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Digital Video Measurements Application Note

QAM Digital Measurements
Using the CR1200 QAM Monitor

Topics Covered

Introduction
Quadrature Amplitude Modulation (QAM)
Forward Error Correction (FEC)

Digital Video Measurements
Average Channel Power
Constellation Display
Modulation Error Ratio (MER) - Signal-to-Noise Ratio
Bit Error Rate (BER)
Adaptive Equalizer Response
Spectrum Analyzer Display

Introduction

Many cable systems are now implementing Digital Video Broadcast via Cable. Digital video offers the advantage of higher picture quality and more efficient use of valuable bandwidth, allowing for significantly more channels to be carried. This application note covers the various tests that cable systems should be making during the installation and maintenance of these new digital video signals in order to provide reliable high quality service to subscribers.

Figure 1 shows a basic block diagram of a digital video transmission system from the analog video source, to the set top converter. The video can originate from either a local headend, or from a centrally located headend known as Headend in the Sky (HITS). First the signal is digitized converting the analog signal to data bits. The bit stream is then compressed using MPEG-2 compression. Forward error correction (FEC) coding is added to help correct errors that may be added as the signal passes through the system. To feed the cable system the bit stream is modulated using QAM modulation and then up-converted to a channel to be carrier on the cable system. The set top converter receives this signal and the process is reversed to provide the original analog video and audio to feed the television. The set top converter contains two key blocks that help remove the effects of signal impairments from the signal. The adaptive equalizer compensates for any distortions that the cable system may have added and the FEC decoder corrects as much as possible any digital errors that may have been added to the digital stream.

Figure 1: Digital Video Transmission System

Quadrature Amplitude Modulation (QAM)

Quadrature Amplitude Modulation (QAM) is a modulation method that has been used in analog television since the beginning of color television. QAM is a method of amplitude modulation that allows 2 channels of information to be transmitted at the same time on a particular frequency. The color subcarrier of the analog video signal for example uses QAM to send 2 color difference signals on the color subcarrier enabling the original RGB signals to be reconstructed at the receiver. Digital QAM signals also carry 2 signals which are referred to as the I and Q components. Its important to understand how Digital QAM signals function in order to understand one of the more important test functions, the constellation display.

Normally we think of digital signals as either on or off. Unfortunately if we were to simply turn the carrier on or in relation to the ones and zeros of the bit stream we would need huge amounts of bandwidth to transmit the required data. To increase the amount of data transmitted for a given bandwidth, we can break the amplitude of a carrier up into intermediate levels that represent specific combinations of bits. The more levels, the higher the amount of bits that can be carried, but the more susceptible the carrier will be to noise and other interference.

Figure 2 shows that the data rate can be increased by a factor of four by breaking up the amplitude of the carrier into four levels rather than just two and by changing the carrier phase by 180 degrees.

Figure 2: Two Level and Four Level Amplitude Modulation With 180 Degree Phase Shift

Earlier we said that a QAM signal allows for two components to be carried at the same time on a single frequency. This is accomplished by having two carriers at the same frequency, but out of phase with each other by 90 degrees. By splitting six bits of data in half and sending 3 bits on the I channel and 3 bits on the Q channel, 6 bits of data or 64 levels can be sent on a single frequency. This is commonly known as 64 QAM. Figure 3 shows a basic block diagram of a QAM modulator. 256 QAM breaks 8 bits of data into two 4 bit I and Q channels.

Figure 3: Basic Block Diagram of QAM Modulator

Since the I and Q components are 90 degrees out of phase from each other, a way to visualize the QAM modulation is to plot the I and Q components on an X Y graph with the X axis representing the I component and the Y axis representing the Q component as shown in figure 4. Because there are 8 levels of I component and 8 levels of Q component there are a total of 64 different possibilities on the graph. This graph when displayed on a test instrument is known as a Constellation Display.

Figure 5 shows how the QAM data is plotted on the Constellation Display graph. As the data on the I and Q channels change ,the resulting data point that is plotted is located at one of the other 64 possible locations on the graph. The instrument displaying the constellation display can store the position of each of the data points and as the data is plotted, a history of the data is recorded eventually displaying multiple data points at all of the 64 different possibilities. Ideally each of these 64 different plots should be at exactly the same location each time they are plotted. In practice however noise and other problems can shift the plot from the ideal location and a display with 64 (or 256 for 256 QAM) clusters, like the one shown in Figure 6 results. The more noise and interference there is on the signal, the further the plots will be from the ideal location. Analysis of the characteristics of different constellation displays will be covered later in this paper.

Figure 4: I vs Q Components Plotted on a Constellation Display	Figure 5: Constellation Display From CR1200

Forward Error Correction (FEC)

Forward Error Correction (FEC) is a method where additional data bits are added to the digital video bit stream to help identify and correct any errors that may be caused by the transmission system. Data errors if not corrected can cause significant picture impairments that will be readily viewable by the subscriber, and the FEC attempts to correct these errors as much as possible.

A simple way to understand basically how FEC works is to think about filling out an expense report. Most expense reports are organized in a spreadsheet where the rows and columns are totalled and if you entered everything correctly the final total of the rows and columns should be the same and everything checks out. You can think of the information in the table as the data to be transmitted and the sums of the rows and columns the overhead that is added for the FEC. If after transmission one of the numbers in the table changed due to an error, the total of one of the columns and rows would not match what was transmitted and the location of the error can be located and corrected using simple math.

	Monday	Tuesday	Wednesday	Thursday	Friday	Total
Travel	84.77	79.82	73.12	59.58	99.44	$396.73
Meals	32.01	35.11	23.10	25.74	26.21	$142.17
Gas	14.22	15.21	14.22	13.35	11.98	$68.98
Entertain	23.66	23.21	15.63	19.87	17.54	$99.91
Total	$154.66	$153.35	$126.07	$118.54	$155.17	$707.79

Figure 6: Data Before Transmission

	Monday	Tuesday	Wednesday	Thursday	Friday	Total
Travel	84.77	79.82	73.12	59.58	99.44	$396.73
Meals	32.01	35.11	23.10	25.74	26.21	$142.17
Gas	14.22	15.21	14.22	13.35	11.98	$68.98
Entertain	23.66	23.21	15.63	.87	17.54	$99.91
Total	$154.66	$153.35	$126.07	$118.54	$155.17	$707.79

Figure 7: Identification and Correction of Error
Error Can be Corrected by Calculation Using Totals

Another method of reducing the effect of errors is by using Interleaving. Data errors tend to come in bursts and a missing chunk of data can cause more errors than the FEC can handle. By taking the data stream and essentially mixing it up by transmitting the bits in a different order the error burst is spread out over a larger amount of data, making it easier for the FEC to handle.

Errors that exist prior to the FEC circuitry in the set top may be completely removed. It is important to determine if the FEC circuitry is correcting errors because this means that if additional errors caused by worsening system problems where to happen, then the FEC may become overloaded and not be able to handle it all. This is why it is important to determine the amount of errors prior to the FEC (pre FEC) and after the FEC (post FEC) to see how they compare. If there is significantly less errors post FEC, then it may be difficult or impossible for the FEC to handle many additional errors and the picture quality will suffer.

Digital Video Measurements

Average Channel Power

Just as in analog systems, maintaining the correct carrier power levels on digital carrier throughout the system is crucial for optimum system performance. Digital carrier levels that are too low will suffer from poor carrier-to-noise ratio, resulting in errors. Carrier levels that are too high can cause intermodulation distortion that can negatively effect other carriers on the system.

Carrier power on digital carriers is measured differently than analog carriers. On an analog carrier the peak carrier power occurs only during the sync pulses of the video signal and at the video carrier frequency. At all other times and frequencies, the carrier power of the analog carrier is much lower, so it is the peak carrier power at the video carrier frequency that you are interested in. On a digital carrier ,the power is spread out more evenly over time and frequency so to get an accurate representation of the power you must measure the total power across the frequency band.

Figure 8: Analog Channel Spectrum	Figure 9: Digital Channel Spectrum

The CR1200 measures average power using a 6 MHz filter and an average power detector. If you try to compare this reading to a signal level meter that was designed to measure only analog signals, you will find there is a significant difference because of the peak detection and measurement bandwidth. Also depending on the shape of the digital carrier and where the measurement was taken on the carrier there will also be differences. It is not really practical to add a correction factor to an analog signal level meter ,as this difference will not be consistent.

In addition to measuring Average Channel Power the CR1200 can also measure peak power on analog carriers. This mode eliminates the need to carry a second instrument to test analog carriers. Both the video and audio carrier levels are displayed graphically and numerically.

Figure 10: Digital Signal Level Meter	Figure 11: Analog Signal Level Meter

Constellation Display

The constellation display is a very effective troubleshooting tool that can help isolate system problems. By observing the shape of he display and how the points are plotted, you can quickly determine how the QAM signal is being impaired and in some cases determine what could be causing the problem.

As stated earlier, the constellation display is an XY plot of the I and Q components. Ideally there should be 64 well defined points making up a perfect square. Noise, interference and gain problems can cause the display to change.

Figure 11 shows a low noise QAM constellation display with reasonably well defined points. Figure 13 shows the same QAM signal with significant amounts of noise added. Notice that the points vary significantly as a result of this noise. Also notice that the measured bit error rate has changed significantly as a result of this noise.

Figure 12: Low Noise Constellation	Figure 13: High Noise Constellation

The gain of the I and Q components can also effect the display. Determining whether one or both of the components are effected can help isolate the location of the problem. Figure 14 shows a constellation display where the I and Q components are not balanced ,indicating only one component is being effected. Notice that the display is no longer a square but more rectangular. The width of the trace is narrower than the height. In this case the QAM modulator or baseband amplifiers and filters should be checked.

Figure 15 shows a QAM signal where both the I and Q components are affected equally. In this case the higher amplitude, outer points are pulled in towards the center while the central points are not affected. This indicates gain compression on both the I and Q signals and can be caused by IF and RF amplifiers and filters, Up/down converters, clock recovery circuits and IF equalizers.

Figure 14: I Q Imbalance	Figure 15: Gain Compression

Figure 16 shows a constellation display with coherent interference. Notice how the clusters take on a circular shape. Examples of coherent interference include CTB, CSO, ingress and spurs. Figure 17 shows a constellation display with excessive phase noise. Phase noise problems are caused by problems with the headend equipment.

Figure 16: Coherent Interference	Figure 17: Phase Noise

Modulation Error Ratio (MER)- Signal-to-Noise Ratio

Modulation Error Ratio is analogous to the Signal-to-Noise measurement made in analog systems. As we mentioned earlier, noise on the system will cause the points on the constellation display to change randomly from their ideal location. If the noise is high enough, the points could extend into the range of adjacent points causing errors. MER is a measure of the ratio of the error power to the average power in an ideal QAM signal. Poor MER is an early indicator of channel impairment. The manufacturer of the set-top will specify the minimum amount of MER or Signal-to-Noise ratio the set-top converter can handle before significant errors occur. Ideally you should have at least 4 or 5 dB of margin from this point to allow for system degradation.

MER measurements are useful for early detection of non-transient (noise) impairments. BER should be used for testing for transient impairments. Examples of non-transient impairments include: system noise, CSO, CTB, Ingress, distortion due to laser clipping and modulator problems.

Figure 18: MER Display

Bit Error Rate (BER)

Bit Error rate measures how often an error occurs in a given amount of data. The more errors there are, the more difficult it will be for the FEC circuitry to correct those errors before causing picture problems. The CR1200 measure the BER pre and post FEC allowing the user to determine how hard the FEC is working to correct errors.

BER is displayed as a ratio of the number of errors to correct data bits and is given in scientific notation. For example, if 1 out of every 1000 bits was an error, this can be written as a ratio of 1/1000. Another way of writing this would be 1/(1 X 103) or 1 X 10-3, so a signal with 1 out of every 1000 bits in error would be displayed on the CR1200 as 1.0E-3.

BER is useful for measuring long term system performance and periodic transient impairments that can occasionally effect system performance.

Figure 19: Pre and Post FEC BER Display

Adaptive Equalizer Response

Digital Carriers can be very susceptible to reflections in the cable caused by a variety of problems such as bad splitters, bad connectors, and kinks or damage to the cable. Reflections can cause errors in the signal because a reflection of a particular bit of data can overlap data bits that come later. To compensate for these reflections set top converters have an adaptive equalizer that senses the amount of reflections and compensates for them. It is important to measure how hard the adaptive equalizer is working to compensate for reflections, because if it is working hard, any further degradation in the cable may be too much for the equalizer to handle and errors may result.

You can think of the Equalizer Response like the spectrum of a filter, only instead of frequency being displayed along the horizontal axis, time is displayed. Each bar represents one-half the symbol rate or 100 ns in time. The vertical line next to the tallest bar indicates the time when the bit or symbol should occur. Bars prior to this line indicate time prior to the symbol and bars after this line indicate time after the symbol. If one or more bars were high after, or to the right of the tallest bars, then the equalizer is adding compensation at that time. It would be like a frequency being modified using a frequency equalizer, only in this case the level is not changed at a frequency, but at the particular time after the symbol ,when the reflection exists, reducing the gain of just the reflection.

Figure 20 shows an Equalizer Response display of a good signal that needs very little compensation in the set top. Figure 21 shows and Equalizer Response display of a signal with a reflection. Notice that the fourth and fifth bar (400 and 500 ns) to the right of the zero line are significantly higher than in the normal display. This indicates that the equalizer is providing significant equalization at those times. The reflection is so high in this case that it is within a couple dB of causing the system to fail.

Figure 20: Normal Adaptive Equalizer Display	Figure 21: Adaptive Equalizer Display With Reflection

The various set top converters available have a recommended maximum amount of equalization that should be added so that the equalizer is not overloaded. The amount of maximum compensation varies by time after the symbol time and you should consult the manufacturer of the set top converter for recommended maximum equalization at the particular times. If the recommended equalization is exceeded at the set top, you should work backwards towards the drop to see where the equalization needed is reduced. This will indicate the location of problem components such as cable, connectors or splitters.

Spectrum Analyzer Display

The Spectrum Analyzer Display is a useful tool for identifying the type of signals at a particular frequency. Figure 23 shows a spectrum analyzer display with a QAM Digital carrier and figure 22 shows an conventional analog video channel. The spectrum analyzer display can also be used to find non-desirable signals such as spurs. A cursor is available to measure the level at a desired frequency. The knob controls the marker frequency and the frequency and level of the marker are indicated on the left of the screen.

Figure 22: Analog Channel Spectrum	Figure 23: Digital Channel Spectrum

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