Progress in compression technology is offering broadcasters new opportunities in production, content management and transmission of SD and HD content.
High Definition Technology Update spoke with Matthew Goldman, director of technology for TANDBERG Television, about the impact of advanced data compression algorithms, such as MPEG-4 AVC (H.264), and Windows Media 9 Video (proposed SMPTE standard VC-1).
Goldman, who helped to write the MPEG-2 standards and is working on the standardization of VC-1 with the Society of Motion Picture and Television Engineers, offered his insight on a range of related topics. In this edition, we focus on transmission opportunities.
HDTU: What’s the significance of emerging advanced data compression algorithms for broadcasters?
Matthew Goldman: One of the nice things about the newer technology that applies to everything is the fact that in general the new technology delivers a 2-to-1 improvement in compression over MPEG-2 when it comes to video. We (industry experts) are all learning still with MPEG-4 AVC and Windows Media 9 Video implementations, but we know SD is going to be in the area of 2x bit rate savings and HD in the range of 2x to 3x when compared to MPEG-2 video.
HDTU: That must translate into financial savings.
MG: Of course. If you are leasing bandwidth on satellite or telco lines, it is a huge gain. You can get two times the content, or half the cost for your backhaul links or a combination of the two of them.
If you are using satellite, there’s an even bigger gain. Advances in channel coding have occurred, such as turbo coding or similar methods. For example Digital Video Broadcasting-Satellite version 2 (DVB-S2) will increase the channel coding efficiency more than 30 percent. So a combination of 30 percent channel coding savings and 2x to 3x times gain in video (with advanced data compression), and there could be a huge savings in backhaul or SNG - easily a 3x savings overall.
HDTU: Given that the commission mandates an MPEG-2 transport stream for broadcasters’ DTV transmissions, how can they take advantage of greater compression with their signal emissions?
MG: For emissions, it’s a little more difficult. For free over-the-air TV, the FCC Report and Orders mandate that the main video service has to be MPEG-2. But there’s a belief by some broadcasters that additional services are not required to use MPEG-2 video and therefore may use bandwidth not required for the main video service for other purposes.
Within the 8-VSB transmission there is 19.4Mb/s of DTV channel. Since there must be accommodation for audio and control information as well, the maximum video bit rate is approximately 18.5Mb/s.
A few years ago, you needed all of that for HD. The FCC did not mandate HD; it mandated DTV. Regardless, with the advances in video coding it no longer takes 18.5Mb/s. You can get good quality HD using a lower bit rate.
For some broadcasters, their position is that they can use that spare bandwidth any way they want. The FCC has required them to broadcast a free over-the-air service equivalent to their NTSC service as DTV. But the spare bandwidth – that’s where things get interesting.
Some broadcasters plan to use that spare bandwidth for additional DTV services, using MPEG-2 video. Others believe they can pick a different video format because it’s not meant for free over-the-air service. As long as they have something that’s compatible with the DTV receiver – that is, doesn’t cause fielded receivers to malfunction – they can use the rest of the bandwidth for other purposes.
HDTU: So MPEG-2 is flexible enough to accommodate advanced data compression technology subsequently developed?
Yes, the transport layer – known as the MPEG-2 Transport Stream – contains in-band control information that enables a receiver to determine the contents of the transport. In the case of terrestrial emissions broadcasting, the ATSC Program and System Information Protocol (PSIP) expand the “control” information. Think of it as a directory, a way of locating what is needed to find the main TV channel and ignoring other things that are not needed for the free over-the-air television service.
You may have multiple programs in the MPEG Transport Stream. The MPEG-2 receivers will recognize some programs, and others they may ignore, if the format is not recognizable. The latter can be used for a pay service, and that doesn’t have to be MPEG-2 video. As long as the bit stream follows the syntax rules for the MPEG-2 Transport Stream and ATSC PSIP, fielded DTV receivers will not fail.
As such, while a normal DTV receiver would ignore the additional service, a special receiver that recognizes the format will notice the additional service and be able to decode the MPEG-4 AVC or VC-1 data stream contained within the same MPEG-2 Transport Stream as the main DTV service.
This is one example of how to use over-the-air transmission for these types of services. Other ways are possible. In fact, over the past year there were some announcements of planned premium over-the-air broadcast services. Two come to mind: USDTV and Emmis Communications. These systems may be using a mechanism similar to what I described here.
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