Modern automotive infotainment systems involve an increasingly rich variety of content sources, including front and rear seat displays targeting individual occupants, auxiliary inputs for personal content from portable devices, and Internet access for portable computing devices.
Vehicle occupants want to share such content at times, but not all the time, so the audio distribution system must be able to distribute multiple streams of content; with each stream going to selected occupants, in such a way that each occupant has control over the stream selection as well as control over any interactive options that stream may offer. This environment demands certain characteristics of the audio distribution system.
Modern automotive infotainment systems have a variety of built-in audio sources such as CD players, DVD players feeding multiple displays, and various types of broadcast radio receivers. Individuals carry around their own sources of content such as portable audio/media players and smartphones that can be connected to the infotainment system via auxiliary inputs. Internet access is being introduced in vehicles offering yet another source of personal content that often involves audio.
Despite the rich variety of content sources being made available in automotive infotainment systems, it is a fact that the built-in speakers of the car require that everyone in the vehicle listen to the same audio stream at the same time. Obviously, the ability of individual occupants to enjoy their own preferences requires the use of headphones.
Headphones for automotive infotainment can be wired or wireless. Of course, in the relatively confined spaces of the automobile, the disadvantages of a tangle of wired headphones are obvious, leading the automotive OEMs to look to wireless solutions.
Wireless technologies: Infra-red (IR) versus digital RF
The main technology candidates for wireless headphones are infra-red (IR) and radio frequency (RF). There are various trade-offs associated with these alternatives.
Generally, most IR solutions transmit analog audio. The audio is transmitted using frequency modulation of the IR carrier with a dynamic range of about 70 dB. Thus the quality is similar to FM broadcast radio which is considerably lower than the 96 dB dynamic range of CD and DVD quality audio.
Furthermore, there are sources of IR interference in the vehicle, not the least of which is sunlight. The transmission of analog audio offers no opportunity to correct any errors that occur during transmission so any small corruption of the IR channel results in an audible artifact in the audio stream, often referred to as "static.”
Analog RF solutions suffer from the same problems of lower audio quality and susceptibility to interference. Arguably, the RF interference problem can be even worse in vehicles that support WiFi and Bluetooth connectivity which share the same spectrum as the RF wireless audio technology.
Whether IR or RF, no wireless transmission is free of interference. Digital wireless transmission of audio is therefore preferred because it offers the ability to detect transmission errors, correct them before they reach the listener, and take steps to avoid future errors.