This page moves from practical explanation toward a more Wikipedia-like view of the moving parts inside modern codecs.
Spatial redundancy: nearby pixels are similar.
Temporal redundancy: nearby frames are similar.
Perceptual limits: humans do not notice every detail equally.
This is where the site stops being mostly buying advice and starts becoming a technical reference: compression tools, trade-offs, and structure.
Video: prediction, transforms, GOPs, entropy coding.
Audio: psychoacoustics, masking, bit allocation.
Delivery: rate control, profiles, decoding limits.
Modern codecs differ in details, but many share the same broad pipeline: predict, transform, quantize, and entropy-code the remaining information.
Instead of coding every pixel directly, codecs often predict a block from nearby pixels in the same frame. Only the prediction error needs to be stored.
Video frames are usually similar over time. A codec can point to a block in a previous or future frame and store a motion vector plus the remaining difference.
Once a prediction is made, the codec stores what is left over. Good prediction means a smaller residual and fewer bits spent.
When you see a technical page or spec, scan for these questions:
Many codecs follow a similar broad process: prediction, transform, quantization, and entropy coding. The exact tools differ by codec.
Quantization reduces precision to save bits. Stronger quantization usually creates smaller files but can also create visible or audible artifacts.
Prediction lets the encoder describe what changed instead of storing everything directly. Good prediction leaves a smaller residual, which takes fewer bits to encode.
Entropy coding is the final packing step that stores common patterns with fewer bits and rare patterns with more bits, making the compressed stream smaller.