The secret to maintaining reliable and high-quality services over different digital television transmission systems is to focus on critical factors that may compromise the integrity of the system. Modern digital cable, satellite, and terrestrial systems behave quite differently when compared to traditional analog TV as the signal is subjected to noise, distortion, and interferences along its path. Today's consumers are familiar with simple analog TV reception. If the picture quality is poor, an indoor antenna can usually be adjusted to get a viewable picture. Even if the picture quality is still poor, and if the program is of enough interest, the viewer will usually continue watching as long as there is sound.

DTV is not this simple. Once reception is lost, the path to recovery isn't always obvious. The problem could be caused by MPEG table errors, or merely from the RF power dropping below the operational threshold or the cliff point. RF problems can include any of the following: satellite dish or Low-Noise Block Converter (LNB) issues, terrestrial RF signal reflections, poor noise performance, or channel interference; and cable amplifier or modulator faults. There are a couple of ways to solve DTV reception problems. One solution is to make set-top receivers more tolerant to degraded signals. A better solution is for the network to maintain a clean, high-quality RF signal. To ensure this, test & measurement provides critical RF measurements for 8-VSB, 8PSK, QPSK, COFDM, and QAM modulation schemes, integrated with real-time MPEG monitoring in a single instrument, the MTM400A. This instrument can be economically deployed at various points within the transmission chain from downlink and encoding, through multiplexing and re multiplexing to final delivery via uplink, head-end, and transmitter sites. Using the MTM400A, an operator can make critical RF measurements at a fraction of the cost of dedicated RF test equipment. Web-based remote control allows the correct measurements to be made at the appropriate signal layers throughout the transmission chain, thus ensuring that cost-effective results can be guaranteed.

Bit Error Rate

This is the ratio of bits in error to total bits delivered. Early DTV monitoring receivers provided an indication of bit error rate as the only measure of digital signal quality. This is simple to implement since the data is usually provided by the tuner demodulator chipset and is easily processed. However, tuners may often output BER after the Forward Error Correction (FEC) has been applied. It is better to measure BER before FEC (pre-Viterbi) so that an indication is given of how hard the FEC is working. After the Viterbi de-interleave process, Reed-Solomon (RS) decoding will correct error bits to give quasi error-free signal at the output.

This is applicable when the transmission system is operating well away from the cliff point, where few data errors occur and pre-Viterbi bit error rates are near zero. As the system approaches the edge of the cliff, the pre-Viterbi BER increases gradually, the post-Viterbi more steeply, and the post-FEC (after RS) very steeply. Therefore, FEC has the effect of sharpening the angle of the cliff. As a result, very sensitive bit error rate measurements do give a warning, but usually too late for any corrective action to be taken.

How to Improve on BER—use MER

The TR 101 290 standard describes measurement guidelines for DVB systems. One measurement, Modulation Error Ratio (MER), is designed to provide a single figure of merit of the received signal. MER is intended to give an early indication of the ability of the receiver to correctly decode the transmitted signal. In effect, MER compares the actual location of a received symbol (as representing a digital value in the modulation scheme) to its ideal location. As the signal degrades, the received symbols are located further from their ideal locations and the measured MER value will decrease. Ultimately the symbols will be incorrectly interpreted, and the bit error rate will rise; this is the threshold or cliff point. Figure 1 shows a graph, which was obtained by connecting the MER receiver to a test modulator. Noise was then gradually introduced and the MER and pre-Viterbi BER values recorded. With no additive noise, the MER starts at 35 dB with the BER near zero. Note that as noise is increased the MER gradually decreases, while the BER stays constant. When the MER reaches 26 dB, the BER starts to climb, indicating the cliff point is near. MER indicates progressive system degradation long before reaching the cliff point.

Modulation and System Variations

The signals used in satellite, cable, and terrestrial digital television transmission systems are modulated using quadrature modulation schemes, where phase and amplitude are modulated to represent data symbols. The most common modulation schemes used in digital television transmission are all variants of Quadrature Amplitude Modulation (QAM). For example, in commonly used terrestrial digital modulation schemes, COFDM (as used in DVB-T transmissions) uses 16-QAM or 64-QAM and 8-VSB (as used in ATSC transmissions) uses an 8-column system. In satellite, the digital modulation scheme used is Quaternary or Quadrature Phase Shift Keying (QPSK), which is equivalent to 4-QAM. QPSK is a very robust modulation scheme, and has been in use for several years. QPSK is also used for contribution feeds and makes a more efficient use of the available bandwidth, but needs a better carrier-to-noise ratio. Cable systems build on this, and have a wider range of schemes, which are still evolving. Additional modulation levels (16-QAM, 64-QAM, 256-QAM and 1024-QAM) improve spectral efficiency, thereby providing more channels within a given bandwidth. In U.S. systems, 64-QAM can transmit 27 Mb per second, allowing the transmission of the equivalent of six to 10 SD channels or 1 HD channel within a 6 MHz bandwidth. 256-QAM can transmit 38.8 Mbps or the equivalent of 11 to 20 SD channels or two HD channels within a 6 MHz bandwidth. New compression techniques can provide up to three HD channels over 256-QAM. In European systems, the 8 MHz bandwidth allows up to 56 Mb per second over 256-QAM.



It is better to predict system problems long before critical revenue earning services go off the air, rather than cure them. MER measurements are able to measure small changes in transmitter and system performance and are one of the best single figures-of-merit for any cable and satellite transmissions system. EVM and more traditional BER are useful for standard cross-equipment checks and as an aid to identify short-term signal degradation. Constellation displays help provide a reliable health check for RF transmission systems by indicating arti-facts, distortion, or equipment drift. By combining these critical RF measurements with comprehensive MPEG transport stream monitoring and alarming in a single probe, system problems can be detected at an early stage, before viewers are affected. With the MTM400A, T&M players like Tektronix is able to provide all the critical RF measurements and interfaces, integrated with MPEG measurements in a single cost-effective monitoring probe.