Tuesday, April 14, 2015
Engineer's HELP: What is a Conditional Access System ?
Engineer's HELP: What is a Conditional Access System ?: What is a Conditional Access System ? It is any system that is used to limit the access of TV signals, to only those whom are authorised...
Modulation in CATV
Modulation
Cable television signals are then mixed in accordance with the cable system's channel numbering scheme using a series of cable modulators (one for each channel), which is in turn fed into a frequency multiplexer or signal combiner. The mixed signals are sent into a broadband amplifier, then sent into the cable system by the trunk line and continuously re-amplified as needed.
Modulators essentially take an input signal and attach it to a specific frequency. For example in North America, NTSC standards dictate that CH2 is a 6 MHz wide channel with its luminance carrier at 55.25 MHz, so the modulator for channel 2 will impose the appropriate input signal on to the 55.25 MHz frequency to be received by any TV tuned to Channel 2.
Digital channels are modulated as well; however, instead of each channel being modulated on to a specific frequency, multiple digital channels are modulated on to one specific NTSC frequency. Using QAM (Quadrature Amplitude Modulation), a CATV operator can place usually up to eight channels on one specific frequency so channel 2 may actually be carrying channels 1 - 8 in a viewer's city. STBs (Set top boxes) orCableCards are required to receive these digital signals and are provided by the cable operator themselves.
Friday, April 10, 2015
Frequency Polarization for Antenna
Digital signals are transmitted from the satellites on either Vertical (V) or Horizontal (H) polarity for linear feeds, or on Right (R) and Left (L) polarity for circular feeds. Standard big dishes are most likely to have feed horn that can receive linear (H/V) polarity. Other system such as Dish Network and Direct TV use circular polarity.
If you have a big dish, most manufacturer of feed horns such as Chaparral and ADL provide what is called a "Teflon Slab" that can be added to the feed horn which would allow it to receive both linear and circular polarity signals. This will cause a loss of about 1 dB in signal level on the linear polarity signals. A Wideband Chaparral feed horn can receive both linear and circular (Ku band) polarity signals.
In order to receive the digital TV signal, you must have the feed horn set to the correct polarity. For LNBFs, the polarity is controlled automatically by a voltage transmitted form the receiver to the LNBF via the coax cable. The receiver will send the LNBF 18 volts for horizontal polarity, and 13 volts for vertical polarity. For standard LNBs, the polarity is controlled by a motorized motor. In this case, odd channels represent one polarity, and even channels represent the other polarity. Standard satellites have the even channels set the polarizer to horizontal polarity, and the odd channels set it to vertical polarity. If the Satellite Polarity is inverse, then the even channel set it to the vertical polarity, and the odd channels set it to the horizontal polarity. Furthermore, some LNBFs such as Dishnetwork's DP LNBFs control the polarity change by shifting the frequency rather than using 13/18 volts. In fact the new shifted horizontal polarity is 25600 minus the original horizontal polarity.
If you are unsure of which channel represents which polarity, simply set the digital receiver to display the signal strength meter, then switching between odd and even channels on the analog receiver will generate a marked difference in signal strength.
Polarity can also play an important role in whether you are able to receive the digital signal at all. Digital signals are not as forgiving as analog signals, it is either you receive it or you do not. If the signal meter shows a high signal (over 85% on the Sat Cruiser), and you are still unable to receive the digital signal, then adjusting the SKEW will almost always help.
Most analog receivers have auto tune feature, it will automatically find the best dish position and the best polarity for the analog signal. Be forewarned that this is not always the best setting for digital signals. However, in most cases it does produce the best setting for the reception of digital signals.
If you are able to receive only the odd or even channels on a given satellite, then your polarizer is defective and need replacement. The polarizer is a motor located on your feedhorn which switches the unit to receive the vertical or the horizontal polarity. Since every time you switch channels the motor moves, eventually it will wear out and will need replacement. It is connected by three wires from the back of your analog receiver to the dish (the wires are normally (Red '5VDC', White 'pulse', and Black 'ground'). Before replacing the polarizer make sure to turn the receiver's power off. It would be prudent to remove the power cable from the electric socket since most receivers continue to provide power to the dish even when the unit is off.
- Right Circular Frequencies
- Left Circular Frequencies
- Horizontal Frequncies
- Vertical Frequencies
low-noise block downconverter (or LNB)
A low-noise block downconverter (or LNB)
is the receiving device mounted on Satellite dishes used
for satellite TV reception, which collects the radio waves from the
dish. Also called a low-noise block
The LNB gets its power from the receiver or STB inside the house. This power is sent "up" the same coaxial cable that carries the received signals "down" to the receiver, eliminating the need for a separate power cable.
Amplification
and noise
The signal received by the LNB is extremely weak
and it has to be amplified before downconversion. The low noise amplifier section of the LNB
amplifies this weak signal while adding the minimum possible amount of noise to
the signal.
Low noise block downconverter (LNB) diagram
The diagram shows the input waveguide on the left
which is connected to the collecting feed or horn. As shown there is a
vertical pin through the broad side of the waveguide that extracts the vertical
polarisation signals as an electrical current. The satellite signals
first go through a band pass filter which only allows the intended band of
microwave frequencies to pass through. The signals are then amplified by
a Low Noise Amplifier and thence to the Mixer. At the Mixer all that has
come through the band pass filter and amplifier stage is severely scrambled up
by a powerful local oscillator signal to generate a wide range of distorted
output signals. These include additions, subtractions and multiples of
the wanted input signals and the local oscillator frequency.
Amongst the mixer output products are the difference frequencies between the
wanted input signal and the local oscillator frequencies. These are the
ones of interest. The second band pass filter selects these and
feeds them to the output L band amplifier and into the cable. Typically
the output frequency = input frequency - local oscillator frequency. In
some cases it is the other way round so that the output frequency = local
oscillator frequency - input frequency. In this case the output spectrum
is inverted.
Examples of input receive frequency band, LNB local
oscillator frequency and output frequency band are shown below.
C band is 3.4 - 4.8 GHz. Ku band is
10.7 - 12.75 GHz. Ka band is 19.2 - 21.2 GHz.
The expression low noise refers
the the quality of the first stage input amplifier transistor. The
quality is measured in units called Noise Temperature, Noise Figure or Noise
Factor. Both Noise Figure and Noise Factor may be converted into
Noise Temperature. The lower the Noise
Temprature the better. So an LNB with Noise Temperature
= 100K is twice as good as one with 200K. C band LNBs tend have the
lowest noise temperature performance while Ka LNBs have the highest (worst).
The expression Block refers to the
conversion of a block of microwave frequencies as received from the satellite
being down-converted to a lower (block) range of frequencies in the cable to
the receiver. Satellites broadcast mainly in the range 4 to 12 to
21 GHz.
How
to test an LNB:
Check with a current meter that it is drawing DC
current from the power supply. The approx number of milliamps will be
given by the manufacturer. Badly made or corroded F type connections are
the most probable cause of faults. Remember that the centre pin of the F
connector plug should stick out about 2mm, proud of the surrounding threaded
ring.
Use a satellite finder power meter. If
you point the LNB up at clear sky (outer space) then the noise temperature
contribution from the surroundings will be negligible, so the meter reading
will correspond to the noise temperature of the LNB, say 100K (K means degrees
Kelvin, above the 0 K absolute zero temperature). If you then point
the LNB at your hand or towards the ground, which is at a temperature of approx
300K then the noise power reading on the meter should go up, corresponding to approx
400K (100K +300K).
Note that LNBs may fail on one polarisation or on
one frequency band and that the failure mode may only occur at certain
temperatures.
If you choose to try a replacement LNB in a VSAT
system check the transmit reject filter and supply voltage - you don't want to
be one of those people who keeps blowing up LNBs trying to find a good one !
LNB Outputs
As we said above, each satellite receiver needs its
own feed from an LNB. This picture shows a Quad LNB – an LNB with four outputs:
Quad LNB – with four outputs
Twin Tuner boxes? Some
satellite receivers, such as Sky+, Sky+HD and the Foxsat Freesat receiver, have
two tuners – these let you record one satellite channel while watching a
different satellite channel. This means that they need two feeds from the LNB,
not one.
Sky+ boxes need two LNB connections
Before picking an LNB, you need to work out how
many boxes you want to feed. The most you can feed is 8, using an Octo LNB. An
Octo LNB will feed 4 twin-tuner boxes or 8 standard satellite boxes (or any
combination).
Can I split one satellite feed into two feeds?
This is a fairly common question. What should you
do if you only have one feed from the dish, and two sockets on a box? Sadly,
you can’t split a single feed from an LNB to let it feed two tuners at the same
time.
Why? Satellite receivers send voltage and signals
to the LNB to get the LNB to change frequency and ‘polarisation’ (some channels
are Vertical and some are Horizontal). If you have two receivers both trying to
switch a single LNB, one box will win, and one will lose.
You can get an LNB switch box that will let you
switch your single feed to either of two receivers – just not to feed both at
the same time. These are no substitute for having two dedicated feeds from the LNB
though.
Need an LNB, cables or connectors?
We recommend Maplin.co.uk.
LNB Problems?
Sometimes, but rarely, LNBs develop a fault.
Typically, this may mean that you can’t get any channels, or you can only get
channels from one polarity (Horizontal-only, or Vertical-only).
If you’re with Sky and can’t get your channels,
call them for advice – they may need to come out and replace your LNB.
Otherwise you can replace an LNB yourself. You can buy LNBs suitable for Sky
and Freesat from Maplin.co.uk.
http://www.radioandtelly.co.uk/
Sunday, April 5, 2015
Satellite Antenna best document for study
I am sharing a web link , it has some animation for better understanding how it is work. so you can study and give some food to your brain. satellite antenna best document.
http://sv1bsx.50webs.com/antenna-pol/polarization.html
http://sv1bsx.50webs.com/antenna-pol/polarization.html
Thursday, April 2, 2015
Headend compression
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.
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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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