How to use a teleprompter app with a beam-splitter rig

a beam-splitter rig puts a piece of 45-degree glass in front of your camera. your phone lies flat under it, screen up, and the glass reflects that screen back at you. you read the words and look straight down the lens of a separate camera at the same time. that's the whole trick, and it's why a phone teleprompter app is still the right tool even when the "real" capture is happening on a mirrorless body behind the glass.

this is how to drive that rig with open prompter.

the phone is the source, not the camera#

on a serious rig the mirrorless body behind the glass is doing the real capture. the phone's only job is to display the script and follow your read. that's exactly what open prompter is built to do, and it does it without making you babysit a copy of your script.

you point open prompter at a folder once. iCloud Drive, an Obsidian vault, Dropbox, any folder iOS can see. it reads the markdown files in that folder directly. no paste, no import. you write the script on your mac, save the file, and the phone catches up on its own. headings, bold, bullets, and blockquotes render on screen; frontmatter and visual-direction cues like [B-roll: ...] get stripped at display time, without touching the file. so the script under the glass is always the one you last saved, formatted for reading, with the production scaffolding out of the way.

flip the text so the glass reads right#

the glass reverses your text left-to-right. so the first thing you do is mirror it.

open prompter cycles the mirror with one chip: off, then horizontal, then vertical, then both, then off again. horizontal is the beam-splitter standard. set it once and the reflection reads normally.

vertical and both exist for the rigs that aren't standard. periscope setups, upside-down mounts, anything that bounces the image an extra time. open prompter does horizontal AND vertical independently, so an odd rig isn't a dealbreaker. flip the axes until the reflection looks right and leave it.

go hands-free so your hands stay on the camera#

on a beam-splitter rig your hands belong on the camera, not on the phone buried under the glass. so you don't touch the phone at all.

pair a Bluetooth remote or keyboard and bind the buttons you reach for: play, pause, scroll, speed up or down, next or previous section, mirror toggle, and jump to the start of the take. that last one is the one you'll use between takes on a rig. fluff a line, thumb the button, and the script snaps back to the top so you can go again without leaning under the glass to scrub manually.

now you start the scroll, pull focus, adjust your framing, and stop the scroll without ever reaching under the rig. if you'd rather drive it from the volume buttons, that's a setting you turn on yourself; it's off by default, so your volume rocker stays a volume rocker unless you say otherwise.

while you're at it, turn on focus mode. it dims the chrome down to the words, so the glass reflects clean text instead of buttons and chips. a single eye button brings the controls back when you need them. clean glass, clean read.

voice tracking that hears you from across the room#

here's the part that most beam-splitter setups can't do.

voice tracking listens to your delivery and scrolls to keep your place, on its own, no thumb on the screen. it runs on-device, so nothing leaves the phone. that alone is useful on a rig. but the detail that matters here is where it listens.

voice tracking listens on the recording audio path. that means it hears whatever your microphone hears. and your microphone doesn't have to be the one built into the phone.

route a lavalier into the phone over USB-C, or over a 3.5 mm TRRS input, and voice tracking follows that mic instead. clip the lav to your collar and step back to your mark. you can stand well past where the phone's built-in mic would ever pick you up, and the script still tracks every word you say, because the audio is coming down the cable, not across the room.

think about what that unlocks on a rig. you're not crowded up against the glass to stay in mic range. you can frame a wider shot, give yourself room to gesture, hit a mark a few feet back, and the prompter keeps pace with your actual delivery the entire time. almost no other professional beam-splitter teleprompter app can do voice tracking from an external mic source. that's not a small thing. it's the difference between a prompter that works at arm's length and one that works at performance distance.

voice tracking is in beta, and it leans on two draggable lines you can tune to taste. the READ line is where the word you just said lands. the FEATHER line is where the scroll eases out of catch-up into a smooth glide. drag them close together for tight word-for-word follow, or apart for looser momentum. if you stop talking, it notices the silence and pauses so you can scroll back and re-read a line.

set it up once#

a working pass, start to finish:

1. mount the phone flat under the glass, screen facing up into the splitter.
2. mirror horizontal so the reflected text reads normally.
3. pair your Bluetooth remote and bind play, pause, and scroll.
4. plug your lavalier into the phone over USB-C or 3.5 mm TRRS and select it as the mic source.
5. turn on voice tracking, drag READ and FEATHER to a feel you like.
6. switch on focus mode for clean glass.

now step back to your mark, look down the lens, and start talking. the remote is in your hand, the lav is on your collar, and the script follows your voice from wherever you're standing.

why this matters#

a beam-splitter teleprompter has always meant looking down the lens while you read. the part that was missing was being genuinely free of the device. hands on the camera, eyes on the lens, standing where the shot wants you instead of leashed to the phone's microphone.

that's what the wired-mic voice tracking gives you. a real hands-free, eyes-on-lens, far-from-device prompter on a rig you can build for cheap. and it's free, MIT-licensed, with no account and no subscription. the rest of the category charges a monthly fee for mirror mode alone. this is the whole thing, for less than the cost of the subscription you were about to start paying.

if you want to see the reading geometry before you mount anything, drag the lines here.