Two MP4 files, one smooth and one stuttering

Why two files with the same extension behave differently

Save a video from one page and a video from another, put both on the same phone, and one plays cleanly while the other jerks and loses frames. Both end in .mp4, so the extension is not what separates them.

MP4 names the outer box. The picture inside it was squeezed down by a codec, a set of rules for turning a moving image into far less data, and your device runs those rules in reverse to put the picture back on screen. The rules a publisher used in 2015 and the rules another used last year are not the same, and your device knows some better than others.

Hardware decoding is what keeps playback smooth

Phones, laptops, televisions and consoles carry a small dedicated circuit that undoes specific codecs and nothing else. When the file matches it, the picture is rebuilt there, the main processor stays out of it, and the battery barely notices.

When the file does not match, the general processor and the player software do the job instead. A current machine absorbs that. A phone from several years ago gives you frozen frames, sound drifting away from the lips and a warm case. That gap is the whole reason one MP4 stutters and another does not.

The three compressions you meet inside an MP4

Three methods cover nearly every video file on the web, and a box labelled MP4 holds any of them.

• H.264, in use since 2003 and undone in hardware by practically every device sold since. Publishers reach for it when they want a file that works everywhere.
• H.265, also written HEVC. It stores the same picture in fewer megabytes, and the circuit for it reached devices around 2015. Older hardware falls back to software.
• AV1, the newest of the three. It saves more space again, and its circuits only reached phones and televisions in recent years, so it troubles an old device most.

Signs that your device is decoding in software

You can settle this without any tool. Play the saved file and watch the machine rather than the video.

• The picture halts for a fraction of a second every few seconds while the sound carries on.
• Fast movement turns to a smear and cleans up again the moment the shot goes still.
• A laptop fan spins up, or the back of a phone becomes warm to hold.
• The battery figure falls faster than during an ordinary video of the same length.

Choosing a row for hardware that is years old

Take a lower row. A 720p file asks far less of a processor than a 1080p or 2160p file whatever the codec, and on an older screen the detail you give up is small. One step down the list turns an unwatchable file into a watchable one.

When the top row is the one you want, put it on the newest screen in the house. The file is identical wherever you copy it; only the decoder changes.

What the row tells you before you commit

Each row carries a height and a size, both read off the source. The list is an inventory of what that page publishes, so it cannot hand you an H.264 version of a video published only in AV1.

Where two rows sit at the same height and one is much smaller, the smaller file is the newer compression doing its job. Of two rows at that height, the larger is more often the older and more widely supported method.

What free use adds to the decision

Free accounts get three downloads a day, and the count goes back to three at midnight UTC. Each single file has to come in under 150 MB. Pro takes off the daily count, lifts the size ceiling and clears the advertising.

Those limits push in the same direction an old device does. A smaller row fits under the ceiling and lands in hardware decoding more often, so the row that saves megabytes is frequently the one that plays smoothly.

Related pages

• What rides inside an MP4 box
• Containers and codecs in plain words
• When the list offers no MP4 row
• Resolution weighed against file size
• Convert a saved WEBM file