommunications Sciences (ITS) has been
working on the question for several years. ITS has come up with a set of digital signal
processing algorithms to evaluate the quality of audio transmissions.


The algorithms compare two audio signals—one over the public switched telephone
network, for instance, and one from an IP voice gateway—and estimate the human
response to the two, said the institute’s executive officer, Val O’Day.


Netrix Corp. of Herndon, Va., has contracted with ITS to test its Network Exchange
Voice Over IP and Voice Over Frame Relay gateways.


The five-year Netrix contract will be ITS’ first foray into objective evaluation
of computer telephony quality, O’Day said. Initial results will come out late this
year.


The institute is the chief research and engineering arm of Commerce’s National
Telecommunications and Information Administration in Boulder, Colo.


About 60 percent of its $11 million budget in fiscal 1997 came from work for federal
agencies and commercial clients. Its mission is to assist U.S. industry through contracts
and technology transfer, such as the new audio assessment techniques.


The objective assessment techniques have received approval from the International
Telecommunications Union as a standard. They also garnered positive results in first
ballots by the American National Standards Institute’s working group on audio
standards, O’Day said.


The objective algorithms are only one component in assessing audio quality, however.
The final judgment always rests on human opinion, ITS electronics engineer Steve Voran
said.


“There will always be a need for some subjective testing,” he said.


Standards for conducting subjective tests are well-established but time-consuming,
expensive and hard for product developers to justify. The ITS algorithms can serve as
preliminary test tools before a final product is submitted for authoritative subjective
testing.


“We consider the subjective test to be the truth,” Voran said. “The
objective test is an estimation of the truth.” But it is a pretty good
estimate—objective tests show 90 percent to 98 percent correlation with the
subjective tests, he said.


The institute calls its approach to objective assessment perception-based.


It reduces a signal to the components that are relevant to a listener’s perception
and analyzes them.


ITS also has developed prototype test instruments: PCs with custom software and 16-bit,
analog-to-digital and digital-to-analog converters.


Under its cooperative R&D agreement with Netrix, ITS will do both subjective and
objective testing of the company’s gateway products using Vodex software.


The institute will compare Netrix’s 5.5K Algebraic Code Excited Linear Predictive,
compression as well as public switched network transmissions at the standard 65-Kbps
rate. ITS will work with Netrix for five years, testing new technology from both the
institute and the company.

About the Author

William Jackson is a Maryland-based freelance writer.

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