To answer the question – what is video transcoding – we first need to understand a little bit about video lifecycles and compression.
When we generate or capture an image, we generate a huge amount of data. If we consider a High Definition (HD) picture, there are over 2 million pixels, and for each pixel we need to capture data on color and brightness. In the case of video, depending on the frame rate we are capturing up to 60 or more of these images per second.
The amount of data soon becomes unmanageable so we throw some of it, in fact eventually quite a lot of it, away by compressing it. At a high level, there are three levels of compression:
- Lossless – where we throw away data that we can completely recreate later
- Visually lossless – where we throw away data, but we can’t detect what we’ve thrown away
- Lossy – where we throw away data and can see the difference, although in many cases, viewers aren’t aware of this.
We combine these different types of compression and use them to “encode” the video using algorithms known as “codecs” - short for (en)code, decode. Depending on where the video is in its lifecycle, we use different codecs and different levels of compression. Early in the lifecycle, we want to store as much image information as possible to enable processes such as:
- color correction
- green and blue screen compositing
- similar editing and grading.
For example, you might want to adjust the picture to find details in the shadows or highlights. If you have shot your scenes using a codec that stores a lot of data, e.g. specifies a high bitrate, you will have the information available in your low and highlights for your creative decision how to adjust and present the video file.
A production facility will compensate for these higher bitrate requirements by using high performance storage and fast data networks between editing stations in order to provide the video data in real-time to the editors.
To make the material usable downstream, for example viewable on a web client, these master files need to be converted to use a different codec and/or more compression.
This conversion is called video transcoding.
Video transcoding happens many times in the video lifecycle and almost every device you consume video on requires a different codec or compression, resulting in many video transcoding processes. If done well, video transcoding has little impact on the perceived viewing quality. That amazing 4K HDR picture on your new TV from your favorite streaming service probably has a bit rate of around 12Mbps while an uncompressed 4K image can exceed 12Gbs – more than 1000 times more!