HD transition and standards
01 September 2006
The switch to digital broadcasting provides broadcasters the freedom to either transmit one HD channel (during primetime) or multiple SD (Standard Definition) channels (during non-primetime) in the same bandwidth.
The switch from analogue to digital television transmissions started in 2001 with the Digital Television Project. Today, 70 per cent of UK homes have DTVs (digital televisions) or digital settop boxes. Furthermore, this summer has seen the beginning of widespread highdefinition broadcasting with the World Cup and Wimbledon both being broadcast in high definition by the BBC and Sky. Currently, two per cent of UK homes own HD-ready televisions, but a million more HD-ready
Broadcasters need to transmit both analogue and digital transmissions for the same channel simultaneously (called simulcasting) and to switch from analogue to digital transmissions individually.
Thus the challenge was given to video equipment manufacturers to introduce multirate and multi-format systems running various video protocols, from SMPTE (Society of Motion Picture and Television Engineers) to DVB-ASI (Digital Video Broadcast-Asynchronous Serial Interface), which will support both HD and SD formats simultaneously.
High-definition growth
Moving away from the studio into the consumer premises, sales of HD peripherals, HD DVD players and HD game consoles are growing worldwide. The HD content running on these devices is being protected by HDCP (high-bandwidth digital content protection) using HDMI (high definition multimedia interface), a spin off of DVI (digital video interface).
HDMI has already expanded into most HDDVDs and is the HD interface of choice for set-top boxes. In fact, HDMI or DVI is required in all HD set-top boxes in Europe and the US. HDMI is also required to support both SD and HD resolutions to aid the transition from analogue to digital formats at the consumer end.
Professional video equipment manufacturers need to transform all of their equipment (video production switchers, editing platforms, or storage servers) that store, process, or distribute DTV to accept and deliver either SD or HD streams. As shown in figure 1, different components of a professional broadcast studio need to carry uncompressed digital video serially from one piece of equipment to another.
Transport specifications
The two specifications for transporting serial digital video within the studio for the SD format are SMPTE259M, which allows 480 or 576 interlaced lines per frame, and the HD format SMPTE292M, which allows transport of 720 progressive, and 1,080 interlaced, lines per frame.
The higher resolution of the HD format is one of the major selling points for consumers. The benefit for broadcasters, on the other hand, is the move to digital, which allows them to multi-cast multiple channels of SD or fewer channels of HD. At the edge of the broadcast studio, both SD and HD streams need to be compressed and transmitted using an MPEG2 transport stream via a serial interface protocol called DVB-ASI. This requires MPEG2 ENDECs that can handle both SD and HD streams.
The implementation of a multi-rate video system to deliver video data from studios to broadcast stations presents many challenges. Most professional video systems prefer serial rather than parallel connections between the delivery and acceptance systems. Serial connections are usually necessary because parallel connections are not feasible for distances greater than 50m for SD and 20m for HD, due to issues involved in delivering multiple bits of data at high speeds with a common clock over a single bus. The parallel data needs to be converted to a single serial stream before delivery and the serial data needs to be converted to a parallel bus after receipt.
Parallelisation
A serialiser/deserialiser (PHY) is an important part of the SDI (serial digital interface), present at any point where serial data is converted to 10bit video data or 10bit video data is converted to serial digital data. The SD-SDI for parallel component video consists of 10bit data transmitted at 27MHz resulting in a serial data rate of 270Mbit/sec. The HD-SDI parallel component video consists of two 10bit datastreams, a 10bit luminance channel and a 10bit colour-difference channel transmitted at 74.25MHz resulting in a serial data rate of 1.485Gbit/sec. SDI is used as the standard serial communication interface within broadcast studios for professional video cameras, video format converters, SDto- HD up-and-down converters, production switches, broadcast routers, and non-linear editors.
There are different types of video PHYs. Some can be just serialisers (transmitters), deserialisers receivers), or a SerDes (serialiser/deserialiser transceiver). Many PHYs also have multiple channels enabling the PHY to integrate the serialisationdeserialisation function across multiple SDI inputs and outputs. An independent multichannel PHY will have the ability to handle different video signals that operate at very different data rates simultaneously on the same IC (e.g: SD-SDI in one channel and HDSDI in another channel). Designers of serialisers and deserialisers have the most difficulty designing the transmit and receive PLLs and their associated VCOs.
Current solutions are also capable of integrating high-speed components such as voltage-controlled oscillators and loop filters into the same package. There are also SerDes devices that can handle DVB-ASI on the same device that handles SD- and HDSDI. This allows the same port to be used for transporting uncompressed SDI video or compressed DVB-ASI video.
When the video data is serialised and ready to be transmitted over the SDI from the PHY, a separate cable driver IC is used. The purpose of the cable driver is to drive the signal over a coaxial cable at a set output voltage swing with minimised reflections, while operating at both SD and HD data rates. This guarantees that the output signal will abide by the SMPTE and DVB-ASI specifications.
The cable driver will usually provide a differential output. This enables broadcasters who use the SMPTE standard (which is polarity-free) to have two redundant outputs per channel. The video signal can be broadcast to more than one location, a requirement in many professional studio environments. However for incumbent DVB-ASI solutions only the positive end of the differential output can be used for transmission. Recently, some semiconductor companies have introduced cable drivers that have two positive serial outputs to allow redundant DVB-ASI outputs.
Consumer Impact
As HD content becomes more abundant many manufacturers and broadcasters are hoping to use HDCP to control piracy. HDCP, a controlled mathematical algorithm, verifies that the receiver is authorised to receive HD content. This will hinder splitters or multiple connectors from sending HD content to multiple receivers (eg a TV and an HD DVDRecorder). Unlike composite, S-Video, or component interfaces, HDMI was designed to handle the high-resolution 720p, 1080i, and even 1080p of HD content.
Currently DTVs can support a limited number of HDMI receiver ports because their processors can only handle one input. This causes problems for consumers who have HD set-top boxes, HD DVD players and game consoles using HDMI but only one input on the DTV. Many semiconductor companies are developing HDMI switches or multiplexers that have the ability to handle two or three inputs while outputting one HDMI output to the DTV processor.
DTV manufacturers can increase the number of HDMI ports without a big design overhead. To support PiP (picture in picture) or PaP (picture and picture), some switches have two outputs with multiple inputs, anticipating when DTV processors will expand their HDMI receive ports. Other semiconductor companies have developed independent HDMI receivers similar to the PHYs developed in professional video systems.
Implementations of the HDMI interface into home theatre peripherals such as HD DVD players and A/V amplifiers look as though they are enticing consumers to adopt HDMI. As semiconductor vendors such as Cypress Semiconductor develop high-speed interconnect solutions that solve the issues involved in taking high-speed digital serial video from point-to-point over long distances of cable, the practical and physical problems and costs of HD broadcasting and display systems will steadily be ironed out.
ADAM EADES is an associate product manager and PALANI SUBBIAH is a senior staff applications engineer, Cypress Semiconductor.
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