Encoding turns raw audio, video, or subtitle data into a compressed stream. Transcoding takes an already encoded stream, decodes it, and encodes it again—possibly after processing it. If you only change the container while copying the encoded streams unchanged, that is usually called remuxing or stream copying, not transcoding.
Encoding, transcoding, and remuxing at a glance
| Operation | What goes in | What happens | What comes out |
|---|---|---|---|
| Encoding | Raw audio samples, video frames, or subtitle data | An encoder compresses the data into a coded stream | An encoded audio, video, or subtitle stream |
| Transcoding | An already encoded stream | The stream is decoded and then encoded again; it may also be filtered or otherwise processed | A newly encoded stream, potentially with different properties |
| Remuxing or stream copying | Already encoded streams | The encoded packets are copied into a different container without decoding and re-encoding | The same encoded streams in a different package, when the target container supports them |
FFmpeg describes transcoding as decoding a stream and then encoding it again. Its documentation also explains that stream copy can pass encoded packets through without that decode-and-encode path: FFmpeg documentation.
What encoding does
An encoder converts raw media data into a format represented by a codec. A codec is the method used to encode and decode a stream; the resulting data is carried in packets that can be stored or delivered.
Encoding is often lossy: it discards some information to reduce the amount of data needed. Lossless encoders also exist, but their output may be much larger than lossy output. Whether a particular encode is lossy depends on the codec and settings, not simply on the word “encoding.”
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What transcoding adds
Transcoding starts with media that is already encoded. The workflow decodes that stream, may alter the decoded material, and encodes a new stream. A video service, for example, might resize a video or convert it to a codec accepted by a destination. AWS describes a typical video transcoding flow as demultiplexing streams, decoding and post-processing, encoding, and multiplexing the result: AWS: What Is Video Transcoding?.
Because transcoding includes a new encoding step, converting lossy source material to another lossy stream can reduce quality further. It does not guarantee a visible loss: the outcome depends on the source, codec, settings, and processing. A transcode can also be useful or necessary when the destination cannot accept the original stream or when you need to make a content change.
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Codec and container are different things
A codec describes how an individual audio or video stream is encoded and decoded. A container is the file or delivery format that packages one or more streams together, often with timing and other information. A filename extension may hint at the container, but it does not establish which codecs are inside or whether a particular player can handle them.
Compatibility depends on the combination of container, video codec, audio codec, and destination requirements. AWS MediaConvert’s supported-input reference lists container-and-codec combinations, illustrating why checking the actual streams matters: MediaConvert supported input formats. Support shown for one service should not be assumed to apply to every device or application.
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How to decide whether to transcode or remux
- Identify the destination. Check the platform, playback device, or delivery specification that must accept the media.
- Inspect the source streams. Find the container, video and audio codecs, and relevant properties such as resolution or frame rate. Do not rely on the extension alone.
- Check compatibility. If the destination accepts the existing streams and you do not need to alter them, stream copy or remuxing may be enough, provided the destination container can carry those streams.
- Transcode only when needed. Choose transcoding if a codec is incompatible or you need to resize, deinterlace, add an overlay, resample or mix audio, or make another change that requires processing.
- Check the result against the delivery target. Compare compatibility, quality, output size or bitrate, and processing time. A platform or delivery profile may impose specific requirements; Apple’s HLS authoring specification is one example of a destination-specific standard: Apple HLS authoring specification for Apple devices.
What the choice means for quality, size, and time
- Quality: Copying streams avoids a new encoding generation. A lossy transcode can add quality loss, but the extent depends on the input and encoding choices; it is not possible to promise a fixed amount.
- File size and bitrate: Re-encoding can change them, but the result depends on the codec, settings, and source. A smaller output is not automatically a better one if quality or compatibility suffers.
- Processing time: A transcode requires decoding and encoding and may include filters. Stream copying avoids that full re-encoding path, though actual processing time depends on the media and tool.
- Compatibility: The target’s supported codec and container combination is decisive. Changing only the container cannot make an unsupported encoded stream compatible.
Examples of the difference
Making a new encoded file
When raw video frames and audio samples are compressed into encoded streams for a media file, that is encoding. The specific codec, quality, and output depend on the chosen tool and settings.
Changing bitrate or frame rate
FFmpeg’s documentation includes conversion examples that change output properties such as bitrate or frame rate. If the change requires decoding and creating newly encoded streams, it is transcoding. Exact output depends on the installed FFmpeg build, selected codecs, settings, and input.
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Changing a container without changing streams
If the existing encoded packets are copied into a different compatible container, the operation is remuxing or stream copying. Since the streams are not decoded and re-encoded, it is not transcoding. This approach is useful only when the destination accepts the codecs and the target container supports them.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common mistakes and how to avoid them
- Calling every format change a transcode: A container-only change can be remuxing. Check whether the encoded streams were actually decoded and encoded again.
- Treating the extension as a codec: An extension generally identifies a container, not every stream inside it. Inspect the video and audio codecs as well.
- Assuming every encode loses quality: Lossy encoding is common, but lossless encoding exists. The method and settings determine the trade-off.
- Assuming transcoding always improves compatibility: It helps only if the output codec, container, and properties suit the destination.
- Re-encoding without a reason: If the current streams already meet the delivery requirements and no processing is needed, copying them may avoid an unnecessary encoding generation.
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