Why do many TV broadcasters have multi-megawatt transmitters, high-performance antennas and tall towers? Most broadcast engineers would say it is to replicate the station’s analog service contour. As longtimers and newcomers to the UHF band have learned, brute force is not always enough to fill in a station’s RF signal shadows and gaps.
In terms of electricity, the ongoing cost of brute force is rapidly increasing out of control. In many markets, a station’s TV service contour can include cows, crops, deserts, forests or mountains between the population concentrations they serve. Is there a way to focus the electrical energy in your signal on geographically diverse population centers in your market, wasting less power covering farmland and forests? You bet there is.
A lesser-known benefit of digital broadcasting and ATSC is the ability to synchronize multiple transmitters operating on the same channel to form a single frequency network. Distributed transmission (DTx) is a single frequency network (SFN) using ATSC transmitters operating on the same channel. Synchronized analog AM stations have been around for years, but it was impossible to fully synchronize analog TV transmissions.
The concept of DTx involves keeping all the transmitters on the same frequency and synchronizing symbol emission. Doing this requires some serious digital finesse.
Two exciters from the same manufacturer, fed with the same SMPTE 310 stream, will likely generate dissimilar symbol sequences. ATSC modulators insert frame sync information every 624 MPEG packets, but the decision as to which packet contains the information is arbitrary. Also arbitrary are the timing of 24 trellis coder and 12 precoder bits, making a total of 36 arbitrary bits assigned by the modulator.
What are the odds of two modulators fed with the same SMPTE 310 stream sending identical symbol sequences? The answer is 1 in 624x236, or approximately one in 43 trillion. 624x236is also the exact number of diverse symbol sequences that can represent the same signal. If you were to restart the same two modulators once a second to achieve random synchronization, it would statistically occur about once every 1.4 million years. In a typical DTx system, exciters are automatically resynchronized once per second or as assigned by the user.
Fortunately, SMPTE 310 and ATSC allow data to be inserted into a reserved packet in the SMPTE 310 stream that provides synchronization information, uniquely addressed to all transmitters in the network. The packet is the distributed transmission packet, also known as DTxP. A placeholder packet with the correct packet ID (PID) is a part of the SMPTE 310 stream. When a distributed transmission adaptor (DTxA) encounters the placeholder, it overwrites it with the data that forms the DTxP.
What DTx requires
A good consulting engineer can predict with accuracy the delays needed at individual DTx transmission facilities. Factors such as antenna locations and height are used to calculate delays at a predetermined receiving site. Field adjustments of delays can be made remotely at any time to tweak the system.
A DTx system requires GPS receivers at the signal origination point and each transmitter site, a DTxA at the studio or master transmitter, and a DTx-capable exciter at each slave transmitter. The GPS provides frequency and time references, using 10MHz reference to lock the IF and local oscillators. It also uses a one pulse/second GPS time reference to provide the timing reference for the network.
The DTxA is the mechanism that allows remote adjustment of delays at each transmitter exciter. It has SMPTE 310 in and out, and it inserts data into the DTxP. When DTxA sees the placeholder, it overwrites it with the data that forms a valid DTxP.
DTxP contains the data that synchronizes trellis coders and frame sync insertion points, so all transmitters emit the same symbols at the same time. It also contains timing information, with individual settings addressed to each transmitter. The timing information allows transmitters to be fine-tuned to maximize synchronization in specific geographic areas.
Cadence sync is separate from DTxP, but is also a necessary component in the stream. It is a bitwise inverted MPEG sync byte that synchronizes where the ATSC modulator inserts the normally arbitrary frame sync. It is inserted once in every 624 MPEG packets.
RF watermarking
An optional component of a DTx system makes it easier for broadcast engineers to field tune the timing in a multiple transmitter network. When a DTx system synchronizes bitstreams and frequencies, it can be difficult to identify signals from specific transmitters in the field.
The option, known as RF watermarking, is a low-level code in the DTxP, buried beneath the ATSC symbols. It appears as noise on receivers but typically at about 0.1dB, it has no practical effect on the digital threshold. Because it looks like noise, it establishes a signal-to-noise measurement floor.
The RF watermark signal is typically approximately 30dB below average transmitter output. When integrated over one ATSC field, the coding benefits from a 54dB coding gain. This makes it possible to identify the code from a signal up to 24dB weaker than another.
When transmitted, an RF watermark can have an effect on adaptive equalization systems. Some transmitters know what the watermark sequence is, and subtract the watermark in the demodulated and ideal reference signals, making more accurate transmitter performance measurements possible. Most RF watermark systems offer several selections of injection levels, including none at all. To see weaker transmitters, increase the level of the watermark.
Most ATSC modulators use frequency correction to make the IF frequency independent of the SMPTE 310 clock. A DTx system handles SMPTE 310 frequency slew errors, which need to be within 0.5Hz of the normal IF frequency. Such systems use a very slow PLL with a time constant of about 20 minutes to avoid hiccups.
Each bit in the DTxA and transmitters slaved to it (DTxN) must be identical. An unlocked transmitter will jam reception of overlapping transmitters on the network. Mixing a DTx system with repeaters on the same channel can be problematic, although some manufacturers offer some creative solutions.
DTx is not a complete replacement for brute force, because it takes power to communicate with mobile DTV devices that generally have tiny low-gain built-in antennas. That seems to be about the only downside. The benefits of DTx are many, including potentially lower power bills and the fact that some viewers may be able to receive off-axis DTx signals without adjusting their antennas, depending on system design. DTx is much easier to implement with a start-up, but as energy costs continue to rise, it may be a viable alternative worthy of consideration for many digital television broadcasters.
The author wishes to thank Axcera and Joseph Seccia P.E. of Harris Broadcast for their assistance in the preparation this tutorial.
