Digital Television Environment

Digital technology has been employed for more than two decades in television production and post-production systems. Digital products have been designed to operate with the same picture and sound format as their analog counterparts, but provide better signal performance (particularly after multiple recording generations) as well as powerful manipulation and editing capabilities that are impossible in an analog domain.

Today, digital technology provides program producers with almost limitless creative tools and techniques. It allows audio and video quality to be controlled through every part of the creative process, ensuring that the final product meets the requirements of its producer. However, once this material is delivered to the distribution channels (terrestrial broadcasters, satellite, cable services or videotape), it is converted to its analog equivalent and quality control is at the mercy of the delivery system used. Terrestrial broadcasts are plagued by poor signal reception properties, cable systems often suffer from cross-modulation problems and analog consumer receivers deliver less than perfect results to the viewer, regardless of input signal quality.

After years of debate, many countries have elected to replace their analog delivery systems with new digital services. These new digital delivery services provide a level of flexibility that could not have been considered in an analog environment. Such flexibility includes choice of picture quality and aspect ratio, audio channel choices, and data services such as electronic program guides and interactive responses. All of the new services include the provision to deliver High Definition Television (HDTV), Standard Definition Television (SDTV) and in some cases, Enhanced SDTV (EDTV) programming, with choices for line and field rates, picture aspect ratio and audio format.

Implementing systems to provide suitable output formats for the intended delivery medium presents a number of challenges:

  1. New digital services may need to accommodate multiple sound and picture formats.
  2. Different compression techniques will need to coexist within a single system.
  3. Compression systems may generate complex signal latency problems.
  4. Surround sound audio will bring new signal management constraints.
  5. Analog services will continue to exist for many years (probably ten or more).

Picture and Sound Formats

Until now, the world has been used to two basic picture formats, 525/59.94 and 625/50, both with 2:1 interlace. Compatibility with existing systems, technical issues, and politics are factors that have determined the next generation of formats. Technically, we would all like to see a single standard adopted worldwide. Our lives would be much simpler and we’d need to know less. However, this is not to be; standards are much more complex and have probably provided us with ongoing job security.

Two bodies, the ATSC (Advanced Television Systems Committee) in the USA and the European DVB (Digital Video Broadcasting) project, have each developed recommendations for picture, sound and signal formats. These have been adopted and ratified as standards by the various government agencies that manage national communications (e.g. the Federal Communications Commission (FCC) in the USA).

DVB formats provide a simple choice: one HDTV format, one EDTV format, and two SDTV formats. The ATSC system offers a large choice of picture formats that can be delivered to the home and ATSC receivers must be able to display them all.

Signal Transport Formats

Can all possible picture formats be carried by the agreed-upon digital transport standards? The answer is no, not at standard sample rates. Refer to the following example:

A standard definition, 525/29.97, 4:2:2 component signal has a sample rate of 13.5MHz. Therefore; 13.5MHz / 29.97 = 450450 samples per frame 450450 / 525 = 858 samples per line However, the same signal with a 30Hz frame rate works out like this; 13.5MHz / 30 = 450000 samples per frame 450000 / 525 = 857.1428571429 samples per line This does not work. In order to distribute standard definition signals via a normal serial digital transport stream (SMPTE 259M/ITU 601), only frame rates of 29.97 may be used. In fact all 525 line signals with a 13.5MHz sample rate must use frame rates based on 30/1.001 (29.97). Further investigation reveals that the only 640×480 rates that could be transported would have to have a 29.97 frame rate and be re-mapped into SMPTE 259M.

High definition signals can be derived from two sample rates: 74.25MHz for integer frame rates and 74.25MHz/1.001 for 525 line compatibility. This means that, unlike SDI formats that share a common data rate (270Mbits), HD-SDI signals have two possible rates. It is important to remember that the generic reference to 1.5 Gbits for HD transport does not tell you what the actual data rate is.

Program production will require equipment that is format specific or multi-format, as most picture formats are not directly compatible or interchangeable. Any system design needs to accommodate camera, storage and editing equipment that is specific to the picture and sound formats supported, even though the connectivity requirements can be common.

So far we have only mentioned video, but the audio portion of the digital television environment presents some new and difficult issues. In both the ATSC and DVB systems, the audio capacity is 5.1 channels and can include mono mix, stereo, pro logic, multiple language, and full surround sound.

Signal compression systems have been developed and standardized for delivery to the consumer (Dolby Digital (AC3) and MPEG2), but the transport, storage and editing of baseband surround sound signals requires a minimum of six channels (three AES signals). Currently, standard DVTRs have no more than a four channel capacity (with the exception of some highly specialized HDTV recorders). Furthermore,

if video storage devices exist (i.e. digital disk recorders) that can accommodate the channel requirement, maintaining accurate phase alignment presents some complex difficulties.

Interfaces are defined and hardware is under develpment for managing multi-channel audio as a single data stream at baseband or compressed at a "mezzanine" level. Obviously, the choice of signal formats to be managed is determined by each application. A single standard may suffice for certain applications, while others may need to handle a wide variety.

Production and Post Production

Any facility providing production or post-production services is faced with a difficult dilemma. In order to provide the services required, several formats may need to be managed. Ideally, a common production format from which all other formats could be output would present a perfect solution. Today, the only common format is film (it may vary in size but the Telecine operator can handle that), but editing celluloid and creating optical effects is uneconomic, undesirable and in many cases impossible.

Currently, dual-format post-production (525/625) is commonplace for international distribution. It is not uncommon for film to be transferred to video twice (once for each format) so that video standards conversion is avoided. Achieving acceptable conversions at a high quality is a very specialized task and many producers still prefer the look of a program that is transferred from film in the final video format. This requires that most programs are transferred and edited twice, therefore the costs are significantly higher than programs produced for local distribution only. With the rapid expansion of world communication systems, the requirement for international program distribution is increasing. The addition of new picture formats could potentially add significant costs to the post production portion of program generation, as well as require the design of very complex technical systems to accommodate all of the required standards.

Discussions within the post community have focused on generating a single electronic ‘film’ format that could be used during all production and post-production stages and then be converted to the desired delivery format for each distribution channel after the product is complete. The ideal candidate for this single format is 1080P/24; it is the direct electronic equivalent of film and is easily converted to all of the possible frame rates, without the introduction of undesirable motion artifacts. Simple line interpolation can be used to derive any of the possible line rates.

Broadcasting Networks

In most broadcasting systems, only one or two picture formats will be output for delivery to the viewing audience. So, system requirements will be simpler than those of the post production facility. However, most broadcasting to date has been single standard, and so we are considerably increasing the complexity with the addition of DTV/DVB.

Managing HD and SD within the same system could be as simple as including up and down converters at the facility input and output, or as complex as separate distribution and production layers for each format.

Naturally, the choice of formats and operational methods is largely up to the broadcasting organization, as the viewer will be able to receive the broadcast regardless of chosen format. Implementing a system designed to manage the processing and distribution of the desired signals could be very complex and costly and serious consideration needs to be given to the design.

Digital Evolution

Digital television is not digital broadcasting. Any television can be fully digitally equipped yet still broadcast in analog. However, because of many advantages of digital broadcasting all countries worldwide are making a transition from analog to digital television broadcasting.

Digital television brings not only better quality but also new services that were not possible in the analog world. When color was added to the television picture (end of 1950s in USA and beginning of 1960s in Europe) this was considered as a big advantage. All the color systems (NTSC, PAL and SECAM) were designed to be compatible with the black and white television so the transition was very smooth. Unfortunately, the transition from analog to digital changes everything. And to further complicate things any change in audio/video codecs in many cases makes the old digital equipment useless.

Fortunately, the broadcasting part is independent of the production part so the digitalization of a television can be achieved gradually in few steps. And being digital is not the end of the road. The television technology development will never end.