What a Preview Bus and a Program Bus Mean on a Video Switcher

Two rows of buttons, two different jobs

Look at almost any video switcher, hardware or software, and you will find two rows of source buttons. The top row is usually called program, the bottom row preview. In the trade each row is a bus: a path that carries one chosen camera or graphic to a destination. Program is what the audience sees. Preview is what you are about to show them.

I teach audio routing, so I came to this from the sound side, where a similar idea exists: you listen on headphones to a channel before you push its fader up. The switcher works the same way. You select the next shot on preview, look at it on its own monitor, and only then move it to program.

Learners in the video switching module practise this from the first hour, because the habit matters more than any single button. A person who watches preview before every cut rarely puts a wrong shot on air.

• Program bus: the live output going to the encoder and the audience
• Preview bus: the shot you have lined up but nobody has seen yet
• Both buses are fed by the same inputs, so any camera can sit on either

Why you never cut blind

The most common error in a multi-camera show is cutting to a camera that is not ready. Someone has bumped a tripod, a presenter has stepped out of frame, or a lens cap went back on during a break. If you cut straight on program, the audience sees it. If you check preview first, you see it and move on to another source.

This is not about being slow. A calm operator glances at the preview monitor while the current shot is still running, decides, and then acts. The glance takes a second, and over a ninety-minute panel it prevents a handful of small embarrassments.

In rehearsal we deliberately set a trap: one camera is framed badly on purpose. Learners who use preview catch it every time. Learners who tap source buttons directly on program find out in front of their classmates, which is a gentle way to learn it.

Cut, mix and the transition button

Once a shot is on preview, you send it to program with a transition. The simplest is a cut: an instant swap between the two buses. A mix, often called a dissolve or fade, blends one into the other over a set time. Many switchers also offer wipes and stingers, but plain cuts and short mixes cover most small studio and panel work.

The trade also uses the word auto for a transition that runs for a preset duration when you press one button, and take or cut for the instant version. Some panels have a fader bar that you move by hand, which gives you control of speed but takes practice.

We suggest learners settle on one default, usually a cut for speaker changes and a half-second mix for moving to a graphic, and keep it consistent through the show. Consistency reads as polish.

• Cut: immediate swap, best for changing speakers
• Mix or dissolve: gradual blend, useful for graphics and slides
• Auto: runs the chosen transition at a fixed duration
• Fader bar: manual control of the transition speed

Where scenes and overlays fit in

Beginners sometimes assume that every item on the buses is a single camera. In practice a source can be a built scene: two cameras side by side, a presenter with a lower-third name bar, or a slide with a small speaker window. You build the scene once, name it clearly, and then it behaves like any other button on preview and program.

A clear naming scheme saves you under pressure. Scene names such as host wide, guest close, slides plus host tell an operator what will appear without opening anything. Vague names like scene 4 invite mistakes at the worst moment.

In the curriculum we have learners build their scene list before the rehearsal, and then trim it. Most shows need fewer scenes than people first create, and a shorter list means fewer chances to tap the wrong one.

Keeping the preview honest

Preview is only useful if you trust it. Check that the preview monitor shows exactly the source you selected and is not delayed or showing a stale frame. Some software setups add latency to the preview window, which makes cuts feel late. If your preview lags, say so during testing and adjust, rather than learning about it live.

Also confirm that the program monitor reflects what the encoder is actually sending. The switcher's output and the stream the audience watches can differ if a cable is unplugged or a platform setting is wrong, and the stream platform's own monitor is the final check.

Results here depend on the equipment you have agreed to use and the venue connection, so we always treat these checks as part of setup and not as extras.

A short drill to build the habit

Here is the routine we ask learners to repeat until it is automatic. Select the next camera on preview. Look at the framing, focus and whether anyone is ready. Wait for a natural pause in the talk. Take the transition. Then immediately choose the following shot on preview, so you are always one step ahead.

It sounds mechanical at first and feels natural after a few sessions. Daniel Brooks, our lead instructor for switching, runs this drill with a recorded panel so that learners can replay their cuts and see where one landed mid-sentence.

The aim is a show where the audience never thinks about the switching at all. Clean cuts are noticed only when they go wrong.

• Line up the next shot on preview
• Check framing and readiness
• Cut during a pause, not mid-sentence
• Choose the following shot straight away