Audio technology for car entertainment systems

The new in-vehicle infotainment system has a variety of built-in audio sources, such as CD players and DVD players that provide content to multiple displays, as well as various types of broadcast radio receivers. Each passenger carrying a portable audio/media player and smartphone has its own source of content and is connected to the infotainment system via an auxiliary input. In addition, the internet feature provided in the car also provides another source of personal content, often associated with music.

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

Wireless audio transmission technology

Figure 1 Rear seat display of the headset.

Headphones for in-car entertainment systems can be designed with wireless or cable. Of course, the disadvantages of using wired headsets in the limited space of a car are obvious, so automotive OEMs are turning to wireless solutions.

Wireless technology: infrared and digital radio

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

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

In addition, in addition to sunlight, there are other sources of IR interference in the car. There is no chance of correcting errors during the transmission of analog audio, so any small IR channel problem will cause audible noise in the audio stream, often referred to as "static noise."

Analog RF solutions also have the same low audio quality and are subject to interference. Moreover, in cars that support Wi-Fi and Bluetooth connectivity, the RF interference problem is exacerbated because Wi-Fi and Bluetooth connections share the same frequency band as RF wireless audio.

Regardless of whether it is IR or RF, any kind of wireless transmission will have interference. Therefore, digital wireless audio transmission technology is a better solution because it has the ability to detect transmission errors and correct the signal before it is delivered to the listener, and can take action to avoid future errors.

Line of sight: IR requires an undisturbed line-of-sight channel from the source to the earphones, but it is difficult to establish such a channel from the dashboard to the rear seat passengers, especially for larger three-row seat vehicles. Words. Therefore, manufacturers are attempting to install IR emitters in multiple locations to ensure that at least one of them is capable of establishing a line of sight channel with the headset. Of course, this solution will increase cost and power consumption. Moreover, this channel is interrupted as long as the passenger with the headset turns the head.

In contrast, RF solutions do not require line-of-sight channels, and in a limited car space, only one transmitter can be delivered to all headphone positions.

Multi-audio channel: In general, the IR only provides a single broadcast channel. Multiple transmitters attempting to transmit different content will interfere with each other. Therefore, analog IR technology can only handle a single audio stream, which is clearly a limitation for new cars with multiple displays and other sources. Some IR solutions solve this problem by multiplexing several audio channels on a single digital IR link. The disadvantage of this scheme is that all audio content must be sent to somewhere before being multiplexed before IR transmission.

The RF solution has the ability to assign individual audio channels to different audio streams. The wireless source will be transmitted as much as possible in the car (see Figure 2) to the audio source, and the headset can select the audio content that you want to listen to by receiving the radio channel.

Figure 2 Heavy source with multiple headphones.

Broadcast: In addition to allowing each passenger to enjoy the music they want to listen to, they also allow multiple passengers to share content, for example when they are watching the same screen.

Infrared technology is essentially a broadcast medium, so all headphones with line-of-sight channels between the transmitters can share the same audio stream.

Digital RF technology differs in this regard. For example, a streaming audio solution using Bluetooth only supports point-to-point connections. Therefore, each headset requires a transmitter, which causes an increase in cost and power consumption. Sharing content means connecting the desired audio stream to all the headphone transmitters that want to share this content. In contrast, SMSC's Kleer technology allows up to four headsets to be connected to the same audio source while allowing the headset to be selected from different sources.

Back-channel communication: The headset provides an ideal location for the user to control audio channel switching, pause playback, and more. However, this requires establishing a return channel from the headset to the audio source, referred to herein as the reverse channel.

Unfortunately, the IR connection contains different transmitters and receivers, so the headset is often only received, and playback control is not available on the headset.

Most digital RF devices have both transmit and receive functions. This is the minimum condition so that the headset can send back an acknowledgment signal to the source, telling the audio packet that it has been received correctly and does not need to be resent. This reverse channel established from the headset to the source can also be used for other purposes, including playback control, battery status, and identification of the headset brand to confirm support.

Many infotainment systems also include a handheld remote control (see Figure 3) that controls various system functions from anywhere in the car. Like wireless headsets, these devices typically use IR technology, but they all suffer from the same problem. A digital RF solution that supports reverse channel can use this reverse channel as a remote control. Thus, an RF component built into a head-unit for streaming audio data to one or more wireless headsets can also be used to receive control commands from one or more remote controls. As a result, there is no need to add a special receiver for receiving remote control commands to the audio host.

Figure 3 Remote control device for the entertainment system.

Another interesting application of the reverse channel is voice communication. This allows the headset to be a headset with a microphone for personal voice calls through the hands-free voice system in the car, eliminating the need to switch to another headset.

RF technology choice

In view of the shortcomings of wired and IR headsets, automotive OEMs are turning to digital RF technology as a solution. There are a variety of RF solutions available on the market for car headsets, including Wi-Fi, Bluetooth, and a variety of proprietary wireless technologies. SMSC's Kleer is one of them. Which RF technology should an automotive OEM choose for this application?

Audio quality is a top priority. In particular, RF technology should not be a limiting factor in the audio quality of headphones. If the car has a high quality audio source for the CD/DVD player, the same quality should be transferred to the headphones. This is one of the main drawbacks of Bluetooth in this application. The limited bandwidth of Bluetooth makes it necessary to use lossy compression to transmit audio, so only audio quality similar to FM radio or IR can be obtained.

The second is to consider power consumption. Since the car headset is mainly placed in the car, it is very inconvenient to replace the battery or charge frequently. Therefore, the power consumption of the RF solution must be as small as possible to extend battery life. This requirement basically eliminates the possibility of using Wi-Fi technology. Because Wi-Fi devices support higher bandwidth and longer transmission distances, this means 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 also begin to have built-in Wi-Fi access points to support wireless laptops and Bluetooth hands-free voice devices. The RF technology chosen must be able to generate no audio drop in this environment, 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 check the radio spectrum it occupies. All 2.4 GHz radio technologies must find an available spectrum of approximately 80 MHz between 2.40 GHz and 2.48 GHz. The spectrum occupied by Wi-Fi is half of this value, and Bluetooth is one-fourth. In contrast, narrowband radios consume only less than 3MHz of frequency, so there is a higher chance of finding available spectrum.

This RF technology must support simultaneous transmission of multiple audio streams, as the primary purpose of the headset design is to allow each passenger to hear what they like. Since these audio streams are not necessarily from the same location (for example, one or more transmitters in a car audio host, each rear-seat display may have its own RF transmitter), RF technology must be able to support Multiple transmitters with different locations. There are a number of options to meet this need, such as Wi-Fi Carrier Sense Multiple Access/Collision Detection (CSMA/CD), a more complex mechanism that simply splits the spectrum into multiple narrowband channels. And dynamically select the available channels.

When choosing a wireless audio transmission solution for an in-vehicle infotainment system, different wireless technologies have their own advantages and disadvantages. In general, digital transmission is more popular than analog technology, and RF transmission is better than IR. However, there are still differences between various digital RF technologies, so it is necessary to carefully and comprehensively evaluate different technologies in terms of audio quality, power consumption, multipoint-to-multipoint connectivity, playback control, audio delay, voice call support, and Wi-Fi and Features such as Bluetooth coexistence.

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