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I just found out that the opening ceremony of the Paris Olympics will be screened live in IMAX theaters and this had me wondering what the quality would be like. I know for movies, theaters get a DCP with 250mbps bitrate max but what about live broadcasts??
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If an event is going to be screened live, it's probably going to be broadcast by satellite. Satellite is a whole different animal!
It's been a while since I have shown a satellite broadcast but, as I remember, the important things are the symbol rate and the FEC. (Forward Error Correction) You have to set your satellite receiver for these parameters (and a few others) or else you'll get nothing. Just an empty picture.
There can be multiple bits per symbol, depending on the transmission method being used. It's often 16 bits per symbol.
FEC is important because satellite transmissions are unidirectional. Data comes down from the sky but you can't send any data back up. What happens if there is an error? You can's send a message back to the satellite and ask for data to be resent. If one single bit gets flipped during transmission, you're screwed. FEC builds in extra, parity bits that can be used to check for errors and reconstruct crunched data. (Up to a point.)
As I remember, my satellite feeds came down at a symbol rate of 22 with an FEC of 5/6. I never really thought about bitrate as a separate thing. If you want to translate symbols and FEC to bitrate, there's a little math to do.
Again... Going by memory...
Symbol Rate X 2 X FEC gives you the data rate.
Thus: 22 x 2 x 5 รท 6 = 36.66 MB/sec. x 8 = 293.33 Mbit/sec.
So, I guess that would be comparable to the data rate of your DCP.
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Not really following the math. Using a modulation method like QAM allows multiple bits per symbol, so you could multiply the symbol rate by 16 or whatever the QAM bits/symbol value is. You can get more bits per symbol on a low noise circuit. FEC adds overhead bits, decreasing throughput, but decreasing the error rate. Another error correction technique is time interleave where the data is "spread out" so a momentary loss of the circuit does not cause a loss of data since it is retransmitted later.
I do think that the bitrate of satellite streaming is considerably lower than in a DCP since the DCP uses JPEG encoding where each image is complete, while streaming uses MPEG (or similar) where the image changes between frames are transmitted instead of the full frame image (with a full frame image being sent now and then to resync the decoder).
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They are using MPEG2 or h.264 (DVB-S/S2), and typical bitrates in the 30-50MBit/s range. Plain consumer DVB-S/S2, just with a larger dish. So, it's closer to Bluray as far as codec and bitrates go.Comment
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When I was doing work in satellite.
DVB, The max is about 22mbit using typical 4:2:2 at best but more likely 4:2:0 and a h264 type compression, maybe a h265 now. The DVB channel HAS TO BE setup into channels of certain width. You cannot dial them up and down easily. So they typically dial them up to do a few HD and then as many SD channels as they can so they can sell it to multiple entities.
From memory, it was like 24mbit with 22 usable was the wides channel. But that may have changed since I was in that game.
No matter how you look at it, real time terrestrial is CRAP compared to a DCP, 120mbit(Video only)/Uncompressed audio. Remember, the 250mbit limit was designed for perceptively clean images in 3D (2 image streams) so 120mbit was approved by the original DCI committee as acceptable.
22mbit/h265 including compressed audio is no match for 120mbit J2K PLUS on top, uncompressed audio (Not included in the 120mbit J2K video)
There are work arounds now as satellite providers are moving completely to IP based distribution, they can widen the bitrate on demand and also add uncompressed audio by using more advanced MP4 containers/packaging of the data. But typically the real time satellite providers for cinema use the cheapest option, the older technology described above.
Personally, I am not a fan of live. Best to be 8-12 hours behind in that you can create a DCP (Resolve can easily render 4x real-time) and distribute it, not in real time, but at quality.
Finally, the satellite method is also compromised by good old streaming. Gamers/eSport do HUGE high quality streams to 100s of thousands of endpoints all the time. All based on open standards. The cinema live event incumbents in this game would like you to think they have the only viable solution. And pretend this technology does not exist. But it does. And (DCI Player) vendors are looking to add the ability to attach to these open standard endpoints like the eSports people use. No custom STB required. No proprietary lock in. Anyone could set it up easily as the industry has popped up to service the eSports community.
The only problem is you do need to have a good quality Internet connection to utilise this.. But I can see, the most traction of this content is likely in areas where such internet connectivity does exist. And every day it's getting better. It will completely undermine the use of satellite, if not already, in the near future.Comment
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The exact bandwidth used for Satellite delivery for organisations like Cinema-Live. Really all depends on how much they want to pay. But in my experience (In my region) it was very low, under the 20mbit range, for a HD signal. I also have a memory about talking to the satellite company and how, they cut them up a lot into as many channels as possible to maximise profits. And they cannot generally flex up and down the mbit easily. (The one way nature and the crap STB on the other end would all have to be rescanned every time they did it.)Originally posted by Carsten Kurz View PostComment
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If you, realistically, want to fill an old-school IMAX screen, you're not going to get there with 20 Mbps, even when doing H.265 on it. Those compression artifacts are going to be really nasty on such big screens.
Realistically, you'd want a 4K stream at the least at bitrates closer to 100 to 150 MBit/s with H.265 HEVC or at least H.264 AVC to have a somewhat acceptable picture at those sizes. Since this is an IMAX thing, I guess it will use somewhat more sophisticated infrastructure than just DVB-C for those broadcasts.
Depending on where you want to deliver this, you could alternatively realize this using terrestrial networks instead of satellite.Comment
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