Standards FAQ

EDID

Q: In EDID, how do you count the week value? Do you start counting on the first Monday or the first Sunday of the new year?

A: One way is to count January 1-7 as week 1, January 8-15 as week 2 and so on. This will give consistent results, regardless of the year, and will always yield a value of 53 or less. You can also count based on the week number (Sunday-Saturday).

Q: Where do I obtain the ID Manufacturer Name?

A: This information is obtained from Microsoft. Plug and Play device ID for a monitor/LCD consists of seven characters. The first three characters represent the Vendor ID which is assigned by Microsoft. The four-character Product ID, which is a 2-byte hexadecimal number, is assigned by the company producing the monitor. Companies can apply online for a Vendor ID at http://www.microsoft.com/whdc/system/pnppwr/pnp/pnpid.mspx. Microsoft does not maintain Product IDs. It is the individual company’s responsibility to assure that they do not assign the same Product ID to two different devices.

Q: Has a common address been set for the EDID memory in the DFP, P&D and DVI standards? In the past A0h was the only address, until P&D introduced A2h.

A: The Enhanced EDID and DDC standards refer only to A0h. This is the address used by DFP, and DVI will follow E-EDID and E-DDC as well. It is anticipated that mainstream support for P&D will fade, and use of A2h will diminish.

Q: For monitors using DVI connectors, how do you set the Video Input Definition byte? Doesn’t the digital port share the same EDID with the analog one on a DVI-1 connector?

A: You must have two switches, as you cannot have only one EDID. Displays that support both an analog and a digital interface should supply a separate EDID for each. However, the display can only be one or the other at a given time.

Q: On LCD monitors, the RGB inputs are 714mV, and on CRT-based monitors, it is 700mV. Why is it higher on LCDs and which VESA standard has this information?

A: E-EDID allows this to be specified at part of the Video Input Definition. However, it depends on the manufacturer, so you should check with them. This information is in E-EDID Release A, 1.0 on page 13.

Q: In E-EDID (specifically 2.2.1.4 on page 8 it says ‘some extensions are not described’. Is there any documentation for 02h, which is the tag for an Additional Timing Data Extension Block?

A: As this time it has not been defined, and there is no documentation to describe this Block.

Q: What is relationship between EDID Version 1 Revision 0, EDID Version 1 Revision 1 and EDID Standard Version 2 Revision 0?

A: EDID standard document Version 2 Revision 0 contains definitions for 2 alternate data structures: a) EDID structure Ver. 1 Rev 0: This is the original data structure defined in DDC Standard Version 1 Revision 0. b) EDID structure Ver. 1 Rev. 1: This is a new data structure introduced in EDID Standard Ver. 2 Rev 0.

Q: If bits 5 & 6 = 0 what should bit 0 equal?

A: Bits 5 & 6 = 0 when there is no stereo image present. In this condition bit 0 should be set to 0, bit 0 =1 is reserved. Ref.: Table 3.17

Q: Is the following true? Horizontal sync offset = Horizontal front porch, if Horizontal border = 0 A: Yes. Ref.: Section 3.10.2 Q: What is the meaning of ‘code page # 437’?

A: ASCII has multiple code pages to allow for national language variations; code page # 437 corresponds to American English. Ref: 3.10.3

Q: Does ‘Horizontal active pixel’ = the total number of pixels on a horizontal line?

A: The Horizontal component of timing consists of the Horizontal active + the Horizontal blanking periods. Ref: Sections 3.10.2 and 3.12. Q: Is ‘Image aspect ratio’ = (Horizontal active pixel) / (Vertical active pixel)? A: Yes. Ref.: Section 3.9

Q: If calculated aspect ratio is not 16:10, 4:3, 5:4 or 16:9, what should be used?

A: Only applies to standard timings defined by VESA. All match except 720×400. Ref.: Section 3.9

Q: How should VESA standard timings not listed in the ‘established timings’ section be handled?

A: The ‘standard timing identification’ fields (2 bytes each) provide for a coded way to identify timings not included in the ‘established timings’ section. It is also possible to fully describe a required timing in a ‘detailed timing descriptor’. Ref: Section 3.8

Q: If I want to use the ‘standard timing identification’ fields, where do I get the ‘Horizontal active pixel’ and ‘image aspect ratio’ for a particular timing?

A: VESA timing standards include these parameters. Ref.: 3.9

Q: If I want to use a ‘detailed timing descriptor’ block, where do I get the detailed information?

A: If it is a standard VESA timing, then all details are part of the standard. If it is a proprietary timing, then details need to be established by the developer. Ref.: Section 3.10

Q: Section 3.10.3 Descriptor Description, Definition # 5 Color Point: “An index number of 00h means that no color data follows”. Does this mean that only white gamma follows or neither white chromacity nor gamma follow?

A: An index value = 00h means that neither white chromacity nor white gamma values follow.

Q: What binary index value should the white point index start from, or is it arbitrary?

A: It is arbitrary and left to individual manufacturers. However, there is white color and gamma data stored in bytes 24-27 (decimal) with no explicit index number. Implementers may wish to assume that this is implicit index number of 1 and hence, the explicit index numbers in a descriptor block should start at 2. Ref.: 3.10.3

Q: How many color point monitor descriptors are allowed? One or up to four?

A: Up to four. There are no restrictions on the number of blocks that may be redefined to a particular type of descriptor. Ref.: Section 3.10.3

Q: What is the most reliable way for a graphics sub-system to determine the operating range of the attached monitor?

A: For EDID structure Version 1 it is recommended that the Monitor Range Limit Descriptor (if provided) be used. Monitor operating range limits cannot be reliably inferred from any other source within the EDID. Ref.: Section 3.10.3

Tools

Q: I am designing a tool that will automatically design data tables for EDID data structures based on the capabilities of a monitor (entered by a designer). I have several questions: a) Are there any development tools that will facilitate my design? b) Which standards should I have to get all the information I need to do the design? c) I want the tool to be backward compatible with earlier versions of EDID; will I face any difficulties in trying to do this?

A: The answers to your questions are as follows: a) While many companies have tools developed internally for this purpose, we know of none that are available commercially. b) E-EDID should provide enough information to create version 1, revision 2, or version 1, revision 3 EDID. As extensions are defined you will want to have those as well. To date, however, VESA has not released any EDID extension documents. c) Creating backward compatibility with earlier EDID 1.x data should not pose a problem, as field definitions remain the same. Newer versions added fields and refined requirements for the use of existing fields in order to improve the consistency of EDID throughout the industry. However, VESA does not recommend using previous versions of the EDID standard to create new data.

Timing

Q: Where can I find a description or algorithm to compute all GTF data from a given resolution and refresh rate?

A: The GTF Standard provides formulas and the companion spreadsheet contains GTF calculations.

Q: I recently ordered the Monitor Timing Specification Standard dated September 1998. The highest resolution listed is 1920×1440 @75Hz. Is there an update with timings of a higher resolution, e.g. 2048?

A: You have the most current version of the standard. At this time, VESA has not defined any discrete timings for formats higher than 1920×1744. However, the GTF Standard can be used to calculate timings for this purpose.

Q: Which standard has the video timings for the following display modes? 640×512, 800×600, 1024×768 @ 50Hz / 100Hz and 1280×768 @ 50Hz / 75Hz / 100Hz

A: VESA does not define any discrete timing for those particular formats and refresh rates, but the GTF Standard can be used to calculate timings for these and other desired formats.

Q: Where can I find plasma display timing standards? Specifically, 848×480, 852×480 and 1360×765.

A: VESA has not published timing standards for plasma displays. However, you can generate these timings using the GTF Standard.

Q: When I looked at the DMT Standard, the resolution list was incomplete. There is no reference to 720p and others, as well as no reference to what a resolution is called; for example — VGA= 640×480, SXGA=1280×1024, etc. Where can I find this information?

A: Although VGA, SXGA and other terms have been commonly used to refer to particular display formats, in reality they refer to particular graphics adapters that may support multiple formats. Because of this, VESA does not use these terms when referring to particular display formats.

Q: I am using the GTF companion spreadsheet to calculate timing information such as Horizontal Front Porch, Horizontal Back Porch, etc. However, I see a difference in the information obtained from the GTF spreadsheet and the Monitor Timing Standard (DMT). Which set of data should I use? Additionally, on modes that are not covered in DMT, should I use GTF?

A: GTF was not designed to produce the same timings as DMT; it cannot do this within the restrictions of linking the three variables of pixel clock, horizontal timing and vertical timing. For that reason, timings created by GTF will differ from those of DMT. It is your choice as to which timings to support. You will probably need to support at least the more common DMTs, which requires that you store the timing parameters for each mode. GTF allows new timings to be created at whatever refresh rate you want, for example, if you have a display properties panel that allows you to select refresh rates, you can use GTF to create the new timings through ‘on-the-fly’ calculation, rather than by storing lots of different mode timings (though you could still do it that way using GTF). You can even create new formats instead of being fixed at the regular 640×480, 800×600, etc.

Q: What is the correct input impedance value of the H-sync signal when we design a monitor?

A: 2.2 to ground or higher

Q: In looking at GTF V1.1 and Monitor Timing Specifications V1.0, I find that the format of composite sync signal is not described, though it is mentioned in GTF. Can you provide me details about this signal?

A: The composite sync is not specified in VESA standards, though the timings could still apply.

Q: Why are the timing parameters that define the sync signal so important?

A: Sync signals for displays drastically affect the quality, performance and even reliability of CRT displays. Even small differences in timing parameters can significantly affect image position and size, causing problems for the user. Difference in blanking times can lead to excessive power dissipation and electrical stress in the scanning circuits, or at the other extreme, incomplete or distorted images being displayed.

Q: Why can’t the monitor automatically set itself to any timing format?

A: Multimode monitors can measure the incoming sync signal frequencies and thus sync to any frequency within their range of operation. But the sync signals carry no information regarding the relationship of the video signal to the sync pulses. Each timing format, up until now, has had a unique definition of these parameters, and so it has not even been possible for the display to accurately ‘guess’ where the image might be, though modern displays do make an attempt to do this, based usually on the preset timing information that it does have.

Q: How will GTF help the monitor automatically set itself to any timing format?

A: GTF defines the relationship between syncs and video signals at any frequency of operation. The display can measure the incoming sync frequency, and thus can predict where the image will start and finish, even though it may not have been preset at that operating point.

Q: Will this require all monitors to have complex microcontrollers?

A: In order to identify the mode, most present day multiple frequency monitors use a simple microcontroller to measure syncs. GTF will simplify the microcode in that it will eliminate the guesswork presently required when an unknown sync signal is applied. However, if required, even analog multiple frequency monitors can use simple frequency to voltage conversions to compute the new image size and position within a reasonable accuracy

Q: Is this GTF system compatible with existing monitors?

A: The GTF algorithm was intentionally chosen to create timings that lie centered in the range of a large number of existing formats. However, as older monitors do not have knowledge of the GTF system, they may still require some user adjustment typical of any other new format.

Q: There are many standard timing formats, including those created by VESA. Will GTF replace these?

A: Standard timings, such as the standard VGA 60Hz timing, will continue to exist and be used, but may eventually be replaced by GTF versions. VESA will still maintain its existing Display Monitor Timings (DMT) and may even produce new discrete timings in the future that do not comply with the GTF method. This may be necessary when particular circumstances dictate precise parameter definition to meet a number of simultaneous constraints. An example would be when a specific timing signal is required that defines pixel clock, vertical refresh rate and horizontal rate simultaneously, say for a HDTV format. In contrast, the GTF fixes the relationship between these parameters in order to guarantee its predictable nature, which is usually of much greater importance to the average user.

Q: Will VESA produce a matrix of GTF formats and timings at, say, 5Hz refresh rate intervals (i.e., 75Hz, 80Hz…)?

A: No. GTF permits a common “standard” timing to be calculated for practically any format and refresh rate, and so it would be meaningless to try to develop a restricted matrix of relatively few timings using this method. The need for standards for specific, EXACT refresh rates, such as might be needed for video compatibility or other applications, are better handled by discrete monitor timing standards, and will be developed by VESA as needed.

Q: GTF allows very high vertical refresh rates to be generated, e.g. above 150Hz. Is VESA implying that faster refresh rates are better?

A: By allowing a standard method of defining refresh rates, any rate can be computed as required by the industry and/or user. Thus VESA is neither limiting nor recommending the use of higher refresh rates, though most users generally seem to prefer higher rates. Also, there are emerging applications that need high refresh rate, such as stereovision. GTF will allow a method of defining these.

Q: For best compatibility with video systems, programmable pixel clocks should be synthesized from a master oscillator with a base frequency that is a multiple of 2.25MHz. Why doesn’t the GTF specification mandate this?

A: GTF can generate timings based upon any given pixel clock frequency. The Nx2.25MHz base reference is a hardware consideration that will determine the available set of actual pixel rates in conjunction with the complexity of the pixel clock synthesizing hardware. These constraints must be determined before hand by the system software to determine which pixel clocks can be used and GTF should be used accordingly, specifying the pixel clock as the defining parameter. However, generally such system restraints also call for precise vertical refresh rates to be used (i.e. TV, HDTV etc.). In those limited applications, VESA Discrete Monitor Timings (DMT) is a better choice.

Q: How can I use GTF to give me an exact refresh rate and an exact pixel clock frequency?

A: GTF cannot be used in this way. Either an exact refresh rate can be specified, an exact horizontal scan frequency or exact pixel clock. In most cases the pixel clock will be subject to some quantization due to the limitations of the clock synthesis hardware. In that case, the desired refresh rate should be used as the input in a first pass GTF calculation to find the ideal pixel clock. The pixel clock should then be rounded to the nearest achievable frequency and then used as the defining input to a second calculation of the GTF. Because of this rounding error, the actual final refresh rate will be slightly different than the desired refresh, the error being set by the accuracy of the pixel clock.

Q: Why doesn’t GTF allow me to set both refresh and pixel clock accurately? VESA discrete timings achieve this!

A: DMT achieves this by varying the blanking times. However, this destroys the predictability of the blanking time from a simple measurement of the horizontal frequency and hence prevents the monitor from being able to accurately ‘auto align’ to an unknown new mode. Where precise refresh rate and pixel clock requirements are mandatory in the system, DMT should be used.

Q: What is Secondary GTF?

A: Secondary GTF is a method that allows a monitor and graphics controller to use a blanking ratio that is different from the blanking ratio that is produced by the default parameters for GTF. This enables the use of shorter blanking ratios that in turn can produce timing modes that have a higher pixel resolution for a given pixel frequency.

Q: How does the monitor avoid confusion between the 350-line VGA mode and a secondary GTF timing?

A: The monitor knows that all timing modes with a horizontal frequency below the “Second GTF start frequency” shall be treated as legacy timings. The Second GTF start frequency shall always be set to a value higher than the horizontal frequency used for any supported legacy timings that have horizontal positive and vertical negative sync polarities.

VSIS

Q: In the VSIS Standard the diagram in figure 6 shows the voltage ripple and noise, but does not spec numbers. The numbers would be useful. Is this an error?

A: The diagram is for visual reference only. The spec is in the tables at the beginning of the document.

DDC

Q: What makes a monitor DDC compliant?

A: If you design your product following the requirements of the DDC standard, it should be ‘compliant’.

Q: I have found a possible inconsistency in DDC/CI version 1. On page 13 at the bottom of the page, there is an example of a DDC2Bi message (host read of a display). On page 16 at the bottom of the page, there is a further example of this. The problem is to do with the second byte ‘source’. On page 13 this byte gives the address of the display (6E), but on page 16 the byte gives the address of the host (51). Shouldn’t the 51 be 6E?

A: Yes, you are correct. The example on page 16 has a typo; the second byte is the ‘data origin’ (6E).

Q: Why is +5 volts on pin 9 mandatory for system units and/or graphic cards.

A: It is becoming common for system management software to interrogate peripheral devices for product type and serial number as a form of asset management. This is usually scheduled for nighttime but if the monitor is powered off then no data can be collected. Providing +5 volts allows the DDC circuit in the monitor to remain active even when the monitor itself is powered off. Ref.: Section 4.1.4.1

Q: Is +5 volts mandatory for portable computers?

A: Portable computers that want to be able to claim compliance with VESA DDC standard version 2 must provide the +5 volt output. Ref.: Section 4.1.4.1

Q: Is +5V from the host required to be continuously active?

A: +5V should be provided whenever the host PC video port is active, or when the host PC is attempting to read the monitor’s EDID data. In particular, mobile PCs running on battery power may disable the +5V output when the system video port is not active, to conserve battery power. Ref.: Section 4.2.

Q: What are the intended uses for the +5V output?

A: +5V is needed as a power source for the DDC and EDID circuits of a display monitor when the host is attempting to read the monitor’s EDID data and the monitor’s own local power supply is not yet turned on. This capability is especially important when a PC is initially booting up or when a mobile PC user enables the external video port, so that the PC can properly detect the type of display attached. This is the main use of +5V defined in the Enhanced DDC Standard. (The EDID and DDC circuits in a display monitor should always be powered from the monitor’s own local power supply whenever it is available.) The E-DDC Standard does not directly define or constrain other possible uses for the +5V output, provided that the specified loading limits are not exceeded. For example, when the display monitor is powered on, +5V may be used as a logic signal to control display power states, i.e., signaling to the display whether or not the host PC video port is active, so that the monitor can decide when to go into a reduced power mode. This is the reason why the +5V must be provided continuously whenever the host PC video port is active. Since the host PC might be a mobile PC running on battery power, the total load on the +5V from the host PC must be limited to 1 mA maximum when the display has local power. Ref.: Section 4.2 & 4.4.

Q: Is clock stretching required?

A: Yes, compliance with VESA DDC standard requires that the requirements of the I2C specification be met. Ref. Section 2.2.