Delivering quality DTV

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.

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.

Some test and measurement products store data offline for later analysis, which may lead to uncaptured errors, but a new concept of real-time demodulation to perform BER measurements and complete analysis online has many advantages. Another advantage when using real-time demodulators is that demodulated analog video and audio signals, as well as digital MPEG transport streams, are available for further processing. In addition, a real-time architecture provides an excellent MER performance (See Figure 2.), while the frequency-dependent curve of the modulation error can be calculated within the analyzer and displayed on its screen. The wide variety of TV-specific measurements should be complemented by measurements in the spectrum, such as shoulder attenuation in accordance with ETSI TR 101 290 or other standard-specific spectrum mask requirements, channel power and adjacent-channel power.

Speed is an advantage not only when carrying out measurements, but also when analyzing TV signals. In addition to innovative, real-time demodulators, high-speed signal processing makes it possible to detect short-duration interference and to perform adjustments in real time. The high-speed performance becomes especially apparent when displaying the constellation of digitally modulated TV signals, when displaying the channel impulse response for orthogonal frequency-division multiplexing (OFDM) signals or when measuring the frequency response, group delay and phase in the TV channel.

A special feature of a modern TV analyzer is the MPEG option. This option offers an in-depth analysis of the MPEG baseband, providing a powerful tool for verifying the integrity of the broadcast content. This had only been possible by using separate, highly specialized MPEG analyzers. It also allows a TV picture to be displayed on the screen, rounding out the varied analysis functions covering everything from RF and modulation to the baseband. The quality of a TV picture often provides an indication of the quality of the entire transmission path and its components.

Optimize SFNs

OFDM transmitter networks are able to broadcast several programs at a single frequency. SFNs make efficient use of the scarce frequency resource, easing frequency planning and contributing to cost-efficient operation, particularly in areas with difficult geographic conditions. OFDM signals in SFNs are received at different times because of distance-dependent path delays. The signals have a time-specific guard interval so the receivers can deal with different path delays. All the signals have to be received during this guard interval.

To ensure trouble-free operation within an SFN, certain criteria have to be precisely met. For example, all transmitters in a network must broadcast their signals at exactly the same frequency, with permissible deviation not exceeding 1Hz (VHF/UHF). Greater deviations will result in time-variant channels in the area of reception, with the consequence of a poorer BER in the case of stationary receivers, accompanied by a decrease in coverage and range.

Measures taken to optimize SFNs include defined delays being set on each transmitter to ensure that the guard interval will be maintained at any location within the network. Violation of the guard interval in the order of a few microseconds can cause problems similar to those encountered in the case of deviations from the transmit frequency, especially in large coverage areas.

The CIR, or echo pattern, measurement window of the latest TV analyzers reveals at a glance whether the mentioned criteria are complied within an SFN. It provides straightforward time-domain display of the individual single-frequency transmitters and of reflections. To ensure that all transmitters within an SFN operate at exactly the same frequency, each transmitter of the network is locked to a GPS reference signal. To verify whether all transmitters actually transmit at the same frequency, it was previously necessary to measure the frequency at each and every transmitter location — a time-consuming method. Now, the SFN frequency offset option indicates, for each echo signal, the frequency deviation relative to the main pulse with an accuracy of less than 0.3Hz. Because the frequency deviation is determined as a relative value, a reference frequency is not necessary, which greatly assists in measurements. (See Figure 3.)

Universal test platforms ensure high-quality TV networks

Universal multistandard platforms for in-depth and fast analysis of TV signals are a prerequisite for high-quality networks. The desired platform should be designed for the commissioning, installation and servicing of TV transmitters and for carrying out coverage measurements on terrestrial TV networks. Ideally, it combines TV test receiver and spectrum analyzer functionality in a single unit while providing a wide frequency range, high measurement accuracy and high speed based on real-time demodulation. A future-ready instrument concept allows new TV standards to be implemented easily on a software and hardware basis. In addition, a compact and robust design is a great benefit for portable or mobile applications, which greatly simplifies network coverage measurements.

Christoph Balz is head of R&D for broadcasting test receivers at Rohde & Schwarz.