Electronic Design

A MOST Promising Solution For HD Video In The Car

With many people spending an increasing amount of time in the car, it’s only natural that they’d like to use their consumer electronics at points during the ride. No doubt, then, automotive infotainment systems will try to meet that demand by offering greater functionality. In turn, these systems become ever-more powerful.

For example, rear-seat entertainment and high-definition video, or DVB-T, in the car are stepping out of pure science fiction and into the real world. One major challenge, though, is that as the number of applications multiplies, so do the requirements to the network that performs the communication between the devices and the user.

To support more complex video applications, the latest generation of MOST (Media Oriented Systems Transport) technology—MOST150—contains an isochronous transport mechanism in addition to higher bandwidth, for a data-transfer rate of 150 Mbits/s. This allows for the transmission of highly bandwidth-efficient audio and video signals that aren’t saddled with the overhead for addressing, collision detection/recovery, or broadcast.

Consequently, a MOST network can simultaneously transmit multiple high-definition (HD) video streams, single-definition (SD) video streams, and multichannel surround sound with premium quality of service. Isochronous channels are provided to support streams that aren’t synchronized to the MOST frame rate.

A typical use case is the transport of MPEG streams over a MOST network, since MPEG streams generally use variable bit rates. Now, thanks to the new isochronous-transport feature, cost-effective, extensive video applications are possible. In fact, MOST150 also supports HD audio and video content from Blu-ray discs or HDTV (Fig. 1).

In addition to image and sound quality, the synchronization between image and sound (lip sync) is essential. When adjusting audio and video to each other, transmission via the synchronous MOST network is a perfect match because the delay between sound and image is absolutely minimum and at the same time constant.

Another advantage is that when operating a number of displays within a system, no special measures are required to synchronize the various displays. That’s because the MOST network already incorporates this synchronization, rendering any further synchronization protocol unnecessary. These technology qualifications pave the way to rear-seat entertainment systems, digital sound amplifiers, TV tuners, video displays, and other entertainment via the MOST network.

Is 150 Mbits/s Enough For HD Video Transmission?

Depending on the frame rate and resolution, the uncompressed transmission of HD video data generates approximately a 2- to 8-Gbit/s data rate for a single video stream. Meanwhile, the MOST network allows the transfer of 150 Mbits/s, which doesn’t seem to be sufficient in any way.

So, to meet this challenge, we have to look at it in a different context—right at the device where the data is processed for storage on the Blu-ray disc. Despite the seemingly immense 50-Gbyte storage volume, the storage capacity would actually suffice for only a few seconds. The solution, then, is data compression. If compressed according to MPEG2, MPEG4, or VC1 standards, the high-definition video data stream will be condensed into a maximum video data rate of 40 Mbits/s. And, remarkably, there’s no noticeable deterioration in quality.

Thus, the best option for Blu-ray videos is to transmit compressed data via the MOST network, which also saves precious bandwidth in the process. Thanks to MOST150, roughly three times the data capacity is now available. The transport stream (TS) of an HD Blu-ray video has a volume of around 54 Mbits/s. It contains perfectly synchronized video signals in HD format and multichannel audio signals.

One method of transmission, therefore, involves feeding this transport stream into the MOST network in an optimized way, meaning without heavy additional protocol overhead. Here, the fact that MOST, as the name says, is optimized for (streaming) media applications. Then it’s tapped in the sink, such as a rear-seat entertainment unit (RSE), and decoded accordingly. Since the data stream contains both sound and images, the RSE unit does virtually the same job as the Blu-ray player—images and sound are decompressed and fed to the correct sink, namely the screen or headphones.

In consumer devices, this process usually happens in a Blu-ray controller that’s been specially adapted for this purpose; one that also controls the drive. At the output, the controller delivers the fully processed video signal, which is then transmitted via HDMI to the “video sink,” i.e., the television/monitor or the projector. Similar solutions for automotive MOST-based systems are currently in development (Fig. 2).

A Closer Look At The Blu-ray Standard

The Blu-ray standard specifies not one, but two HD data streams: one for picture-in-picture (PIP) applications and, as can already be seen in today’s first applications, another one for 3D movies. In addition, one more HD signal can be added, in the form of “BD-J” objects. The Blu-ray standard also supports interactive graphic content in HD format, which in extreme scenarios may produce a third complete HD graphic stream.

It seems that transmitting all of this simultaneously via MOST could become a bit tight. However, a further aspect involves the system interior, where the various HD video streams (a maximum of three) are mixed together.

Home Blu-ray players and PlayStation consoles already contain a Blu-ray controller, since it’s needed to control the drive. The Blu-ray controller incorporates the decoder for all of today’s standard formats, such as DVD, CD, MP3, DivX, BD, etc. The situation is very similar for automobiles, with the Blu-ray disc controller being in the drive. Using these methods would require installing a second Blu-ray chip or equivalent in the RSE unit, significantly raising cost.

Besides the methods described so far, there’s another to consider. Basically, stored data would be decoded directly on the drive, as is the case with consumer devices. The complete image content—including the main video stream and, if required, a second video stream for PIP and menus—would produce an HD picture. Subsequently, the resulting video stream is compressed and encrypted again, transmitted over MOST, and decoded and decompressed on the display. As soon as the stream is decompressed during this process, the resolution of the particular display connected is taken into account.

Due to the principle involved, the compression causes certain minimum losses. However, these are negligible in view of the conditions that prevail in cars (relatively small screens, reduced dynamics, and color intensity), provided that a proper compression method is chosen.

This slight drawback must be weighed against the correspondingly lower transmission bandwidth and the reduced amount of equipment required 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. In addition, this method makes it possible to connect any number of displays.

Digital Transmission Content Protection

Perfectly synchronized and optimized sounds and images aren’t the only key factors for designers. It’s equally important to ensure that the data digitally transmitted in original BD quality isn’t intercepted illegally and used to produce pirate copies. The same strict regulations apply in cars (and in aircraft) as they do for similar networks used in the home.

From a technical standpoint, the problem has long been solved for MOST. MOST was the first network to be fully approved by the DVD Copy Control Association (DVD CCA) to carry digital transmission content protection (DTCP) protected content. This was made possible by adapting DTCP to the MOST standard. By doing so, HD-DVD and Blu-ray content on MOST is also supported, since Advanced Access Content System (AACS) licensing allows DTCP-protected digital outputs.

DTCP requires source and sink devices to authenticate each other. In addition, multimedia streaming data must be encrypted before sending it over a digital network. A sink device, therefore, has to be able to decrypt protected digital content. DTCP on MOST also supports point-to-multipoint connections.

The Digital Transmission Licensing Authority (DTLA) has also approved encryption using DTCP for audio and video transmission via MOST. On top of that, this method is used for the transmission of audio and video signals in the Blu-ray Disc application. In fact, several available IC solutions already integrate DTCP encryption and decoding facilities.

Consequently, the MOST network is establishing itself as a multimedia backbone in the age of high-definition video (Fig. 3). Scalable solutions, including copy protection, are being developed by various manufacturers. It’s pleasing to see that, in spite of the much-cited automotive crisis, we’re still experiencing new technical momentum.

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