25, 30, 50 or 60 Frames per Second and How to Avoid Interlace Combing

Frame rate is one of the first settings you meet on a camera, a capture card and in your streaming software, and it is one of the easiest to get subtly wrong. Pick a rate that fights the lights in the room and the picture flickers. Mix two rates in one chain and motion judders. Send an interlaced signal to a progressive encoder and every moving edge grows a comb of thin lines. This guide explains where the common rates come from, how to choose one for your stream, and how to keep combing out of your picture.

Where the common frame rates come from

The rates you see on your camera menu are not arbitrary. Television in most of Europe, Africa, much of Asia and Australia grew up around a mains frequency of fifty hertz, and its frame rates are twenty-five and fifty. North America, parts of South America and some other regions use sixty-hertz mains, and their rates are thirty and sixty.

The odd numbers such as twenty-nine point nine seven are a leftover from analogue colour television in sixty-hertz countries. Your camera still offers them for broadcast compatibility. For streaming on the internet, a whole rate is simpler, and nothing about your picture gets worse when you choose it.

Why the lights decide more than you think

Most indoor lighting flickers with the mains, twice per cycle. Your eye does not notice, but your camera does when its shutter speed does not match that flicker. The result is a slow band rolling through the picture, or a brightness that pulses, which no encoder setting can remove later.

The rule that prevents it is about the shutter, not only the frame rate. Under fifty-hertz lighting, use a shutter of one-fiftieth or one-hundredth of a second. Under sixty-hertz lighting, use one-sixtieth or one-hundred-twentieth.

That is why crews in fifty-hertz countries traditionally shoot at twenty-five or fifty frames: those rates pair naturally with a flicker-free shutter. You can still shoot thirty or sixty frames per second under fifty-hertz light, as long as you set the shutter yourself instead of leaving it on automatic. Thirty frames with a one-fiftieth shutter, or sixty frames with a one-hundredth shutter, both stay clean.

LED walls, projectors and computer screens in the shot have their own refresh rates. If one of them flickers or shows rolling bars on camera, adjust the shutter in small steps while you watch that screen on your monitor, because the right value depends on that particular display.

Choosing a rate for your stream

Think about what moves in your picture and how much upload you have.

• Talks, worship, lessons and interviews look natural at twenty-five or thirty frames per second. Motion is modest, and the lower rate leaves more bits for detail in faces and slides.
• Sports, dance, gaming and fast camera moves benefit from fifty or sixty frames. Motion looks continuous, and fast action stays sharp instead of smearing.
• A cinematic music video or a short film may use the cinema rate on purpose. That is a creative choice, not a technical requirement, and it is usually better kept for recorded material than for a live show.

A higher rate needs more bitrate for the same sharpness, because the encoder has to describe twice as many pictures each second. If your venue upload is tight, a lower rate with a clean picture beats a higher rate that falls apart every time someone moves.

Keep one rate from camera to viewer

The judder and dropped frames you see almost always come from a mismatch somewhere in your chain. The camera sends one rate, the capture card captures another, and your streaming software is set to a third. Each conversion has to repeat or drop frames, and the result is motion that stutters in a regular rhythm.

Before every show, check these four places in order:

1. The camera output, often in an HDMI or SDI settings menu separate from the recording format.
2. The capture card, which must capture that exact rate and not only accept it.
3. The video settings of your streaming software, where the common frame rate values are set.
4. The output settings of the encoder itself, so it does not override the video settings.

If several cameras feed one switcher, set every camera to the same rate. A switcher can convert a camera that disagrees, but you pay for it in motion quality on that camera, and the difference shows every time you cut to it.

What interlace is and why it combs

If your camera sends interlaced video, it does not send whole pictures. It sends half pictures, called fields, one carrying the odd lines and the next carrying the even lines, a fraction of a second later. Old television screens drew them in turn, and your eye merged them into smooth motion.

Computer screens, phones and streaming encoders work with whole progressive frames. When two fields from different moments are woven into one frame, anything that moved between them shows a pattern of thin horizontal teeth along its edges. That is combing, and on a live stream it is most visible on hands, faces turning and text that scrolls.

Your camcorder or older broadcast camera may offer interlaced outputs, often labelled with an "i" after the line count. Some cameras send interlaced over HDMI even when they record progressive internally, so check the output setting rather than the recording setting.

How to get rid of combing

The best fix is to never create it. Set the camera's HDMI or SDI output to a progressive format, labelled with a "p", at the rate you chose. If the camera offers a progressive output, nothing later in the chain has to guess.

If the camera can only send interlaced video, deinterlace before the encoder. OBS Studio and most switching software offer deinterlacing per source, and the better modes rebuild a full frame from each field, which keeps the motion of a high field rate. Simpler modes that blend or discard a field are lighter on the processor but soften the picture.

A hardware converter or a switcher that outputs progressive can also do the conversion for you, so your computer does not have to. Whatever you use, check a moving edge on your monitor at full size, because combing is easy to miss in a small preview window.

Frame rate on DCAST

DCAST encodes the frames it receives as they are, so a picture that arrives interlaced reaches your viewers with its combing. Make sure your encoder sends progressive video; there is no deinterlacing step on the way to the player.

For a live broadcast, set your encoder to a whole thirty or a whole sixty. When the source is exactly sixty frames per second, DCAST keeps that rate on the top quality level of the live ladder and encodes the lower levels at thirty, which helps viewers on weak connections. A source below sixty, including a fifty-frame source, is delivered at thirty on every level.

That makes the lighting rule above practical: in a fifty-hertz venue, choose thirty frames with a one-fiftieth shutter or sixty frames with a one-hundredth shutter, and you get a flicker-free picture at a rate DCAST carries unchanged.

Uploaded videos follow a different rule on DCAST. A file keeps its own frame rate on the top quality level, so a twenty-five-frame or fifty-frame recording stays at that rate, and the lower levels run at no more than thirty. If you need a fifty-frame master for a European audience, record locally at that rate and upload the file after the event.

For bitrate figures that match each rate, see how much bitrate your live stream needs, and for where the encoder settings go, see connecting OBS to your stream.