Compression technology
Jun 1, 2008 12:00 PM, BY CARL FURGUSSON
Figure 2. Comparison of 3 compression formats at low bit rates
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Figures 2 and 3 show example PSNR curves for MPEG-4 AVC compression formats at low and high bit rates, respectively. The three formats are 8-bit 4:2:0, 8-bit 4:2:2 and 10-bit 4:2:2. The 10-bit 4:2:0 format was not included in this analysis because it is assumed that 4:2:2 coding will be considered more important for contribution applications than 10-bit coding.
It can be seen that 4:2:2 coding provides picture quality improvements compared with 4:2:0 at bit rates above 4Mb/s whereas, on average, more than 20Mb/s are required before 10-bit 4:2:2 coding pays off. However, for pictures with low spatial complexity, 10-bit coding could improve picture quality at lower bit rates as well. Interestingly, there is no crossover point between 8-bit and 10-bit coding, i.e., 10-bit video codes as efficiently as 8-bit video at low bit rates. Similarly, the crossover point between 4:2:0 and 4:2:2 is at very low bit rates as has been observed in MPEG-2 systems.
MPEG-4 AVC artifacts
Figure 3. Comparison of 3 compression formats at high bit rates
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A key difference between MPEG-2 and MPEG-4 AVC is the type of artifact introduced in the compression process. The industry is familiar with MPEG-2 macroblocking effects, which are not present in MPEG-4 AVC because of the use of in-loop de-blocking filters. However, in MPEG-4 AVC, there is a tendency toward posterization or contouring on large plain areas of the same color.
This effect is masked in MPEG-2 by noise introduced by DCT inaccuracy and the mismatch control introduced to compensate for DCT inaccuracy. This has the effect of introducing dither into the picture, which appears as a small amount of noise on the plain areas. The dither in effect masks the boundaries between macroblocks to which human eyes are extremely sensitive.
Figure 4. Comparisons between MPEG-4 AVC 4:2:2 (8- and 10-bit) and MPEG-2 4:2:2. The result here can be summarized as showing a marked improvement using MPEG-4 AVC 4:2:2 over MPEG-2 4:2:2
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Posterization is caused by the fact that there is only a luma level difference of one or two (in 8-bit terms) between neighboring blocks, but because the entire macroblock is so plain, the eye is very sensitive to this change. When this data is presented to the in-loop de-blocking filter, there is nothing the filter can do to hide this boundary. However, in the 10-bit case, the in-loop de-blocking filter has two extra bits of precision. Therefore, intermediate levels can be created and hence soften the macroblock boundary. This indicates that certainly for contribution applications, 10-bit encoding will provide considerable benefits in masking the contouring artifacts.
This effect is even more apparent in contribution networks, as the source material is unlikely to have significant levels of noise that could serve to mask the contouring effect.
These results are further confirmed in Figure 4, which shows considerable improvements found in the use of MPEG-4 AVC 8- and 10-bit encoding over MPEG-2 and summarized for a particular scene that has large areas of a single color.
1080p broadcasting and contribution networks
A key component in a master control room is the management and monitoring of MPEG encoding.
To date, no broadcaster has announced plans to broadcast in 1080p. There are considerable barriers to doing this, particularly in the management of the uncompressed video. Here it would require studio and networking infrastructure capable of operating in the bandwidth range of 3Gb/s per video channel. Camera and video editing equipment would also need to be suitably upgraded.
In terms of carrying 1080p compressed video across either direct-to-home (DTH) or contribution networks, there is likely to be a delay in this implementation until the prerequisite video decoding technology becomes widely available, economically viable and implemented in decoder devices. It is also most likely that it will permeate the DTH market before being used for contribution and distribution, as was the case for MPEG-4 AVC. There are a number of choices to be made prior to broadcasting in 1080p format, including whether to use MPEG-4 scalable video coding (SVC).
The SD case
There are already several thousand SD MPEG-4 AVC channels in operation worldwide commonly using bandwidths from 1.5Mb/s through 3Mb/s. Very few SD MPEG-4 AVC broadcast encoders are optimized to operate above 5Mb/s.
Figure 5. Bit rate saving of MPEG-4 AVC SD
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Figure 5 shows that above the 6Mb/s operating point, there are additional gains in bandwidth that can be achieved. Again, there appears to be a point of minimum efficiency for SD at around 6Mb/s.
The important point to take into account here is that this is not because of any particular encoder optimization. Rather, it is a result of the fact that the gain in PSNR is constant, and the gradient of the rate-distortion curve reduces toward higher bit rates. Therefore, constant PSNR gains translate to higher percentage bit rate savings at higher bit rates. It is for this reason that it can be seen that MPEG-4 AVC has significant advantages for contribution and distribution applications.
Figure 6. MPEG-4 AVC gains over MPEG-2 for SD
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It has been proven in the past that MPEG-4 AVC has significant gains over MPEG-2 at lower bit rates, and this analysis shows the gains continue at higher rates as well. Figure 6 shows that comparative bandwidth gains continue to be available as the bandwidth increases, as indicated in previous charts. It also confirms that the maximum percentage gains are in the very low bandwidth area. The least are in the 6Mb/s area for SD video, and the maximum absolute bandwidth savings increase at the higher bandwidths. Here only standard definition is used to illustrate the principle, although it is similar for high definition.
Summary
From the above information, it can be seen that MPEG-4 AVC is a technology that can be applied to low-bandwidth applications such as DSNG today. In the future, it will provide worthwhile bandwidth savings at the higher bit rate contribution applications. Moving to 10-bit encoding for HD applications will also provide significant improvements to video quality at these higher bit rates by removing posterization artifacts. Smaller gains are forecast in the adoption of MPEG-4 AVC 4:2:2 AVC, although the gains in chroma quality may still make its use advantageous.
Carl Furgusson is vice president of compression product management at TANDBERG Television.
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