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Over the past few weeks a lot of people have been complaining that the signal levels between the Kordia multiplex is higher than the signal from the TVNZ multiplex. It would seem that they are right there is no doubt that the signal from 12456 is stronger than 12483. The second complaint has been the picture quality on some channels is now lower than when the service were launched.
Kordia leases a full 54 MHz transponder from Optus. This transponder is located on Optus D1 satellite located @ 160 degrees east. As with most transponders with bandwidth over 36 MHz it is customary to halve the transponder into two 27 MHz half transponders. (NZ national B beam)
Why is this done? Most modulators that set the broadcast parameters normally only operate to a maximum of 31,000 M/s (Mega Symbols) so to use this configuration would use a lot of bandwidth and therefore severely limit the amount of channels that could be allocated.
By halving the 54 MHz transponder into two 27 MHz half transponders, each now has a symbol rate of 22.5000 M/s and using a FEC rate of ¾ a satellite receiver threshold of approximately 6 Db CNR. However what one must remember is when halving the transponder, you loose at least 4 Db of signal strength per 27 MHz. So if the Optus full transponder signal is 53DBw over all of New Zealand it reduces to 49DBw.
Using this configuration our transponder is capable of generating 31 Mbits of usable data. How this data is allocated is up to the operator. If statistical multiplexing is used it could provide an increase in data making the usable data allocation 34 Mbits.
Most operators opt to allocate nine channels per half transponder making a total of eighteen most channels allocate 2 to 4 Mbits for video and up to 128 Kbits for audio. In the case of radio the allocations are anything from 100Kbits to 128Kbits.
Statistical multiplexing works on the principle that not all the channels in a group have the same amount of motion at the same time. The stat mux system efficiently uses unwanted data and allocates it to the channels which require more data due to high motion scenes .Each channel has preset values upper and lower limits so the stat mux knows exactly how much data to allocate. Not all the channels may fall into the stat mux group, for example Stratos, Cue, TV3, TV2 and C4 have higher data allocations indicating that these channels might have fixed data allocations.
The current configuration is as follows:
TV1: 2.68Mbs video 197 Kbits Audio, TV2:5.31Mbs video 194 Kbits Audio,
TV3: 4.00Mbs video 198 Kbits Audio, C4: 3.86Mbs video 198 Kbits Audio,
MTV: 2.24Mbs video 196 Kbits Audio, Te Reo 1.23 Mbs Video 194Kbits Audio ,
Parliament 1.25 Mbs Video 194 Kbits audio, TVNZ 6 2.44Mbs video 199 Kbits audio,
TVNZ 7 2.00Mbs video 197 Kbits audio, TVNZ sport extra 1.15Mbs video 194 Kbits audio
TV3+1 3.00Mbs video 196 Kbits audio Stratos 4.08 Mbits 197Kbits audio,
CUE TV 4.69 Mbits 193Kbits audio
Radio NZ 198Kbits, Concert Programme 160 Kbits, George FM 160 Kbits, Base FM 160Kbits
It can be seen from both off screen pictures of signal strength and quality that Free View is lower than Kordia.
The spectrum analyzer displays clinch it
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| Freeview to 60cm dish | Kordia signal to 60cm dish |
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Freeview Signal -59dBm |
Kordia signal -56dBm |
As can be seen from the above illustrations taken off a 60cm, dish and a .7 db LNBF, people are right, the kordia signal at 12456 GHz (1183) MHz is stronger than Free view’s 12483 GHz (1156) MHz half transponder.
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TV1: 2.68Mbits video 197 Kbits audio |
Stratos 4.08 Mbits 197Kbits audio |