In order to understand the different signals that can (directly or indirectly) be plugged into a digital cinema projector, we first need to understand the terminology that describes these various signals. The first major distinction that we can make is between analogue and digital video signals.

In analogue video signals, the information is conveyed by continuously varying electric signals, quantities being represented by levels or voltages. Analogue is typical of nature and the human senses. Analogue video signals are prone to interference and distortion during transmission.

Digital information is conveyed by discrete states (on/off) and its quantities are represented by numbers. Digital information is fundamental to Information Technology and is more robust for storage and transmission than analogue signals. Digital technology is often described as 'bits and bytes', without the person using really understanding what that means.

A bit is a single digital state (on / off).

• One bit can represent values from 0 - 1
• Two bits can represent values from 0 - 3
• Three bits can represent values from 0 - 7
• 16 bits can represent values from 0 - 65535

A byte is a group of eight bits and it can represent values from 0 - 255. A byte is typically used to hold a single printable character and is commonly used as a basic measurement for storage. In digital video signals, these 'bits and bytes' are used to form an image. This image is often described in pixels or resolution.

Pixels are picture elements and represent the smallest part of a digital image that can be individually controlled. An image is a rectangular block of pixels. The resolution of a digital image is the number of pixels contained in an image. It is usually expressed as Horizontal Pixels x Vertical Pixels, for example 1920 x 1080 (HDTV), 1024 x 768 (PC XGA). A pixel doesn’t have a specific size; a large screen can contain as many pixels as a small screen. Every pixel is described by 4 bytes (32 bits) in order to be able to represent all colours ranging from black to white.

Now that we have an image, in cinema we need to produce a moving image. These are the terms used to describe the moving image.

A frame (of a digital image) can be compared to a digital photograph in time, containing all pixels required to reproduce the image.

The frame rate is the number of frames displayed in each second. For example: film has a frame rate of 24 fps, TV is 25 fps, US TV is 29.97 fps. The abbreviation fps means frames per second.

An important term that one also comes across in digital cinema is colour space. This describes the way in which the colour of the image is described, also giving limitations as to the range of the colour. More formally it is called Colour Specification System.

Coulour space is the way in which colour information is represented for storage, transmission and reproduction of digital images. There are several types in common use for images, for example R'G'B', Y'Pr’Pb (Rec. 709) and X'Y'Z' (the standard colour space for digital cinema). An image will only be reproduced correctly if it is decoded in the colour space in which it was encoded.

Interlaced is a term that we find in tv's, but as a remnant it is still found in professional video equipment. It is a technique of improving the picture quality of a video signal, without consuming extra bandwidth. Historically, in television the image was portrayed in 2 fields, one showing half the image and the second field the second half, generally in odd and even lines. The switching happened so fast, that the human eye didn't even notice and just saw the one image. You can say that half the information was processed at twice the speed. The reason for using interlacing, was that the original bandwidth was limited and by processing only half the image at a time, the quality could be improved.

A field (of digital image) is a portion of a frame in an interlaced image. It usually holds half of the vertical pixels (lines), typically all odd or all even numbered lines.

Progressive Modern video images are 'progressive', showing one frame after an other. We also find this back in video terminology: for instance 1080i refers to an image which is high definition and interlaced, where as 1080p tells us that the images are progressive. When interlaced video is shown using a video scaler, we have to de-interlace the image to improve the quality. This might be necessary as interlaced video is not very good at showing fast moving images, as these show fuzzy edges to the moving objects, instead of straight lines. The difference between interlaced and progressive, combined with multiple frame rates, can provide particular problems when transferring film to video.

Video players (DVD, Blu-ray, game consoles ...) can have different types of outputs / connectors. The projectionist who wants to connect one of these drives to its d-cinema projector, must choose the connectors that will enable him to get the best picture quality and the best sound. There are two types of connectors of peripheral equipment: digital and analogue.

Digital outputs

Digital outputs are generally the best option when connecting a video signal to a digital cinema projector. During transport from the source to the projector, analogue signals can generally degrade for instance due to bad connectors or long cables, causing interference, drop of quality etc. A digital link virtually provides a guarantee that the signal emitted by the player will experience very little degradation during transport to the projector.

Analogue outputs (in the white circle) and digital outputs (in the red circle) on a Sony Blu-ray player

SONY's SRW5000 HDCAM player has 2 HD SDI outputs on its rear panel.

The HD-SDI interface can carry a high-definition signal. It requires a high quality coaxial cable; the data rate that passes through it can reach 1.485 Gbit/s and has a high resistance to interference signals. The BNC connector characterizing this interface ensures a solid connection with little risk of a loose contact.

A characteristic of the HD-SDI outputs is that it transports only image, and no sound, which in cinema is generally not a problem as we use separate systems for video and audio. However, there is a technical solution for transporting the audio signal with the video through HD-SDI by adding an extra box called ‘SDI/AES De-Embedder’. When using HD-SDI a more common way to handle the sound is by using XLR connectors (reserved for professionals) which then handles transportation of the sound to the processor.

DVI (Digital Video Interface) outputs.
Designed to carry uncompressed video images of a player to a projector, the DVI connection is a high quality connection, boasting a high data rate that can carry high definition signals (HD). Like HD-SDI, DVI transports only images, not sound.

The DVI connector has been used a lot in the past on computers, video projectors and DVD players. Nowadays HDMI, which can handle both video AND audio has often taken the place of DVI, which limits the number of cables required. However, an advantage of DVI over HDMI - especially in professional use -is that the connector is much more robust, as it is fitted with screws, guaranteeing a secure connection.

There are 3 types of DVI connectors, that can be recognised by the number of pins in the connector:

• DVI-D (DVI-Digital) : transports only digital signals

• DVI-A (DVI-Analog) : transports only analog signals

• DVI-I (DVI-Integrated) : can transport digital and analog signals, but not both at the same time

DVI is partially compatible with HDMI in digital mode (DVI-D), and with VGA in analogue mode (DVI-A or DVI-I).

DVI cable with connectors.

HDMI cable.

The success of HDMI originates in the fact that it carries both image and sound signals, as opposed to DVI which only carries image. This is a great benefit for consumers because all they need is one cable for connecting peripherals to their television screens. But HDMI does not simplify the job of the projectionist, because in cinemas video and audio signals are treated separately: the first in the projector and the latter in the sound processor.

There are many successive versions of HDMI (versions 1.0, 1.1, 1.2, 1.3, 1.4), all with different characteristics, becoming more advanced with each version number. Today, the most common HDMI connector is 1.3 which features a maximum data rate of 10.2 Gbits p/s maximum. The upcoming version 1.4 will be able to handle 3D content in HD (1080P) and 4K content (4096 x 2160 pixels). HDMI and DVI are compatible with each other, so converters can be used to connect HDMI video sources to projectors with DVI connectors and vice versa.

HDMI is compatible with the HDCP copy protection standard, which protects high definition (consumer) content like Blu-ray discs against unlawful copying. This can present problems in cinemas, as many of the first generation of d-cinema projectors did not support the HDCP-standard, so HD-content cannot be played on their digital external inputs. In this case, an analogue connection will do the trick, but we’ll return to this issue later on.

Analogue outputs

The use of an analogue interface for connecting a device to a d-cinema projector may not be an optimal solution since the signal carried between the two devices may deteriorate during transport, especially if the cable used is not of premium quality. But that does not mean that one should systematically exclude the use of this type of interface.

Please note: unlike HDMI, and with the exception of the SCART connector that is not used in cinema, all analogue connections only transport images. It is therefore imperative to use other (extra) cables for sound.

RGBHV.
RGBHV uses five cables to transport a video signal and is generally only found on professional video equipment. Three cables are used to transport the primary colours (Red, Green, Blue) and the two remaining ones are used for the horizontal and vertical synchronisation of the image.

RGBHV can be used to transport high definition signals between a player and the projector. As it produces a high quality signal, it can replace digital signals if necessary.

RGBHV cable.

Component video output on a Blu-ray disc player.

There are different types of RGB / component video, but the term generally refers to YPbPr video with sync on luma. The ability to transfer video using the RGB interface stops at the 1080i standard (interlaced images of resolution 1920 x 1080).

VGA (Video Graphics Array).
This connector is a version of the RGB analogue video component standard, that is generally found on computers, laptops and computer monitors. As it is so common on most computers, in cinemas this connector is generally favoured for simple PowerPoint presentations, as the quality is considered good enough to display these images.

The connection between the source and the projector (or scaler) is generally secured with two screws. However, there are VGA outputs on the market, generally found on small laptops, that got rid of the screws in order to save space. When used for presentations, these connections should be secured using Gaffa tape.

S-video.
When using an S-video interface (max 720 x 576 pixels), the image signal passes from the player to the projector via a single cable. Within this, the luminance and chrominance are separated through separate conductors which only partially minimizes quality loss. However, this does not prevent the occurrence of interference, especially considering that the vast majority of S-video cables are of poor quality. This connector is therefore to be avoided for the large screen.

VGA cable.

S-video connector (here in high quality).

Cinch (RCA) cable.