Format and Timing Standards (Display Interfaces) Part 2

Format and Timing Standard Development

As noted, the primary markets served today by electronic imaging are, broadly, entertainment video (broadcast television and other consumer-oriented entertainment services) and the computer industry. To be sure, there are other electronic imaging and display applications, but more and more the standards developed for TV and PC displays have come to dominate the field. Several organizations, operating in one or the other fields (or in some cases both) are responsible for these.

Due to the nature of broadcast communications, many of the standards in the television industry derive from regulations established by the government agencies in various countries, such as the Federal Communications Commission in the United States, the Radiocommunications Agency in the UK, the Ministry of Posts & Telecommunications in Japan, etc.. As radio waves do not respect national borders, though, there is also a need for the various national regulations to be coordinated, especially in terms of frequency allocations but also to some degree the specifics of the technical details of various standards. This task falls primarily on the International Telecommunications Union, or ITU, originally established in 1865 to coordinate telegraph (and then telephone) standards, but since 1947 an agency of the United Nations charged with the overall coordination of electronic communications standards. Many ITU standards were originally published under the name of the Comité Consultatif International Teléphonique et Telégraphique (CCITT), a subsidiary group which ceased to exist as a separate body following an ITU reorganization in 1992.

In terms of industry standards organizations, or professional associations which do a significant amount of standards development in this field, the leading players include the Electronic Industries Association (EIA) and Consumer Electronics Association (CEA, formerly the Consumer Electronics Manufacturers Association), along with the Society of Motion Picture and Television Engineers (SMPTE) in the television market. In the computer graphics and display field, the leading organization in the creation of format and timing standards has been the Video Electronics Standards Association (VESA), which was founded in 1991 to address just this need. However, it should also be noted that a considerable number of PC video “standards” were developed prior to the establishment of this group, and were in fact originally simply “de facto” standards which came through the success of various manufacturers’ products. This has resulted in some obvious incoherence in the standards for display timings first introduced in the 1980s, a problem which is industry is just now addressing.

While the details of the various television and computer-video systems and standards differ considerably, the basics in terms of format and timing standards are directly comparable. All of these systems are based on raster-scanning systems, with the details of the timings generally based on the needs of CRT-based display devices. (Only recently has either industry begun to consider the problem of timing standards oriented specifically to non-CRT displays.) The television industry has traditionally employed 2:1 interlaced scanning (as a method of reducing the bandwidth required for the broadcast transmission of a given image format), but even this is now changing to some degree with the introduction of digital television systems.

An Overview of Display Format and Timing Standards

While a truly exhaustive list of current format and timing standards used in electronic displays and imaging devices is nearly impossible to produce, at least an overview of those in most common use is of interest for comparative and reference use. Table 7-1 provides a list of image and display formats from a number of industries and applications, along with some brief comments on their origins and usage. Table 7-2 then provides some details of the timing specifications for the more common computer and television formats, where such exist.

Some explanations of the terminology of these standards is in order at this point. First, the pixel formats given in Table 7-1 refer to the active or addressable image space. The complete video signal, whether analog or digital, will include information corresponding to the image itself, but also will generally have additional “overhead” requirements as previously mentioned. In the simplest situation – that of an analog video signal – the “overhead” is in the blanking period, that portion of the signal which is intentionally left free of active content, or of information corresponding to a part of the image itself. The requirement for such periods is imposed by the needs of the various imaging and display hardware technologies, which must have some idle time between each scanned line and frame or field in order to reset and prepare for the next. This can amount to 25% or more of the total signal time, especially when dealing with (or having to accommodate) CRT-based display systems. The blanking period almost always contains the signals which provide synchronization information to the display – those pulses or other signals which identify the start of a new line, field, or frame. It is common, then, to divide the overall blanking period into three sections, as shown in Figure 7-3. The period prior to the synchronization pulse is the “front porch,” a name resulting from the appearance of a typical video signal as displayed in standard television practice (in which it is presented at it appears at the cathode of a CRT, with the sync pulses positive-going). The remainder of the blanking period is then divided into the sync pulse itself and the “back porch” (which is any remaining blanking time following the end of the sync pulse). It is common in analog video signal and timing standards to use either the beginning of the blanking period or the beginning of the sync pulse itself as the reference point from which the rest of the line or frame timing is defined.

Table 7-1 An overview of common image and display formats.

Pixel format (H x V)	Name/description	Controlling standard/organization	Comments
176 x 144	Quarter-CIF; video teleconferencing and similar low-res video apps.	CCITT/ITU H.261	Viewfinders,
320 x 240	Quarter-VGA (“QVGA”)	PC industry	other low-resolution
352 x 288	“Common Image Format” (CIF); video teleconference standard	CCITT/ITU H.261
640 x 480	“VGA” (Video Graphics Adaptor) standard; “square pixel NTSC”	PC industry (VESA standards)
720 x 480	Standard format for digital 525/60 video; 13.5 MHz sampling1	CCIR-601	Non-square pixels
720 x 576	Standard format for digital 625/50 video; 13.5 MHz sampling1	CCIR-601	Non-square pixels
768 x 483	4fsc samplinga of 525/60 video	SMPTE 244M	Non-square pixels
768 x 576	“Square-pixel” 625/50 digital video
800 x 600	“Super VGA” (SVGA) PC standard	PC industry (VESA standards)
854 x 480	Widescreen (16:9) 480-line format	LCD/PDP TV displays	b
948 x 576	4fsc samplinga of 625/50 video		Non-square pixels
1024 x 576	Widescreen (16:9) 576-line format
1024 x 768	“Extended Graphics Adaptor” (XGA) PC standard	PC industry (VESA standards)
1152×864	Apple Computer 1 Mpixel standard	Apple Computers
1280 x 720	16:9 HDTV standard format	ATSC
1280 x 960	4:3 alternative to SXGA (below)	PC industry
1280 x 1024	“Super XGA” (SXGA) PC standard, originally used in workstations	Unix workstations	5:4 aspect ratio
1365 x 768	“Wide XGA”; 16:9, 768-line format	LCD/PDP TV displays	b
1440 x 1050	“SXGA+”; first seen in notebook PC LCD panels	PC industry
1600 x 1200	“Ultra XGA” (UXGA) PC standard	VESA
1920 x 1080	16:9 HDTV standard format	ATSC
1920 x 1200	16:10 widescreen PC displays	PC industry	c
2048 x 1152	16:9 European HDTV format	DVB-T
2048 x 1536	“Quad XGA” PC display format	VESA

^a For details on digital video sampling standards, see chapter 10.

^b Both of these formats exist in several versions, differing in the exact pixels/line count. For instance, the 854 x 480 format is also seen as 848 x 480, 852 x 480, and 856 x 480. (An exact 16:9 format would require 853.33 pixels/line.)

^c The PC industry has begun to standardize on 16:10 displays for “widescreen” applications, in contrast to the standard 16:9 formats of HDTV.

Table 7-2 Comparison of selected TV and computer display timing standards. Used by permission of VESA.

Format	V. rate (Hz)	Pixel rate (MHz)	H total (Hs)	H FP (us) HS (μβ)		H BP (μ3)	H rate (kHz)	V total (lines)	VFP (lines)	VS (lines)	VBP (lines)
525/60(1)" (orig.)	60.000	n/a	10.7	1.5	4.7	4.5	15.750	262.5	3	3	16
525/60(1)" (color)	59.94+	n/a	10.7	1.5	4.7	4.5	15.734	262.5	3	3	16
625/50(1)"	50.000	n/a	12.05	1.5	4.7	5.8	15.625	312.5	2.5	2.5	20
					pixels
640 x 480b	59.94+	25.175	800	16	96	48	31.469	525	10	2	43
	75.000	31.500	840	16	64	120	37.500	500	I	3	16
	85.008	36.000	832	56	56	80	43.269	509	I	3	25
720 x 480 (I)c	59.94+	13.500	858	16			15.734	262 263	3 3.5	3	16 16.5
720 x 576 (I)c	50.000	13.500	864	12			15.625	312 313	2.5	2.5	19 20
800 x 600b	60.317	40.000	1056	40	128	88	37.879	628	I	4	23
	75.000	49.500	1056	16	80	160	46.875	625	I	3	21
	85.061	56.250	1048	32	64	152	53.674	631	I	3	27
1024 x 768b	60.004	65.000	1344	24	136	160	48.363	806		6	29
	75.029	78.750	1312	16	96	176	60.023	800	I	3	28
	84.997	94.500	1376	48	96	208	68.677	808	I	3	36
1280 x 720“	60.000	74.250	1650	70	80	220	45.000	750		5	25
1280xl024b	60.020	108.000	1688	48	112	248	63.981	1066	I	3	38
	75.025	135.000	1688	16	144	248	79.976	1066	I	3	38
	85.024	157.500	1728	64	160	224	91.146	1072	I	3	44
1600xl200b	60.000	162.000	2160	64	192	304	75.000	1250	I	3	46
	75.000	202.500	2160	64	192	304	93.750	1250	I	3	46
	85.000	229.500	2160	64	192	304	106.25	1250	I	3	46
1920×1080“ (I)	60.000	74.250	2200	45	88	148	33.750	562.5	2 2.5	5	15.515

H total, total duration of one line, in microsecond or pixels; H FP, horizontal “front porch” duration; HS, horizontal sync pulse duration; HBP, horizontal “back porch” duration; V total, total duration of one field or frame, in lines; V FP, vertical “front porch” duration; VS, vertical sync pulse duration; V BP, vertical “back porch” duration. (I) denotes an interlaced timing. The vertical values given are for one field; some parameters may have two values given, one for each field. Note: Due to the nature of standard for interlaced television systems, the above values for the television-related standards should be used for comparison purposes only. The reader should consult the appropriate standards for detailed information in each case.

" Analog broadcast television standards; nominal values given, some derived.b Per current VESA standards.c Per CCIR-601 sampling standard. d HDTV nominal values derived from SMPTE-274M/296M, EIA-770.3-A.

Figure 7-3 Standard nomenclature for the portions of a video signal. In this example, the signal shown corresponds to one complete horizontal line of the image. In terms of defined signal amplitudes, three levels are key: the maximum amplitude the signal may normally attain (which is referred to as the white level), the blank level (which is typically the reference for all other amplitude definitions), and the sync level (the level of the commonly negative-going pulses that provide the synchronization information). (There may also be a fourth defined level, the black level – which is generally defined as the lowest signal amplitude that will be encountered during the active image time. If this is omitted, it may be assumed to be identical to the blank level.) In time, the line is divided into active and blanking periods; the latter is further divided into the front porch, the sync pulse itself, and the back porch. The division of the blanking period into these three sub-periods is commonly done, even for those systems which do not provide a sync pulse as part of the video signal (i.e., those with physically separate sync signals). The timing specifications relating to the vertical synchronization (the frame or field timing) uses the same nomenclature (active and blanking periods, front and back porch, etc.), but will normally be specified in terms of line times (e.g., “3H”).

In the case of digital video systems, there is often no need for an explicit blanking period, as the information will be placed in digital storage and/or further processed before being delivered to the display itself. However, many “digital” video systems and standards are based on the assumption that an analog signal will be “digitized” in order to create the digital data stream, and so include definitions of the blanking period, sync pulse position, etc., in terms of the sample or pixel period. Doing away with such things entirely, and treating the image transmission as if it were any other digital data communication, is generally not done except in such standards as also support packetization of the video data.

Algorithms for Timings – The VESA GTF Standard

To address the need for supporting an ever-growing array of formats and rates in the personal computer industry, the Video Electronics Standards Association introduced a new means of “standard” timing generation in 1996. This was the Generalized Timing Formula, or GTF, standard. GTF does not explicitly define timings, in the traditional sense, at all. Instead, this standard defined an algorithm and established a set of constants which could be used by PC systems – both in host computer firmware and software – and by the display manufacturers to generate timings for any arbitrary combination of image format and frame rate.

At the heart of the GTF system is a definition of a standard “blanking percentage” curve for CRT monitors. You may note, from the timing standards presented in the previous section, that CRT-based displays generally require more blanking time, as a percentage of the total horizontal period, as the horizontal or line rate is increased. At the low end of the standard PC timing range – around 30 kHz horizontal – a typical CRT display might require that 18-20% of the horizontal period be spent in blanking. As the horizontal rate increases, this percentage goes up to 25% and higher. The GTF curve assumes that, within the range of horizontal frequencies likely to be encountered in practice, no more than 30% of the line time will be required for blanking. The curve is therefore defined, using the original, default constants, as:

where fh is the horizontal frequency in kHz. This basic curve is shown as Figure 7-4, with some standard timings plotted on the same axes for comparison.

With a standard blanking-percentage curve defined, there is now a fixed relationship between the pixel clock and the horizontal frequency for any timing. Since the horizontal period is the inverse of the horizontal frequency, and the pixel clock may be defined as the number of pixels in the active (or addressable) video period divided by the duration of that period, the relationship – again using the default parameters as used in the above equation -becomes

 

 

or

so

where fclk is the pixel clock rate, fh is the horizontal or line rate, and Nh is the number of pixels in the active or addressable line time.

Figure 7-4 The default GTF blanking curve as discussed in the text. This sets the target blanking percentage for the horizontal timing parameters produced under the standard GTF algorithm. Several VESA-standard timings are plotted along with the curve, for comparison. (A) 640 x 480, 60 Hz; (B) 800 x 600, 60 Hz; (C) 800 x 600, 85 Hz; (D) 1024 x 768, 75 Hz; (E) 1280 x 1024, 60 Hz; (F) 1280 x 1024, 85 Hz; (G) 1600 x 1200, 75 Hz. More recent VESA timing standards have attempted to follow the GTF guidelines as closely as possible.

This fixed relationship is the key to GTF; with either the horizontal frequency or the pixel clock given, the other is also defined. By applying this relationship and a few other simple rules, GTF can then produce a “standard” timing for any format/rate combination.

The GTF algorithm proceeds as follows: first, it must be determined if the timing is to be driven by a requirement that the pixel clock, the horizontal frequency, or the vertical rate. In other words, the standard permits any one of these to be “locked down”, at which point the algorithms will set the others as needed to achieve the desired timing as closely as possible. The details of the algorithms are too complex to be covered here; the reader is directed to the VESA Generalized Timing Formula standard for these. However, it is important to note that only one of these three parameters may be selected as the “driving” factor, the one which will be preserved at all costs by the system (assuming that a timing is even possible with the chosen value). This limitation means that the GTF formulas will not be suitable as a replacement for explicit timing standards in all applications. If, for example, an exact frame rate must be achieved, and this must be done while still using the finite number of clocks which would be available from a frequency-synthesis clock generator (as described earlier), the GTF system may not be able to produce a viable timing.

Assuming that the constraints of GTF are acceptable, and the formulas have successfully been used to determine the pixel clock, horizontal frequency, and horizontal and vertical blanking requirements for the desired timing, the choice of the remaining parameters be comes a simple task. The GTF standard also established some very simple rules for the positioning and duration of the synchronization pulses within the blanking periods. The horizontal sync pulse is always made as close as possible (within the constraints of the hardware) to 8% of the total horizontal period in duration, and positioned such that the end (the deassertion) of the sync pulse be located as closely as possible to the center of the horizontal blanking period. The rules for the vertical sync pulse duration and position are even simpler – GTF vertical syncs are always 3 lines long, and follow a 1 line “front porch.” (In an effort to harmonize GTF timings with the more traditional “discrete” timing standards, VESA has recently been applying these same rules to all new timing standards developed by that organization, and further attempting to make the horizontal blanking times used in the those standards fall as closely as possible to the GTF default curve.)

The basic aim of GTF was a simple one. If a monitor declares itself to be “GTF compliant” in the ID information provided to the host system, the host may then generate timings for any given format and rate using this method, with the expectation that the image will still be properly sized and centered by the display. This will occur whenever the monitor detects a “GTF” timing being produced by the host (the sync polarities are used to identify such timings; “GTF” video will have the horizontal sync pulse negative-true, as opposed to the VESA standard for traditional timings of having both syncs positive). The monitor, upon seeing GTF operation flagged by the syncs, “knows” that the timing complies with the GTF defaults and therefore may itself derive the expected blanking times, sync positions, etc., and adjust size and centering accordingly.

The discussion so far has treated the GTF default blanking curve as the only possible choice, and the values used to produce it as mandatory constants. This is not the case, however; in a fully compliant GTF system, these values must be implemented as variables, and are calculated from a set of parameters which may be provided by the display. (If the display does not provide such an alternate, or “Secondary GTF” set, then the defined defaults are to be used. This feature of the GTF standard permits a very wide range of “standard” blanking curves to be defined, through mutual agreement between the host system and the display currently in use. This is intended to free the system from what may be excessive blanking requirements imposed by the default curve (which is admittedly very CRT-oriented, and conservatively so at that), or even to increase the blanking if need be. This “programmable” feature of GTF may only be used if both the host and display support it, and then only if a bidirectional communications channel has been established between the two.

At this point, we have covered the basic concepts of electronic imaging, the operation of the common display technologies, some of the fundamental practical requirements for display interfaces in general, and now the basic standards of image formats and rates used by these displays. The next three topics now focus on how the specific standards for several key markets and applications were developed, and the details of each. These include the analog video standards which have been developed for both broadcast television and the personal computer market, followed by the newer and still rapidly growing field of purely digital video interfaces and transmission systems.