One of my former students has emailed me with some questions about an assignment. So to be fair I will publish my answers for all to see in this blog. You can select more to see the questions and answers, I also would love to have more questions on this subject in the comments…
Q: I’m currently writing an assignment about the uses of satellites in broadcast, was just hoping you might be able to help. I’m interested in he more current ways in which you operate, I do have some books that indicate methods that can be used. However I’m not sure how up to-date they are, they don’t seem to include much about current data rates. No need to answer them all, but if you can pull anything from the back of your head would you know any of these:
1. Common Symbol Rate that you may expect.
[bob] Symbol rates vary slightly between satellite operators and those who lease space on them. But it all comes down to how much RF bandwidth you have been allocated. Typically 7.5MHz is a standard slot (plus or minus 0.5MHz) and you could always book a double slot if you wanted more from life. Once you have the RF slot you will pick a symbol rate, or you might just pick from the standard values which are nominal in presets. Symbol rates for SNG work are typically in the range 4.111 to 6.138MSym/sec. However the most common values are 5.632MSym/sec, 6.111MSym/sec and 6.138MSym/sec (the last really pushing the envelope provided by the RF roll-off in a PSK modulated system for the bandwidth).
2. Modulation Method Used — 8PSK/16-QAM?
[bob] In most all standard cases QPSK will be used as the lowest common denominator, most downlink stations have bought between half-a-dozen and two dozen receivers and many of them in lower budget places are QPSK only, places like the BBC/Siemens Satops however have a wide range of receivers and often use 8PSK for improved data-rates. DVB-S2 is also on the rise not only for it’s standardisation of improved modulation but also it’s efficiency of error correction. 16-QAM is only really used on long term OBs where not only is the weather good, but you have a backup. 16-QAM also requires the use of a large high quality receive antenna because it is very sensitive to phase noise (due to the high density). Choice of modulation is mostly dictated by equipment availability and the link-budget (often standard link budgets are kept for standard situations not done for each broadcast).
3. The compression method used & common data rate?
[bob] Compression is nominally still MPEG 2 video, but mostly uses 4:2:2 chroma sampling for improved quality in the successive transmission chain. MPEG 4 AVC (h.264) may be comming to SNG but I did not see it in my time. Data-rate for the video is a balance of the modulation used, the FEC overhead choice (determined by a balance of how robust the signal should be vs how much more video you want) and the bandwidth available. You may also be able to gain more data-rate by changing the RF envelope of the transmission, the only effective way of doing this is to tighen the ‘roll-off’ factor which is by default 35% in most QPSK modulators. A tighter roll-off requires better reception hardware such as LNBs and decoders. All in data-rate is more a result of the choices you make than a common value. The satellite operator will often suggest transmission parameters (based on a known link budget) and you will end up with a data-rate based on that.
4. Who owns/provides the satellites links and how you book a ransponder? Also be good if you know which satellites tend to be used for SNG links.
[bob] Satellites typically cost between $150mill and $400mill and they only last for between 17-25 years (nominally). Plus they have a relatively high failure rate, either at launch or during their life. So owning a satellite is a very expensive thing, typically the most satellites are owned by publically owned consortia like Intelsat, Eutelsat and others. They may also be owned by telco’s, governments and NGOs. They may also be leased in their entirety such as is the case of Intelsat 10-02 and Telenor. You would not typically book a whole transponder unless you had a very large event as most transponders are about 30MHz wide. A company will lease transponders wholesale and sell SNG slots on them of 8MHz for a period of time, per minute charging with a sliding scale rate-card. How much business you do will dictate your price, some satellite operators will also lease their space directly as they can re-sell space which is kept as disaster recovery backup for permanent services (DTH television); most people buying SNG time don’t realise that the operator can ‘pre-empt’ their transmission and cancel their booking without notice if they wish. You pay alot extra for ‘non-pre-emptable’ space.
6. Which Band is used? I’m assuming only the C Band?
[bob] C-Band is only used in legacy and tropical regions where you might want to cover a wide area with a difficult geography and probably high level of rain-fall. C-Band is less sensitive to rain absorbtion but requires dishes of much larget size compared to the same dish at Ku-band. In my time doing links work I think only twice did I take bookings that included a C-Band element and they were very exotic situations. Some work has been done with lower Ka-band SNG (DBS band) at about 20GHz, this allows for very small SNG kits but is very tricky with the link budgets.
7. Information that you require when downlinking or uplinking?
[bob] First, which satellite (you will need to work out the point angles for your dish to use for the location you are operating from), then which polarisation to set your dish to (polarisation angle varys over the arc of the horizon tilting clockwise or counter-clockwise as you pan away from due south, additionally Intelsat requires about 50dB of isolation between the polarisations for transmitting), you will need to know the frequency you are receiving from the satellite and if you are uplinking the transmit frequency. Further you will need the symbol-rate and probably the FEC choice (some newer receivers may auto-detect the FEC). The rest of the values can be either determined from the received signal or calculated internally within the receiver from the other parameters.
8. Hows the job going?
[bob] Really well thanks, just busy at the moment with a product launch but things are looking up.