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Glossary · concept

What is Lossy compression?

Lossy compression discards some data permanently to make files smaller. MP3, AAC, JPEG, H.264, H.265, AV1, and VP9 are all lossy formats — they throw away detail the human eye or ear is unlikely to notice. The compression is one-way: the discarded data cannot be reconstructed from the compressed file.

Also called:lossy encoding · lossy compression

Every consumer media file you have ever played is lossy. The math is simple: uncompressed 1080p video is ~1.5 GB per second; uncompressed CD audio is ~1.4 Mbps. Storage and bandwidth at those rates is unworkable. Compression algorithms pick which information to drop by modeling what humans perceive — they can throw away high-frequency audio above 16 kHz (most adults can't hear it), they can blur visually-similar pixels (the brain reconstructs them), they can store only differences between frames (consecutive frames are 95% identical).

The amount thrown away is controlled by bitrate. A 320 kbps MP3 throws less away than a 128 kbps MP3 at the same source. A 4K H.264 video at 25 Mbps throws less away than the same source at 5 Mbps.

The catch: every time you re-encode a lossy file, you compound the loss. Downloading a YouTube video then converting to MP4 with different settings produces a file that's visibly worse than either the original or what YouTube would have served. This is why VidPickr does lossless muxing — we never re-encode the codec data, only repackage it.

Common questions

Is YouTube lossy?
Yes. Every video uploaded to YouTube is re-encoded to a lossy codec (H.264, VP9, or AV1) for delivery. The version you watch is never the bit-identical file the uploader uploaded — quality is roughly halved on the encoding step in exchange for ~10× smaller files.
How can I avoid lossy compression?
You can't for most consumer pipelines — every streaming service, every consumer codec, every smartphone camera output is lossy. Lossless formats (FLAC for audio, lossless H.264/AV1 modes for video) exist but produce files 5-10× larger and aren't supported by most playback hardware.

Related terms

Lossless compression

Lossless compression makes files smaller without discarding any data — the original is perfectly reconstructed when decoded.

Codec

A codec is the algorithm that encodes (compresses) and decodes raw audio or video into a smaller stream.

Bitrate

Bitrate is the amount of data a video or audio stream carries per second, measured in bits per second (bps) or kilobits (kbps) and megabits (Mbps).

Transcoding

Transcoding is the process of decoding a compressed media file and re-encoding it into a different codec, container, bitrate, or resolution.

VidPickr is a free, browser-based YouTube downloader. Every term in this glossary either describes how YouTube delivers video or why your downloads behave the way they do. Try the downloader →