Signal processing techniques have now advanced significantly since these early times, when processing to reduce noise often had the undesirable side effect of making speech sound distorted. State-of-the-art noise reduction techniques, developed in part for Formula One and NASCAR race teams, can reduce unwanted broadband noise typically by 95% (12dB), leaving the voice quality largely unaffected.
Even within the demanding environment of a race car or convertible sports car, such noise reduction techniques can remove sufficient noise to make the speech intelligible, as illustrated here.
Such techniques can also be adaptiveas the background noise changes, the algorithm adapts to continuously cancel the noise to give a more consistent performance. For example, as a car accelerates, the clarity of the conversation usually decreases as the noise level rises. Using adaptive noise reduction techniques, however, the speech can remain intelligible, and the listener is less likely to be distracted by the changes in the background noise, as shown below.
In addition to noise, which without the use of reduction techniques can tend to drown out the speech, automotive hands-free communications are often degraded by the previously mentioned use of muting, which allows only one person to speak at a time. Such muting is necessary to prevent the far-end speaker from hearing his own voice as it comes out of the loudspeaker in the car and is picked up by the microphone. While muting (turning off the microphone in the car when there is speech coming out of the loudspeaker) makes an interactive conversation difficult, hearing your own voice as echo is much more disturbing.
Alternative solutions to muting, such as acoustic echo cancellers, are now starting to appear on the market. Such acoustic echo cancellers reduce the need for muting and effectively allow both parties to talk at the same time and have a natural, interactive conversation.
Illustrations below show the basic operation of an acoustic echo cancellation algorithm which continuously monitors the loudspeaker signal in the car and effectively subtracts this from the signal picked up by the microphone. The remaining microphone signal consists only of the driver's speech, which is transmitted to the far-end listener via the hands-free phone kit.
In response to the increasing demand for information while on the move, some automotive manufacturers are now offering the option of integrating into the cabin a complete driver and passenger information system incorporating audio, climate information, onboard computer, and more recently, phone and satellite navigation. Even the performance of these very sophisticated hands-free communication/infotainment systems can be degraded by unwanted background noise and echo.
DSP technology, which incorporates noise and echo cancellation, can be simple to integrate into both hands-free car kits and driver and passenger information systems. There is considerable interest from automotive manufacturers who wish to incorporate noise reduction and echo cancellation into their next generation of in-car communication systems, and they are seeking companies who have expertise in these technologies to provide the solutions.
The challenge for these companies is to provide consistent performance across all types of automobiles, including those in which noise can cause extreme interference, such as convertible sports cars. Towards the end of this decade, in-car, hands-free communication systems could become standard in all automobiles, because of the quality of communication improvements that are being achieved through state-of-the-art noise and echo cancellation DSP technology.
Gillian Davis is managing director of NCT (Europe) Ltd developers of the noise reduction and acoustic echo cancellation algorithms.
Glossary of terms from NCT:
Algorithm: A prescribed finite set of well defined rules or processes for the solution of a problem in a finite number of steps. The mathematical formula for an operation.
DSP (Digital Signal Processor): A specialized computer chip designed to perform speedy and complex operations on digitized waveforms.
Echo: The echo you get when using a hands-free kit is caused by the microphone picking up sound from the loud speaker and relaying it back to the far-end speaker (i.e. to the person not in the car). As a result, far end users hear a delayed version of their speech. This is referred to as "acoustic echo." The delays in mobile communications systems make this echo particularly noticeable.
Far End: The party remote from the hands-free car kit.
Full Duplex: Transmission in two directions simultaneously or more technically, bidirectional simultaneous two-way communications. The best two-way phone conversations take place on four-wire circuits, two for transmission in one direction and two for transmission in the other. All long distance circuits are four wire. Most local lines are two wire, which means they are a compromise. Most speakerphones are half-duplex, meaning they only transmit in one direction at one time. The speakerphone flips its direction based on who's talking or more precisely, who's talking the loudest. Many hands-free car kits are also half-duplex.
Half Duplex: A circuit designed for transmission in both directions, but not at the same time. Telex is an example of a half-duplex system.
Near End: The party using the hands-free kit.