Why does converting an audio or video file reduce quality?
Changing the container can preserve the encoded audio and video. Encoding them again with a lossy codec can introduce another generation of loss.
Three different operations
| Operation | What changes | Quality effect |
|---|---|---|
| Rename the extension | The name only | No conversion; it may confuse the player. |
| Remux / stream copy | The container around copied streams | No additional codec loss in the copied streams. |
| Transcode | Audio or video is decoded and encoded again | Depends on the codec, settings and processing; lossy encoding can reduce quality. |
FFmpeg documents stream copy as copying packets without decoding or encoding. It is fast and avoids encoding loss. The destination container still needs to support those streams; subtitles and metadata may not all transfer.
Why does lossy-to-lossy conversion matter?
MP3 to AAC, AAC to MP3, or repeated H.264 encoding works from audio or video that has already lost information. A later encoder can introduce further changes. Higher output bitrate may limit additional damage but cannot restore information discarded earlier.
Likewise, converting MP3 to FLAC preserves the decoded result without making it equivalent to the original lossless recording. A larger file is not evidence of recovered detail.
How can I preserve quality?
- Keep the original as your master copy.
- Use remuxing when only the container needs changing and the new container supports the streams.
- If conversion is necessary, encode once from the best source you have.
- Avoid unnecessary resizing, frame-rate changes, sample-rate conversion and audio processing.
- Check the result on the device you actually intend to use.
What if I need a more compatible file?
Some devices cannot decode the original codec. In that case, remuxing alone will not fix playback; a supported codec may require transcoding. Treat it as a compatibility copy and retain the original.
A converter can copy video while encoding audio, or the reverse. Assess each stream separately rather than describing the entire operation as lossless.
Read what is remuxing?, what is transcoding? and M4A vs MP3.