Wireless Audio Transmission Technology for In-Vehicle Infotainment Systems

Modern in-car infotainment systems include an increasingly diverse array of content sources, including front-seat and rear-seat displays designed for passengers, content from portable devices, and Internet access from portable computing devices. Passengers may want to share the same content sometimes, but not always, so the audio delivery system must be able to deliver multiple content streams and target each stream to a specific passenger. That is, each passenger has control over the content selection and all of the interactive options that are available with that content. To meet the demands of such an environment, the audio delivery system must have certain specific features.
Headphones

Modern in-car infotainment systems have multiple built-in audio sources, such as CD players and DVD players that deliver content to multiple displays, as well as various types of broadcast radio receivers. Each passenger with a portable audio/media player and smartphone will have his or her own content source that is connected to the infotainment system through an auxiliary input. In addition, the availability of Internet access in the car provides another source of personal content, often music-related.

Despite the rich content sources in the in-car infotainment system, the car's built-in speakers force each passenger to listen to the same audio information at the same time. Obviously, if each passenger wants to listen to their favorite music, they must use headphones (Figure 1).

Wireless Audio Transmission Technology


Figure 1 Rear Seat Display with Headphones.

Headphones for in-car entertainment systems can be either wireless or wired. Of course, the disadvantages of using wired headphones are obvious in the confined space of a car, so automotive OEMs turn to wireless solutions.

Wireless Technology: Infrared vs. Digital RF

Infrared (IR) and radio frequency (RF) are the two main technologies used for wireless headphones, and each has its own advantages and disadvantages.

Audio Quality: In general, most IR solutions transmit analog audio, and the audio is transmitted over a frequency modulated IR carrier with a dynamic range of about 70dB. Therefore, its quality is similar to FM radio, which is significantly lower than the 96dB dynamic range of CD and DVD audio.

In addition, in addition to sunlight, there are other sources of IR interference in the car. In the transmission process of analog audio, there is no opportunity to correct errors, so any minor IR channel problems will cause audible noise in the audio stream, often referred to as "static noise."

Analog RF solutions also suffer from the same low audio quality and susceptibility to interference. Furthermore, in cars that support Wi-Fi and Bluetooth connectivity, RF interference issues are exacerbated because Wi-Fi and Bluetooth connectivity share the same frequency band as RF wireless audio.

Any wireless transmission, whether IR or RF, will have interference. Therefore, digital wireless audio transmission technology is a better solution because it has the ability to detect transmission errors, correct them before the signal reaches the listener, and take action to prevent future errors.

Line of sight: IR requires an interference-free line-of-sight path from the source to the headset, but it is difficult to establish such a path from the dashboard to the rear seat passengers, especially in larger vehicles with three rows of seats. Therefore, manufacturers try to install IR transmitters in multiple locations to ensure that at least one of them can establish a line-of-sight path to the headset. Of course, this solution increases cost and power consumption. Moreover, this path is interrupted whenever the passenger wearing the headset turns his head.

In contrast, RF solutions do not require a line-of-sight path, and only one transmitter is needed to transmit to all headset locations in the limited space of the car.

Multiple audio channels: Generally speaking, IR only provides a single broadcast channel. Multiple transmitters trying to transmit different content will interfere with each other. Therefore, analog IR technology can only handle a single audio stream, which is obviously limited in new cars with multiple displays and other audio sources. Some IR solutions solve this problem by using time-division multiplexing of several audio channels on a single digital IR link. The disadvantage of this solution is that all audio content must be sent somewhere to be multiplexed before IR transmission. RF solutions

have the ability to assign individual audio channels to different audio streams. Wireless audio is transmitted as close to the audio source as possible in the car (see Figure 2), and the headphones can select the audio content they want to hear by receiving the radio channel.

Figure 2 Multiple sources and multiple headphones.

Broadcast: In addition to allowing each passenger to listen to the music they want through their headphones, it also allows multiple passengers to share content, such as when they are viewing the same screen.

Infrared technology is essentially a broadcast medium, so all headphones with a line-of-sight channel to the transmitter can share the same audio stream.

Digital RF technology is different in this regard. For example, streaming audio solutions using Bluetooth only support point-to-point connections. Therefore, each headphone needs a transmitter, which increases cost and power consumption. Sharing content means connecting the desired audio stream to all headset transmitters that want to share this content. In contrast, SMSC's Kleer technology allows up to 4 headsets to be connected to the same audio source, while also allowing the headset to select between different sources.

Back-channel communication: The headset provides an ideal location for users to control functions such as switching audio channels, pausing playback, etc. However, this requires a return channel from the headset to the audio source, which is called the back-channel.

Unfortunately, IR connections involve different transmitters and receivers, so headsets are often only able to receive, and playback control functions cannot be implemented on the headset.

Most digital RF devices have both transmit and receive functions, which is the minimum requirement, so that the headset can send a confirmation signal back to the sound source, telling it that the audio data packet has been received correctly and does not need to be resent. This reverse channel established from the headset to the sound source can also be used for other purposes, including playback control, battery status, and identifying the brand of the headset to confirm whether it is supported.

Many infotainment systems also include a handheld remote control (see Figure 3) that can be used to control various system functions from anywhere in the car. Like wireless headphones, these devices usually use IR technology, but they also suffer from the same problems. Digital RF solutions that support a backchannel can use this backchannel as a remote control. Therefore, an RF component built into the car head-unit that transmits the audio code stream to one or more wireless headphones can also be used to receive control commands from one or more remote controls, eliminating the need for an additional receiver in the head-unit specifically for receiving remote control commands.

Figure 3 Remote control for infotainment system.

Another interesting application of the backchannel is voice communication. This allows the headset to become a headset with a microphone, allowing personal voice calls through the car's hands-free voice system without having to switch to another headset.

RF Technology Selection

Given the many shortcomings of wired and IR headsets, automotive OEMs have turned to digital RF technology as a solution. There are many RF solutions available for automotive headsets, including Wi-Fi, Bluetooth, and several proprietary wireless technologies, such as SMSC's Kleer. Which RF technology should automotive OEMs choose for this application?

Audio quality is the primary consideration. In particular, the RF technology should not be the limiting factor in the audio quality of the headset. If the car has a high-quality audio source in the form of a CD/DVD player, the same quality should be transmitted to the headset. This is one of the main disadvantages of Bluetooth in this application. Bluetooth's limited bandwidth requires lossy compression to transmit audio, so the audio quality can only be comparable to that of FM radio or IR.

The second consideration is power consumption. Since the in-car headset is mainly placed in the car, it is very inconvenient to frequently change the battery or charge it. Therefore, the power consumption of the RF solution must be as small as possible to extend the battery life. This requirement basically eliminates the possibility of using Wi-Fi technology. Because Wi-Fi devices support higher bandwidth and longer transmission distance, it means that it consumes more power than other technologies. Even Bluetooth consumes more power than SMSC's Kleer solution because Kleer is designed for battery-powered wireless audio applications.

Coexistence with other radio technologies is also an important consideration, as automotive systems are beginning to include built-in Wi-Fi access points to support wireless laptops and Bluetooth hands-free voice devices. The selected RF technology must be able to operate in this environment without causing audio drops and without interfering with Wi-Fi throughput and Bluetooth voice quality. One way to evaluate whether a radio technology can coexist with other radios is to examine the radio spectrum it occupies. All 2.4GHz radio technologies must find about 80MHz of available spectrum between 2.40GHz and 2.48GHz. Wi-Fi occupies half of this spectrum, while Bluetooth occupies a quarter. In contrast, narrowband radios consume less than 3MHz of the frequency band, so they have a higher probability of finding available spectrum.

The RF technology must be able to support the simultaneous transmission of multiple audio streams, because the main purpose of the headset design is to allow each passenger to hear their favorite content. Since these audio streams do not necessarily originate from the same location (for example, each rear-seat display may have its own RF transmitter in addition to one or more transmitters in the car head unit), the RF technology must be able to support multiple transmitters in different locations. There are several approaches to this requirement, such as Wi-Fi's Carrier Sense Multiple Access/Collision Detection (CSMA/CD), which is a more complex mechanism that simply divides the spectrum into multiple narrowband channels and dynamically selects an available channel.

The RF technology must be able to support multiple headphones connected to the same source. Of course, one way to circumvent this requirement is to use multiple RF transmitters (one for each headphone), but this is not only expensive but also power inefficient. A better approach would be to allow each headphone to receive the audio source that the passenger wants to hear, regardless of whether others are also listening to the same audio.

The RF technology must be able to transmit audio with a latency of no more than 70 milliseconds, and preferably less than 45 milliseconds. If the latency is too long, the passenger will experience a desynchronization between the video and audio when watching a video. Depending on the type of headphone, latency of less than 25 microseconds may sometimes be required. If the headset has a long latency, a lot of ambient sound will be transmitted to the headset wearer's ears, which will cause a low hollow sound if the listener is listening to the same audio channel as the car speakers are playing.

The RF technology must support two-way communication, allowing information from the headset to be transmitted back to the audio source. This information may be information about the headset, such as its remaining battery life, or it may include audio playback commands such as play/pause and track forward.

The RF technology must support secure transmission to ensure that passengers in the car cannot hear the audio source of other vehicles. This requirement eliminates "broadcast" RF technologies such as FM radio. In addition, if two different automotive OEMs choose the same RF technology, they may need to establish a security mechanism to ensure that only headsets purchased from that OEM can be used in the OEM's car.

Different wireless technologies have their own advantages and disadvantages when choosing a wireless audio transmission solution for in-vehicle infotainment systems. Generally speaking, digital transmission is preferred over analog technology, and RF transmission is preferred over IR. However, there are still differences between various digital RF technologies, so different technologies must be carefully and thoroughly evaluated in terms of audio quality, power consumption, multipoint to multipoint connectivity, playback control, audio latency, voice call support, and coexistence with Wi-Fi and Bluetooth.