What are the Best Frame Rates for Live Streaming?
One of the questions we get most often is: what frame rate should I use for streaming? There are a lot of options, and a lot of reasons for using different options, both from a technical and stylistic point of view.
So, you’ve set up your camera, your streaming software, or encoder, and you’re ready to go live. First, though, you’re most likely presented with a choice in your camera and encoding settings for your choice of frame rate.
So, what is frame rate?
Frame rate, often abbreviated as FPS, measures the speed that individual images or frames are captured and displayed to form a continuous video. The term actually comes from the days of film cameras where frames of a celluloid strip would travel behind the lens and be exposed through a gate at a certain speed.
As you may know, a video file is just a series of individual pictures played back in succession to form the representation of continuous motion. Think of an old-fashioned flipbook you might have played with as a kid. In this case, the number of pages that you have, and the speed at which you flip them, defines both the “capture” and “playback” frame rate in a way. If you have 30 pictures in your flipbook, and you flip the book in one second, your frame rate would be 30 frames per second.
Early animators found the perception of motion to start taking place somewhere between 12 to 16 frames per second. Enter Thomas Edison (that’s right, did you know he was a filmmaker?) determined that 46 frames per second was ideal for the human eye. Unfortunately, between the high cost of film and technical limitations with the stock, filming at high frame rates was not possible at the time. Instead, most films were shot between 16-18 fps and projected between 20-24. This led to the often “jumpy” nature notorious in early silent films.
As time went on, 24fps became the de facto standard for film, striking a balance between how much film stock would be needed and natural-looking motion. Later, in the 1950s, another factor was introduced in the advent of home television. The first TVs, based on cathode ray tube technology, required that the display refresh at AC line frequency, or the flow of electric power running to a display. This is 60 Hertz in the US, and 50 in Europe. Because of this, standards for NTSC (or 60 frames per second) and PAL (50 frames per second) were established for broadcast television.
What’s does interlacing have to do with frame rates?
In order to conserve bandwidth and present the most natural motion without flickering, engineers invented a creative method known as interlacing. Interlacing simply means that two video fields make up a single frame, similar to Venetian blinds. With interlaced video, odd and even fields flash, one after the other. For example, with video filmed at 60i (or interlaced), you’re watching a half-frame every 1/60th of a second and a full-frame every 1/30th of a second. Progressive scan, on the other hand, means that each frame is displayed in its entirety sequentially, consuming double the bandwidth but providing for a much cleaner image. Generally, this is captured as either 30 or 60 frames per second. While interlacing was helpful for broadcast for many years, even standard practice up to the mid-2000s, progressive scan has quickly become more popular as displays such as LCD and OLED have replaced CRTs.
So that brings us to today. 24 frames per second has continued to generally be the standard for cinema, while 30 and 60 are used for broadcast. However, with the latest in digital capture and playback technology, videographers can break free from typical conventions and record in whatever frame rates are most suitable for their desired style.
While the three most common rates (24, 30, and 60), are characteristically displayed as whole numbers, you might also be familiar with seeing them represented as decimals (dang… do we really need to get math involved?). That’s right, as color broadcast television replaced black and white, a very slight slowdown was required in the way that frames are measured in order to display effectively on older systems and prevent issues with sound. This is referred to as drop-frame measurement, and does not affect the visual image in any way, but is why you may see video frame measurement described as 23.98, 29.97, or 59.94 instead of whole numbers.
How do you choose a frame rate?
Now that you know the history of frame rates, let’s talk about how to choose between them for video. As we mentioned before, you have the creative liberty to determine both the frame rate at which your video is captured and displayed. Back to that flipbook analogy – if we added extra pages, but flipped them at the same rate (30 pages per second), the animation would slow down, almost as if we were displaying it in slow motion. That’s why typical “slow-mo” shots are filmed in framerates like 120fps but played back at 30 or 60. However, if we flip faster, the animation will appear in a normal speed, but have smoother motion overall, because more frames of it are displayed.
In practical purposes for streaming, you should always try to make sure that your display framerate, or what you actually have your streaming software or encoder set to, matches the input framerates of your devices. For example, if your cameras are set to 1080p60, your encoder should be set to stream at 1080 resolution at 60 frames per second.
From there, it’s all about the style that you are going for, and the amount of bandwidth you have available to use. Your framerate is part of a larger equation that considers the bitrate and resolution of the cameras, both of which we will get into in other videos. If you have limited bandwidth or CPU processing, it may be worth limiting your bitrate to 30 instead of 60, for instance, to make sure that frames don’t need to be discarded, or “dropped” due to the inability to process or transmit them in time. Using a technology like Resi’s can also come in handy, which transmits on a short delay through Resi’s Resilient Streaming Protocol, to ensure that every frame makes it to the destination perfectly without dropping, even on troubled internet connections.
Most of the time, we would recommend starting at 30. This strikes a good balance between the natural motion of what most are accustomed to seeing, and not being too intensive on your or your viewers’ connection. If you are streaming a faster event, like a race, hockey game, or dance competition, a higher frame rate will give your audience the best viewing experience with the smoothest motion. If you are looking for a more cinematic, dream-like visual appearance for your stream, consider using 24fps.
Many churches and live music concerts, for example, like to use 24fps to achieve a more film-like experience with their broadcasts, usually in tandem with using cinema cameras and lenses like the Canon C300 or Varicam. This is where a new tool like VFR, or variable frame rate, can be beneficial, which adds a third element to the equation – within the camera, a framerate is captured; but packaged into another output framerate which the camera then passes to the video switching infrastructure. This allows camera operators to dynamically choose between styles while maximizing compatibility with the system and can be used to record in multiple “looks” within a single stream (for example, using a cinematic style for music and a more natural, full-motion look for speaking elements).
Be sure to check out the video above! We spoke to our friends at Summit Integrated Systems, who specialize in video system design for major house of worship venues around the world, about their recommendations when installing video streaming systems to achieve the style and goals that their clients are looking for.