Digital Cable Television An Overview

This article describes the technological building blocks of a present day Digital Cable TV System. It also explains how the existing coaxial cable distribution networks can be upgraded to modern bi-directional hybrid fiber coaxial networks. After describing how this new infrastructure could be utillised by cable service providers to offer to their subscribers a variety of new enhanced/ interactive services it concludes with a brief review of TRAIs recommendations on digitalization of Cable TV Networks in India.

Introduction 

Recently Telecom Regulatory Authority of India ( TRAI) has recommended a national plan for digitalisation of Cable TV Networks in India in a phased manner. The first phase will commence in April 2006 and will end in 2010 commencing with the Commonwealth games to be held in India during that year. During this phase TRAI wants digital services to be made available in all cities/urban areas with a population of 1 million plus. 

It is in view of this development that this article has been prepared. In it we first describe the various technologies used in a digital Cable TV System. The objective here is not to burden the readers with too many technical details but at the same time convey to them the essential functional features of each of the technology blocks and to show how each of these fit into the overall system architecture of a present day state of the art digital cable TV system.

Having done this, it is now time to have a close look at the existing analog Cable TV Systems, primarily to ascertain what needs to be done to upgrade them during the digitalisation. Naturally this process would require each cable operator to make a large investment. The actual size of the needed investment will depend on the current state and size of his network. Looked at from Cable operators point of view, the question is: What do they get in return for these huge investments?

It is this question which is dealt with next in the article. Digital Cable operators will not only be able to deliver very high quality video/audio signals but also 10 to 13 times more TV programme channels to their subscribers. In addition to this, the new infrastructure will enable them to deliver several new value added services, thus reaching out to new segments of the population which in turn will result in new revenue streams.

And now the most vial question ‘ how fast will be the take up of digital cable services? The answer to this question will naturally depend upon the type of services offered. These have to be carefully designed keeping in mind the fact that people do not buy a technology, they buy services. A few of such services are described next. The article concludes with a brief discussion of some of the regulatory issues. 

Digital Cable TV- the building blocks 

Just like in analog Cable TV system, the digital cable TV system is also made up of two main constituent parts : The headend and the distribution system. However there are considerable differences between the two systems. A digital headend have many more functional blocks compared to its analog counterpart and a digital distribution system has a much higher bandwidth of 860 MHz and even approaching 1 GHz, and is thus a hybrid fiber coaxial system with coaxial cable being restricted to subscriber drops.

At the digital headend, the video/audio components of all incoming ‘analog’ TV channels are first converted into the digital formats. This process is carried out in accordance with well established international standards. In the digital format the signals are represented as streams of binary digits (bits for short) , each bit having a value of either ‘I’ or ‘O’. An important parameter of a stream is the rate at which the bits are generated in the analog to digital converter. This is called ‘bit rate’ and is measured in ‘bits per second’. The bit rate depends upon the type of signal being converted to digital format for example the video component of a TV signal has a bit rate of 216 Mbits. In an analog cable TV system, each 7 or 8 MHz wide cable channel carries one analog TV programme channel, however in the case of a digitised signal, the RF Bandwidth required for transmission depends upon the ‘spectral efficiency’ of the selected digital modulation scheme. For example, for transmitting a 216 M bit/s video stream over cable using the best available modulation scheme ( 256 QAM) having a spectral efficiency of 8 bits/s/ Hz) would have required 27 MHz wide channels.

A solution to this problem is provided by the digital video /audio compression technology as laid down in a well known international standard called MPEG-2 ( Moving Picture Expert Group). This process works by removing from the incoming bit stream all redundant bits and thus bringing down the bit rate. For this reason it is also called a ‘bit rate reduction’ technology. It is obvious that compression will always result in some loss of resolution as well as introduce some distortion in compressed signals. However the video/audio compression process is based on the ‘psycho visual properties of the human eye as well as the ‘psycho acoustic’ properties of the human ear. Put in simple words, what this means is, the viewers and listeners will fail to notice the resulting loss in resolution or increased distortion. 

At the digital headend a MPEG-2 encoder carries out this process. The output bit rate of an encoder is usually set by the cable service provider and is usually in the range of 4 to 6 Mbit/s per channel. Thus a compressed bit stream can now be transmitted easily through cable channels of 8 MHz. In reality, after compression more than one digital TV channel could be transmitted through a cable channel. Let us illustrate this by an example - using a 256 QAM modulation scheme we can transmit a total bit rate of about 50 Mbit/s through a 8 MHz cable channel. This means we can multiplex or combine together some 8 to 12 digital TV channels and then transmit that multiplex through a single cable channel.

Thus whereas an analog Cable TV channel transmits single TV programme, in digital cable, each channel carrier a ‘ multiplex of 8 to 12 TV channels. Each MPEG-2 encoder compresses the input video/audio signals and multiplex these together to form for what is known as a ‘single programme transport stream’ ( SPTS) at its output. SPTS from a number of Encoders as required are connected to the inputs of another device called the ‘MPEG-2 Transport Stream Multiplexer”.

As the name implies, it is the transport MUX which generates the final ‘multiplex’ of digital cable TV channel meant for transmission through each of the cable channels.

The multiplexer generate a MPEG-2 transport stream (TS) which is a stream of fixed length ( 188 bytes) packets. Each packet has a 4 byte ‘header’ and a 184 byte payload. The header contains all the information needed for uniquely identifying the packet as well as its contents ( i.e whether it contains video, audio or other data etc.).

Another important functional unit is called service information ( SI) generator. This could be either built into the multiplexer itself or it could be a stand alone external unit. The SI is arranged in tabular form and is carried in the TS. The purpose of SI could be explained as follows unlike analog cable TV digital cable TV carries many many more programmes and it would be very difficult for a subscriber to memorise all the channel numbers. In order to enable him to navigate through the umpteen number of available channels, every digital cable Set Top Box ( STB) runs an application called a built in navigator or an Electronic Programme Guide ( EPG). In its simplest form an EPG displays a ‘list of all available programmes alongwith some other information about those programmes. All this information is available in SI tables. Once the subscriber selects a programme the STB should extract the selected programme and display it to the viewer. First of all the STB should de-multiplex each received multiplex into its constituent TV programmes. The de-multiplexer in the STB does this job by making use of the information contained in some other SI tables carried in the TS. 

The MPEG-2 TS outputs from each of the multiplexer next goes through an FEC ( Forward Error Correction ) coding process. The FEC coder computes and appends with each packet 16 check bytes thus increasing the packet length to 204 bytes. ( In the STB, the ‘FEC decoder’ uses these ‘check’ bytes to detect errors in transmitted packets which occurs during transmission, and then corrects those errors ). The TS next goes through another process called ‘ time interleaving the propose of which is to protect the stream from lengthy error bursts. 

The FEC coded transport stream then goes as input to a digital QAM ( Quadrature Amplitude Modulator ) unit. The user can select from a list of available options for this modulation the choice being 16 QAM, 32 QAM, 64QAM , 128 QAM, and 256 QAM. The 16 QAM unit works like this : it takes from the incoming TS , 4 bits at a time. This combination of 4 bits is called a 16 QAM symbol and it can have 16 possible values. The modulator charges the amplitude and instantaneous phase of a RF carrier in accordance with value of the transmitted symbols. Finally the outputs of a bank of QAM modulators are combined and fed into the distribution network. 

It is obvious from what has been stated above that unlike its analog counter part, the digital cable headend deals with 10 to 12 time more incoming TV programmes channels and there should be adequate arrangements for routing /switching these many channels. 

While most of the content is acquired by downlinking Satellite distributed signals, some content may also be receive via other media such as Telco networks. IP networks etc. Content may also be acquired from a local studio playout.

Satellite distributed content may already be in MPEG-2 digital format. It is also possible that some times one full multiplex received from a transponder may directly be transmitted in one Cable channel. If this is the case, then a digital receiver having the capability of outputting a de-modulated and FEC decoded MPEG-2 TS is used and its output fed directly to the transport multiplexer of the desired cable channel. In other cases, some of the channels from the received satellite multiplex are replaced by some local channels.

Another important functional unit in the headend is the Conditional Access System ( CAS) CAS is the technology which enables a cable service provider to implement ‘pay TV’ services. From the transmitted bunch of channels some premier channels are named as ‘pay channels. Whereas all other channels will be available to al subscribers, the pay channels will be only made available to those subscribers who purchase them at the operator quoted price.

The CAS have two main parts. The first part is a process used to make the pay channel ‘unintelligible’ by ‘scrambling it. The second part is a methodology used to make the signal intelligible again by transmitting certain encrypted messages to the ‘set top’ boxes of paid subscribers. Although these messages will be available in the TS, only authorized STBs will be able to decrypt them.

Source: http://cablequest.org/articles/broadcast-technology/item/1300-digital-cable-television-an-overview.html