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Comment by opticalflow | Hacker News Reader

All and all a good intro tutorial that gets into some of the common professional use cases. On "constant bitrate" assumptions and some of the subsequent discussion here, ANY transform-and-entropy codec like VP9 or H264 will ALWAYS be variable bitrate internally. In the pro broadcast world, where you can't go over your FCC-allocated radio frequency bandwidth allocation by even one iota (or nastygrams and fines ensue), this is "handled" by actually having the encoder understoot the actual target (say it's 5mbit), and then the stream is padded with null packets to get a nice even 5mbit/s. This also happens with LTE broadcast as well. The encoders that do this reliably are fabulously expensive and of a rackmount flavor.

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Reading this reminds me of all of the days spent using Manzanita MPEG software to get streams to "work". While its true that the bit rate may/will fluctuate, it was the muxing software that saves the stream. The muxer would introduce those null packets. The studio I used to write automated workflows for would get work specifically because we could make streams work that other facilities could not. Manzanita software was always the difference. Rarely, did an encoding software/hardware output work directly. However, remuxing the same elementary streams with Manz software would pretty much always solve the problem. If that didn't do it, it was probably a VBV buffer under/overflow issue around a scene change somewhere in the stream.

Oh gawd how I don't miss those days.

I'd love to hear more details about those things. I'm guessing it's not as simple as wiggling the quality around to keep the output within a certain size.

That's basically it. All of the digital broadcast streams (over the air, cable, satellite) use a fixed bitrate per physical channel, and send an mpeg transport stream. A transport stream is built of fixed length packets. Within the transport stream you can multiplex different programs. OTA gets 20Mbps per channel, if you use that for one program, it's likely you may not use the whole thing, so you include null packets as needed to fill the stream. If you send multiple programs, you probably will fill the stream, so you have to reduce quality and/or play games with timing of i-frames and possibly adjusting program start times or commercial break lengths to avoid having multiple programs needing high bitrate at the same time.

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