Scatmag.com

February 2019

DISH DOCTOR
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BOUQUETS BY LCOS ?

Q: Can LCOs create their own Pay channel and FTA bouquets for their consumers? We interact with our consumers and best understand what they want.

B. Shah, via WhatsApp

ANS: Last-mile Cable Owners (LCOs) are required to transport signals from the MSO's Headend to consumers.

Creating bouquets is done at the headend, by the MSO, for both FTA and pay channels. The LCO does not have access to creating bouquets or allocating channel numbers (placement) of channels.

Pay channel broadcasters charge the MSO for their content, based on the number of actual subscribers for that channel. This report is generated by the Subscriber management System (SMS) also located at the digital headend and controlled by the MSO.

The MSO is responsible for payments to the Pay Channel broadcaster.

There is no direct interaction between the Pay Channel broadcaster and the LCO.

Hence, LCOs cannot create their own bouquets. They can of course advice / request their MSO to create certain bouquets that consumers request.




SYMBOL RATE FOR DAS HEADENDS

Q: In a digital headend, the Symbol rate is most often set as 6875 or 6900. Is this compulsory or can we also set a symbol rate of 6700?

Thank you.

Siva Ram, Andhra Pradesh, By WhatsApp

ANS: You have asked a very interesting and technically significant query. Setting up the parameters of a digital headend is an elaborate task that requires good, in-depth knowledge of the DVB-C specifications and the practical consequences of each parameter. A separate technical article would be ideal which we plan to carry in the future. However, let us take a quick overview and answer your query.

The amount of digital information carried on a digital Cable TV network depends on:

 

1. The Digital Modulation used: QAM64 or QAM256

2. The Symbol Rate (SR)

QAM MODULATION

Cable TV uses QAM Digital Modulation. For Cable TV networks in India, QAM 64 & QAM 256 are used. Theoretically there is also QAM 128 but very few meters are programmed to measure QAM 128 signals. (I have seen only a Deviser meter provide this facility.) Hence for practical reasons it is best to avoid QAM 128.

QAM 256 allows for almost 33% more digital TV channels to be carried in the same bandwidth 8 MHz as QAM 64. In India we follow the European system for TV transmission ie. the analog channel bandwidth is 8 MHz for all frequencies above 300 MHz (UHF).

SELECTING THE QAM

Since QAM 256 compresses much more digital information in the same 8 MHz channel bandwidth, it is more prone to corruption by a poor-quality transmission networks of the MSO or LCO, due to extra noise and distortion from the network. This results in frequent picture 'freezing' for the consumers.

THE HOME STREAM

A digital cable TV network transmits content in multiple data streams.

Each data stream occupies 8 MHz.


The Home Stream Is Like The Index Page Of A Book. It Indicates The Total Number Of Channels Transmitted And Where The STB Will Find Each Of These Channels.


The most important data stream is the 'Home Stream' which carries all the information necessary for the STB to identify and tune into the various digital channels. It is like the Index page of a book which tells the reader the page numbers of different articles or chapters in the book. Similarly, the Home Stream tells the STB the total number of channels transmitted and where the STB will find each of these channels.



If the Home Stream is corrupted during transmission, the STB will not even be able to tune in to the digital signal from the MSO.

SELECTING QAM

Since it is very important to carry the Home Stream data to STBs, the Home Stream is best carried as a QAM 64 signal.


The Home Stream Is Always Carried In QAM 64


Typically, the channel allocation data will not fully occupy the data capacity of the QAM 64 home stream. Hence a few digital channels are also carried in the Home Stream.

All the other digital channels are carried in separate streams, which can be at QAM 64, or if more channel capacity is required, the non-Home Streams are carried as QAM 256.

Carrying all the digital channel streams also as QAM 64 will provide the most robust transmission, but the channel carrying capacity will be 33% lower for the same digital bandwidth used.

QAM 64 FOR POOR QUALITY NETWORKS

Often, MSOs in DAS-3 and DAS-4 areas prefer to transmit all the channels in QAM 64 because the quality of the distribution networks is not good and a lot of noise and distortion is generated in the LCO network, which causes picture freezing.

As mentioned earlier QAM 64 is very robust and will ensure no picture freezing even with poor quality networks of the MSO and LCO.

The disadvantage for using QAM 64 is that fewer digital channels can be carried in each 8 MHz slot or Digital Stream. Hence many 8 MHz slots will be required to transmit all the digital channels. Typically, digital Cable TV networks prefer to utilise 300 MHz to 550 MHz. This bandwidth may not be enough to carry a large number of (e.g. 500 channels) digital channels using QAM 64. However, most DAS-3 and DAS-4 areas rarely provide more than 250 digital channels.

SYMBOL RATE

The DVB-C standard requires a 15% "Guard Band" to be maintained in each 8 MHz channel.

The Symbol Rate can be calculated using the simple formula:

Symbol Rate = 8000/1.15 = 6956.

Hence the maximum theoretical Symbol Rate is 6,956 Msym/s.

PRACTICAL SYMBOL RATE

If this maximum Symbol Rate of 6,956 Msym/s is used, there is a significant chance of picture freezing in case of poor transmission of the digital signals by the MSO/LCO.

Setting a lower symbol rate will make the signal more resistant to errors during transmission.

However, as you reduce the Symbol Rate, it also reduces the number of digital channels that can be carried in a 8 MHz digital stream.


The Maximum Theoretical Symbol Rate Is 6,956 Msym/s.


WHY 6875 SR ?

It is interesting how and why most cable networks now have adopted a SR of 6,875 Msym/s. Most new digital networks simply copy what is already being successfully used earlier.

Hathway, when it set up one of India's first digital Cable TV networks, decided to reduce the Symbol Rate from 6.95 Msym/s to 6,875 Msym/s. They observed that in practice, a sample rate of 6,875 Msym/s provides the best compromise between robust signal transmission & good channel capacity.

6700 SR?

Digital Headends can set their Symbol Rate to anything below 6,950. Yes, you can also set it to 6,700 as mentioned by you. 6,700 will provide even more tolerant digital transmission (no freezing of picture with even poor quality LCO networks) but the channel carrying capacity will reduce.



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