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Cure for concatenation

Mar 1, 2009 12:00 PM

Use precision filter technology to preserve 4:2:0 chroma integrity in concatenated operations.

Figure 1. Degradation of the 4:2:0 chroma component in concatenated operations
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Since the early days of transcontinental and intercontinental networking of TV content, several intermediate communication links have generally been required to complete end-to-end delivery. This is referred to as “concatenation” and has been an area of significant attention regarding the maintenance of video and audio quality and integrity. In earlier analog systems, demodulation and remodulation of the TV signal at each en route microwave or satellite terminal contributed to signal degradation (particularly video signal-to-noise), which, in many instances, reduced the image at the point of delivery to less than that normally acceptable for broadcast.

The transition to video compression and digital networking has created new challenges in mitigating the effects of degradation caused by concatenation in the multiple encode-decode process associated with digital turnaround over satellite, wireless and terrestrial links. One particular problem is maintaining integrity of the 4:2:0 chroma component where an HD-SDI interconnect is used between concatenated decoders and encoders. This relates equally to both the widely used MPEG-2 and newer MPEG-4 (H.264/AVC) codec standards. (See Figure 1.)

Figure 2. Comparison of 4:2:2 and 4:2:0 formats
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HD-SDI is inherently a 4:2:2 interconnect, which requires that a 4:2:0 stream be upsampled and then downsampled at each digital turnaround. This can, within as little as four to five concatenated links, result in visual blurring of the color image to the point where the quality and integrity of HD content can be severely compromised.

This is of particular importance in applications such as HD digital electronic newsgathering, where the use of lower data rate encoding and 4:2:0 chroma sampling can provide operational and economic advantages. In HD-DENG, a significant number, possibly as many as 10 or more, of concatenated operations can take place before final delivery to network affiliates and others.

Chroma encoding formats

The two most widely used chroma encoding formats in broadcast TV applications are 4:2:2 and 4:2:0. (See Figure 2.)

Figure 3. Conventional and non-degraded 4:2:0 concatenated links
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The original HD 1080i picture shown in Figure 3 has 1920 pixels in the horizontal scan and 1080 pixels in the vertical. The components of the picture comprise the luminance, or Y component, plus the two U and V chroma components. It is commonly accepted that the human eye is less sensitive to chroma than it is to luminance, which enables chroma resolution to be reduced to less than that for luminance in the encoding process.

In a 4:2:2 format, the two chroma components are therefore downsampled by a factor of two from the horizontal luminance component, resulting in a pixel ratio of 960 × 1080; whereas in the 4:2:0 format, the U and V chroma components are downsampled by a factor of two in both the horizontal and vertical, resulting in a pixel ratio of 960 × 540.

Chroma sampling and MPEG-4

The recent introduction of the MPEG-4 (H.264/AVC) compression standard has delivered yet another improvement in digital network use. Most MPEG-4 content can be carried in less than 50 percent of the bandwidth required for similar MPEG-2-encoded content. MPEG-4 HD video performance at encoded data (bit) rates below 10Mb/s is generally accepted as sufficient to meet many broadcast TV operations, including HD-DENG.

Figure 4. Interoperability with conventional MPEG-4 decoders
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For lower encoded video data rates, those at or below 10Mb/s in MPEG-4 (H.264), 4:2:2 chroma sampling may not produce any significant improvement in performance compared to that of 4:2:0. It also requires more bandwidth for a specific encoded data rate. Content encoded at 10Mb/s 4:2:0 requires approximately 20 percent less bandwidth than 4:2:2-encoded content.

This presents both an opportunity and a challenge: whether to use 4:2:2 chroma sampling and accept about a 20 percent increase in bandwidth cost, or to use the more efficient 4:2:0 format with potential chroma degradation in concatenated links.