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December 2017

MEASURING DIGITAL CATV SIGNALS IN THE FIELD


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


INTRODUCTION

Modulation is essential for transmission of 2 or more signals simultaneously. Modulation avoids any interference between the 2 signals and also ensures that signal errors are avoided during transmission.

We are all familiar with the common Analog modulation techniques of Amplitude Modulation (AM) and Frequency Modulation (FM). It may come as a surprise for us to realise that the first modulation of electrical signals was really digital modulation. Samuel Morse invented the Morse Code where every English alphabet and number was coded as a sequence of a dot (.) and a dash (-). This is similar to translating each character of the English alphabet into either a zero (0) or a one (1), of the digital code as we know it today.



THE DIGITAL BIT

Before we dwell further on the topic lets take a brief look at the term "Bit". All digital information is transmitted as a series of zeros (0) and ones (1). These two symbols are called Bits. The number of Bits could be considered also as a unit to express the amount of information that is being transmitted. As an example, if a million symbols (0 & 1) were transmitted, it would imply 1 Mega Bit.

To help transmit digital Bits over any significant distances, different modulation schemes have been devised. However, the requirements for broadcasting a satellite signal would be rather different from the requirements for transmitting a digital signal over a cable network. We will take a look at these shortly.


DVB

We have been used to referring to Digitally compressed Video Broadcasts simply as DVB. Usually we have referred only to Satellite DVB broadcasts. Satellite DVB broadcasts are more correctly DVB-S. Similarly DVB broadcast for cable are correctly referred to as DVB-C, and for Terrestrial Digital broadcasts it is DVB-T.


MODULATION BASICS :

We are all familiar with the 2 most common forms of Analog Modulation viz. Amplitude Modulation (AM) and Frequency Modulation (FM). For Amplitude Modulation, the signal strength of the carrier is varied or modulated. For FM, the frequency of the carrier is modulated.

For Digital Modulation, instead of varying either the Amplitude or Frequency of the carrier, it is preferred to vary or modulate the Phase of the carrier (for a brief explanation of the Phase of a signal, refer to the next page).

Phase Modulation is preferred for Digital transmissions because it offers better protection for transmitting signals which carry binary information i.e. are either 0 or 1 i.e. fully ON or fully OFF. If AM or FM was used with digital signals, the modulation would vary between two extremes of full modulation or no modulation only. This would further complicate accurate demodulation as well as affect the spectral density of the transmitted signal. Often a combination of Both Phase & Amplitude Modulation is used.



UNDERSTANDING PHASE

A cyclically varying electrical signal ( Fig i ) can possess 3 variations viz.:

Changes in the signal strength or level. These are also referred to as Amplitude Changes. Two signals with different amplitudes, but otherwise identical are shown in Fig. A.

Alternatively, the signal level or amplitude may remain fixed but its frequency may be varied. This is shown in Fig. B.

Modulation methods that change the frequency or amplitude of a signal are quite common in consumer electronics and referred to as Amplitude Modulation (AM) and Frequency Modulation (FM) respectively.

Phase is a property which compares the starting point of two signals. A complete signal cycle is shown in Fig. i and has a total phase variation of 360 Degrees.

Two signals are said to be in phase when the timing of their starting points coincide.

Two signals are said to be out of phase by 90 degrees if one of them is at its starting point (or zero crossing) and the other is Out of Phase or delayed by 90 degrees.

From this it is apparent that signals can be characterised and therefore separated, based on their phase differences between each other. This property is used in phase modulation which is a key technique for modulating digital signals.


DIGITAL MODULATION:

There are two major categories of Digital Modulation. One category uses a constant amplitude carrier and carries the information in Phase or Frequency variations, known as Phase Shift Keying or Frequency shift keying (FSK).

Digital (DVB-S) satellite broadcasts universally use Phase Modulation - actually QPSK.

The other category conveys the information in carrier amplitude variations and is known as amplitude shift keying (AMP). A combination of FSK & AMP are employed for CATV Digital Transmissions.


DIGITAL MODULATION:

The accuracy of the transmitted digital signal is measured by "Bit Error Rate" (BER). Simply put, Bit Error Rate is:

            The number of Error Bits

BER = ----------------------------------

            The total number of Bits

A lower Bit Error Rate implies that the signal has been more accurately transmitted and demodulated. As we shall see later, a Bit Error Rate of one error in 10,000 Bits transmitted is quite normal for modulated signals. After error correction is applied, the Error further falls down to one part in 100,000 Million Bits !


CATV TRANSMISSION - Q.A.M :

Quadrature Amplitude Modulation (QAM) systems utilise changes of BOTH, Phase Shift Keying and Amplitude Shift Keying to increase the amount of information carried i.e. the number of states per symbol. Each state is defined with a specific variation of BOTH - Amplitude AND Phase.

QAM modulation is ideal for use in CATV networks.

A cable system provides different transmission characteristics compared to satellite transmissions. A system such as QAM must be able to address the following needs, if it is to be successfully employed for Digital Modulation in a CATV system.

The bandwidth allocated per channel is restricted - just 6 to 8 MHz (depending on the TV system such as PAL, NTSC or Secam. Hence the Digital Modulation system must densely pack the digital data in a small bandwidth (unlike a satellite based transmission).

♣ The signal levels are significantly higher than for satellite transmissions. Since the Carrier (signal strength) is larger, the Carrier to Noise (C/N) ratio is always fairly good in a CATV network.

♣ A large number of channels are modulated and carried simultaneously on the same cable. Hence the modulation scheme should provide good Inter Channel Interference suppression.

QAM comfortably meets all these requirements.

Since the Phase and Amplitude are varied in QAM Modulation, a large number of states or possible discreet values can be created to provide dense Digital Modulation. Hence designers have created 16 Bit, 64 Bit and higher QAM Modulation. Fig.2a & 2b shows graphically 16 QAM and 64 QAM.

Each time the number of states or options per symbol is increased, the bandwidth efficiency also increases. This bandwidth efficiency is measured in bits per second/Hz.

As higher density modulation schemes are adopted, the Decoder or Demodulator gets progressively more complex. A benefit of digital technology is that higher complexity does not necessarily mean a higher cost to the customer, since Large Scale Integration (LSI) ICs can be mass produced at reasonable cost, if a large demand exists. Consumers can look forward to fairly sophisticated QAM Receivers or Demodulators, at reasonable prices.


ERROR CORRECTION

Digital Data like any other data is prone to errors during transmission. As explained earlier in this article, Digital Modulation aims to minimise the errors (BER). Even fairly high BERs can cause visible deterioration in the picture. Hence some additional methods for further correcting errors during transmission have been devised.

Most Digital Error Correction relies heavily on the pioneering work done by 3 mathematicians - Reed, Solomon and Viterbi. The Error Correction schemes have been named in their honour, Reed- Solomon (R-S) and Viterbi Error Correction.

An Error Correction system transmits a small amount of extra data which provides some indication (e.g. Checksum) of the previous Bits of information. If the extra Bit does not tally with the previous data, that particular data stream is considered to be in error and is rejected.

As a simple example of checksum - supposing the numbers 2,3 & 4 are to be transmitted, an extra Check-sum digit 9 will also be transmitted, viz : 2,3,4,9. The last digit 9 is a sum (total) to check the 1st 3 digits.


FORWARD ERROR CORRECTION (FEC)

When Digital data is transmitted over a system that provides two way communication i.e. from the sender to the receiver and back to the sender (This is often the case when computers exchange digital data). The receiver can request the sender to resend a packet of Bits that has not been received well.

However, in Digital Broadcasting (either DVB-T or C or S) there is no return path. Hence the Error Correction system must be powerful enough to recognise and recover the correct data even from a packet of corrupted Bits. Since the Error Correction has to function purely over a One Way or Forward Path, it is called Forward Error Correction (FEC).

As explained earlier, all schemes of Error Correction transmit extra data for the correction. An FEC system denoted as 3/4 implies that one extra Error Correction Bit is transmitted for every 3 data Bits. Similarly for a 7/8 FEC, one Error Correction Bit is transmitted extra for every 7 Bits of data. As one can imagine, an FEC of 3/4 provides better correction and is less prone to error and noise than an FEC of 7/8. To that extent, an FEC of 1/2 would be extremely robust since it would carry one correction Bit for every data Bit. Ofcourse an FEC of 1/2 would be extremely wasteful on transmitter capacity but could be considered where critical data is to be transmitted. An FEC of 1/2 is never utilised for broadcasting entertainment programmes.


DIGITAL TRANSPORT STREAMS

In an analog CATV system each channel is modulated on a specific frequency.

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).

In practice the total number of digital channels is usually divided among multiple Transport Streams. This provides many benefits viz:

1. Different Transport Streams may be transmitted in different frequency bands.

2. Multiple Transport Streams can be created, each with different QAM ratios.

3. If one Transport Stream is not received well, the other Transport Streams may continue normal transmissions.

The first Transport Stream is usually referred to as the "Home Transport Stream" (Home TS). This carries all the reference information (NIT: Network Information table) for all the digital channels on that Cable TV network, including information on channels carried in other data streams. As a result it is extremely important that the Home TS should be accurately transmitted by the digital CATV network and received by the consumers' digital STB. When the STB searches for digital channels, it actually looks for the "Home TS”. Once the "Home Transport Stream" is received, the STB has all the necessary information of all the digital channels on the network.

To ensure that the "Home TS" is accurately transmitted, networks often use basic 64 QAM for the "Home TS", which also carries a few digital CATV channels. All other Cable TV channels may be carried in separate digital streams that can use more dense QAM e.g. 256, 512 or even 1024 QAM.

In poor quality CATV distribution networks, the QAM, 256 and higher data streams may not be received well by the STB. However a QAM-64 based "Home TS" will certainly be received, enabling the STB to be initialised and start searching for the other data streams.

Some networks include the NIT information on each of the Transport Streams. This facilitates update of STBs automatically, with the program information when additional channels are added to the network.

Next Month, Part-2 of this article will provide details of measurements & their interpretations, along with detailed screen shots. n