Scatmag.com

February 2018

MEASURING DIGITAL CATV SIGNALS IN THE FIELD


Part - I I
Based On Repeated Readers Requests, We Bring You This 2 Part Article, Explaining How Field Technicians Can Measure & Interpret Digital CATV Signals.
Last Month, Part-I Provided A Technical Background On Digital CATV Signals.
Part-II This Month, Will Provide Details Of Measurements & Their Interpretations, Along With Detailed Screen Shots.


CATV MEASUREMENTS

Measurements of the transmitted Cable TV signal are essential for setting up and maintaining any network, professionally.



ANALOG LEVEL

Analog Cable TV networks essentially require measurement of the signal level to ensure that at least 60 dBU reaches the TV set. This is an absolutely basic requirement.

In addition the quality of analog signal can be evaluated by measuring other parameters such as C/N (Carrier to Noise Ratio), S/N (signal to Noise Ratio), CSO & CTB distortion, etc. Measurement of these quality evaluating parameters is far more difficult than simply measuring the signal level. As a result, in the field, typically only signal levels are measured. Sometimes, C/N is also measured in the field. However, since analog CATV provides for gradual deterioration of the picture, simply viewing the picture quality, provides and indicator is all is well.

Since most of the analog CATV signal power per channel is transmitted in the video carrier, analog signal level (dB) meters usually only measure the video carrier strength. This is shown in figure 3.


DIGITAL LEVEL

A digital Cable TV signal (figure 4) consists of a very large number of carriers, most of which are approx equal. Hence measurement of the digital CATV signal requires measuring and averaging multiple digital carriers. Digital signal level meters will provide a direct read-out of the digital signal level / strength. Even most digital STBs will provide a reading of the digital signal received in dBU, on one of its menu screens.


BIT ERROR RATE (BER) & QUALITY

To evaluate the quality of a digital CATV signal, it is necessary to measure the B.E.R. (Bit Error Rate), which is similar to the C/N measurement in an analog Cable TV network. Part-1 of this article (last month) explained BER.


Good Signal BER 1.0E-8
Threshold for visible degradation BER 1.0E-6
Dangerous BER 1.0E-5
Bad BER 1.0E-4
Table-1 : Acceptable B.E.R
Note: BER = 1.0 E-4 represents an error of 1 bit in 10,000 (1.0 E4 = 1 followed by 4 zeros = 10000.) Similarly, 1E- 6 represents 1 bit of error in 1 million bits.



PRACTICAL BER TESTING

Although it is a popular quality indicator, BER is often not tested correctly. Because they require several million bits of data, BER tests take time to complete. The QAM (64/128/256) and the symbol rate used ultimately determine time required to test BER. A more complex QAM results in more data transmitted in a given period of time, users must wait longer for a BER test to complete on QAM 64 than on QAM 256.

As an example, if a technician wants to measure BER to 1E-8 (1 error in 100m bits) he must wait for the test equipment to receive at least 100 million bits. On a Cable TV network with QAM 64, it will take 3.3 seconds. For QAM 256 it will take 2.3 seconds. Testing BER to 1E-9 on QAM 64 increases the test time to 33 seconds!

As a result individually testing QAM channels becomes very time consuming. Also considering the fact that a BER of 1E-8 will result in very good picture, most field technicians are instructed not to test for BER higher than 1E-8.


M.E.R

The M.E.R. (Modulation Error Ratio) measurement in a digital CATV network is similar to S/N measurements in an analog network. Like SNR, MER is usually expressed in decibels (dB).


Table-2: Results Of M.E.R. Changes In A Digital CATV Network
Level
Typical MER Results
64QAM
256QAM
Outstanding
36
37
Excellent
35
36
Very Good
31
34
Good
28
31
Marginal
24
29
Dangerous
23
28

Both BER and MER can be directly read out on a digital CATV analyser.

NOTE:

Using BER for trouble-shooting and fault location is not repeatable and very inaccurate.

• BER is best used to verify that the digital picture is being delivered with adequate performance margin, and hence will not pixelise or freeze, during regular operation.

• A small variation in MER (+/- 1 dB) will cause a large variation in BER measurement.


ERROR LIMITS

A digital TV picture will appear almost perfect, tolerating certain BER deterioration. However, once the BER increases (more errors) beyond a certain threshold, the picture suddenly pixelises or freezes. Beyond that limit the picture disappears altogether.

This behaviour is quite different from that of analog CATV where the picture gradually deteriorates and even a very, very poor transmission will yield a faint (but unacceptable) picture.


FEC

As explained in Part 1 of this article, Forward Error Correction (FEC) is a mathematical system used to automatically correct errors in digital transmission i.e. where a '1' is transmitted instead of a '0' or viceversa.

FEC can improve B.E.R by 10 times or more. As a result, FEC is always used for transmitting Digital CATV signals.


NOISE & DISTORTION ON A DIGITAL NETWORK

All Digital CATV networks use analog CATV amplifiers & optical nodes during transmission, which will produce noise and distortion, if their signal levels are not properly set.

The distortion and noise present on a digital CATV network can only be assessed in the field, by taking a closer look at the constellation diagram displayed by a digital signal meter. This will be discussed below.


DIGITAL CONSTELLATIONS

Part 1 of this article explained the concept of a digital constellation and also indicated a perfect digital constellation. This has been shown once again as figure 6-A.


CONSTELLATION PROBLEMS & POSSIBLE CAUSES

Every digital analyser used in the field by digital CATV technicians displays a square with series of dots. This is referred to as a digital constellation. It is up to the Cable TV field technician to closely look at the digital constellation and recognise the error and its cause. This needs practice and a sharp eye. We detail below various digital system problems that will show up in the constellation, and how to recognise them.


CONSTELLATION PROBLEMS & POSSIBLE CAUSES

In a digital CATV system, channels are transmitted digitally in a Transport Stream (TS). Transport streams can contain one or more than one video program. A transport stream that offers just one video program (it can have multiple audio) is called a single program transport stream (eg in a DVD for Home Theatre.) If a transport stream offers more than one video program, it is a multiple program transport stream (eg Digital CATV).


SYSTEM NOISE

Figure 6-B shows the digital constellation of a digital CATV transmission which suffers from severe system noise. The dots are spread out and may not have a sharp outline. If there is excessive noise, the digital analyser will not lock into the signal and no constellation will be displayed at all!


PHASE NOISE

Figure 6-C shows a constellation of a Digital CATV networl, which has a high level of Phase Noise. The constellation appears to rotate at the extremes.

Phase noise is normally gererated by digital Headend equipment such as Transmodulators. It is usually not caused by te distribution network. Hence you will rarely see this problem except if the Digital Headend is not properly set up.


DISTORTION

Even digital cable TV networks deploy analog CATV amplifiers. These generate distortion (e.g. if the amplifier level is set too high) which in turn will also distort the digital CATV signal. Figure 6-D shows a constellation diagram of a CATV distribution network with high levels of CTB and CSO distortion. Each of the dots have a 'hole' in the centre, i.e. they are donut / medu vada shapped. This constellation pattern can also be generated if there is strong signal ingress / plunging due to poor quality or long lengths of coaxial cable used in the network. (It is good practice to try and ensure that each subscriber receives signals to their STBs with less than 300 m of coaxial cable between the headend and the STB.)

Digital interference signals include pagers, Wi- Max transmissions, automobile spark plug noise, etc.

Sometimes only a couple of the constellation dots may be displaced. This indicates laser clipping (in the Optical transmitter) or sweep interference, if a sweep generator is being used to detect and locate signal leakage and ingress.


GAIN COMPRESSION

Figure 6-E shows a digital CATV network constellation affected by 'Gain Compression' caused by Amplifiers either at the Headend of in the distribution chain. Filters, IF equalisers and up/down convertors or Transmodulators can also cause these problems.


INTERMITTENT INTERFERENCE

Intermittent interference will show up as additional dots in the constellation, as shown in fig. 6-F. Such problems are usually caused by laser clipping (in the optical transmitter) or due to intermittent signal ingress, such as that caused by an intermittent transmitter such as a paging system.


I-Q IMBALANCE

All constellation diagrams shown in this article are created by indicating the I channel on the vertical axis and the Q channel on the horizontal axis (See part-1 of this article carried last month). If the I&Q channels are not balance, the constellation diagram will not be a square with equal side. A rectangular constellation is shown in Figure 6-G. This indicates a problem at the headend with the amplifiers / processors used in the baseband signal.


SUMMARY

Analog CATV meters (dB meters) cannot be used on a Digital-only CATV network.

Digital CATV analysers will provide you a direct digital readout of the digital signal level, the BER and MER. These parameters will clearly indicate if there is a problem on the network, even before the digital picture at the consumer premises freezes or goes blank. However, just the signal level, BER and MER readings will not point to the cause of the problem. To identify the cause of problem in the digital transmission, one needs to examine the digital constellation.

Digital CATV analysers provide a display of dots, called the Digital Constellation. A close look at the digital constellation will indicate the cause of the problem. However, one needs to know how to interpret these dots. This article provides basic information on Digital CATV measurements, and broad pointers of how to use a Digital CATV analyser in the field, and interpret digital constellations. The field technician needs to gain actual experience which over time will enable him to rapidly locate problem areas on the network, and resolve reception problems at the digital STB. n