Hi there,
Thanks for checking out a 'Explained' blog! In these; my aim is to give you some details on what some common technical terms mean in digital video and film. I hope you find something useful here.
Please note; I do not consider myself an expert on the topics - if you believe I've made a mistake somewhere please do let us know! I'd love for this to be a learning experience for me as much as anyone else.
Bit Depth
Okay, maybe not barnacles. But if you've always wondered what the heck a bit or byte is; be it your download speed or storage space, this bit’s for you (no pun intended!).
A bit is a piece of digital information, the smallest possible. It contains a single value, either a 0 or a 1. Think of it like a light switch, either on or off. They are generally represented by an electrical voltage. These single bits (either 1 or 0) can be put into combinations/sequences (known as permutations) which will eventually create information that is meaningful.
For example, if we have a sequence of 8 bits, that could be 256 different potential sequences (ie, 256 different orders of 1 & 0). This can be calculated by 28, where 2 represents the two different states (1 or 0) and 8 is the number of bits we have. If it were 12 bits, the number of potential sequences would jump to 4096 (212).
So that's a bit, but what about bytes? A byte is just a sequence of 8 bits, ie, 8 values of either 0 or 1, for example: 10101001. Note that bytes isn't a term typically used in bit depth; this is a term used more for speeds and storage space.
And that's really all there is to it, at a basic level at least! It's sequences of many, many bits in a certain order that generates usable data.
You may also be wondering what a kilobit, megabit and gigabyte is. Well all these prefixes (the bit at the start) come from the International System Of Units, and they mean the same, regardless of the suffix. Now, technically these terms are incorrect when referring to binary such as bits and bytes, but due to marketing and other influences they have stuck. Traditionally, this means:
kilo mega giga tera peta | 103 106 109 1012 1015 |
BUT when dealing with bits this is not strictly accurate because bits are binary values, not decimal. A ‘kilobit’ is actually 1024 bits, and a megabit would be approximately 1,048,000 bits.
To explain bit depth, we are going to stick with an RGB Colour Model (LINK) based format. This means each pixel is made up of a channel for red, green and blue; the combination of which gives us the final colour value of the pixel.
If our footage has a bit depth of 8-bit, we would have 8-bits of information for red, green and blue. For example:
RED | GREEN | BLUE |
So that's bit depth. Before we move on to colour space/gamuts; I want to cover where these two aspects can become confused with one another. So here it is: No matter what bit depth you have, the range of colours available does not vary. Bit depth changes the resolution of the colour values (ie, how many increments there are), leaving the range unaltered. A diagram is best to help explain this:
Colour Space/Colour Gamuts
![Picture](/uploads/2/4/5/7/24574909/4032087.png?219)
The triangular-esque colour gradient shape is our visible spectrum - that is, all the colours we can see. The red triangle within that is the sRGB colour space/gamut whilst the other outline is CYMK.
sRGB is very commonly used within digital video whilst CYMK is used in print.
As you can see, humans are capable of seeing a far greater range of colours than these gamuts can show. Bare in mind that both these gamuts could exist within an 8-bit world. They'd both have 16.8 million colours to use, but from a different range. So technically, a gamut with a smaller range of colours would have a higher resolution of colours in the same bit depth compared with a gamut of a larger range. This is because both would have the same amount of colours to use, but one would have to spread those colours over a great range compared to the other.
But what happens to those colours outside the range of the gamut? As you may expect, these are all pulled back within the gamut; so even a shade of green right on the top tip of the visible spectrum will be brought to just within the gamut on the green end. As you can see from the diagrams; this can cause large colour inaccuracies but is not something we specifically notice when watching video.
Rec. 709 - The Standard
Rec. 709 has it's own colour space, based on, and very similar to sRGB.