What Is Definition?
Before we talk about what high definition is, let's start by figuring out what definition means. Definition itself, refers to detail in one or more dimensions. Unlike the real world, where there are three spatial dimensions, video only has two. However, like the real world, an additional dimension of time may also be considered.
Digital Video resolution
In digital media, resolution normally refers to the number of samples played back at a single moment in time. For example, a film frame converted to progressive video for an NTSC DVD will have a frame (group of samples from a single moment in time) with dimensions of 720x480 (480 rows of 720 samples each). For interlaced video, resolution actually refers to samples from two moments in time, which are considered together because it takes both fields to fill a television display.Temporal resolution
The often forgotten resolution factor is temporal resolution. This is a reflection of the number of frames per second for digital video or sampling frequency for audio. Unlike spatial resolution, which is theoretically always degraded by a decrease, temporal video resolution can be improved by lowering it to the framerate a clip was originally shot at. For standard film or progressive video shot to match film standards this will be 24fps. Traditional television technology isn't able to support film framerates because neither PAL nor NTSC is natively 24fps or designed to allow variance in signals.Digital Audio resolution
Like video, audio consists of multiple samples taken at regular intervals many times per second. Unlike video however, there is no discrete analog model (like individual film frames) to put samples in large groups that occur relatively few times per second. Instead, resolution is measured by the number of bits per channel in an individual sample. This is also referred to as the Data Word size. Digital audio also has a temporal resolution, normally represented in Hz, which means cycles (samples in this case) per second. For example, CD-Audio has a Data Word size of 16 bits (per channel) and is sampled at 44.1kiloHertz (44,100Hz).Digital resolution vs. Perception
You will sometimes see references to resolution in the analog world. resolution is often used to describe analog signals, such as saying that VCD has a resolution of 352x240. In reality, video resolution in the analog world doesn't consist of pixels. It's actually a measure of how much detail the human eye can perceive in a given signal transmitted across a particular media and reproduced on a particular display. In other words, not only is it completely subjective, it's also dependent on factors beyond the signal itself. High definition video is also affected by transmission media and display technology. For example, given a small enough screen, 1080p doesn't provide any perceptible advantage in detail over 720p at a normal viewing distance.Notation
There are standard ways of noting information relevant to the resolution of digital video or audio. For video, this information is normally in the form of [horizontal lines] x [vertical lines] followed by i for interlaced or p for progressive. When video is described this way, standard television resolutions are generally assumed. Instead of spatial resolution, sometimes video is also described in terms of framerate, with the frame type (i or p) appended, such as 24p or 29.97i. Although NTSC framerates are often rounded up (ie 23.976->24), but in order to avoid confusion this guide won't interchange round and NTSC numbers. Audio is described by a combination of data word length and sampling frequency in kiloHertz (thousands of samples per second). For example, 24/48 represents 24 bit samples taken 48,000 times per second.
