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Streaming bitrates are a measure of how many bits of information it takes to represent one second of video, expressed as kilobits per second (Kbps) or megabits per second (Mbps). Ultimately, streaming video bitrate is the amount of data being sent in a video stream. Bitrate is an important consideration when streaming, because while it may seem logical that higher bitrate equals higher quality, this is only true so long as there is sufficient end-to-end bandwidth to support it. When bandwidth is insufficient, the viewer will begin to experience jitter and other buffering related quality issues. This sort of latency (delay) is also very problematic when one or more parties are interacting in real time.
Because stream bitrate is just one of four principal components of streaming video quality, you can’t just look at bitrate alone. You must also consider the interrelated topics of resolution, frame rate and compression. In this article, I will help you understand what these things are, how they relate to one another, and why it matters to you.
Because stream bitrate is just one of four principal components of streaming video quality, you can’t just look at bitrate alone. You must also consider the interrelated topics of resolution, frame rate and compression. In this article, I will help you understand what these things are, how they relate to one another, and why it matters to you.
Resolution is probably the easiest of the three to understand: It simply represents the height and width of the video image. The unit of measure is a pixel and the relationship between the height and width is called the aspect ratio. A common high-definition video format is 1920 (pixels) x 1080 (pixels) which has an aspect ratio of 16:9. Common 16:9 video resolutions are 1280 x 720 (HD), 1920 x 1080 (Full HD) and 3840 x 2160 (4K). When a video’s resolution is insufficient for the device it is being played on, quality suffers. A 640 x 480 SD video for example, may look okay on a phone but terrible on a big television screen.
Frame rate is defined as the frequency (rate) at which consecutive images (frames) are displayed or captured. Digital video works just like a movie made on film. A series of still frames are presented in sequence, at a certain speed, creating the illusion of motion. This speed is the frame rate, which is expressed as frames per second (FPS) and set at the point of capture (camera). Frame rate can have a big impact on the aesthetic quality of the video. 24 FPS, a motion-picture carryover, is considered to be the most natural—you might see a little bit of blur or slurring in the motion—a certain softness. For subject matter with a lot of motion, sports for example, a higher frame rate might be more desirable. Slow motion video typically requires a higher frame rate to allow the video to look smooth when slowing down the speed at which it plays. The most popular frame rates today are 24, 30 (broadcast standard) and 60. Uncompressed, 60 FPS will require twice the bitrate of 30 FPS.
If we ignore the larger and more complex topic of video compression, video bitrate is simply a function of frame rate and resolution. For example, a video with higher resolution and/or higher frame rate means a higher bitrate. The inverse is also true: decreasing the frame rate and/or the resolution would decrease the bitrate.
Virtually all video is compressed and internet streaming simply would not work without it because the file sizes would be too large. Compression is the process of encoding the video stream in a way that requires less bandwidth than the original—optimizing it for transmission. Most compression algorithms work by removing data that is redundant or falls outside of human perception. For example, if the background is not changing from frame to frame, there is no need to send that information over and over again. This is why video sequences without a lot of action can absorb a lot more compression (without noticeable impact) than say a football game. So really, bitrate is a function of resolution and framerate—less whatever is taken out through the process of compression. Typically, video codecs (the hardware or software component responsible for encoding and decoding a digital stream) will let you select the frame rate and resolution and a bitrate cap. Then the video is compressed until it meets this requirement.
In streaming, the bitrate is important because when it exceeds available bandwidth (anywhere along the route) quality will suffer in the form of the video stream stuttering, buffering, or cutting out entirely. If the video signal starts breaking up, it won’t be long before your audience begins tuning out. Streaming video is a little bit of an art form—you are always looking for just the right balance, between size (bitrate), quality, and deliverability, in a given situation. In many cases, it simply isn’t possible to deliver an interactive, realtime, experience with Netfiix video quality. This is especially true when depending on the public internet outside of the United States—and the challenge becomes greater as participant numbers rise—especially if synchronicity (everyone seeing the same thing at the same time) is important. You will need to assess the situation and then manipulate the parameters discussed here to arrive at a workable bitrate. These are some of the questions you will be asking:
One thing is for certain—the commonly held misconception that higher bitrate is always better for video, or that higher bitrate is always better for streaming, can get you into trouble. In fact, a low bitrate, even if that means lower frame rate or resolution, is often the only viable choice.
Is your head spinning? Here’s a summary of some of the comparisons between bitrate and other components of streaming video quality we’ve covered:
You can use these general guidelines to determine your end-to-end bandwidth needs:
If you wanted to stream something fast moving that would benefit from a high frame rate of 60 fps, and deliver that in Full HD (and your target devices will display it), you will need end-to-end bandwidth between 4.5 and 6 mbps —at a minimum. Some experts recommend that you pad this number by at least 50% to protect against instability. This is end-to-end—which must consider your upload and download speeds (often slower than what your ISP advertises), the network, and each of your participants’ upload and download speeds.
If you are reading this, it is likely that you are interested in enabling your website or app with real time engagement (RTE) capability. You can take a DIY approach using WebRTC, or use an established RTE platform like Agora. One of the biggest advantages of the platform approach is that you leave most of these details to the experts while you focus on your core business proposition.
Interested in building something with real-time, interactive streaming video? Agora offers SDKs and building blocks that make it easy to add real-time engagement like video and voice chat, interactive live streaming, and real-time messaging to your application. Get started here: Agora Developer Resources
