Video Interface Options & Tradeoffs

For automotive infotainment applications, the video interface solutions can be divided into a number of categories. First, the solution could be wireless or wireline. However, considering that the primary application in the vehicle is for stationary displays and sources, and the fact that a wireless system could introduce issues of interference, the need for more complicated and expensive interfaces to support a dynamic channel, and the fact that wireless links are typically more limited in bandwidth due to regulatory constraints, the consensus of the automotive engineering community is that currently either wireline or optical fiber should be used. Table 3-1 provides an overview of the advantages and disadvantages of choosing a wireless, wireline, or optical fiber as the physical layer of an automotive video link.

Once the designer has limited his focus to wireline or cable solutions, another segmentation to consider is the type of electrical interface: analog versus digital. And for a digital link there is the additional consideration of whether or not the data should be compressed or uncompressed, or equivalently, how much bandwidth the electrical interface should be required to support. These design choices are graphically presented in Figure 3-1, and Table 3-1 provides a summary of the advantages and disadvantages

Figure 3-1: Video Transmission Alternatives

Compressed data digital networks have many advantages, including high picture quality, the potential for significant cable size and weight savings, and reduced bandwidth requirements due to compression of the video and audio data streams.

Source coding, or compression, can significantly reduce the required data rate to support video and audio transmission. For example, uncompressed NTSC D1 Format (720x480 pixels, 4:2:0, 30 frames/s) video has a data rate of approximately 125Mbps, and uncompressed digital audio (44.1K samples/s, stereo with 20 bits/channel sampling) can have data rates that exceed 1.7Mbps. To reduce this required data rate, typically requires preprocessing of the original source data by means of linear transforms such as the Discrete Cosine Transform (DCT). DCT based source coding algorithms such as MPEG can reduce the typical data rates of standard definition video to the 2 to 8Mbps range, and in the case of MP3, typical digital audio data rates can be reduced to 96 to 384Kbps. However, these reduced data rates do not come without a penalty. The key disadvantage of networks which rely on compressed audio/video transmission is increased node cost due to the requirement that a source encoder and/or decoder be present. But another equally important issue is forwards compatibility with new video compression standards. Although MPEG-2 source coding is common today, the future may see new standards such as MPEG-4, WMV9, and H.264. Unless the in-vehicle network node source decoders can accept that format video, or unless a transcoding requirement is placed on the source device, then compatibility with the large variety of video sources formats can become an issue. For these reasons, networks such as MOST and IDB1394 which require source coding of video due to limited bandwidth, are not as attractive as baseband point to point video interfaces.

In the case of baseband digital point to point video interfaces, the choices with a significant market presence are limited to LVDS family devices, HDMI, or GVIF. Each interface standard is capable of transporting uncompressed video for all classes of in-vehicle LCD displays, including XGA. However, there are some differences as shown in Table 3-3. In the case of LVDS, the key limitation is the lack of support for content protection. This is a “must have” feature for digital representation of DVD video content. In addition, LVDS family products typically achieve higher data rates via the use of multiple channels and a separate clock line. This leads to large cable sizes and significant added weight. HDMI uses transition minimized TMDS as its electrical standard, and has similar problems with cable size and weight. While the advantage of HDMI is that it is widely adopted in consumer grade electronics devices, for automotive applications HDMI has the additional problem that automotive quality grade (QS9000 / TS16949) IC’s and modules supporting this format are not generally available.

GVIF, on the other hand, does support content protection (HDCP) for delivery of uncompressed video from source to sink. In addition, due to the small connector and thin cable consisting of one shielded twisted differential pair, GVIF interface cables are small size and low weight. GVIF IC’s are also available with applicable automotive quality standards. Figure 3-2 shows the difference between an LVDS family video interface and GVIF, and Figure 3-3 shows a picture of an HDMI versus a GVIF cable.

Figure 3-2: LVDS Family Interface versus GVIF Block Diagrams

Figure 3-3: GVIF versus HDMI Cable and Connector

An overall assessment of the tradeoffs between competing video interface standards for in-vehicle distribution of video has led to the conclusion that Sony’s GVIF is an attractive solution. GVIF makes use of a small connector and a thin cable that is well suited to automotive cable harness size constraints and weight limitations. GVIF also exhibits high noise immunity and low EMI, providing the designer an assurance that target bit error rates will be achieved and that unintended interference with other equipment will not occur. The ten meter supported interface distance easily meets the bulk of the fleet requirements.