Trim videos and recordings, then fix the sound

To trim a video, choose Fast, which doesn’t re-encode, if a start a few seconds early is fine, and Accurate, which re-encodes, when the start has to be exact. Audio cut without a fade keeps its quality, and a quiet recording is safest to boost in dB only as far as its peak stays at or below −1 dBFS. This guide assumes you’re working with files you made or have the right to use.

Why a Fast cut snaps back to a keyframe

A video doesn’t store every frame as a full picture. According to MDN, video compression typically compares frames and records just enough information to update the previous frame so it looks like the next one. So a frame in the middle can’t be decoded without the frames before it, and files include periodic key frames (also called intra-frames or i-frames): full frames that also make seeking faster.

That’s why a cut made without re-encoding can only start at a keyframe. The FFmpeg documentation explains that in most formats it can’t seek to an exact position, so it seeks to the closest seek point before it. When transcoding, the extra part is decoded and thrown away; with stream copy, it’s kept. The Mediabunny documentation likewise says a copied region may differ from the requested range because of key frames and packet boundaries, and its “expand” policy widens the region to include everything you asked for.

Here’s how the two methods in Cut Video compare on our test files (measured in desktop Chrome; results vary by device and browser). The 1080p file has a keyframe every 2 seconds; the WebM file has one only at 0 seconds.

Original	Range · method	Result
1080p MP4, 30 s, 5.8 MB	10–20 s · Fast	MP4 1.9 MB, 10.0 s, not re-encoded
Same file	11–20 s · Fast	From the keyframe at 10.0 s, 10.0 s long
Same file	10–20 s · Accurate	MP4 2.3 MB, 10.0 s, H.264 + AAC
WebM, 10 s, 829 KB	2–6 s · Fast	Cut from 0 s, 5.98 s long

Fast keeps the quality and format, and when the start moves back, the result says “Cut from (n)s to match a keyframe”. Accurate cuts at the exact point but re-encodes to MP4, which takes longer and adds another round of lossy compression. Use Fast when a few extra seconds don’t matter, and Accurate when the clip must begin on a specific scene.

Trimming recordings and making ringtones

Cut Audio doesn’t re-encode unless you add a fade, so the quality stays the same. But compressed audio such as MP3 or M4A is stored in blocks (frames or packets), so the cut widens to the block edges to include your whole selection. Cutting 2 to 5 seconds from a 1 kHz test tone gave exactly 3.000 s for WAV, which is cut sample by sample, and 3.048 s for MP3, still at 128kbps (measured in desktop Chrome; results vary by device and browser).

For a ringtone or alert sound, set the start and end with [Play selection], then add a 1- or 2-second Fade out so the end doesn’t stop abruptly. A fade means the audio is re-encoded: MP3 at 192kbps, M4A as AAC at 192kbps. For how bitrate affects file size, see 128 vs 192 vs 320 kbps MP3. Ringtone formats and lengths vary by phone, so check the maker’s instructions.

Boosting a quiet lecture or call recording: how dB works

Digital audio levels are handled as values from −1 to 1. The W3C Web Audio API specification treats this as the nominal range of audio signals at the output, and converts between linear gain and decibels like this:

dB = 20 × log10(gain)
gain = 10^(dB ÷ 20)

If the largest value, 1, is 0 dBFS, a sound with an amplitude of 0.1 peaks at −20 dBFS. Multipliers for each boost (calculated):

Boost	Multiplier
+3 dB	about 1.4×
+6 dB	about 2×
+10 dB	about 3.2×
+15 dB	about 5.6×

Boosted values past 1 are cut off at 1 (as this tool does with Prevent clipping off), and a waveform with sliced-off tops sounds distorted. The W3C specification calls this clipping, or distorting, the audio output. That’s why Volume Booster has Prevent clipping on by default and only boosts as far as the peak stays at or below −1 dBFS. Leaving 1 dB of room makes it harder to go over the limit if the sound shifts slightly when it’s saved again with lossy compression.

Original (peak)	Choice	Result
WAV, −20 dBFS	+6 dB	−14.0 dBFS
Same file	Normalize to max	+19 dB, −1.0 dBFS
WAV, −3 dBFS	+6 dB, Prevent clipping on	only +2.0 dB, −1.0 dBFS

Measured in desktop Chrome; results vary by device and browser. For a lecture or call recording, start with the default, +6 dB. Background noise gets louder by the same ratio. If one loud sound, such as a cough or a bump on the mic, sets the peak, the rest can’t be raised further. If that sound is at the start or end, cut it off with Cut Audio first.

Removing the sound from a video

To keep only the picture, use Mute Video to remove just the audio track. The video isn’t re-encoded, so quality and length stay the same. A 1280×720, 10-second MOV (1.1 MB) became a 1,011 KB MOV with no audio track, still 10.0 s long (measured in desktop Chrome; results vary by device and browser). A video with no sound shows “This video already has no sound”. You can’t mute only part of a video, so cut first if you need just some scenes.

How to do it with ZEKILO Studio

1. In Cut Video, drag and drop one video or pick it with [Choose files]. The video and a range strip appear in the edit preview.
2. Drag the handles or type seconds into the Start and End fields (90 for 1 minute 30 seconds), choose Fast or Accurate under Cutting method, then press [Start].
3. Check the actual length and message on the result card, then save with [Download]. The result is named like clip-cut.mp4.

For recordings, use Cut Audio and Volume Booster; for the sound in a video, pull it out first with Video to MP3. Everything runs inside this browser, and your files never leave your device.

Summary

• Fast keeps the quality, but the start may move back to the previous keyframe. Accurate cuts at the exact point but re-encodes.
• Audio cut without a fade keeps its quality. MP3 is cut by frame, so the result can run slightly long.
• dB and gain convert with 20 × log10; +6 dB is about 2×. A peak at or below −1 dBFS prevents clipping.
• Muting doesn’t re-encode the video, so the picture stays the same.