Advances in DTV transmission technology: Getting the most bang for your digital buck

[Gregg Lengling]

BY DAVE GLIDDEN

Broadcast Engineering, Jul 1, 2003

 
As broadcasters near the completion of the initial on-air phase of DTV, they must manage new challenges to ensure that the entire digital transition is successful. Compared to the initial scope of DTV planning, the challenges in 2003 include:

An increased emphasis on maximizing UHF coverage area
An extended period of uncertainty about broadcast interference due to later dates for maximization and replication
Additional costs of operating dual facilities for a longer period of time
A need to operate aging analog transmission plants past 2006
Fewer trained technical personnel to maintain both analog and digital facilities.
Unless managed carefully, these challenges could overwhelm the resources of many broadcasters in this critical stage before DTV generates significant revenue. Fortunately, recent technology and product advancements offer broadcasters effective ways to address and overcome each of these transmission issues.

Maximizing DTV coverage area
--------------------------------------------------------------------------------

 To minimize interference between TV stations, the FCC requires all stations to limit the amount of power they radiate outside of their designated 6MHz channels. This restriction is specified by a power vs. frequency curve known as a "mask" (see Figure 1). In 1998, the FCC tightened its DTV mask requirement and, as a result, broadcasters operating at the limits of the old mask found themselves in violation of the new mask. To comply with the new mask, they had to reduce their transmitter power output by about 10 percent, which reduced their coverage area. Many of these broadcasters responded to this by purchasing more powerful transmitters (which have a wider linear operating range) to maintain the same coverage area while conforming to the new mask.

But recent advancements allow broadcasters to achieve greater power output without additional amplification. Combining a temperature-compensated sharp-tuned filter (STF) with appropriate correction techniques in the DTV exciter increases transmitter operating power and efficiency while also attenuating the intermodulation products in adjacent channels. The result is full compliance to the DTV mask requirement and enhanced interference protection.

Sharp-tuned filtering
Power amplifiers have a finite linear operating range. At the upper portion of this range, the harder an amplifier is driven, the more distortion it will create. A DTV broadcast station typically uses three or more linear amplifiers in parallel to achieve its desired output power. To avoid distortion that could create out-of-band emissions, the station drives these amplifiers well below saturation. But, by placing a sharp-tuned filter at the output section of the amplifier, a station can eliminate unwanted out-of-band emissions before they are transmitted. Thus, sharp-tuned filtering frees the broadcaster to drive the amplifiers harder because the filtering will eliminate the out-of-band emissions caused by the increased distortion. This increases the station's operating efficiency. It may also allow the station to use fewer amplifier sections in the amplifier to reach its required output power, reducing power consumed by cooling and other ancillary systems.

Sharp-tuned filters also provide the level of isolation needed for upper-adjacent combining into a single antenna without serious degradation to the lower analog channel. With this level of isolation, it is also possible to combine two DTV signals or a lower-channel DTV and upper-channel NTSC signal.

However, while the sharp-tuned filter effectively suppresses out-of-band radiation caused by transmitter nonlinearities, it can increase in-band nonlinearities such as ripple and group delay as the power output is increased. These nonlinearities can degrade the in-band signal-to-noise performance in a way that cannot be corrected by a receiver's adaptive equalizer. Consequently, broadcasters must use adaptive pre-correction techniques to limit and pre-correct these artifacts if the transmitter is to achieve the minimum signal-to-noise ratio of 27dB.

Adaptive pre-correction
Since transmission chains are subject to environmental and operating anomalies that can affect the quality of the RF transmission, adaptive pre-correction techniques are critical to ensuring that stations are radiating only within their licensed channel. Adaptive pre-correction continuously samples the DTV signal at the output of the channel combiner or the mask filter. If the pre-correction system detects any distortions, it feeds them back to the exciter and automatically corrects them without interrupting transmission.

Until recently, 8-VSB exciters provided effective techniques for adaptive linear correction, but little in the way of nonlinear adaptive correction. New exciters can provide continuous sampling of the transmitted signal before it is filtered, and then add the needed adjustments automatically to correct nonlinearities that could result in non-mask-compliant, out-of-band performance.

Nonlinearity correction ensures compliance with the FCC's DTV RF mask in the two critical regions 500kHz inside the lower and upper limits of the 6MHz channel – regions that a standard mask filter cannot protect. Prior to these recent advancements, transmitter engineers had to perform tedious manual adjustments or difficult computer-to-exciter interface adjustments. The latest techniques are self-contained, using intelligent algorithms to perform all the underlying digital filter adjustments on a continual basis in a way that is essentially transparent to the user.

As more and higher-power DTV transmitters come online, adaptive correction techniques can help mitigate the effects of a crowded RF environment by ensuring ongoing DTV transmitter compliance with the FCC's DTV RF mask.

Higher-efficiency transmitters

 
In their initial DTV planning, broadcasters never anticipated that they would operate both analog and digital transmission plants for an extended period. As the period of dual transmission stretches out, it is critical that broadcasters find ways to reduce operating costs. In addition to the positive impacts of sharp-tuned filters and adaptive correction, new amplifier technologies are promising dramatic improvements in transmitter efficiency. For digital high-power UHF operations, a new generation of multi-stage depressed collector (MSDC) devices promises to improve the operating efficiency of inductive-output tubes (IOTs) as much as 50 percent compared to IOTs with conventional collectors.

Inside the MSDC
In a conventional IOT collector, the kinetic energy of the electrons striking the collector electrode is converted into heat, which is wasted energy. If the collector could slow the velocity of the electrons before collecting them, it would convert their kinetic energy into potential energy, which could then be returned to the power supply. This might be simple if all the electrons traveled at the same speed, but they don't. During IOT operation, signal modulation varies the speed of the electrons as they travel from the cathode to the collector. In fact, the velocity of the electrons entering the collector ranges from zero electron volts (corresponding to the cathode voltage) to several thousand electron volts (corresponding to the anode or collector voltage), and an infinite number of velocities in between. If the collector could reduce the kinetic energies of all the electrons to zero velocity by using electrodes with corresponding voltages, it would collect them with perfect efficiency. But, of course, it is impossible to build a collector with an infinite number of collector electrodes. Even implementing more than just five collector electrodes in a tube significantly increases its design complexity and cost.


--------------------------------------------------------------------------------

Figure 2. This cutaway view of a four-stage MSDC shows the five electrodes that create the electrostatic field and collect the incoming electrons.
--------------------------------------------------------------------------------
 
Several manufacturers are developing MSDC IOTs that typically have five collector electrodes instead of the one found in conventional IOTs (see Figure 2). Such an MSDC has an electrode at the back of the collector that is set at cathode (ground) potential and four conical-shaped electrodes that are set at higher voltages. The conical-shaped electrode at the mouth of the collector has the highest voltage. The next one has a lower voltage, and so on. Together, the five electrodes create an electrostatic gradient, albeit a very uneven one. As the electrons enter the MSDC, the electrostatic gradient slows them down, converting their kinetic energy to potential energy (voltage). The conical-shaped electrodes collect the electrons at four usable voltage levels and return them to the power supply, thus saving energy that would otherwise be wasted as heat. Initial figures from tube manufacturers show that an MSDC increases an IOT's beam efficiency by 50 percent while reducing the power required to drive the tube by one third.

continued::::