(Part 1 of this feature looked at the trade offs between infrared and radio frequency technologies for wireless automotive connectivity.)
Back-channel communications with headphones
Headphones offer an ideal location to put user controls to switch audio channels, pause playback, etc. However, this requires a transmission path from the headphone back to the audio source, referred to here as a "back channel."
Unfortunately, IR connections involve distinct transmitters and receivers and therefore headphones tend to be receive-only, eliminating the possibility of having playback controls on the headphone.
The majority of digital RF devices have both transmit and receive capabilities. At a minimum, this is required so that the headphone can transmit an acknowledgement back to the audio source to confirm that the audio packet was received correctly and does not need to be re-transmitted. This creates a back-channel from the headphone to the audio source that can be used for other functions, including playback controls, battery status, and identifying the brand of headphone to ensure that it is supported.
Many infotainment systems include handheld remote controls (see figure below) that allow control over many system functions from anywhere in the vehicle. As with wireless headphones, these often use IR technology and suffer from many of the same issues.
Digital RF solutions that support a back-channel offer the potential of using that back-channel for handheld remote controls. Thus, an RF device in the vehicle head-unit that streams audio to one or more wireless headphones, can also be receiving control commands from one or more handheld remotes, eliminating the need to have an additional receiver in the head-unit dedicated to receiving commands from the remote controls.
Another interesting application for the back-channel is voice communication. This allows the headphone to become a headset that includes a microphone to be used for private voice calling over the vehicle hands-free voice system without switching to a separate headset.
Which RF technology?
Given the difficulties associated with wired and IR headphones, the automotive OEMs are looking to digital RF technology for a solution. There are a variety of RF solutions that can be considered for automotive headphones, including WiFi, Bluetooth, and several proprietary radios such as SMSC’s Kleer
. What should the automotive OEM look for in selecting an RF technology for this application?
Audio quality is a key requirement. Specifically, the RF technology should not be the limiting factor in the audio quality that can be delivered to the headphone. If the vehicle has a high quality audio source such as an audio CD or DVD player, it should be possible to deliver that quality to the headphone. This is one of the primary drawbacks of Bluetooth in this application. The limited bandwidth of Bluetooth requires the use of lossy compression to transmit audio, resulting in audio quality that is more like FM broadcast radio or IR.
The next need is low power consumption. Because headphones for the car tend to stay in the car, frequent replacement (or recharging) of batteries is highly undesirable. Therefore the RF solution must consume as little power as possible to preserve battery life. This requirement tends to eliminate WiFi technology. The higher bandwidth and longer range supported by WiFi devices also means that they consume a lot more power than the alternatives. Even Bluetooth is relatively high power compared to a solution like SMSC’s Kleer that was designed specifically for battery-powered, wireless audio applications.