dcineco - Digital Cinema Consultants


	Technical Glossary The dcineco glossary contains explanations of some of the terms used on our site and reflects their common use in the world of digital cinema. We understand these terms are not unique to digital cinema and may have different meanings in other applications. Anamorphic An anamorphic image is one that has been horizontally scaled so it can be accommodated in a frame with a smaller aspect ratio. As an example, widescreen television images have an aspect ratio of 1.78:1 and they are horizontally squeezed to fit the standard television frame of 1.33:1. Correct reproduction requires the image to be horizontally expanded to the original aspect ratio. As an alternative to anamorphic processing, the image can be stored in its original aspect ratio but utilising less than the full height of the frame. This format is usually known as letter-box. Manipulation of the image aspect ratio can be performed by resizing equipment and format converters. Digital cinema projectors usually incorporate image resizing capability that will perform any necessary conversion provided they are adjusted to suit the format of the content. Top Close Aspect Ratio The aspect ratio of an image is the ratio of its width to its height. It is usually expressed as a ratio to unity by dividing the width by the height. So a standard TV image may be shown as 4:3 or 1.33:1. Many different aspect ratios have been used in the past. Film, in particular, has utilised a variety of aspect ratios to create new experiences. Digital cinema systems are most likely to need to accommodate the following ratios: 1.33:1 Academy Silent (full-gate) ratio for film, traditional TV images (4:3) 1.38:1 Academy Sound ratio for film - exact ratio is 1.375:1 1.66:1 Early European Widescreen ratio for film 1.75:1 UK Widescreen standard film ratio during 1960’s and 70’s 1.78:1 Widescreen TV and High Definition standard (16:9) 1.85:1 US Widescreen film standard - now effectively World Widescreen standard 2.39:1 Panavision Anamorphic film format - often called "Scope" - it was changed from 2.35 in 1995 Top Close Automation There are many forms and degrees of automation in cinemas. Simple automation systems may just need a connection to the lighting system to bring up the lights when the show finishes. Full automation may have the entire performance scheduled and controlled by a computer. Digital cinema systems can be fully integrated into existing automation schemes through good system design. Digital cinema servers generally have their own event-driven automation which may connect directly to an existing film-based system. During the design process, it is important to identify any incompatibilities between systems and specify any additional equipment required to achieve translation of the signals. Top Close Colour-space (also see Colourimetry) Colour-space refers to the form in which the colour information of images is encoded for electronic storage and transmission. Different colour-spaces are used for different applications. Incorrect management of colour-space can result in subtle or gross errors in colours so it is frequently necessary to perform a colour-space conversion to correctly reproduce an image. Digital cinema projectors usually contain colour-space converters which will perform the task provided they are correctly adjusted for the content and the effect of other system components. Typical colour-spaces include: · SMPTE 170M for standard-definition television · ITU-R BT.709 for high-definition television · DLP Cinema P7 for digital cinema Top Close Colourimetry Digital cinema projection systems contain powerful colour processors that, when correctly adjusted, ensure the projected images look exactly as the content creator intended. Subtle errors in the reproduction of colours will spoil carefully crafted content and lessen the audience’s experience. The process of adjustment requires accurate measurement of the whole projection system with specialist instruments. Once calibrated, the system should not require further adjustment unless there is some change to the projector or projection path. When provided with appropriate colour mapping data, digital cinema projectors can emulate the performance of other types of projector and even the whole chain of film printing, processing and projection. This is a really valuable feature when working with digital intermediates. The ability to get instant visual confirmation of the affect of changes in colour-correction and dynamic range dramatically improves the accuracy and speed of this part of post-production. Top Close Compression The uncompressed digital data for a typical full-length feature film consumes Terabytes (millions of megabytes) of storage space. Data of this size is difficult to transport and store so methods of reducing the size have been developed. Compression processes reduce the size of the data without noticeably impairing the appearance or sound of the content. The processes frequently exploit our understanding of human sensory systems by reducing or removing portions that convey little or no value. The quality requirements of digital cinema necessitate very high performance compression schemes. While some existing methods have been extended to cope with the requirements, the best results have been demonstrated by schemes that were designed to specifically meet the needs of high-quality, large-screen projection. Top Close Content The term “content” is widely used to refer to a specific piece of material consisting of visual and/or sound images or to a collection of such pieces. Content generally refers to the images rather than the form in which they are embodied so it is important that the physical format is also specified. The content may be a particular feature film, a rock concert or a TV programme. This could be delivered on film, videotape, disk, or by network transmission. In each case there is more than one possible format so this must also be specified. Other information may also be required to ensure the content can be accurately reproduced. For example: A classic feature film may be available on HD-D5 tape with 1920 x 1080 format frames containing anamorphic 2.35:1 aspect ratio images for replay at 24 frames/second. We would also need to know the colour-space for which the content was mastered and the number, format and encoding of the soundtracks. Top Close Digital Cinema Digital cinema is the electronic projection of content with a level of quality that meets or exceeds that of 35mm film. The content used for digital cinema may have originated on film or have been electronically captured. Reproduction of the content should preserve the full range of the original image resolution, colours, contrast and sound tracks. This level of quality can only be achieved by equipment and systems designed to stringent performance objectives. Film supports a huge range of colours and a wide dynamic range. Normal electronic reproduction and projection systems cannot reproduce these subtle variations so special equipment is required. A digital cinema system typically includes a server, a projector, a sound processor and an automation interface. Top Close Distribution In the world of digital cinema, distribution refers to the way in which content in a digital form is delivered to the point at which it is to be presented to an audience. There is no ideal method for all circumstances and the best solution for a particular application is one that meets the economic, security and time goals. Small-scale distribution is often best achieved through the use of physical media – such as high-capacity data tapes. At the receiving end, the content is loaded from the tapes onto a server for eventual playback. The tapes can be encrypted to reduce the risk of unauthorised use and are small enough to be carried by normal or secure postal services. The cost of a tape is very small compared with that of a film print. Where there are many sites that need access to the content, distribution by satellite broadcast may be the best option. A high level of encryption would be employed to ensure that the content was only of use to the intended recipients. The content could be presented, live, as it is being received or stored on a server for later showing. The cost of the satellite bandwidth necessary to transmit Gigabytes of data may be high but when shared over a large number of receiving sites, it makes economic sense. Sites in major cities may be able to employ fibre-optic links for fast – even real-time – transfer of digital content. Networks of “dark-fibres” have been laid between many cities and a connection point at or near a site may already exist. This type of link is point-to-point so unauthorised reception is less likely but some level of encryption would still be advisable. The cost of this approach varies according to the bandwidth of the link and the amount of time it is required. It is most suited for time-critical applications. It is probable that a practical distribution strategy will employ a combination of techniques and will evolve to accommodate the number of recipients whilst exploiting advances in digital connectivity. Top Close Format Conversion Content is delivered in many, increasingly, different forms – in terms of the both the physical medium and the way in which the images and sound are encoded. The form in which it is available may not always be ideal for the intended use and a conversion step may be necessary. Typical conversions include: · Physical media · Resolution up or down conversion · Aspect ratio · Frame rate · Colour-space · Image encoding · Sound encoding Top Close Luminance The apparent brightness of a projected image is referred to as its luminance. There are recognised standards for luminance in cinemas and content is produced on the assumption that it will be projected to those standards. If the projection system does not conform to the standards then the content will not be reproduced correctly and the audience will not enjoy the experience as the director intended. Luminance is dependent on the whole projection path from the lamp to the screen. Each component in the path has an effect on the light which must be considered for accurate calculation of screen luminance and required lamp power. The luminance of a 35mm film projector is usually measured by running it with no film in the gate and measuring the light reflected from the screen with a spot light-meter. The commonly accepted standard for this “open-gate” reading is 16 foot-Lamberts (55 candelas per square metre). The actual luminance of a projected image would never be this high as even clear film would reduce it by about 25%. Digital cinema projectors have no equivalent to an open-gate reading so luminance measurements are made during projection of a full-frame, peak-white test pattern. As this corresponds to the mechanical projection of clear film, the luminance standard for digital cinema is 75% of the open-gate figure (12 foot-Lamberts) for comparable brightness with actual images. Top Close Masking Digital cinema projectors use electronic masking so that the image can be adjusted to exactly fit the screen dimensions. Recent models allow independent adjustment of the position, angle and curvature of each edge of the projected image. This is equivalent to an electronic version of a film projector aperture plate but is a lot more accurate and can be changed in an instant. Correct adjustment of the electronic masking can dramatically improve the appearance of the image and improve the audience’s experience of the content. Adjustment of the projector masking must take into account all the aspect ratios that may be required as well as the physical properties of the screen, its fixed or variable masking and compatibility with film projection. Top Close Projectors To be suitable for digital cinema, projectors must meet stringent objectives for image quality. The electronic reproduction of images to, and beyond, the performance of 35mm film projection requires special capabilities. In particular, a digital cinema projector should meet or exceed the characteristics of film projection in the following areas: · Colour gamut – the range of colours that can be individually perceived · Contrast ratio – the dynamic range of brightness between extremes of black and white · Sharpness – the visibility of fine details in the projected image · Motion portrayal – depiction of movement within the image · Light output – to achieve optimum image brightness at required image size Digital cinema systems also offer noticeable advantages over film projection in the stability of the image (absence of film-weave and focus-flutter), repeatability (no wearing or scratching of the film) and lack of visible flicker. Industry standards are still under development but hundreds of cinemas are already benefiting from the technology. These sites all have systems based on DLP Cinema technology from Texas Instruments. This technology was the first to be approved by the major Hollywood studios for projection of feature films and has been deployed in real cinemas across the world for five years. DLP Cinema projectors – incorporating the technology – are available from three manufacturers and their worldwide representatives: Barco Digital Cinema Christie Digital Systems Digital Projection Cinema Top Close Security Content for digital cinema is stored and distributed in very high quality digital form. The owners of the content, quiet reasonably, want to ensure that their product is protected from unauthorised reproduction or modification. This requires appropriate levels of security for each situation where the content is stored or transported. Wherever content is present on servers or storage media, physical access should be limited to authorised personnel. Server access should be controlled through the use of individual user identification and passwords – which should be subject to periodic, mandatory change. Content may be encrypted so it can only be loaded and played on servers when a security key is present. The key – either physical or in the form of software – can be set to be valid for limited periods of time. The encryption may also encompass the link between server and projector to prevent electronic copying of the content during playback. One area where content is vulnerable – just as much with film as with digital cinema – is with copying through the use of camcorders in the auditorium. There are some promising developments in image processing that could, in future, prevent this form of piracy for digitally projected content. Until then, it is common for content to be unobtrusively marked (watermarked) by the owners so that illegal copies can be analysed and the source identified. This technique can be enhanced, with digital cinema projection, by including in the watermark details of the date and time of the screening in addition to the location of the cinema. Top Close Server A digital cinema server provides the means to store and play digital content. Servers generally employ arrays of hard disks to store the content. High-quality content requires a lot of disk space – even when compressed – so server storage capacities will be several hundreds of Gigabytes (thousands of megabytes) or a number of Terabytes (millions of megabytes). Servers usually provide a number of ways in which content can be loaded into storage. Digital tape cartridges are frequently used to transport content but optical disks (DVD-R) and removable hard drives are also often supported. In addition it is common for servers to have Ethernet ports to allow content to be transported by various types of network. Digital cinema content is generally compressed to avoid the need for impractical sizes of storage media. Servers incorporate hardware and software to decompress the content during playback. Servers usually offer high levels of security to prevent unauthorized copying or playback of restricted content. In some cases the content will only play on a server if a time-sensitive, site-specific key value is provided. Server suppliers include: Avica Technology EVS Digital Cinema GDC Technology Technicolor Digital Cinema QuVIS Digital Cinema Top Close