Delivering quality DTV
Apr 1, 2009 12:00 PM, By Christoph Balz
Broadcasters need versatile test and measurement equipment with multistandard support.
Figure 2. The important MER value is displayed enlarged.
Click to enlarge
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.
Figure 3. An SFN frequency offset option expands the echo pattern (amplitude) graph and indicates deviations from the center frequency with high precision — DVB-T/H, for example.
Click to enlarge
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.
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