To allow the automobile manufacturer to react quickly to changes in technology, there are two possible solutions. Firstly, there is the multimedia gateway, which offers the advantages of great flexibility and shorter product-development times. Alternatively, the head unit requires a substantially greater integration effort, but is much more cost-effective. Both solutions employ custom-programmable processors that support multimedia applications and are open for software updates.
Until recently, the multimedia gateway has provided the only multimedia solution for vehicles. While the head unit represents a general user interface, with a huge number of functions geared towards radios and CDs, communications, navigation and general vehicle alignment, the gateway is initially restricted to multimedia applications.
Multimedia gateway
The main tasks of the multimedia gateway are the adaptation and conversion of proprietary and standardised communication protocols, for example Bluetooth and USB, as well as the decoding of digital audio formats. For maximum flexibility, the gateway software can be upgraded at any time.
The gateway typically communicates by means of a CAN or MOST bus. The CAN bus has a bandwidth of approximately 1Mbit/s and merely serves as a control interface; it is not suitable for multimedia-data transmission. Multimedia data is transmitted as digital data (raw PCM data) using the MOST bus at up to 25Mbit/s, or as analogue data. The multimedia interface also provides access to terminal equipment such as USB, SD cards or wireless Bluetooth (Figure 1).
The vehicle side will support the necessary MOST and CAN network services. The operating system guarantees compliance with critical real-time requirements. On the terminal-equipment side, the gateway must support standardised communication protocols as well as proprietary protocols to enable the use of popular devices such as the Apple iPod. USB OTG, SDIO and Bluetooth are among the most important standard protocols.
Figure 1: The multimedia gateway provides isolation between audio equipment and automobile electronics.
In the future, wireless will play an increased role in the transmission of audio and video data between portable devices and vehicles. New protocols will be supported, such as the Bluetooth-based A2DP profile for playing audio files on a mobile phone via the vehicle's audio system.
Figure 2: The software framework of the multimedia gateway.
Demands on the multimedia processor
A closer look at user scenarios shows which demands the multimedia processor must cope with (Figure 2). The most complicated application would be a combination of the MOST network services with A2DP Bluetooth. At the same time, several audio files from different sources must be decoded along with their respective digital-rights management.
In the future, video applications will place extra demands on the processor, with data rates of several megabits per second as opposed to the few hundred kilobits per second that are required for audio. The MOST standard (MOST-150), which is still being defined, will operate at up to 150Mbit/s. The Bluetooth standard will also approach its limits as far as video is concerned since the maximum rate of 2Mbit/s allows to replay only small video files. For short-range wireless networking, Bluetooth will be replaced by a new wireless USB standard capable of transmitting up to 480Mbit/s.
The head unit
The head unit is the user interface integrated into the dashboard that controls communications, entertainment, air conditioning and navigation. It may be controlled by speech-recognition commands or with a special input device. Output is provided by a display or by text-to-speech functions.
The HMI (human-machine interface) is typically based on a separate processor with a direct interface to the display. The HMI processor can communicate with control devices via the CAN interface. User commands are transferred directly to the respective functional unit, where they are processed. To lower total cost, it will be possible, in the future, to combine functional blocks in one processor. This approach is mainly used for components that belong to the same functional group, and it requires a powerful processor that can combine DSP and control functions. Software and hardware requirements are higher than for the multimedia gateway, since the processor will have to execute the HMI and - if necessary - a digital radio standard as well as the typical functions already described for the gateway.
Example - Audi A5
The HMI of the Audi A5's audio system uses up to four Blackfin processors (Figures 3 and 4). The system includes DAB digital radio, an MP-3 compliant 6-disk CD changer, and the music interface for connecting portable media players and using the multifunction controls that are mounted on the steering wheel. Since the Audi's radio equipment is based on software-defined-radio technology, it is easy to implement new radio protocols with new software. Software flexibility is important in automotive multimedia applications because of the constant change of media formats and communication standards.
