A Time to Decompress

IP video can feel easy delivering higher quality digital video with H.264

• By Barry Keepence
• Apr 13, 2007

THE latest official video compression standard, H.264, follows on from the highly successful MPEG-2 and MPEG-4 video standards and offers improvements in both video quality and compression. Where IP video systems will see the most benefit is in the ability to deliver the same high-quality, low-latency digital video with savings of up to 50 percent on bandwidth and storage requirements. To put it another way, it delivers significantly higher video quality for the same bandwidth.

What is H.264?
H.264 is a video codec (compressor and decompressor) standard. A video codec is designed to compress and uncompress digital video in order to reduce the amount of bandwidth required to transmit and store the video. This is needed as the raw data rate of uncompressed CCIR-601 active digital video (720 x 480 pixel, 4:2:2 video at 30 fps) is in excess of 158MBps—more than 300 times the capacity of a 512 KBps ADSL connection and just over one hour recording on an 80 GB hard disk.

Simply scaling the video to CIF resolution (352 x 240 pixel, 4:2:0 video at 30fps), and compressing with standard utilities such as WinZip or gzip could achieve 10:1 compression. However, at least 300:1 compression is needed to stream live video over an ADSL connection and to achieve 300 hours recording to an 80 GB hard disk. This level of compression can be achieved with H.264.

Implementing the Standard
It is important, before looking at H.264 in more detail, to understand the difference between comparison and actual implementation of a standard. The two are different, and it’s misleading to say H.264 provides better video quality than MPEG-2.

H.264 is a video compression standard. The H.264 standard defines the syntax of a compliant bit stream to which a compliant decoder must conform exactly, implementing all the necessary tools defined by the standard in order to decode the bit stream.

An H.264 encoder can implement a subset of the syntax defined by the standard, providing it produces a compliant bit stream. Various implementations and algorithms within the encoder also are not defined by the standard and are created by the designer of the encoder. As such, H.264 encoders from different vendors will produce streams of differing quality for the same bit rate.

H.264 provides a richer syntax and toolset than MPEG-2 and, as such, allows the possibility of implementing a superior video encoder that can generate higher-quality video for the same bit rate and can generate the same quality video at a much lower bit rate.

This can be demonstrated using the reference software encoder available from the International Standards Organization. The H.264 reference encoder allows a user to select which tools to use to encode a particular video sequence.

However, it also is clear that the addition of tools comes at the expense of increased complexity—in this case, measured by the execution time of the encoding process. This increase in complexity often causes some tools or algorithms to be omitted from the design of an H.264 encoder.

Relationship to MPEG-4 Part 2
MPEG-4 (ISO/IEC 14496) is a collection of standards defining the coding of A/V objects. The collection is divided into a number of parts describing video compression and audio compression standards, as well as system level parts. The video compression standard found in many products today is the traditional DCT-based, MPEG-4 Part 2 (ISO/IEC 14496-2) standard.

The H.264 video compression standard has been incorporated into MPEG-4 as MPEG-4 Part 10 (ISO/IEC 14496-10). This means MPEG-4 now has two video compression standards available. However, these two video compression standards are non-interoperable, with each standard using different methods to compress and represent the data. An MPEG-4 Part 10 (H.264) decoder cannot decode an MPEG-4 Part 2 bit stream, and vice versa.

IP Video and H.264
The best way to see the benefits of H.264 in IP video solutions is to look at an actual implementation of the standard.

Frames of video are captured from the camera and sent to the internal H.264 encoder to be compressed. Each frame of video is then compressed in one of two ways: as an I-frame or as a P-frame.

An I-frame is a video frame that has been encoded without reference to any other frame of video. A video stream or recording will always start with an I-frame and will typically contain regular I-frames throughout the stream. These regular I-frames, also called intra frames, key frames or access points, are crucial for random access of recorded H.264 files, such as with rewind and seek operations during playback. The regularity of these I-frames is known as the I-frame interval; however, the disadvantage of I-frames is that they tend to be much larger than P-frames.

P-frames are motion-compensated frames. The encoder makes use of the difference between the current frame being processed and a previous frame of video, ensuring that information does not change or that a static background is not repeatedly transmitted. Unlike purely difference-based codecs, such as delta-MJPEG, H.264 not only looks for differences, but searches for motion that has occurred in the video. This means that motion-compensated codecs will typically outperform simple difference-based codecs when there is motion. The process of searching for motion is known as motion estimation.

Within the codec, the motion estimation unit is one of the most computationally expensive parts and critical to the performance of the H.264 encoder. Motion estimation is a complex procedure and often encoders, especially real-time software or DSP-based encoders, will use reduced search areas or use a restrictive search algorithm in order to achieve real-time performance. However, this often can result in poor-quality video and significantly reduced compression.

Compared to MPEG-4, H.264 can achieve savings of typically between 20 and 25 percent in bandwidth usage and in excess of 50 percent during periods of scene inactivity, such as when there is no moving traffic. Not only does this reduce the overall bandwidth requirements of the IP video system, but more importantly, it can significantly reduce the amount of storage required for recording the video—often one of the most expensive items in the system.

Hardware Considerations
It is clear from looking at how H.264 can be implemented that the demands on the processing power of the codec are significant if the full range of features are used and the full benefits of the technology are to be realized. H.264 is a general-purpose video compression standard not specifically designed for digital CCTV applications. However, by using a custom FPGA-based design, the necessary processing power can be provided and the design can be tailored for CCTV applications. For example, extra compression can be achieved when there is low activity in the video—a situation common in many surveillance applications. The custom FPGA approach has a number of other benefits:

• High-quality video can be maintained during fast-moving activity without frames being dropped, regardless of bit rate and motion. This is paramount in applications such as casino gaming table surveillance.

• Low-cost, high-performance encoding of 4 CIF 30 fps video that is fully compliant with H.264.

• Field upgrade to existing installations as compression standards advance.

• Real time analytics algorithms can be executed in high-performance-dedicated hardware rather than in software. Doing this at the edge of the network—at the camera—makes for a truly scaleable solution.

H.264 offers significant benefits to the user and system designer. However, the extra complexity of the implementation comes at an extra cost. So H.264 will not replace MPEG-4 overnight, but it provides a wider choice of solutions to the end user. Even though MPEG-4 and H.264 compression standards are not compatible, IP networks allow both systems to co-exist.

Tools	Bit rate (KBps)	Total execution time of required coding tools only (relative)
I-frame-only encoding	2,279	1
I and P frames but with no motion estimation (0 search range)	1,055	1.5
I and P frames with a +/-16 search using a simplified search algorithm	453	14
I and P frames using a full-search algorithm with differing block-size motion compensation	421	56
This table shows that the more tools and algorithms that are used, the greater the compression achieved for the same quality of video.