goaliebob99 said:
so why is 8psk more benificial than qpsk for hd??? is it because the extra bit's that it provides.. for the increased quality for the picture and audio?
Yes, basically they are providing a higher data rate on the 8psk transponders. Check out the Symbol rates on lyngsat..
8psk example:
Symbol rate = 21500k
Forward Error Correction (FEC) = 2/3
Modulation = 8PSK
So, each second, 21,500,000 symbols are transmitted. Each symbol contains 3 bits. So that is 64,500,000 bits per second, for the entire transponder. However, all of that is not usable data, since FEC is used. You cant retransmit a video signal if it arrives corrupt, so what they do is send the error correction along with the signal. Thats what the 2/3 is. 1/3 of the 64.5Mbit is for error correction, thus 64.5 * (2/3) = 43Mbit of user data per transponder. That is divided by whatever channels are on the transponder.
QPSK example:
Symbol rate = 20000k
FEC = 5/6
Modulation = QPSK
Take 20,000,000 symbols * 2 bits = 40Mbit / s. Apply FEC, 40Mbit * (5/6) = 33.3Mbit/s of user data on a qpsk transponder.
So why dont they just use 8psk for all of them?? Well, you need a decoder than can handle 8psk, which are only found in the HD boxes as well as the 311 I believe. Signal to noise ratios also play a part in all of this. You need a higher SNR to use 8psk.
The transponders with locals on them, some have 12 video channels, thats 2.8Mbit/s for each video channel. This is really kind of the bottom of the barrel in terms of video quality expected from a mpeg2 stream of this datarate.
I might have made some errors here and there, please correct me if I'm wrong!
