Spaced repetition that brings each card back before you forget it

What a spaced repetition app does

A spaced repetition app shows you each flashcard again just before you are likely to forget it, and waits longer every time you remember it. Phosphor is a spaced repetition app that schedules every card with FSRS and builds the cards for you from a lecture video, a topic you type, or notes you paste in.

The idea is old. In the 1880s Hermann Ebbinghaus measured how fast he forgot lists he had learned and found that the loss is steep at first and then levels off: the forgetting curve. Each time you successfully pull a fact back out of memory, the curve flattens and the next review can wait longer. Rereading does far less than this. Recalling the answer before you see it is what does the work.

So a good spaced repetition app has two jobs. It makes you recall rather than reread, and it works out the day each card is due so you never have to keep a calendar of what to revise.

How Phosphor schedules each card

Phosphor uses FSRS, the Free Spaced Repetition Scheduler, through the open-source fsrs package, which implements FSRS-6. After every card you grade yourself Again, Hard, Good or Easy, the same four buttons Anki uses, so the habit carries over if you are coming from there. FSRS keeps an estimate of how stable each memory is and how difficult the card is for you, and from those it predicts the day your chance of recalling the card falls to the target. That day is the due date.

By default the target is 90% recall. If you add an exam date, the last 14 days before it are drilled harder: the target rises to 95%, so reviews come closer together exactly when it matters most. A card you grade Hard returns after a shorter gap; one you grade Easy is pushed further out.

You can see the fade as well as trust it. Each card carries a retention estimate, and a forgetting curve panel flags the cards slipping lowest so you can review those first. Cards you have failed four or more times collect in a separate list called Cards you keep missing, where you can rewrite, reset or suspend them. A card that keeps failing is usually a badly written card, not a bad memory, and reviewing it one more time does not fix that.

Where the cards come from

You do not have to write the cards yourself. Phosphor starts from one of three things.

• A lecture video link. Phosphor reads the video’s captions and writes a summary, notes and a set of flashcards: 10 per video on the free plan, 25 on Pro. Every card is tied to the second of the video it came from. Tap the timestamp and the player jumps there, so checking a card takes seconds instead of a re-watch.
• A topic you type. Type something like “Newton’s laws of motion” and Phosphor writes study notes on it first, then builds cards from those notes.
• Text you already have. Paste lecture notes, a handout or a chapter you typed up, and the cards are built from that text.

If a video has no captions, Phosphor tells you so rather than guessing, and you can paste a transcript in SRT or VTT format to load the captions yourself. A playlist can become an ordered course with chapters, or one deck that you preview card by card, keeping or rejecting each before anything is saved.

That means Phosphor also works as a YouTube video summarizer and a way to turn a YouTube video into notes. The summary is the start, not the product: notes you read once fade like anything else, and the cards are what you keep. We wrote up the longer version of how this works in how AI turns YouTube lectures into Anki cards, and there is a walkthrough for turning Boards and Beyond lectures into an Anki deck.

Review from the notification

A due card can reach you as a notification that shows its question, with four buttons underneath: Again, Hard, Good and Easy. Tap a grade and Phosphor opens with it already applied — the schedule updates the same way it would for a review inside the app, and you land on the next card instead of hunting for the deck. A two-minute review in a queue counts exactly as much as a review at your desk, and that is most of what keeps a deck alive through a busy week.

Different ways to be tested

A card you only ever see in one form gets easy to recognise long before you really know it, so Phosphor varies the format:

• chapter quizzes with multiple choice, true or false and short answers
• cloze cards, where you fill in the blank
• concept maps that lay out the main ideas of a lecture in order
• confusable-pair drills for two ideas you keep mixing up
• free recall, where you write down everything you remember before you look
• teach-it-back, where you explain the idea in your own words

You can also put a question to one lecture or to your whole library, and the answer is drawn from your own notes with a timestamp pointing at where it came from. A weekly recap sums up what you covered.

Your cards stay yours

Export any deck whenever you like: as a native Anki package (.apkg) that imports in one tap, as a tab-separated CSV for Anki, as Markdown for Obsidian, or as a print-friendly PDF. You can also add the review schedule to your calendar. Notion sync, which pushes notes and cards into a Notion database you choose, is part of Pro.

The free plan and Pro

The free plan covers up to 10 video summaries and 3 new flashcard decks a week. Reviewing the cards you have already saved does not count against any limit, so a deck you built in September keeps working in December whether or not you pay. Phosphor Pro raises the limits to 500 summaries and 100 new decks a week, makes 25 cards per video instead of 10, and adds Notion sync.

Who it is for, and when Anki is the better choice

Phosphor fits students who revise from recorded lectures, in medicine and nursing, law, engineering or computer science, and language learners. It also suits anyone who got to the end of an online course and found that most of it had already gone.

If you already have a large Anki deck tuned the way you like, and you enjoy writing your own cards, Anki is excellent and you may not need anything else. Phosphor is for getting from a lecture to a scheduled deck without that setup. Because every deck exports as an .apkg, moving your cards to Anki later costs you nothing.

The same app, a new name

Phosphor was formerly NoteTube AI. It is the same app under a new name. One part of the older app is still there for people who make videos rather than study them: Channel Radar. Track up to 3 channels on the free plan, or 50 on Pro. Their uploads are checked every 15 minutes, and when a new video gets twice that channel’s usual views in its first 48 hours, you get a notification.