Video Cable Basics

[Alex]

Video Cable Basics

Since someone asked in another post for a summary of video cables and their uses I have done a short(ish) article on the subject, for your education. I hope this makes sense and doesn’t just confuse people further!

What is in a video signal?

However your data is displayed, whether on a computer monitor, LCD TV, or projector, it is displayed as a mix of three colours; red, green, and blue. With these three colours we can see all the TV programmes, DVDs and games we have. A video cable then carries data on these three colours, as well as information on the brightness of any of these colours. There are three main ways in which this data is carried:

Analogue composite video – The most basic form of video transmission
Analogue component video – Including s-video, SCART, and what we know as ‘component video’ – YPbPr
Digital video – Including DVI, and more recently HDMI

Composite video

As the name suggests, composite video combines all the data into a single ‘composite’ data stream, which it then sends down a single cable. The bandwidth of a single analogue cable is quite low, so this is compressed down to fit, and then unpacked and the data streams separated at the receiving end. This is essentially the way in which video is transmitted over the airwaves for our analogue TVs. The process of untangling the data streams is never perfect and therefore there is some inevitable degradation of video quality at this end of the transmission as well. This is then the most basic, most compressed, and most ‘lossy’ form of video transmission and a composite video cable should only really be used if there is no alternative.

Analogue component video

With analogue component video, the video is transmitted in, you guessed it, various components. These components are generally split up into luminance (brightness), and chroma (colour). Exactly how these are split up varies depending on the cable being used.

S-Video

S-video is the next step up from composite video in that it transmits video in two components, one set for luminance (denoted Y) and one for chroma (C). There is less compression of the video signal necessary, and the receiving appliance does not have to separate the chroma and luminance signals. There are other cables which further split up the data stream to provide better image transmission, but s-video is a good step up from composite video, and should be used preferentially if possible.

RGB

An obvious method of splitting up the data stream is the separate it out into the component colours which make up all of our images, which is what happens in an RGB cable. Red, with the luminance for the red channel is carried in one cable, blue in another with the blue luminance, and the green in another with the green luminance. RGB can be carried by a 3-phono cable (i.e. what we would think of as a component video cable, see below), but is generally carried in either a SCART cable or an analogue computer monitor cable (SVGA).

SCART

SCART cables can actually carry data in a variety of ways, depending on the output of your source (i.e. VCR, freeview box, console etc). SCART cables have 21 cables internally to carry different signals, and are capable of carrying composite video signals, s-video signals, and RGB signals, as well as two channel stereo sound. The variety of possible transmission methods down a SCART cable is the reason it is possible to buy simple adaptors for composite video to SCART, s-video to SCART, etc etc.

There two further complications with SCART, firstly in that is bi-directional. This means that a SCART cable can carry video signals from a TV to a VCR, as well as from the VCR to the TV. This is not really a complication to use it, just in the explanation of what it is. The second complication, which is more of a complication, is that although a ‘fully wired’ SCART cable can carry any of the video signals listed above, some consoles and other appliances will come with a SCART cable which does not contain all 21 wires of a SCART cable, and therefore can only carry, say, composite video. This is only really done to save money and easy to check – if you look at a SCART cable it is easy to see whether all the pins are present.

This is pretty much as good as YPbPr component video, but is still bandwidth limited and so cannot carry high-definition video signals. The one advantage it does have over component video (other than also carrying audio) is that one of the connectors allows your device (say VCR) the send a signal to the TV when turned on, so that the TV can switch automatically to that channel.

YPbPr Component Video

This is what most people mean when they talk about a ‘component video cable’. This is known as a component video cable as this is the analogue cable in which the video is split into RGB and a separate luminance signal. It does this in quite a clever way – by having firstly a luminance cable (denoted Y), then two other cables these other two cables then carry the difference between the total luminance and the blue (Pb) and the total luminance and the red (Pr). From these three bits of information, it can calculate the green component, so this does not actually need to be sent down a cable. As well as meaning that all the video components are kept separate, this also means that effectively you only need to send two thirds of the amount of data as for RGB video transmission. This allows for more data to be sent down the cable, making it possible to send a high definition video signal through this type of cable.

This is in fact, as far as I’m aware, the only analogue method of sending high definition video signals.

Digital Video Transmission

Digital is the ideal transmission method for sending video since it is not subject to interference and signal degredation in the same way that an analogue cable is. With an analogue cable a signal can be at any level. Say the minimum level is 0 and the maximum is 1. If a particular section of data is transmitted at a level 0.75, then over the course of a 10 metre cable it may lose 10% of the signal, making it roughly now 0.67, ie noticeable different. In a digital cable everything is either 0 or 1. If a signal is transmitted as 1, and loses some of its strength over the length of a cable, it is received as, say, 0.9. Since a digital signal can only either be 0 or 1, this 0.9 is interpreted as a 1 by the receiving appliance, and so no signal is lost.

The other advantage to digital cables, at least as far as film makers are concerned, is the ability to build in copy protection, i.e. both your HD-DVD player and LCD TV have to agree to send and receive the data (known as a ‘handshake’) before a DVD will play on your TV. A HD-DVD player will not handshake with a DVD recorder, so DVDs cannot be copied by a digital cable. This type of copy protection is known as High Definition Copy Protection (HDCP).

Digital Visual Interface

DVI cables carry data separately for red, green, and blue channels, and are very common to go between computers and LCD monitors. DVI cables are available in three types, DVI-D (digital, the most common), DVI-A (analogue – yes, you can get and analogue digital interface, apparently), and DVI-I (which can carry both digital and analogue signals). Most DVI cables available are DVI-D, and DVI-D cables are capable of handling HDCP copy protection. DVI can however only carry video signals, unlike HDMI.

High Definition Multimedia Interface

HDMI is backwards compatible with DVI, as it carries video in much the same way. What HDMI adds is the ability to carry audio, and in the latest version (which is version 1.3 now, but I won’t go into that here) can carry 8 channel high definition digital audio. As well as being able to carry high definition digital audio on HD-DVDs and blue ray discs, since it can handle copy protection, it is the only method of digitally transferring the sound from SACDs and DVD-audio discs.

Since HDMI carries video in the same way as DVI, it is possible to buy an HDMI to DVI or DVI to HDMI video only cable, which can be useful for connecting a PC to a TV or projector with only an HDMI socket and no DVI.

Summary

There are still other methods of transferring video from one place to another, but these are the most common methods used at the moment. The information here is only from my own understanding and may therefore in places not be 100% accurate, but anyone is free to correct me if I have said anything wrong. Also, there is more to these topics than I know or can be bothered to write about, but the above should keep you going!

So, if you can’t be bothered to read the above then:

1. Use a composite video cable only if there is no other available method - it gives the poorest signal.
2. Use a component video cable preferentially over RGB cable, which should be used preferentially over s-video.
3. Use a digital cable if available since these will offer the best video quality transmission, as well as be able to handle copy protection on next generation discs.

Hope this helps!

If anyone spots something wrong with this, please PM me and I shall edit the post.

[UKMuFFiN]

Awsome! Cheers.

[kidzer]

Very cool read, I shall even try and remember some of it!

[ikonia]

can we make this a sticky please

Very good.

thank you

[johnny02004976]

can we make this a sticky please

Very good.

thank you

seconded

[awm]

Excellent article! I have just have a little bit of constructive criticism. In the component video section you do not make it clear why it is preferable to separate luminance as a separate channel as opposed to a more typical RGB system. Also you claim "This is in fact, as far as I’m aware, the only analogue method of sending high definition video signals.". I believe VGA is also a high def analog system. Perhaps what you meant is that is the only high def analog system without separate sync pins? Anyways great job and I'm sure a few little fixes and you'll have a professional quality article.

[Alex]

Will look at that later today - you are of course right that VGA (RGBHV) will send High Def signals - only reason I didn't put it in was that I've only ever seen VGA used in computer applications - but I supopose you can send HD from a computer to HD screen so valid point.

The point about luminance separately is that there are basically 4 components to video (if you exclude syncing - which I don't really understand if I'm honest) - and YPbPr component separates them out into their 4 components, the 3rd colour being inferred from the other 2 colours.

Thanks for the constructive criticism though, will edit the post later

[awm]

I guess what I don't get is why it isn't better to just use RGB as that is what monitors display. OK, I read the Wikipedia article on luma, and I'll quote a bit for anybody else who's curious:

As applied to video signals, luma represents the brightness in an image (the "black and white" or achromatic portion of the image). Luma is typically paired with chroma. Luma represents the achromatic image without any color, while the chroma components represent the color information. Converting R'G'B' sources (i.e. the output of a 3CCD camera) into luma and chroma allows for chroma subsampling, enabling video systems to optimize their performance for the human visual system. Since human vision is more sensitive to luminance ("black and white") detail than color detail, video systems can optimize bandwidth for luminance over color.

(Emphasis added, source)

[Alex]

There is that, but my point was more the bandwidth increase along the cable by splitting off the signal into Luminance, Red and Blue (and not needing to send the green, since it can be inferred from the other 2...) you only need to use up 2/3 of the cable bandwidth for colour, meaning there is more available for HD signalling. That's how I understand it anyway, but I may well do some more research before updating the article.