Backup STL links
Jan 18, 2009 9:14 AM, By Russell Brown
Today’s broadcast TV station depends on high-speed, real-time links to transport audio/video to and from the studio. The most important of these links is the one to the transmitter. The last “Transition to Digital” tutorial covered different types of permanent links. This tutorial will cover backup links that may be used on a temporary basis. A permanent secondary link should always be installed if possible, but other options are available.
Thinking of a station’s transport stream as data, many products and services that have been developed for the IT industry can be used. Usually, we only think of the connection speeds we use in our homes and at work such as DSL (500KB-2MB) or cable company Internet connections (up to about 12MB). But in the IT world, these are pretty slow when compared to the speeds required for businesses and ISPs. Telephone companies have traditionally provided wired high-speed data links at very high costs. Before fiber optics came along, there was a reason for this, but even now that prices have fallen, they haven’t fallen far enough to make it practical to use these data links instead of dedicated broadcast connections.
With some creativity, broadcasters can start using some of the equipment and services of the IT industry to provide low-cost alternatives to getting signals to and from the studio.
Convert to IP
The Society of Motion Picture and Television Engineers has developed a standard that specifies the format for converting ASI to IP (transport stream over Internet Protocol or TSoIP) and back again. This standard, SMPTE 2022, provides for IP encapsulation of MPEG transport streams with forward error correction designed for media. The forward correction is known as 2D FEC for the arrangement of the FEC packets in the IP stream, which allow for better correction of errors in real-time media. The amount of FEC will affect the amount of bandwidth needed to transport the TSoIP, but there is also IP protocol headroom. This is the data needed within the IP network to route the packets to make sure they arrive at their destination.
Figure 1
Click to enlarge
A 19.4Mb ASI signal may need as much as 80 percent extra bandwidth (FEC and IP protocol) to be transported over an IP network; this equals 34.92Mb. Keep in mind that the higher data rate provides protection from data loss, and this data rate is less than half of a 100BASE-T Ethernet’s bandwidth. (See Figure 1.)
Cable companies have been using IP services for years to deliver transport streams — most of their MPEG encoders interconnect with their statistical multiplexers using IP protocol with 1000BASE-T physical cabling and connectors (RJ45 and Cat 6).
There are many advantages to using IP for transport streams, such as the ability to use network routers to switch equipment into service by changing IP addresses. Matrixes (physical switches) are not needed when streams are routed by their IP address; a system can grow with just the addition of another network switch. In addition, there is no need for looping or distribution amplifiers because a single IP address can be routed to several destinations at the same time.
Several manufacturers make ASI to IP (and even SDI to IP) converters, but not all follow the SMPTE 2022 standard, so use caution if attempting to intermix different converters.
Microwave
Most engineers have heard of the unlicensed 2.4GHz and 5.7GHz microwave radios used to transmit audio and video over short distances, but models exist that also will transmit data. Some unlicensed 5.7GHz radios will transmit more than 20mi at a data rate in excess of 45MB/s, more than enough bandwidth for the ATSC ASI transport stream. These are microwave links that require no license, which means that there is no protection from interference encountered over the path used. This also means that it can be set up anywhere, such as during an emergency when a station’s main link goes down. In this case, the pros and cons of possible interference would have to be considered. In more rural areas, there may be no problem with interference (read other unlicensed links), but there could be considerable interference in metropolitan areas. Cities and counties have used unlicensed links for years for surveillance services, to name just one purpose.
Figure 2
Click to enlarge
It’s relatively easy to put together an unlicensed system like this once the signal has been converted to IP. The cost of an unlicensed data radio runs anywhere from $1000 up, and the ASI to Ethernet converters cost approximately $1500 each. At these prices, it becomes a very economical solution to getting the ASI stream to the transmitter, compared with the $50,000 plus for a broadcast-grade, licensed microwave system. (See Figure 2.)
There are a few points to keep in mind when selecting a link:
- The shorter the distance, the better — dishes become smaller and data bandwidths become larger.
- The data bandwidth stated for a radio is for all data, even the bits the radio uses. A typical stated bit rate may be 20Mb, but the actual bit rate for the user would be only 14Mb, which means the radio its self accounts for a 42 percent overhead.
- In a real-world application after a 19.4Mb ASI signal turns into a 34.92Mb IP stream, add another 40 percent (see above) on top of that, which means a radio is needed that can pass 48.88Mb to get the ASI signal out the other end. These numbers can vary and a lower bit rate may suffice, but this is an appropriate rule of thumb.
- Data bandwidth can be traded for distance/interference. A radio that starts out with a data bandwidth of 300Mb that must cover a path of 30mi with a high noise floor may only output 45Mb at the receive end.
- These radios provide a two-way link, which means any extra bandwidth can be used for communication with the transmitter. This includes remote control and monitoring, IP cameras and ENG feeds.
These unlicensed radio links are only suggested as a backup to the main link to the transmitter. For a less complicated link, licensed radios can be used.
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