Just like NTSC transmitters had to deal with linear and nonlinear distortions, so do today’s DTV transmitters. The difference is that instead of the wrong color or brightness, digital distortions will cause complete picture loss. Even relatively small distortions will reduce a transmitter coverage area because the receivers at the fringes already experience a lower input level as well as other impediments to good reception. Understanding, monitoring and controlling the linear and nonlinear distortions within the 8-VSB signal will help ensure that the broadcast signal reaches as many viewers as possible.
Linear distortions
Frequency response and group delay errors within the 6Mhz bandwidth are responsible for linear distortions in the 8-VSB signal because they reduce its signal-to-noise ratio (SNR). Most linear distortions can be corrected for with the use of digital signal processing (DSP) contained within the digital exciter. All DTV receivers are designed to detect and correct linear distortions provided that there is a minimum of nonlinear distortions contained in the signal. Linear distortions are caused by many factors, including the amplifier, the transmission line, the mask filter, the transmitting antenna as well as the path from the transmitting antenna to the receiving antenna. Multipath at the receive antenna will also cause linear distortions, but today’s modern DTV receivers handle multipath much better than the first-generation receivers.
Multipath
With the receive antenna indoors or mounted on the roof, there may be no direct line-of-sight path to the transmitting antenna, and the received signal is the result of reflections, diffraction and scattering. Multipath causes several different problems for the 8-VSB receiver:
- Signal strength variations — These changes occur as furniture and people move about and cause rapid changes in signal level; these are different from fading (see below). As multiple signals are received, phase cancellation and strengthening of the signal will create unstable signal strength. The 8-VSB receiver has to be able to adjust for these wide variations in signal strength.
- Selective fading — As the various reflected signals reach the receive antenna, phase shifts can occur within different sections of the 6Mhz bandwidth. As these signals combine at the antenna, these phase shifts cause attenuation of certain frequencies within the band. As the paths change, so will the resulting attenuated frequency.
- Time delay spread — Differences in the path length of the numerous reflected signals causes a time delay spread between the shortest and the longest paths. The distortion that results can be major if the delays are close to the pulse width or multiple of the pulse width of the 8-VSB signal. There is a correlation between frequency selective fading and time-delay distortion because the smaller the pulses, the wider the bandwidth required.
- Fading — If the receiver is in motion, or when the there are changes in the environment such as trees blowing in the wind or people moving around, there will be fading, either slow or rapid. This fading can include amplitude and frequency distortion, and even time-delay fluctuations.
Nonlinear distortions
Signal envelope compression and incidental phase modulation are the cause of nonlinear distortions in an 8-VSB transmitter. Unlike linear distortion, nonlinear distortions are caused within the transmitter’s power amplifier; also unlike linear distortions, nonlinear distortions cannot be corrected for by the DTV receiver and must be corrected or compensated for at the transmitter. As the input power and/or gain changes in a DTV transmitter, so will the nonlinear distortions, therefore it is very important to monitor these distortions to be sure all viewers are receiving the best signal possible. DTV exciters will adjust for nonlinear distortions either automatically or when programmed to by the transmitter engineer. The best situation is where the DTV exciter automatically adjusts for these distortions as they occur, but not all exciters can do this. In this case, the transmitter engineer must maintain a constant power level and monitor these nonlinear distortions.
The goal
The goal for the transmitter engineer is to have the DTV transmitter produce the smallest percentage of error vector magnitude (EVM) and the highest SNR. Both measurements correspond to the 8-VSB signal accurately hitting the correct amplitude for every symbol level and how easily the DTV receiver can correctly interpret them. The parameters that the station has control of must be monitored and adjusted as required to insure that the 8-VSB signal will reach as many viewers as possible.
Next time
The next Transition to Digital will cover news from NAB2008.
