Have we already covered everything?
On closer inspection, we see that the Blu-ray standard specifies not one, but two HD data streams, namely for picture-in-picture (PIP) applications and, in future, for 3D movies. But that’s not all. A further HD signal can be added, namely in the form of ‘BD-J’ objects. The Blu-ray standard even supports interactive graphic content in HD format, which, in extreme scenarios, can produce a third complete HD graphic stream. Things are looking a little tight again if we try to transmit all of this at the same time via MOST.
An additional aspect involves the place within the system at which the various (up to 3) HD video streams are mixed together. Home Blu-ray players and Playstation consoles already contain a Blu-ray controller anyway, where it is required for controlling the drive. The Blu-ray controller also contains the decoder for all the standard formats available on the market, e.g. DVD, CD, MP3, DivX, BD, etc. In cars, basically speaking, the situation is no different, with the BD controller being in the drive. Using the methods just described would require installing a second Blu-ray chip or equivalent in the RSE unit, which would constitute a considerable cost factor. For this reason, in addition to the methods described thus far, it is possible to envisage an alternative method. With this method, the data stored on the medium is decoded directly on the drive, as is also the case with consumer devices. The entire image content -- that is, the main video stream and, if necessary, a second video stream for PIP and menus -- is processed to produce an HD picture. Subsequently, the then combined video stream is compressed and encrypted again, transported via MOST, and decoded and decompressed on the display. During this process, as soon as the stream is decompressed, the resolution of the particular display connected is taken into account accordingly. As a result, although certain losses are sustained on account of the compression due to the principle involved, these are negligible in view of the conditions that prevail in cars (relatively small screens, reduced dynamics and color intensity), provided that an appropriate compression method is selected. This minor drawback has to be seen alongside a correspondingly lower transmission bandwidth and a reduced need for equipment in the RSE unit (or displays). Compared with a full-scale Blu-ray controller, the decoder in the display can be much less sophisticated and thus cheaper. What is more, this method allows any number of displays to be connected.
Synchronization of image and sound
Apart from the picture, the sound is naturally also highly significant. In particular, the synchronization between the image and the sound (‘lip sync’) is extremely important. Due to the fact that light travels considerably faster than sound in a natural environment, the human brain is, to a certain extent, used to the fact that the sound is slightly slower than the pictures. This is the case, for example, when observing a person shouting something to us from far away. In this case, the lip movements speed ahead of the sound. If, however, the sound speeds ahead of the picture signal, this appears extremely unnatural. If observed for longer periods of time, this can also most certainly cause headaches. When adjusting sound and image to each other, transmission via the synchronous MOST network is extremely helpful, because the delay between sound and image is absolutely constant. The MOST network itself does not add any notable delay. This also has the specific advantage that when operating a number of displays within a system, no special measures are required for synchronizing the various displays – the MOST network already incorporates this synchronization and, consequently, renders any further synchronization protocol unnecessary.
Accordingly, the MOST network accomplishes its function as a multimedia backbone also in the age of high-definition video. Scalable solutions for this, including copy protection, are being developed by various manufacturers. It is pleasing to note that in spite of the much-cited automotive crisis, we are still seeing new technical momentum.
Blu-ray Disk reencoded video transmission over MOST
About the author:
Harald Schoepp is vice president marketing of the Automotive Information Systems division at SMSC. He is a founding member of the MOST Cooperation and representative of SMSC in the steering committee.