Shortly after the compact disc became widely available as a digital-format replacement for analog audio, it became feasible to also store and use video in digital form.
Each compression specification defines various mechanisms by which raw video (in essence, a sequence of full-resolution uncompressed digital images) can be reduced in size, from simple bit compression (like Lempel-Ziv-Welch) to psycho-visual and motion summarization, and how the output is stored as a bit stream.
So long as the encoder component of the codec adheres to the specification, it can choose any combination of these methods to apply different parts of the content.
Each encoder implements the specification according to its own algorithms and parameters, which means that the compressed output of different codecs will vary, resulting in variations in quality and efficiency between them.
Press-releases and amateur forums may sometimes select sequences known to favor a particular codec or style of rate-control in reviews.
Objective methods are classified based on the availability of an original pristine video signal, which is considered to be of high quality (generally not compressed).
Subjective video quality tests are quite expensive with regard to time (preparation and running) and human resources.
Meanwhile, the same pair of codecs may give opposite results if running on an older computer with reduced memory (or cache) resources.
Modern standards define a wide range of features and require very substantial software or hardware efforts and resources for their implementation.
These are mostly for professional (e.g., camera and editing system) applications: Moreover, the standard now also contains three Scalable Video Coding profiles.
For offline storage and viewing, it is typically preferable to encode at constant quality (usually defined by quantization) rather than using bit rate control.