Dealing with the design challenges of in-vehicle hands-free systems

For example, wideband technology being introduced in Europe allows for greater call fidelity, but also, unfortunately, transfers more road noise. Sophisticated algorithms can remove noise from wind or defroster vents, allowing freedom in microphone placement without requiring complete system tuning for each vehicle.

An affliction prevalent in the automotive industry is the "Audio Is Easy" syndrome—also known as AIE syndrome. Symptoms include the illusion that all hands-free speech communications in an automobile requires is a microphone and loudspeaker connected to a network access device.

Delivering high quality hands-free speech communications in an automobile is unfortunately not so simple. It requires a thorough understanding of how to measure performance and of common causes of performance issues, and the application of the best available solutions to solve these performance problems.

Measuring performance
The role and importance of performance measurements is often overlooked. Reliable and valid measurements are needed to specify requirements for suppliers, detect and diagnose problems early, optimize performance and predict end-users' satisfaction.

Understanding speech communications. An understanding of what we want to measure is necessary for recognizing valid performance measurements. Speech communications is fundamentally different than other types of communications systems, because the measure of its quality is, ultimately, subjective.

Data communications over a radio link, for example, can be measured by simply determining the number bits that were successfully transmitted. With speech communications, however, the transmitter and the receiver are people, and transmission performance must be assessed in terms of human perception.

Whether or not we even care about 1% or 3% bit errors in the telecommunications system depends on whether or not there is a perceivable difference to the people communicating. Figure 1 below shows the links involved with speech communications, or the "the speech chain" (Denes and Pinson, 1993).

Figure 1. The speech chain.

When people are geographically separated such that acoustic signals from the mouth of the speaker cannot directly reach the ear of the listener, a telecommunications system is needed to carry the speech signals between speakers and listeners.

This system can be thought of as just an extension of the speech chain (more links in the chain). Therefore, the validity of measures of telecommunication system performance depends on how closely they agree with human perception.

Test methods and requirements. It should be clear from the previous section that objective measurements, such as percent of speech frame loss, frequency response, etc., are indirect measures of what we really care about—performance as perceived by the user.

However, it is not always possible to use perceptual (or subjective) measures of performance, because these measures are expensive, time consuming, and cannot always be used to assess performance at intermediate points along the telephone connection. Therefore, objective measurements must play an important role in delivering high quality hands-free speech communications.

The state-of-the-art in terms of objective test methods and requirements can be found in ITU-T Recommendation P.1100 (Narrowband hands-free communications in motor vehicles). This is a new international standard that was recently approved and can be downloaded for free from the ITU-T website.

Figure 2 below shows the test arrangement used in ITU-T P.1100, with he optional digital injection of background noise shown. The term "speech" in the figure is used generically to refer to any type of test signal used to simulate speech signals.

Test equipment manufacturers already have commercially available turn-key measurement solutions for testing according to ITU-T P.1100. A wideband version of this standard is currently being developed by the Carcom ITU-T Focus Group and is expected sometime latter this year.

Figure 2. Objective testing arrangement used in ITU-T P.1100.

It is important to note that there are limitations with the objective test methods and requirements found in ITU-T P.1100 and other requirements documents, such as VDA 1.6. More specifically, objective test results do not always agree with subjective performance. This is particularly true with measures of echo, attenuation during double-talk, and one-way speech quality predictions (i.e., MOS). Therefore, important decisions should be based on subjective evaluations.

General guidance on subjective testing can be found in Section 13 of ITU-T P.1100. This information should prove useful to those unfamiliar with subjective testing. However, this guidance does not follow the form of a test plan and does not provide requirements. Work has begun within ITU-T Study Group 12 to develop detailed subjective test methods and requirements for automotive, but a new standard ("Subjective requirements for hands-free speech communications in motor vehicles") is not expected until mid 2010.