Audio and Video all start life in the ``Analog Domain''. (Domain is used in this context just to mean before or after some particular conversion). It is important to understand the basic requirements of the media in time and space. The analog domain is usually best understood in terms of the range of frequencies in use for a particular quality. For sound, this means how low and high a note/sound is allowed. For video, this translates into the number of distinguishable colours. For video, we also have to consider the frame rate. Video is similar to film in that it consists of a number of discrete frames. You may recall seeing old films which were shot at a lower frame rate than is used nowadays, and flicker is visible. To refine this point further, we should distinguish between the rate at which a scene is sampled, and the rate at which a frame on a screen is displayed. For many moving image systems, these may be different. For example, films may show the same frame more than once to reduce flicker. Although Cathode Ray Tubes have significant persistence, video systems may refresh different parts of the screen at different rates - interlacing is used in many systems where alternate lines of the screen are refreshed in alternate cycles. This is motivated by the possible reduction in bandwidth, both in the analog and digital domain.
Both sound and image can be broken down at any instant into a set of basic frequencies. This is the so-called ``waveform''. We can record all of the frequencies present at anyone time, or we can choose to record only the ``important'' ones. If we choose to record less than all frequencies, we get less ``fidelity'' in our recording, so that the playback is less like the original. However, the less we record, the less tape/recording media we need.
Audio and Video start as waves, a sequence of compression and rare-faction of air, or the fluctuation of an electric and magnetic field, with time.
Waves need to be captured, by some device, and then sampled digitally. Typically, a ``sample-and-hold'' technique is used: An electro-mechanical or light sensitive device responds to the sound or light, and produces an analogue electrical signal. This can be averaged over a sample period by providing some discrete clock signal, and an averaging circuit. The value during this sample period can then be converted to a digital value of a given accuracy (``quantized'').
We can do this sampling ``perfectly'' by sampling twice as often digitally as the highest analog frequency, or we can take advantage of human frailty and reduce the quality by decreasing the sample frequency (the clock rate above, and/or the quantization (number of bits used per sample).