Digital video generated by computers are converted into analog signals (Red, Green, Blue video signals) by video graphics card and fed to CRT monitor. As present day plasma, LCD flat panels are digital in nature, generated analog signals are once again converted into digital and fed to display devices. This method is inefficient due to following reasons. First, Digital-to-Analog and Analog-to-Digital process causes loss of image quality. Second, a digital interface can make entire conversion process as well as associated hardware redundant. A low cost, universally accepted as well as versatile digital interface evolved and it was called Digital Visual Interface (DVI). This was extended for high end devices and called as High-Definition Multimedia Interface (HDMI).
Before getting into the details of DVI technology we have to learn about the need for the technology.
Resolution Name Pixel Resolution
Video Graphics Array (VGA) 640 x 480
WVGA 854 x 480
Super VGA (SVGA) 800 x 600
Extended Graphics Array (XGA) 1024 x 768
WXGA 1280 x 768
Super XGA (SXGA) 1280 x 1024
WSXGA 1600 x 1024
Ultra XGA 1600 x 1200
High Definition TV (HDTV) 1920 x 1080
Quad XGA (QXGA) 2048 x 1536
Table 1. Resolution Name and Pixel Resolution (Ref. [1], [3])
Resolution name and other details are specified by Video Electronics Standards Association (VESA). The monitor refreshing rates available are 60Hz, 75Hz and 85Hz. Higher the refreshing rate is always better. Now we will calculate the amount of data digital interface has to carry from the computer to display device.
Data carried = No of horizontal pixels x No of vertical pixels x refreshing rate x Blanking
For a monitor with SXGA resolution and 85Hz refreshing rate, will generate 55 Mega pixels(Mp) data per second for one colour. For three colours it will be 155 Mp per second. This will amount to whopping 1.6 Gbps data rate (155 M pixels and each pixel with 10 bit representation; Yes, 10 bits). Beyond two Gbps it is not possible to send through twisted pairs. This phenomenon is called "Copper Barrier". Data generated by QXGA monitor with 85 Hz refreshing rate is 350 Mp per second. The required bit rate exceeds the copper barrier. So two links are used instead of one. Coaxial cables, Waveguide are other transmission media that can handle two Gbps data rate with ease. But they are expensive compared to twisted pair. In DVI 1.0 specification they have not mentioned the term "twisted pair" explicitly. This term is used in the reference material [1].
In April 1999 DVI 1.0 specification was released by Digital Display Working Group (DDWG). Its Promoters are Intel, Compaq, Fujitsu, HP, IBM, NEC and Silicon Image. Transition Minimized Differential Signaling (TMDS) technology used in DVI was developed by Silicon Image Inc and connecters were developed by Molex Inc. The first digital standard "Plug and Play" was developed by Video Electronics Standards Association (VESA). Few years later, "Digital Flat Panel" interface was developed by consortium of Compaq Corporation and its associates. Due to various reasons both standards were not very successful. DVI is backward compatible with analog VGA, Plug and Play and Digital Flat Panel.
DVI have two types of connectors namely DVI-Integrated (DVI-I) and DVI-Digital (DVI-D). 29 pin DVI-I have allotted five pins for analog video and 24 pins for two digital video links. Analog video pins are Red, Green, Blue, Horizontal sync and analog ground. Digital video pins can be grouped into data channels and control signals. There are six pair of data channels to carry two R’, G', B' colour signals. The difference between RGB and R'G'B' will be discussed in upcoming blog post. Remaining 12 pins carry clock signals and other things. 24 pin DVI-D is designed to carry digital video only.
TMDS is an electrical technology used to transmit data from computer to display device. Twisted pairs are susceptible to noise and electromagnetic interference (EMI). In differential signaling, one and zero are encoded not in absolute terms but in relative terms. This makes them to be immune to noise. A sharp spike in one twisted pair can create an EMI in adjacent twisted pair. So it becomes necessary to reduce the steep transition in signals. This is done at the cost of 25 percent increase bit representation (10 bits instead of 8 bits). Earlier to TMDS, Low Voltage Differential Signaling (LVDS) was used in digital interface standards. LVDS was developed by National Semiconductors to transfer data between notebook computer’s CPU to LCD display. This is optimized for short cable length and not for long lengths.
References
Before getting into the details of DVI technology we have to learn about the need for the technology.
Resolution Name Pixel Resolution
Video Graphics Array (VGA) 640 x 480
WVGA 854 x 480
Super VGA (SVGA) 800 x 600
Extended Graphics Array (XGA) 1024 x 768
WXGA 1280 x 768
Super XGA (SXGA) 1280 x 1024
WSXGA 1600 x 1024
Ultra XGA 1600 x 1200
High Definition TV (HDTV) 1920 x 1080
Quad XGA (QXGA) 2048 x 1536
Table 1. Resolution Name and Pixel Resolution (Ref. [1], [3])
Resolution name and other details are specified by Video Electronics Standards Association (VESA). The monitor refreshing rates available are 60Hz, 75Hz and 85Hz. Higher the refreshing rate is always better. Now we will calculate the amount of data digital interface has to carry from the computer to display device.
Data carried = No of horizontal pixels x No of vertical pixels x refreshing rate x Blanking
For a monitor with SXGA resolution and 85Hz refreshing rate, will generate 55 Mega pixels(Mp) data per second for one colour. For three colours it will be 155 Mp per second. This will amount to whopping 1.6 Gbps data rate (155 M pixels and each pixel with 10 bit representation; Yes, 10 bits). Beyond two Gbps it is not possible to send through twisted pairs. This phenomenon is called "Copper Barrier". Data generated by QXGA monitor with 85 Hz refreshing rate is 350 Mp per second. The required bit rate exceeds the copper barrier. So two links are used instead of one. Coaxial cables, Waveguide are other transmission media that can handle two Gbps data rate with ease. But they are expensive compared to twisted pair. In DVI 1.0 specification they have not mentioned the term "twisted pair" explicitly. This term is used in the reference material [1].
In April 1999 DVI 1.0 specification was released by Digital Display Working Group (DDWG). Its Promoters are Intel, Compaq, Fujitsu, HP, IBM, NEC and Silicon Image. Transition Minimized Differential Signaling (TMDS) technology used in DVI was developed by Silicon Image Inc and connecters were developed by Molex Inc. The first digital standard "Plug and Play" was developed by Video Electronics Standards Association (VESA). Few years later, "Digital Flat Panel" interface was developed by consortium of Compaq Corporation and its associates. Due to various reasons both standards were not very successful. DVI is backward compatible with analog VGA, Plug and Play and Digital Flat Panel.
TMDS is an electrical technology used to transmit data from computer to display device. Twisted pairs are susceptible to noise and electromagnetic interference (EMI). In differential signaling, one and zero are encoded not in absolute terms but in relative terms. This makes them to be immune to noise. A sharp spike in one twisted pair can create an EMI in adjacent twisted pair. So it becomes necessary to reduce the steep transition in signals. This is done at the cost of 25 percent increase bit representation (10 bits instead of 8 bits). Earlier to TMDS, Low Voltage Differential Signaling (LVDS) was used in digital interface standards. LVDS was developed by National Semiconductors to transfer data between notebook computer’s CPU to LCD display. This is optimized for short cable length and not for long lengths.
References
- “White paper on DVI”, by Infocus Incorporation, available Online from http://electro.gringo.cz/DVI-WhitePaper.pdf
- DVI specification from DDWG, available Online from http://www.ddwg.org/lib/dvi_10.pdf
- Keith Jack, “Video Demystified: A handbook for the digital engineer”, 5th edition, Publishers:- Newnes , 2007. ISBN: 978-0-7506-8395-1, Indian reprint 978-81-909-3566-1. Rs. 800.
- Pin diagrams of DVI, available Online from http://www.te.com/catalog/Presentations/dvipresentation.pdf
- Video Electronics Standards Association (http://www.vesa.org)
- Digital Display Working Group (http://www.ddwg.org)
