I need Information about Digital Video Broadcasting ..? 1

[mro83]

Hi everyone,
I am working on Video Broadcasting Satellites.
I want to know
1)signal charactreistics of uplink & downlink
2)characteristics of earth station
3)how astellite transponder works

I have elementary information about topics above and I need more detail such as:
1)exact modulation and multiplexing of signals
2)geographical information about earth stations
and ....

thank u

 

[johnwiss]

The DBS system uses several channels per carrier modulated using QPSK shaped (Root-Raised Cosine) modulation with Convolutional and Reed-Solomon Coding to improve the downlink capacity. The transponders are linear repeaters in the 11/14 GHz range on the satellite. There is no "uplink" except thru a phone line. The modulated data to the home is in MPEG format to compress the video NTSC data to about 4 Mbps per channel. The uplink facility encodes and multiplexes the signals together to send via large antennas the encoded video/audio up to the satellite and then it gets filtered, amplified, and translated to the downlink frequency and sent to a dish to the home.

 

[mro83]

Very well
I wanted to know that why is QPSK modulation with Convolutional and Reed-Solomon Coding used ?

and why is not SCPC used ?

 

[johnwiss]

QPSK is robust modulation with reasonable spectral efficiency and the coding works well for satellite channels --there is about 5-6 dB of coding gain which makes for smaller dishes on people's roofs. Reed Solomon coding helps correct burst errors generated by the Viterbi decoder. The number of carriers are limited to increase efficiency so there isn't a lot of frequency wasted packing in more carriers which require guard bands around each one. I think that is why more than one TV channel may be sent on one carrier. It would still work if it was SCPC but perhaps with a little lower capacity since there would need to be over 100 carriers since there are more than 100 satellite channels.

 

[mro83]

Thank Johnwiss
You said that bit rate required for NTSC quality TV is 4 Mbps per channel.I wanted to know if HDTV quality is used for TV broadcasting and I have read in Europian standards that bitrate required for HDTV is 20 - 40 Mbps.

Can you reffer me to somewhere in the www that I could find more about satellite video broadcasting and standards that are used ?

 

[johnwiss]

Just search it in google -- the answer is "yes" for HDTV by satellite. The Direct TV systems does broadcast 1 or 2 channels in High-Def format.

 

[Digitalwoodshop]

Check out this web sight

http://www.loralskynet.com

I am near the Hawley, PA station. They have tech info on the site.

 

[Digitalwoodshop]

Opps it captured the "I" too.

 

[mro83]

Hi everyone
It was very useful.
Here comes another question.
the question is that :
how geographically different Earth Stations can transmit their data to one transponder of a TV broadcasting satellite ?
In other words What is Multiple Access of TV broadcasting satellites?
Because I've seen different TV programs with one center frequency of a satellite transponder , and I know that the earth stations of these TV channels is far from each other so they cannot transmit their video from one place to a specific transponder.

Finally the information in

http://www is

very general and I have not found detailed info.

thank u for ur help.