"Bitrate" (or "bit rate", also known as "data rate") is a measurement of the rate of transfer of digital data. The most basic, fundamental unit of data is the bit, and measurements of the rate of transfer are usually going to describe the number of bits per second (abbreviated as bit/s or bps). This number can get quite large, so it is often converted into a more manageable form like kilobits per second (kbit/s or kbps, with one kilobit being equal to 1,000 bits) or megabits per second (Mbit/s or Mbps, with one megabit being equal to 1,000 kilobits or 1,000,000 bits).
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Bitrate is used throughout computing; for example, it is used to measure the current amount of data being transferred across a network, and also the maximum amount of data that can be transferred across that network. Bitrate is also used to describe the amount of data being processed when a video file is being played. But in the context of live streaming, bitrate is extremely important, as it describes exactly how much data is being transferred to create your stream.
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While some variation is unavoidable, the ideal bit rate for a stream is one that is essentially unchanging. Changes in bitrate can cause buffering issues and quality issues, or even the total collapse of the stream. Bitrate varies for many reasons, but it's important to understand why:
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Due to the nature of audio/video content, the precise bit rate of your stream will shift up and down naturally from moment to moment, as some types of audio/video content are less complex than others and need less data to be transferred. Within your encoder are parameters (either built-in to the device or specified by you) that tell the encoder what its target bitrate should be and how it should try to achieve that target. It will then make adjustments to the actual bitrate throughout the stream, in order to keep it in line with the target parameters.
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Sometimes, natural variations can be large enough to cause problems. The usual culprit here is the inclusion of digital graphics (like an announcement slide show), which can cause a massive drop in the video bitrate. Some encoders are better at compensating for this than others!
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One key parameter is the "rate control" method, which is the set of rules the encoder uses to decide how and when it will make bitrate adjustments. Many different rate control methods exist, but the two most common are "variable bitrate" (or "VBR") and "constant bitrate" (or "CBR"), and their names are fairly self-explanatory: CBR attempts to keep the bit rate constant even if the relative complexity of the current part of the stream warrants an elevated bit rate, and VBR allows the bit rate to shift dramatically from moment to moment in order to send the amount of data that is optimal for each given moment of content. While VBR does offer the potential for higher quality than CBR, that potential is often not achieved; furthermore, variations in bit rate are bad for stream stability and harder to process, and most streaming services (including Subsplash Live) prefer CBR. While there are several options out there for CBR-capable encoders, our favorite is OBS!
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Remember how bitrate literally means "bits of data per second", and so it can also be used to measure the current and maximum data transfer rates over a network? Whether we are talking about the actual bit rate of your stream or the maximum bit rate of a network, the bits being measured are the same. Because they are the same, it follows that a stream (which has to be transmitted over a network) can never have a bit rate that is higher than the maximum bit rate of that network. It would be like connecting a garden hose to a fire hydrant; while your encoder (the fire hydrant) might have a very high maximum transfer rate, your actual transfer rate is going to be limited by the much lower maximum of your network (the garden hose).
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Just as the bitrate of your stream will naturally shift up and down, the achievable bitrate of your network will also shift up and down. Furthermore, that achievable bit rate has to be distributed among all the devices connected to your network. Thus, in order to avoid problematic fluctuations in your stream's bitrate, we recommend trying to stream with a bitrate no higher than 50% of the maximum data transfer rate of your network.