MOST network technology is now entering its third generation: With MOST150, the age of HD video has now finally arrived in the automotive sector. This would signify a giant step forward – but do 150 Mbps really suffice for the transmission of high-definition video signals?
For a single video stream, the uncompressed transmission of HD video data generates a data rate of around 2-8 Gbps – depending on the frame rate and resolution. It is obvious that 150 Mbps are by no means sufficient for this. But is this really a problem? In fact, it isn’t. This is because, in principle, the problem has already been solved in a different context – i.e. where the data is processed for storage on the Blu-ray Disc. In spite of the seemingly immense storage volume of 50 gigabytes, the storage capacity would only suffice for a few seconds. The secret is called data compression. By means of compression according to the MPEG2, MPEG4 or VC1 standards, the high-definition video data stream is compressed into a volume of a maximum video data rate of 40 Mbps – and, what is more, without any noticeable deterioration in quality.
Would it therefore not be a sensible idea, since the data is already available in a compressed format, to also transmit it in this format via the MOST network and save precious bandwidth in the process? It is precisely this principle that can be employed in the data transmission of Blu-ray videos via MOST.
The transport stream (TS) of an HD Blu-ray video has a volume of around 54 Mbps. It contains both the video signals in HD format and also the multi-channel audio signals perfectly synchronized with each other. Thanks to MOST150, roughly three times the data capacity is now available. One extremely elegant method of transmission therefore involves feeding this transport stream in a suitable way into the MOST network and subsequently tapping it in the drain (e.g. a rear-seat entertainment unit) and decoding it accordingly. Since the data stream contains both sound and images, virtually the very same thing happens in the RSE unit as what normally happens in the Blu-ray player - the images and sound are decompressed and fed to the respective drain, namely the screen or headphones. In consumer devices, this usually happens in a Blu-ray controller that has been specially adapted for this purpose and which also controls the drive. At the output, this chip delivers the fully processed video signal, which is then transmitted by HDMI to what is known as the video drain, i.e. the television/monitor or the projector. Corresponding solutions for MOST based systems in cars are currently in development.
If these sound signals are to be further processed and reproduced in a different place, for example on a central sound amplifier, the audio stream has to be fed back into the network, thus allowing it to be tapped again at the corresponding processing unit, such as the DSP sound amplifier. As such, the video signal is transported with exactly the same quality via the MOST bus as it has been stored on the Blu-ray Disc (BD). In addition, it is naturally important to ensure that the data digitally transmitted in this way, in original BD quality, is not intercepted illegally and used to produce pirate copies. Utter nonsense in cars? Maybe. However, the film industry does not make any distinctions here, with the same strict regulations applying in cars (and incidentally also in aircraft) as they do for digital transmission of digital content via other networks, such as in homes, for instance. And from a technical aspect, the problem has long been solved for MOST. Encryption using the DTCP (digital transmission content protection) method has already been approved by the DTLA (Digital Transmission Licensing Authority) for the transmission of audio and video signals via MOST. This method is also used for the transmission of audio and video signals in the Blu-ray Disc application. Even IC solutions are already available that come with DTCP encryption and decoding facilities already integrated.