BASIC ARCHITECTURE OF A DIGITAL CATV HEADEND
Cable TV headends through out the country are now seriously considering the addition of digital CATV channels. Besides the advantages of better picture clarity and multi channel sound as well as the potential to deliver HDTV (High Definition Television), the key necessity to shift to digital is large number of channels and the limited analog channel capacity of 106 analog channels on a cable TV network.
Digital CATV provides for carriage of 6 to even 20 digital channels in the bandwidth of a single analog channel. Hence, if 10 analog channels are vacated, that bandwidth can carry 60 to 200 digital channels.
The fact that CAS roll out countrywide is only a matter of time, further adds to the impetus to roll out digital CATV channels from the headend.
This article provides a simple overview of the basic structure of a digital CATV headend. The aim is to provide cable operators an overview and understanding of a basic digital headend.
LOCAL ENCODING - ANALOG TO DIGITAL CONVERSION
Signals from pay or Free-To-Air (FTA) satellite channels are typically available at the headend through an IRD (Integrated Receiver-Decoder) which provides a composite video (analog) output signal along with separate mono or stereo sound signals. Both, the video and audio signals are analog signals are need to be converted to a digital signal for use in a digital headend.
This conversion of the analog video and audio signals to a digital data stream is done by a MPEG-2 Encoder. The MPEG-2 encoder provides a signal stream ofdigital data that contains both, the video and audio digital signals.
One encoder is required per analog TV channel.
http://www.scatmag.com//image/feb13-techart-1.jpg
Hence, if 20 analog TV channels are to be carried as digital channels, the digital headend will require 20 s e p a r a t e encoders to convert the analog signals to digital signals.
Encoders form a crucial component in the quality of the digital signal. If the conversion of analog to digital is not done well, the picture quality will certainly suffer.
The cost of digital encoders used for local encoding is very high and would typically account for a major part of the headend cost. MPEG-2 encoders will typically cost Rs. 20,000 to Rs. 2 lakhs per channel, depending on the brand, quality and facilities offered.
BIT RATE ALLOCATION
MPEG-2 also permits the user to set the maximum digital bit rate of the digital output signal. An analog channel can be converted into a digital channel with bit rates varying from 1.5 MBps to 5 MBps or even higher. The larger the bit allocated to each analog channel, the better the picture quality. However, larger bit rates imply that fewer the digital channels can be squeezed into the bandwidth of 1 analog channel. On the other and a low bit rate of 1.5 MBps may result in a visibly poor digital picture quality. As technology marches on, it has been possible to achieve good picture quality with lower bit rates using MPEG-2 compression.
TYPICAL BIT RATES
Larger bit rates are required for channels where the picture changes rapidly, such as in a sports channel covering a football game. The camera continuously follows the ball and the entire picture changes rapidly. Such channels require a bit rate of 3 MBps to 5 MBps.
On the other hand a News channel often has very little change in picture content from TV frame to TV frame. The news reader's face and background remains almost constant. Such channels require a much lower bit rate. It is generally felt that news channels can be adequately encoded by allocating them a bit rate of 1.5 MBps to 2.5 MBps.
STATISTICAL MULTIPLEXING
Of course, there will be certain period when the sports channel focuses only on the Commentator's face. At these durations, the lower bit rate applicable for News channels would be adequate for the Sports channel.
Similarly if the News channel shows an outdoor clip, it would require a much higher bandwidth.
It would be extremely wasteful if News channels and Sports channels were allocated fixed data rate. This has led to the advanced development of - "Statistical Multiplexing". This examines the picture content of each channel approximately 20 times every second and continuously allocates different bit rates for different channels, depending on the instantaneous picture requirement for each channel.
If fixed data rates encoding accommodates 6 digitals channels per analog channel, statistical multiplexing practically increases it to 10 or 12 digital channels compressed into an analog channel bandwidth.
MPEG-4 COMPRESSION
Instead of MPEG-2, the MPEG-4 standard can also be used for digitising an analog signal. MPEG-4 offers almost 40% better compression that is 40% moredigital channels in the same analog bandwidth.
A detailed discussion on the MPEG-4 is beyond the scope of this article. However, SCAT has carried a detailed article on MPEG-4 in past issues of the magazine.
The MPEG-4 encoders and decoders (STBs) are currently very expensive and rarely deployed on cable TV networks, currently.
DIGITAL INTEGRATION
As indicated above, the cost of digital encoders is typically very high.
A digital headend can therefore save a substantial amount of money if the digital satellite receiver provides for a digital (ASI) output rather than the audio videoanalog outputs. Many professional digital satellite receivers offer such a facility though typically, the digital IRDs distributed by pay TV channels do not offer an ASI (Digital) output. Further, since pay channels "pair" their IRDs and smart cards, it is also not possible for the cable operator to use an authorised smart card with an independently procured digital satellite receiver with ASI output.
This is an area that the TRAI needs to look into and address, to facilitate lower cost digitisation of CATV headends.
If the satellite receiver directly provides an ASI output, no encoder is required and the digital signals can be directly fed into the digital combiner (multiplexer or MUX, See Figure 2)
http://www.scatmag.com//image/feb13-techart-2.jpg
THE MULTIPLEXER
Encoders provide separate digital output for each TV channel as indicated in Fig. 1.
In an analog headend a channel combiner combines multiple analog channels. Similarly, in a digital headend a multiplexer (MUX) combines multiple digitalcannels and creates a "Transport Stream" (TS)
The Transport Stream not only combines the digital channels but also creates a summary of the digital data contained in the Transport Stream similar to the index page of a book which lists the different content and the location of each content.
Multiplexers are typically available to 'combine' either 12 or 20 digital channels.
Such multiplexers accept ASI inputs upto 200 MBps and offer between 1 to 4 outputs. The block diagram in Fig.2 shows (for simplicity) a single ASI output from the multiplexer.
MULTIPLE OUTPUTS
A multiplexer combines several digital channels to form a single transport stream that will be carried in the bandwidth of a single analog channel. Depending on the capability of the encoders and whether statistical multiplexing is used, the number of channels that can be compressed into the space of a single analog CATV channel (8 MHz for PAL-G) varies from 6 channels to as high as 16 to 20 channels.
However, the amount of digital content (MBps) that can be carried on a single analog channel will also depend on the type of modulation used by the cable TV network.
QAM MODULATION
Quadrature Amplitude Modulation (QAM) provides for carriage of a large amount of digital data in a small bandwidth. QAM however requires strong signal strengths with very little noise. Hence QAM modulation cannot be used for satellite transmission but is used universally for digital CATV networks.
(Note: A past SCAT article has explained in detail various types of digital modulations and their applications. The article is available free on scatmag.com - Satellite & Cable TV).
QAM modulation is typically used as either QAM 64, QAM 128 or QAM 256.
QAM 64 offers the least compression and is most tolerant to external noise injected into the network due to poor quality cables, connectors or tap-offs. On the other hand QAM 256 provides the largest number of digital channels within a single analog channel but requires very good networks to transmit digital pictures to the consumer without freezing or pixelising (picture breaking up into small squares or dots).
InCableNet in Mumbai utilises QAM 256 for some of its channels.
MULTIPLEX CONFIGURATION
Depending on whether QAM 64, 128 or 256 is to be used for digital modulation, the multiplexer is to be configured to offer the appropriate mixing. The multiplexer is configured by connecting it to a PC, through SNMP via an ethernet port.
Table 1 shows the different digital output bit rates applicable for QAM 64, 128 & 256.
Typical Bit Rates For Different Levels of QAM Modulation
QAM 64........................... 38 MBps
QAM 128 ......................... 48 MBps
QAM 256 ......................... 51 MBps
The multiplexer can be used for multiple channel inputs with a total bit rate of upto 200 MBps. Hence if the full 200 MBps input capability is utilised, the multiplexer will have to be configured to provide for separate ASI output data streams each of 50 MBps. The cable network will have no choice but to use 256 QAM digital modulation after the multiplexer.
If the network intents to use 128 QAM it will have to reduce the input data rate to the multiplexer by either:
i) Using more compression per channel (hence more expensive encoders or poorer picture quality) or
ii) Using fewer channels.
SCRAMBLING
CAS requires that pay channels be scrambled and the subscriber's STB decodes/un-scrambles only the channels that they pay for.
Hence a digital headend that carries pay channels will typically have to scramble the pay channels.
Fig.3 shows the location of the scrambler in the digital headend.
http://www.scatmag.com//image/feb13-techart-3.jpg
Each multiplex output requires a separate scrambler. The cost of the scrambler can vary very widely depending on the scrambling system used. As a rough estimate a scrambler could cost Rs. 2 lakhs each. Note that if their multiplexer is configured with 4 outputs, 4 separate scramblers will have to be installed, increasing the cost of digital headend very substantially.
For free-to-air (FTA) channels no scramblers to be used and the output of the multiplexer is fed directly to a QAM modulator as shown in Fig. 3.
QAM MODULATORS
A single QAM modulator will modulate multiple channels. All channels within a single transport stream (e.g. 8 to 12 channels) are modulated by a single QAM modulator which costs approximately Rs. 1 lakh. As a result the cost of digital modulation; per Digital Channel; is not very high and in fact comparable with the cost of good quality analog modulators.
The QAM modulator can be user configured for QAM 64, QAM 128 or QAM 256 modulation.
OTHER OPTIONS
There are a few other options for configuring the digital headend. These will be briefly touched upon and some of these topics will be covered in greater detail, in separate article. On the other hand, topics such as transmodulators have already been covered in detail in earlier articles in this magazine. These earlier articles are available free on scatmag.com - Satellite & Cable TV.
http://www.scatmag.com//image/feb13-techart-4.jpg
TRANSMODULATORS
Satellite transmissions use QPSK modulation. Cable TV transmission use QAM modulation. A transmodulator simply converts the digital signal from the satellite which has QPSK modulation into a digital signal, with the same properties but with QAM modulation. Such a QPSK to QAM converted digital signals can be directly mixed at the final output of the digital headend.
IP DIGITAL SIGNALS
In this entire article we have referred to the digital signal in the ASI digital format. While ASI is very widely used in most digital heads worldwide, it is not the only option.
The wide adoption of the internet had led to widespread use of the IP (Internet Protocol) digital data format. Hence, it is possible to use equipment that converts the analog signals into an IP digital data stream instead of an ASI digital data stream.
The IP signals can then be multiplexed and modulated. However an IP multiplexer will be required.
Some international manufacturers such as Teleste now offer a combination unit of Multiplexer + QAM Modulator for approx. Rs. 15 lakhs.
The use of IP digital signals through out the digital headend enable IPTV to also be easily deployed from the same digital headend. However IPTV is usually deployed by telephone companies using digital modems and IPTV set top boxes.
Cable TV distribution networks typically use DVB-C STBs.
A detailed review of IP digital streams is beyond the scope of this article and may be covered in a separate article, some time in future.
SUMMARY
This article was intended to provide a quick and basic overview of how a digital cable TV headend is configured.
All these past article are available free onscatmag.com - Satellite & Cable TV. Readers are encouraged to review these past articles. n
This Article Was Published Earlier In Satellite & Cable TV Magazine - Ed.
Source: scatmag.com - Satellite & Cable TV
Digital CATV provides for carriage of 6 to even 20 digital channels in the bandwidth of a single analog channel. Hence, if 10 analog channels are vacated, that bandwidth can carry 60 to 200 digital channels.
The fact that CAS roll out countrywide is only a matter of time, further adds to the impetus to roll out digital CATV channels from the headend.
This article provides a simple overview of the basic structure of a digital CATV headend. The aim is to provide cable operators an overview and understanding of a basic digital headend.
LOCAL ENCODING - ANALOG TO DIGITAL CONVERSION
Signals from pay or Free-To-Air (FTA) satellite channels are typically available at the headend through an IRD (Integrated Receiver-Decoder) which provides a composite video (analog) output signal along with separate mono or stereo sound signals. Both, the video and audio signals are analog signals are need to be converted to a digital signal for use in a digital headend.
This conversion of the analog video and audio signals to a digital data stream is done by a MPEG-2 Encoder. The MPEG-2 encoder provides a signal stream ofdigital data that contains both, the video and audio digital signals.
One encoder is required per analog TV channel.
http://www.scatmag.com//image/feb13-techart-1.jpg
Hence, if 20 analog TV channels are to be carried as digital channels, the digital headend will require 20 s e p a r a t e encoders to convert the analog signals to digital signals.
Encoders form a crucial component in the quality of the digital signal. If the conversion of analog to digital is not done well, the picture quality will certainly suffer.
The cost of digital encoders used for local encoding is very high and would typically account for a major part of the headend cost. MPEG-2 encoders will typically cost Rs. 20,000 to Rs. 2 lakhs per channel, depending on the brand, quality and facilities offered.
BIT RATE ALLOCATION
MPEG-2 also permits the user to set the maximum digital bit rate of the digital output signal. An analog channel can be converted into a digital channel with bit rates varying from 1.5 MBps to 5 MBps or even higher. The larger the bit allocated to each analog channel, the better the picture quality. However, larger bit rates imply that fewer the digital channels can be squeezed into the bandwidth of 1 analog channel. On the other and a low bit rate of 1.5 MBps may result in a visibly poor digital picture quality. As technology marches on, it has been possible to achieve good picture quality with lower bit rates using MPEG-2 compression.
TYPICAL BIT RATES
Larger bit rates are required for channels where the picture changes rapidly, such as in a sports channel covering a football game. The camera continuously follows the ball and the entire picture changes rapidly. Such channels require a bit rate of 3 MBps to 5 MBps.
On the other hand a News channel often has very little change in picture content from TV frame to TV frame. The news reader's face and background remains almost constant. Such channels require a much lower bit rate. It is generally felt that news channels can be adequately encoded by allocating them a bit rate of 1.5 MBps to 2.5 MBps.
STATISTICAL MULTIPLEXING
Of course, there will be certain period when the sports channel focuses only on the Commentator's face. At these durations, the lower bit rate applicable for News channels would be adequate for the Sports channel.
Similarly if the News channel shows an outdoor clip, it would require a much higher bandwidth.
It would be extremely wasteful if News channels and Sports channels were allocated fixed data rate. This has led to the advanced development of - "Statistical Multiplexing". This examines the picture content of each channel approximately 20 times every second and continuously allocates different bit rates for different channels, depending on the instantaneous picture requirement for each channel.
If fixed data rates encoding accommodates 6 digitals channels per analog channel, statistical multiplexing practically increases it to 10 or 12 digital channels compressed into an analog channel bandwidth.
MPEG-4 COMPRESSION
Instead of MPEG-2, the MPEG-4 standard can also be used for digitising an analog signal. MPEG-4 offers almost 40% better compression that is 40% moredigital channels in the same analog bandwidth.
A detailed discussion on the MPEG-4 is beyond the scope of this article. However, SCAT has carried a detailed article on MPEG-4 in past issues of the magazine.
The MPEG-4 encoders and decoders (STBs) are currently very expensive and rarely deployed on cable TV networks, currently.
DIGITAL INTEGRATION
As indicated above, the cost of digital encoders is typically very high.
A digital headend can therefore save a substantial amount of money if the digital satellite receiver provides for a digital (ASI) output rather than the audio videoanalog outputs. Many professional digital satellite receivers offer such a facility though typically, the digital IRDs distributed by pay TV channels do not offer an ASI (Digital) output. Further, since pay channels "pair" their IRDs and smart cards, it is also not possible for the cable operator to use an authorised smart card with an independently procured digital satellite receiver with ASI output.
This is an area that the TRAI needs to look into and address, to facilitate lower cost digitisation of CATV headends.
If the satellite receiver directly provides an ASI output, no encoder is required and the digital signals can be directly fed into the digital combiner (multiplexer or MUX, See Figure 2)
http://www.scatmag.com//image/feb13-techart-2.jpg
THE MULTIPLEXER
Encoders provide separate digital output for each TV channel as indicated in Fig. 1.
In an analog headend a channel combiner combines multiple analog channels. Similarly, in a digital headend a multiplexer (MUX) combines multiple digitalcannels and creates a "Transport Stream" (TS)
The Transport Stream not only combines the digital channels but also creates a summary of the digital data contained in the Transport Stream similar to the index page of a book which lists the different content and the location of each content.
Multiplexers are typically available to 'combine' either 12 or 20 digital channels.
Such multiplexers accept ASI inputs upto 200 MBps and offer between 1 to 4 outputs. The block diagram in Fig.2 shows (for simplicity) a single ASI output from the multiplexer.
MULTIPLE OUTPUTS
A multiplexer combines several digital channels to form a single transport stream that will be carried in the bandwidth of a single analog channel. Depending on the capability of the encoders and whether statistical multiplexing is used, the number of channels that can be compressed into the space of a single analog CATV channel (8 MHz for PAL-G) varies from 6 channels to as high as 16 to 20 channels.
However, the amount of digital content (MBps) that can be carried on a single analog channel will also depend on the type of modulation used by the cable TV network.
QAM MODULATION
Quadrature Amplitude Modulation (QAM) provides for carriage of a large amount of digital data in a small bandwidth. QAM however requires strong signal strengths with very little noise. Hence QAM modulation cannot be used for satellite transmission but is used universally for digital CATV networks.
(Note: A past SCAT article has explained in detail various types of digital modulations and their applications. The article is available free on scatmag.com - Satellite & Cable TV).
QAM modulation is typically used as either QAM 64, QAM 128 or QAM 256.
QAM 64 offers the least compression and is most tolerant to external noise injected into the network due to poor quality cables, connectors or tap-offs. On the other hand QAM 256 provides the largest number of digital channels within a single analog channel but requires very good networks to transmit digital pictures to the consumer without freezing or pixelising (picture breaking up into small squares or dots).
InCableNet in Mumbai utilises QAM 256 for some of its channels.
MULTIPLEX CONFIGURATION
Depending on whether QAM 64, 128 or 256 is to be used for digital modulation, the multiplexer is to be configured to offer the appropriate mixing. The multiplexer is configured by connecting it to a PC, through SNMP via an ethernet port.
Table 1 shows the different digital output bit rates applicable for QAM 64, 128 & 256.
Typical Bit Rates For Different Levels of QAM Modulation
QAM 64........................... 38 MBps
QAM 128 ......................... 48 MBps
QAM 256 ......................... 51 MBps
The multiplexer can be used for multiple channel inputs with a total bit rate of upto 200 MBps. Hence if the full 200 MBps input capability is utilised, the multiplexer will have to be configured to provide for separate ASI output data streams each of 50 MBps. The cable network will have no choice but to use 256 QAM digital modulation after the multiplexer.
If the network intents to use 128 QAM it will have to reduce the input data rate to the multiplexer by either:
i) Using more compression per channel (hence more expensive encoders or poorer picture quality) or
ii) Using fewer channels.
SCRAMBLING
CAS requires that pay channels be scrambled and the subscriber's STB decodes/un-scrambles only the channels that they pay for.
Hence a digital headend that carries pay channels will typically have to scramble the pay channels.
Fig.3 shows the location of the scrambler in the digital headend.
http://www.scatmag.com//image/feb13-techart-3.jpg
Each multiplex output requires a separate scrambler. The cost of the scrambler can vary very widely depending on the scrambling system used. As a rough estimate a scrambler could cost Rs. 2 lakhs each. Note that if their multiplexer is configured with 4 outputs, 4 separate scramblers will have to be installed, increasing the cost of digital headend very substantially.
For free-to-air (FTA) channels no scramblers to be used and the output of the multiplexer is fed directly to a QAM modulator as shown in Fig. 3.
QAM MODULATORS
A single QAM modulator will modulate multiple channels. All channels within a single transport stream (e.g. 8 to 12 channels) are modulated by a single QAM modulator which costs approximately Rs. 1 lakh. As a result the cost of digital modulation; per Digital Channel; is not very high and in fact comparable with the cost of good quality analog modulators.
The QAM modulator can be user configured for QAM 64, QAM 128 or QAM 256 modulation.
OTHER OPTIONS
There are a few other options for configuring the digital headend. These will be briefly touched upon and some of these topics will be covered in greater detail, in separate article. On the other hand, topics such as transmodulators have already been covered in detail in earlier articles in this magazine. These earlier articles are available free on scatmag.com - Satellite & Cable TV.
http://www.scatmag.com//image/feb13-techart-4.jpg
TRANSMODULATORS
Satellite transmissions use QPSK modulation. Cable TV transmission use QAM modulation. A transmodulator simply converts the digital signal from the satellite which has QPSK modulation into a digital signal, with the same properties but with QAM modulation. Such a QPSK to QAM converted digital signals can be directly mixed at the final output of the digital headend.
IP DIGITAL SIGNALS
In this entire article we have referred to the digital signal in the ASI digital format. While ASI is very widely used in most digital heads worldwide, it is not the only option.
The wide adoption of the internet had led to widespread use of the IP (Internet Protocol) digital data format. Hence, it is possible to use equipment that converts the analog signals into an IP digital data stream instead of an ASI digital data stream.
The IP signals can then be multiplexed and modulated. However an IP multiplexer will be required.
Some international manufacturers such as Teleste now offer a combination unit of Multiplexer + QAM Modulator for approx. Rs. 15 lakhs.
The use of IP digital signals through out the digital headend enable IPTV to also be easily deployed from the same digital headend. However IPTV is usually deployed by telephone companies using digital modems and IPTV set top boxes.
Cable TV distribution networks typically use DVB-C STBs.
A detailed review of IP digital streams is beyond the scope of this article and may be covered in a separate article, some time in future.
SUMMARY
This article was intended to provide a quick and basic overview of how a digital cable TV headend is configured.
All these past article are available free onscatmag.com - Satellite & Cable TV. Readers are encouraged to review these past articles. n
This Article Was Published Earlier In Satellite & Cable TV Magazine - Ed.
Source: scatmag.com - Satellite & Cable TV
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