DVB-T Transmission Generates New Test Challenges

New Measurements Describe Digital Television Systems

The new digital video broadcast (DVB) standards require a whole new set of measurements to ensure signal quality. Experienced technicians can readily recognise potential problems with today’s analogue signals by looking at an RF channel using a spectrum analyser. Parameters such as the system noise level and ratios between signal component levels (video, colour, and sound carriers) are easy to see, and translate directly into impairments that consumers will view on their televisions.

For digital satellite and cable transmissions, the test methodology and many system measurements derive from similar measurement requirements in the telecoms industry. For example, measurements on wideband quadrature phase-shift keying (QPSK) systems have been used for many years to develop, manufacture, and monitor communications satellites. Similarly, 64-state quadrature amplitude modulation (QAM-64) is the typical modulation scheme that 140 Mbit line-of-site microwave radio systems employ. One challenge for test equipment manufacturers is to adapt existing techniques to products that better suit a new user base, and to train these new users.

Meanwhile, the DVB terrestrial (DVB-T) television standard presents a completely new measurement challenge for RF modulation quality analysis. The coded orthogonal frequency-division multiplex (COFDM) technique that DVB-T uses is a high-density, multi-carrier environment that permits the construction of large networks of single-frequency broadcast transmitters, and combats the high probability of multi-path interference in cities. The transmission signal comprises 2k or 8k carriers within the 8 MHz bandwidth of a normal analogue channel. Data are modulated onto the carriers, typically using low data-rate QAM-64.

The signal will inevitably be distorted to some degree by the time it reaches the consumer’s receiver. Some carriers may be notched out due to multi-path reception or co-channel interference effects. To compensate, some of the data bandwidth provides forward error correction data redundancy, enabling data recovery in the presence of a limited level of transmission interference. The consumer’s picture will appear perfect until the error correction mechanism cannot handle the number of errors being detected, at which point the system will fail and the picture either freezes or vanishes.

The probability of recovering data to provide constant picture availability improves proportionally with increasing transmission signal modulation quality. Engineers who plan and implement networks must assess reliability at the edge of a transmitter’s coverage area, where operating margins are most important. A transmitter’s performance determines its coverage area, and a “cleaner” transmitter reaches further just as surely as a more powerful one. Since frequency planning restrictions severely limit transmitter output power levels in many areas, transmission quality is extremely important.

Measuring Modulation Error

Technicians are becoming familiar with using the constellation diagram display as a “window” into the modulation quality of a digital channel. With training, the constellation diagram provides a channel performance indicator that’s similar to the traditional spectrum of an analogue signal. Technicians can see effects such as noise (points become clouds) and gain compression (outermost points squeeze in from their target). But the constellation diagram does not directly relate to picture quality; it relates to another measurement that’s new to the television industry — bit error ratio (BER). Because the digital signal has forward error correction, BER relates to picture quality only at the point where the picture turns from good to bad (you can access receiver error rate estimates from the service menu of most of today’s set-top boxes).

During DVB’s development, a sub-committee was set up to recommend measurement methods that are now part of the standards. The recommended modulation quality metric for cable and satellite transmission measures the magnitude of the modulation error vector (Figure 1). The modulation error vector is the sum of all impairments, including noise and other channel impairments, together with modulator imbalance and offset effects.

Figure 1. The standard modulation quality metric measures the error vector magnitude using a constellation diagram display.

There are two modulation error measurements in current use. The original measurement is error vector magnitude (EVM), which is the target vector’s rms error value compared with its peak value. The newer measurement is modulation error ratio (MER), where the error vector is compared with the target vector’s rms value to yield a power ratio. For cable technicians, MER resembles familiar carrier-to-noise measurements in analogue systems. MER is a figure of merit that can characterise individual devices such as modulators, yet it is sufficiently universal to monitor the network’s performance in delivering the signal to the consumer.

Evaluating Prototype DVB-T Transmission Equipment

During the design and manufacture of any transmission equipment, measurements from the RF components help you to predict overall system performance. Key parameters include modulation linearity, noise performance, and power hand- ling. But because no measurement equipment for COFDM signal analysis was available at the time, evaluating prototype and early production equipment for DVB-T trials was a challenge. Functional tests on early COFDM transmission equipment relied exclusively on using the same high-quality test receiver (the “golden” receiver) to gain a comparative knowledge of the behaviour of a series of transmitters or modulators.

While the test receiver’s demodulator outputs provide a point for displaying a constellation diagram on an oscilloscope, this approach cannot quantify modulation impairments. But you can extract the data stream from the receiver’s decoder and measure the BER. You can then inject noise into the RF path (in a procedure known as “loss of noise margin”) until a defined BER threshold appears. The noise level that generates this BER threshold provides an indication of the original signal’s “cleanliness”, and allows you to compare different modulators.

But the noise injection technique has limited accuracy, due to the small differences between devices compared with the uncertainty in measuring noise levels. A low threshold on a particular device-under-test does not tell you where the problem source lies — it merely indicates that a problem exists. Further, the noise injection technique does not provide a metric that potential customers can use in terms of device specification. So, observing constellation diagrams helps you with diagnostics, but analysing specific attributes of the spectrum would be extremely useful.

Until now, transmitter operators could only compare equipment from different manufacturers with reception quality tests using their own “golden” receivers. Transmitter operators also need to check the compatibility and sensitivity of any type of set-top box that consumers might use in their service area. Although comparative measurement techniques are time-consuming and have limited accuracy, they were adequate for early prototype development and system trial purposes. Now, volume production demands repeatable measurements to more rigorous specifications.

New COFDM Modulation Analysis Tools

Newly available tools help you overcome the limitations of previous test techniques (see, “Software Extends Spectrum Analyser for DVB-T”). A vector signal analyser with dedicated software allows you to display and evaluate various modulation quality parameters.

For example, the EVM magnitude spectrum shows a plot of EVM versus carrier number across the entire channel, and is ideal for identifying spurious responses and interference in the transmission signal. An error statistics summary table can report channel power and the relative amplitudes of the reference, data, and transmission protocol signalling (TPS) carriers. The error statistics summary can also report overall EVM and MER values, together with the carrier number and the maximum EVM for each symbol (Figure 2).

Figure 2. Hewlett-Packard’s COFDM data analysis software interface shows a constellation diagram, EVM magnitude graph, and summary error statistics.

Figure 3 shows a constellation diagram from a noisy DVB-T modulator taken at its IF output. Notice that the only difference between a 2k and 8k carrier system is the number of samples that make up each constellation point for each symbol period. You can see the constellation points due to the pilot carriers, which you can assess separately. As with any other QAM system, the size of the constellation points indicates the difficulty the decoder faces in correctly recovering the data (indicating the likely BER — bigger points mean more errors and less margin-to-failure).

Figure 3. A constellation diagram display shows the IF output from a noisy modulator.

Software COFDM modulation analysis together with a vector signal analyser’s high quality RF measurements now provide developers and manufacturers with the confidence to specify the modulation quality of their DVB-T transmission equipment. And, for the first time, broadcasters have an independent reference to evaluate competing products and commission their own transmitter sites.