Tuesday, 17 April 2012

4. The Display

What Will Work

If you plan on connecting via SSH or VNC only, then the display is not something you are probably interested in, and you can skip this part, but if you want to use the Raspberry Pi as a regular computer, or to play visual media, then you will want to connect a display. The Raspberry Pi comes with two methods of connecting a display its HDMI port, and its Composite video port. There is an array of ways to connect a display to the Raspberry Pi, I have provided a diagram of methods that are known to work, and it should maximise the chance that you can find some cables lying around to make up a connection. The Raspberry Pi can connect to PAL and NTSC analogue TV's; TV's with SCART sockets, and digital monitors and TV's, it may even connect to some projectors.

What Won't Work

Digital to Analogue Cables

There are a number of cables on EBay which look like they solve problems that don't seem solvable with just a cable, the conversion of a digital signal to an analogue signal or vice-versa. The common cables on the market are HDMI to VGA, or DVI-D to VGA. The HDMI and DVI-D connectors are digital only outputs, and VGA is an analogue standard, the cables exist because there are a select few of displays that can accept a digital signal through an analogue input, but as a percentage, almost all displays with an analogue connector such as VGA can only receive and decode an analogue signal.

Understanding DVI-I, DVI-A and DVI-D

DVI connectors have three signals, digital link 1, digital link 2 and analogue. There are five different connectors to cover the combination of signals that are transmitted by different computers or laptops, although the most commonly used are single link DVI-D, double link DVI-D and DVI-I. DVI-D only ports usually signal that a computer display output (or monitor input) can only transmit (or receive) a digital signal. Another thing to be aware of is that you cannot physically plug a DVI-I male connector into a DVI-D port. You can plug a DVI-D male connector into a DVI-I port, so it's a good idea if you have to buy a converter, to buy a converter with DVI-I for its female side, and DVI-D for its male side, this maximises the cables that you can use to cobble together some working connection.

There is also no need to worry about the differences in number of digital links a converter has, the connection will just default to a single link if any part of the connection only has a single digital link. The importance of double link connections over single link connections is discussed in a later section.

Cables with connectors such as a DisplayPort or DVI-I cable at one end, that connect to a standard VGA cable are extremely useful for Computers with both digital and analogue display outputs, or for Monitors which perhaps have a DisplayPort adaptor and no VGA adaptor, (DisplayPort supports a single analogue signal) these cables do nothing more than connect the VGA connector to analogue pins on the DVI-I or DisplayPort connector. They cannot convert the digital parts of a DisplayPort or DVI-I cable to analogue, these parts of the connector are simply ignored.

If you are using an Analogue connection, then it is important to note the difference between component RGB and composite video, one sends three signals, Red Green and Blue and is not compatible with the other, which is a single serial encoded signal. Some projectors and other displays might have an S-Video, S-Video might work with the composite video signals, as devices with an S-Video port often come with an S-Video to composite video converter cable. It is probably worth a go if you can't find any other cables around. To help I have put together an image demonstrating common cable combinations that won't work.

Finding Parts

To find digital cables, raiding TV and home entertainment packaging is a good first step, as is monitor packaging. Graphics cards can sometimes have adapters, although they are often not HDMI adapters. Behind the TV might also be a good place to look, and older TV or video recorders that might have been put in storage could also have S-video or SCART connectors. A number of TV card or graphics card manufacturer’s ship composite video cables with their products.

Buying Guide

DisplayPort comes with new concepts such as active and passive converters, active converters can be very expensive, and even passive converters can be several times the cost of HDMI to DVI converters. ATI/AMD certifies a handful of high cost converters for converting signals from one input to DisplayPort, but there is virtually no need to buy only certified converters. If you are running several displays then very high bandwidth, very high cost equipment is necessary, otherwise, lower cost parts should be OK. Generally DisplayPort components have fallen to reasonable prices after being marked up by a few hundred percent when they first appeared on the market. DisplayPort is a great technology for professional computer equipment, but it is probably best avoided here if possible due to its cost compared with other connections.

Suppliers

As in previous posts, consider the likely quality of items bought of EBay sellers, and check feedback and specifications, while it is still common to find cables heavily overpriced in retail stores, (just like the multimedia cards in the previous post), some sellers on EBay sell faulty, or very low specification cables and converters. In my experience up to 1 in 3 cables or converters I have bought have been faulty, so spending an extra £2 from a reputable seller is worth the money.

Fair Price Guide
Item EBay Cost Retail Cost
10.8Gbps+ HDMI Cable 99p to £2.00 £1.20 to £7.00
10.8Gbps+ DisplayPort Cable £7.00 to £12.00 £8.00 to £14.50
9.9Gbps+ Double Link DVI-I/D Cable £2.40 to £5.00 £6.00 to £8.00
9.9Gbps+ HDMI Male to DVI-I Female, or HDMI Female to DVI-D Male converter £1.50 to £4.50 £2.00 to £5.50
9.9Gbps+ HDMI Male to DisplayPort Female, or HDMI Female to DisplayPort Male converter £4.50 to £7.00 £8.00 to £12.00
Composite-Video Cable £1.20 to £2.00 £2.10 to £5.00
Composite Video with stereo audio to SCART IN £1.20 to £4.00 £1.20 to £4.50

Table A: List of fair prices for various cables and converters

Brands

Overpriced Cables

There is virtually no benefit to buying branded goods where digital cables are involved. When audio and video switched to digital communication from analogue, there were a number of very expensive digital cables released onto the market, some in excess of £50.00. Because of the differences between a digitally transmitted signal and an analogue signal, there is no need for expensive gold plated connectors, highly conductive cable, or very heavily shielded cables. Any moderately well made cable is going to do an effective job, and it is more effective to trust the seller on the quality of the cable over the brand.

Warnings

The major warnings have already been listed; these are overpriced goods, low quality goods, and faulty branded goods from untrustworthy sellers. It's important to know the high cost of some DisplayPort technology, which is almost certainly not necessary.

Sources

  1. Andrew MacGregor, What video connector do I have, Technology Services Group at the University of Illinois, Published 02/11/2010, Accessed 16/04/2012

1 comment:

  1. Male HDMI to Male DisplayPort will not work.

    ReplyDelete