HD- Taking Compression into Account
Jeremiah Golston
Chief Technical Officer, TI Streaming Media
I’m continually surprised by the applications that are taking advantage of high definition (HD) video. The trade-offs from standard definition (SD) are negligible in some cases, and in other cases HD makes no sense at all. I would not be surprised to see even cell phones requiring some HD encoding support soon. Despite the limited screen size on the phone itself, camera phones will want to capture short high quality video clips to watch later in the home on a higher resolution display.
When start-up company LifeSize first talked to us about bringing HD quality to video conferencing a few years ago, I thought they were crazy. My opinion was the issues facing video conferencing had nothing to do with resolution but instead centered on ease-of-use and bandwidth for good SD transmission. In retrospect, they were exactly right. With HD videoconferencing, the overall experience is much improved. For one thing, meetings flow more naturally with the ability to draw on a physical white board and have the text and diagrams visible to the far end instead of having to use a separate white board application. Additionally, facial expressions are much more discernable in high definition and this is one of the real benefits for adding video on top of a standard conference call. Video conferencing room systems typically already use large screen displays – a natural fit for higher resolution if the compression limits can be addressed.
Raw 1080i60 video has six times and 720p60 more than five times the amount of data as SD video. In terms of basic processing throughput, I/O bandwidth, and memory requirements for video buffering the resolution alone can drive the system requirements up by a factor of six for HD compared to SD. Moreover, HD applications often demand the higher compression efficiency of advanced codecs like H.264. Newer advanced codecs achieve greater compression by employing even more memory and processing so the system requirements become correspondingly even higher. These requirements translate into higher component costs that will diminish over time.
This is something Lifesize took into account and designed around to great effect. They found that 720p30 was currently the best fit for their application. Almost all HDTVs today have 720p max resolution, so encoding at 1080i or 1080p would be a waste. However, I think we will see the video conferencing market move to 1080i or 720p60 over time as a way to offer lower latency. Until then, 720p30 offers the perfect tradeoff.
Normally, digital video is compressed to reduce the enormous bandwidth required, which exceeds 124 Mbps for SDTV broadcast formats and approaches 750 Mbps for 1080i60. Storage is a factor, too, since single-layer DVDs can hold about 4.7 GBytes of data—enough for only short clips of uncompressed video. Single-layer HD-DVD and Blu-ray discs extend storage to about 15 and 25 GBytes, respectively, but they still require a huge amount of compression to hold a full-length HD movie on a disc.
The Main Profile of MPEG-2, the best established and most widely used standard for video compression, can normally provide high-quality compression for difficult content at ratios of around 30:1 for SD and 50:1 for HD, using 4:2:0 color sampling and depending on the source. Since H.264/MPEG-4 AVC, High Profile, roughly doubles this level of compression, the video broadcast and recording industry will be transitioning to the new standard during the next few years. All of the ITU/MPEG standards are lossy, however, so the decompressed image played back is by nature less well defined than the original image before compression. Because the images are in motion, and because the standards are based on a great deal of study about how people perceive images, the loss of image definition is concealed so well that it is generally acceptable provided the compression ratio is not pushed too high.
However, pushing this loss beyond the ~60-100:1 compression ratio supported by H.264’s High Profile risks revealing flaws in the image, and these flaws show up much better with HD displays. If you find the network bandwidth or storage requirements for your application demands compression rates higher than these ratios to support a given flavor of HD, you are likely better off staying with a lower resolution. You may also find that HD displays are not only unnecessary, they may be distracting and even unpleasant to view.
As you can see, the considerations a designer must take into account are extensive and should be tailored to the specific application requirements. However, the applications for HD do extend far beyond the traditional display end equipments typically discussed.