Video compression in transition
Sep 1, 2009 12:00 PM, By Ian Trow
MPEG-2 and AVC can coexist in modern facilities.
Figure 1
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Broadcasters and service providers have a wide variety of options to consider when choosing the correct compression strategy for the contribution and distribution of broadcast content.
With significant investment in MPEG-2, broadcasters are keen to see vendors addressing the need for greater SD compression efficiency for distribution applications. Higher channel densities and the trend for larger flat screens have resulted in renewed interest in re-evaluating MPEG-2 for further compression gains in an environment where bandwidth is both scarce and expensive. HD distribution started with MPEG-2 over cable and satellite, but many HD services are now possible as a result of the compression efficiencies attainable with MPEG-4 AVC.
Current compression systems now demand multistreaming and file-based capability alongside real-time delivery. This article will investigate the issues surrounding the selection of a compression standard, optimum operational bit rate and ideal system configuration.
MPEG-4 compression
Figure 2
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When first introduced, the MPEG-4 compression standard was heralded by many as an MPEG-2 replacement. Since then, a series of profiles and levels has been developed within the MPEG-4 standard that successfully addresses many of the shortcomings of MPEG-2. These include greater compression ratios to facilitate HD carriage, resilience to errors introduced by packet-based IP distribution, and the application of MPEG techniques for mobile, handheld and PC streaming applications.
MPEG-4 has been refined to focus on commercial areas with requirements that eclipse the capabilities of MPEG-2. The result is MPEG-4 Part 10, more commonly known as H.264 or Advanced Video Coding (AVC). In terms of compression, H.264 is now viewed as a possible successor to MPEG-2 in broadcast distribution applications. However, it should not be assumed that applications will switch from MPEG-2 just to track the latest in compression technology. Many compression applications will remain MPEG-2 for the foreseeable future because of the large installed base of set-top boxes. Additionally, for some broadcasters, the gain associated with upgrading the compression scheme does not yield the benefit to offset the downside of upgrading in terms of capital outlay and retraining.
MPEG-4 AVC was initially developed for low-bit rate multimedia applications. This made the standard more robust and appropriate for IP-based networks when compared with predecessor legacy standards like MPEG-2. This advantage carries through to the application of AVC in contribution applications, and consequently makes bandwidth more plentiful and workflow shorter.
Beyond MPEG-2 compression
Figure 3
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MPEG-2 compression is a tough act to follow, so it should not be assumed that broadcast applications will no longer use this standard. Much of the research effort in video compression has been directed toward improving MPEG-2 to exploit the advantages brought about by more capable processing technology.
Where MPEG-2 has met significant challenge is in bandwidth-hungry HD contribution applications, where the savings offered by a move to a more efficient scheme are seen as commercially essential.
In terms of compression, the following are major factors:
A significant move toward HD programming;
Demand for greater compression efficiency;
Greater use of file-based production techniques;
Availability of more transmission bandwidth;
A desire for reduced workflow; and
Support for multiple playout platforms.
These factors make the selection of a suitable compression scheme for contribution far more complex than when MPEG-2 was introduced into this environment.
The bit rates used in a typical production chain are shown in Figure 1 on page 42. The rates shown are typical of those used in HD 1080i/720p environments with distribution over MPEG-2. The main development in terms of bit rate has come about because of a desire to greatly reduce the emission bandwidth using H.264 to rates of approximately 4Mb/s-12Mb/s.
In terms of mastering quality, 1080p acquisition introduces the prospect of 3Gb/s for full frame-rate carriage. So even though there is more capability to carry higher payloads, for the vast majority of applications, there is still a requirement for efficient video compression to bring the bit rate down to manageable levels. So, what constitutes a manageable bit rate in a contribution environment, and what compression standard can be used?
Both MPEG-4 AVC and MPEG-2 have been applied to multiple markets in the typical production chain. (See Figure 2.) This usage in multiple markets has significant workflow benefits in reducing the complexity of each production stage.
In most areas, direct-to-home (DTH) deployments have been based on MPEG-2, creating an established infrastructure and a significant legacy for which replacement of MPEG-2 by AVC equipment is not a practical solution. Broadcasters and service providers need to work further within the constraints of MPEG-2 on infrastructure that needs replacement or upgrade to a more efficient implementation.
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