Delivering quality DTV
Apr 1, 2009 12:00 PM, By Christoph Balz
Broadcasters need versatile test and measurement equipment with multistandard support.
To detect and solve problems in TV networks at an early stage, there is a need for versatile test and measurement functionality with multistandard support for all significant analog and digital TV standards.
Digital TV networks use high-performance components such as fully digital and modular transmitters to ensure high-quality TV signal transmission. The signals are generally distributed to the transmitters via IP networks over redundant paths, resulting in high reliability. But even the most modern technology is subject to aging or other influences, which may lead to degraded signal quality or transmitter failure. Faults or operation outside the specified parameters cannot always be avoided. As a result, program providers may claim recourse, or the number of viewers may decrease drastically — consequences that may prove expensive. Advanced monitoring test and measurement equipment cuts down on these incidents by alerting network operators to problems when they first arise.
Many aspects are critical to ensure that a proper TV output signal is delivered. TV analyzers for broadcasting applications have to support all major standards and allow the measurement of signal-quality parameters that are relevant in analog and digital broadcasting. Related test and measurement equipment is needed to control the most important signal parameters and provide accurate information about the quality of an applied signal. Providing a wide dynamic range and powerful measuring functions, infinitesimal changes in the monitored transmitter parameters can be detected, and network operators can be informed about looming disruptions with enough time to take appropriate countermeasures.
Various parameters affect TV signal quality
To ensure high-quality, reliable digital TV services, a variety of parameters have to be taken into account when measuring and monitoring digital TV signals. In addition to the operating frequency, the modulation standards and the baseband signals should be checked. One of the most important benchmarks is the measurement of the modulation error ratio (MER), but transmission errors including bit error ratio (BER) should also be measured. In addition, the desired test functionality should cover nonlinearity specifications (complementary cumulative distribution function or amplitude probability distribution) and interferences or reflections (MER vs. frequency or channel impulse response [CIR], also called echo pattern). Furthermore, constellation and eye diagrams complemented by spectrum analysis functions are helpful tools to ensure the quality of TV signal transmission. Also, an ideal test solution should support single-frequency networks (SFNs), which are becoming more important, and the analysis of the MPEG transport stream.
The transmitting system must be checked to determine if it affects other channels during operation. Spurious emissions can be analyzed in the upper and lower adjacent channels using the “shoulder attenuation” measurement function. An asymmetrical shoulder and high emissions in the adjacent channels indicate poor signal quality.
Figure 1. Using the constellation diagram, the signal state errors that occur in quadrature modulation can be clearly displayed.
Click to enlarge
The constellation diagram is a graphical representation of the in-phase and quadrature components of a QAM signal in the X and Y axes. In the case of modulation with multiple carriers, the constellation diagram typically forms the sum of the signal states of all of the carriers. A noisy or disrupted DVB-T/H signal will exhibit cloudlike effects. The MER and the error vector magnitude (EVM) are useful for quantitative assessment of the constellation points in the constellation diagram with regard to their deviation from the theoretical location. (See Figure 1.) The greater each MER and EVM value in decibels, the better the signal quality.
DVB-T/H, like many other digital TV standards, specifies two essential error protection mechanisms: the Viterbi decoder and the Reed-Solomon decoder. Both techniques are designed to detect and correct bit errors occurring in the data stream during transmission. Under ideal conditions, the BER before Viterbi (i.e., before any error correction) should equal zero; therefore, to get a clear picture of the transmission quality, BER measurements before the Viterbi and the Reed-Solomon decoder are important.
Advantages of real-time demodulation
To detect and solve problems in TV networks at an early stage, there is a need for versatile test and measurement functionality with multistandard support for all significant analog and digital TV standards, such as DVB-T/H, ATSC/8-VSB, DTMB (China), T-DMB/DAB, ISDB-T and DVB-C (J.83/A/B/C). On the other hand, the test platform has to be flexible so it can quickly be adapted to any future developments.
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