Backhaul contribution

Advances in RF modulation and video compression improve the quality of backhaul.
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With the accelerating spread of MPEG-4 AVC and second-generation DVB modulation for both SD and HDTV distribution to the home (DTH) in many markets, is backhaul of sports and news contributions back to the studio advancing at the same rate?

It depends on the market, as the rollout of HD is the real driver for change. In terms of satellite backhaul, up until the last two years, the rate of adoption of MPEG-4 AVC and DVB-S2 has been at a much slower pace in the contribution market. This was in part because there was not a great enough incentive initially to upgrade to MPEG-4 AVC encoders and DVB-S2 modulators.

In 1997, the DVB-S standard became the standard for digital satellite broadcasting. This specified MPEG-2 compression, and for the RF part, quadrature phase shift key (QPSK) modulation along with a strict error correction format. In 1998, the DVB-DSNG standard expanded the modulation schemes to include 8-PSK and 16-QAM. These higher modulation schemes are more efficient, but for every step up in modulation scheme, almost double the uplink power is required for a given downlink antenna size. For SNG, because of this additional power requirement, modulation schemes beyond QPSK did not gain traction for another decade.

MPEG-4 AVC, the true successor to MPEG-2, was finalized in 2005. While MPEG-4 AVC obviously has its roots in MPEG-2, it is really a step change in the processes used, achieving significant improvements in quality compared with MPEG-2 for a given data rate. However, although the Moving Picture Experts Group includes a wide range of manufacturers that contribute to the development of a standard, it still takes time for the final result to become implemented in real products. MPEG-4 AVC is primarily focused on consumer applications rather than backhaul, so products for the contribution market lagged behind a few years.

Backhaul needs a constant bit rate

A significant difference in backhaul operations compared with DTH distribution is the use of constant bit rate (CBR) coding rather than variable bit rate (VBR) coding. VBR is used for DTH transmission where there are a bundle of services within a multiplex. Statistical multiplexing optimizes bandwidth usage per channel as the data demand can vary on a frame-by-frame basis for any channel in the stream. It therefore makes the most cost-efficient use of data bandwidth on a service-by-service basis.

But the backhaul market operates differently, requiring transport streams at a single continuous rate — CBR coding. Firstly, a significant number of carriers are only carrying a single program channel (SCPC) operating within a fixed bandwidth slot on the satellite. Secondly, even on events where the uplink is operating in multiple channels per carrier (MCPC) mode, there is still often an overriding demand to deliver the highest quality possible, thus requiring CBR coding. Thirdly, contribution bandwidth is sold in fixed bandwidth slots.

DVB-S2

DVB-S2, the second generation of DVB-S, was also finally adopted as a standard in 2005. It is intended to bring the amount of information that could be conveyed close to the Shannon limit — the holy grail for information technologists, where the ideal code would minimize the number of error correction bits while maximizing the chances of correcting errors. Broadly speaking, the inner convolutional error correction coding in DVB-S was changed to low density parity coding (LDPC), and the Reed-Solomon outer coding based on Bose-Chaudhuri-Hocquenghem (BCH) coding was discarded for a different form of BCH coding. The result is a theoretical improvement in efficiency of 50 percent to 80 percent if all the benefits DVB-S2 offers are employed.

The DVB-S2 standard expands dramatically on DVB-S. (See Figure 1.) With either DVB-S or DVB-DSNG, users are locked into using a single modulation scheme for a transport stream. It can be QPSK, 8-PSK or 16-QAM with a given forward error correction (FEC) ratio, but it cannot be changed once transmission has commenced. DVB-S2 brought more tools, most significantly adaptive coding and modulation (ACM), designed for distribution of a broad range of services to the home. Fundamentally, ACM allows both the modulation mode and FEC to be changed on the fly.

This ability to dynamically and independently select both modulation schemes (QPSK, 8-PSK, 16-APSK and 32-APSK) and FEC rates on a frame-by-frame service basis in a transport stream is potentially useful. The aim of ACM was to allow a multiplexed transport stream containing many different services to have its characteristics tailored to the demands of the moment, both in terms of the content and also in environmental conditions at the receiver. If the noise floor level at the receiver increased — due to a rainstorm, for example — a return link from the receiver could automatically request both the distribution encoder and the DVB-S2 modulator to compensate, thus maintaining the link with no breaks with high levels of noise, albeit at reduced quality. If enough receivers made the request, an automatic decision could be made to alter the signal parameters accordingly to compensate for increasing errors. This is a theoretical scenario; few if any broadcasters have actually put such a system into effect, but it is the ultimate aspiration of DVB-S2. The return channel is not defined in DVB-S2, so it could be via anything from a phone line to the Internet.

In a backhaul application, this scenario is easier to manage, as a single SCPC signal could be modified on the fly through feedback on the return channel from the receiving stations to either alter the FEC rate to maintain signal reception, adjust the base information rate (although potentially reducing quality), or both.

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By 2007, MPEG-4 AVC and DVB-S2 products had developed to the point that they could be used for the backhaul market. But MPEG-4 AVC equipment vendors have to compete with a large established user base of legacy MPEG-2 equipment. To begin with, despite strong arguments in favor of migrating to new equipment to achieve bandwidth savings claimed to be up to 50 percent, the backhaul market was not widely convinced. It was also hindered by a lack of professional receivers and serious interoperability issues. The biggest obstacle MPEG-4 AVC has to overcome is latency — the coding delay, which can introduce awkwardness on live two-ways between the studio and the remote location. The higher the data rate, the better the quality but the greater processing and hence latency. Early implementations of MPEG-4 AVC encoders had latency figures in the 400ms-600ms range, but these are steadily decreasing and are currently below 300ms for news-quality contribution. And, of course, this coding delay is in addition to the inherent satellite uplink/downlink delay introduced by a signal traveling 72,000km.

The economic recession naturally put pressures on minimizing satellite usage costs, and the carve-up of satellite transponders into slots suited to “traditional” MPEG-2/QPSK signals was increasingly questioned. The first step in the backhaul market came with use of 8-PSK for MPEG-2/DVB-S transmissions. This was made easier with a new generation of more sensitive satellites, which helps 8-PSK transmission. The conventional 9MHz slot was halved to 4.5MHz to operate with 8-PSK modulation to suit news contributions, while still offering about the same quality as 9MHz QPSK. This coincided with MPEG-2 encoders using the latest algorithms having reached the pinnacle in encoding quality at lower data rates, so it was no longer necessary to encode baseband information at 8.448Mb/s. An acceptable quality could be achieved well below 6Mb/s.

HD backhaul

The next push came with the accelerating spread of HDTV. While MPEG-2 has been used for HD contribution for several years in some markets for high-profile pay-per-view sports and events, its usage had been relatively small across the global contribution community. In the last two years, the accelerating rollout of HD DTH channels in the most prosperous markets has increased the demand for HD contribution. HD is notoriously bandwidth hungry (as it would be, based on a basic SDI information data rate of 1.485Gb/s), and even compressed information data rates can range up to 60Mb/s, depending on the demands of the broadcaster. The transport stream signal needs to be delivered from the field to the studio in the highest quality to allow for ensuing post-production processes; thus, some broadcasters demand high base information rates.

Using MPEG-2/DVB-S, this could lead to whole transponders being required to deliver a single channel back to the studio. For example, in a 36MHz transponder using DVB-S running with minimal error correction (QPSK, 7/8 FEC), the maximum possible information rate is 48Mb/s; but with DVB-S2 running at 32-APSK and again minimal error correction of 9/10 FEC, the rate increases to 133Mb/s — over two and half times greater. This can carry up to six MPEG-2 HD channels depending on the required base information rate (though it should be emphasized that using 32-APSK for backhaul is unlikely). More significantly for backhaul, the earlier assertion that moving to higher modulation schemes requires greater uplink power can be negated with DVB-S2. For example, a DVB-S signal modulated as 8-PSK requires an increase in power of about 2.5dB for a given receive antenna size. But if the signal is modulated as 8-PSK with DVB-S2, the required receive margin decreases by 2.5dB — zero net change in required power.

The transition in backhaul to MPEG-4 AVC and DVB-S2 has now accelerated as backhaul service providers are now forced to start a new cycle of investment. Across a number of markets in the last year or so, an increasing number of SNG units have been equipped as MPEG-4/DVB-S2 uplinks, or at least with MPEG-2 encoders field-upgradeable to MPEG-4.

Microwave links

Lastly, the terrestrial microwave market is still a significant part of the backhaul chain in some markets. It is split into two parts: camera-back-mounted wireless links to connect either back to a news truck or to a central city receive site and mast-mounted microwave links to connect news trucks back to their station. Because the link is directly point-to-point, therefore not involving any third-party service providers as is the case with a satellite, there is no demand to minimize bandwidth to save money.

The demand instead is to save bandwidth because, increasingly, regulatory authorities in a number of countries are reallocating and reducing the frequency bands traditionally allocated to these broadcast backhaul services to 3G and 4G cellular services. Consequently, the move to MPEG-4 AVC is as much as to improve efficiency as meeting the demands for HD. In the terrestrial world, the relevant digital modulation standard is DVB-T, which was developed after DVB-S to meet the demand for converting analog DTH services to digital transmission. Digital microwave link manufacturers conform either to DVB-T or have developed their own proprietary modulation schemes. The second-generation DVB terrestrial standard is DVB-T2, and many of its objectives mirror those of DVB-S2. The latest digital microwave links are now offering DVB-T2, or an equivalent proprietary method, in order to further squeeze even more channels into a scarce resource. In the future, digital terrestrial links may be based on LTE technology.

Backhaul is subject to the need to provide ever-higher quality for lower cost — as in all other sectors of broadcast engineering. Fortunately, the latest compression and modulation tools make that easier.

Jonathan Higgins is managing director of BeaconSeek and author of “Satellite Newsgathering.”