Reelpass
Guide

How to shrink an exam video under 2GB

Re-encode only the picture at a bitrate worked out from the limit and the running time, copy the sound untouched, and change nothing else: no trimming, joining or reordering. Recording at 720p and 30 frames per second usually does the rest.

The arithmetic

A video file is mostly picture. The sound is small, and the container adds a little for its index. So the question is how many bits per second the picture may use for the file to land under the limit. The Reelpass planner works it out in four lines:

  1. Target: the limit multiplied by one minus a safety margin. The default margin is 4 per cent.
  2. Take away the sound. It is copied, not re-encoded, so its size is known exactly.
  3. Take away the container overhead, a fixed header plus a small cost for every frame and sound packet, counted generously.
  4. Divide what is left, in bits, by the running time in seconds. That is the most the picture may use.

Then it picks the picture size. It keeps the source up to 720p, the size two boards advise, and steps down only when the bitrate would give fewer than 0.06 bits per pixel per frame. Frame rates above 30 are halved. If the picture would get under 150 kbps, the recording is too long for the limit and the planner says so instead of making something unwatchable.

Worked example

A 25 minute Grade 8 recording, ABRSM 2GB limit
  • Source: 3.40 GB, 1920 by 1080 at 30 frames per second, 25:00 long.
  • Target: 1.92 GB (2GB less 4 per cent).
  • Sound at 128 kbps, copied: 24.0 MB.
  • Container overhead: 1.9 MB.
  • Left for the picture: 1.89 GB, which is 10 Mbps over the running time.
  • Picture size: 1280 by 720 at 30 frames per second.
  • Picture bitrate chosen: 5.5 Mbps.
  • Expected file: about 1.06 GB.

Why 5.5 Mbps and not 10 Mbps? The planner caps the picture at 0.2 bits per pixel per frame. At 1280 by 720 and 30 frames per second that ceiling is 5.5 Mbps. Past it, a camera on a stand filming one player gains nothing a viewer can see, so the file stops at 1.06 GB with room to spare.

What compression may change, and what it may not

Every board asks for one continuous, unedited take. Compression must respect that. It may change how the picture is stored: its bitrate, its size (for example 1080p down to 720p) and, above 30 frames per second, how many frames are kept. It may not trim the start or the end, join clips, reorder anything or touch the sound. Reelpass copies the sound track across bit for bit, so the examiner hears exactly what the microphone recorded.

The iPhone HEVC problem

iPhones record HEVC (H.265) by default. The fast route decodes the picture with the browser's own decoder, and not every browser can decode HEVC. When the check page meets such a file, it offers these fixes:

The fallback encoder is ffmpeg compiled to run in the browser. It works on files the fast route cannot read, but it is much slower, because it does all the work in software rather than with the computer's video hardware. Leave the tab open and the computer awake while it runs. For some files it re-encodes the sound as AAC, and the check page tells you when it does.

Step by step

  1. Keep the original. Copy the original recording somewhere safe. Compression makes a new file; it never changes the one you recorded.
  2. Check the file. Open the video in the check page and choose your board, so the planner knows the limit and the running time.
  3. Read the plan. The planner shows the target size, the space the sound takes, the bitrate left for the picture and the picture size it will use.
  4. Compress. Start the compression and leave the tab open. The picture is re-encoded in the browser and the sound is copied across untouched.
  5. Watch the result. Play the new file from start to finish with sound, and confirm it is under the limit before you upload it.

Common mistakes

To see the plan for your own file, check and compress your exam video in the browser. Nothing is uploaded.

Sources

Checked on 27 September 2026. Every rule here is paraphrased from the page named, read on the date shown. Boards change their guidance; the board's own page is the authority, so check it before you upload.

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Frequently asked questions

Does compressing count as editing the performance?

Not in the sense the boards mean. Compression lowers the picture's bitrate and leaves every moment in place, in order. Nothing is cut, joined or reordered, and the sound is copied bit for bit.

Why not use the whole budget?

Above about 0.2 bits per pixel per frame, extra bits buy nothing visible for a static exam camera, so the planner stops there and the file ends up smaller than the limit allows.

Why is my iPhone video refused by the fast encoder?

iPhones record HEVC by default and some browsers cannot decode it. Set the camera to Most Compatible for the next recording, open the file in a browser that decodes HEVC, or use the slower fallback encoder.

Is my video uploaded to compress it?

No. Compression runs in your browser with WebCodecs, or with the fallback encoder in the same tab. The file stays on your device.