7.5 Structure of HD Component Digital

7.5 Structure of HD Component Digital

Given the large number of HD scanning standards, it is only possible to outline the common principles here. Specific standards will differ in line and sample counts. Those who are accustomed to analog SD will note that in HD the analog sync pulses are different. In HD, the picture quality is more sensitive to horizontal scanning jitter and so the signal-to-noise ratio of the analog sync edge is improved by doubling the amplitude. Thus the sync edge starts at the most negative part of the waveform, but continues rising until it is as far above blanking as it was below. As a result 50% of sync, the level at which slicing of the sync pulse is defined to take place, is actually at blanking level. All other voltages and gamuts remain the same as for SD.

The treatment of SD formats introduced the concept of the digital active line being longer than the analog line. Some HD formats have formalized this by describing the total active pixel array as the production aperture, and the slightly smaller area within that, corresponding to the unblanked area of the analog format, as the clean aperture. The quantizing standards of HD are the same as for SD, except that the option of 12-bit resolution is added.

SMPTE 274M 6 describes 1125 lines per frame 16:9 aspect ratio HD standards having a production aperture of 1920 — 1080 pixels and a clean aperture of 1888 — 1062 pixels. The standard uses square pixels, thus 1080 — 16 = 1920 — 9. Both interlaced and progressive scanning are supported, at a wide variety of frame rates: basically 24, 25, 30, 50 and 60 Hz with the option of incorporating the reduction in frequency of 0.1% for synchronization to the traditional NTSC timing.

As with SD, the sampling clock is line locked. However, there are some significant differences between the SD and HD approaches. In SD, a common sampling rate is used for both line standards. This allows both a common interface data rate and an interface that works in real time, but results in pixels that are not square. In HD, the pixels are square and this causes the video sampling rate to change with the frame rate. In order to keep the interface bit rate constant, variable amounts of packing are placed between the active lines but the result is that the interface no longer works in real time at all frame rates and requires buffering at source and destination. The interface symbol rate has been chosen to be a common multiple of 24, 25 and 30 times 1125 Hz so that there can always be an integer number of interface symbol periods in a line period.

For example, if used at 30 Hz frame rate interlaced, there would be 1125 — 30 = 33750 lines per second. Figure 7.9(a) shows that the luma sampling rate is 74.25 MHz and there are 2200 cycles of this clock in one line period. From these, 1920 cycles correspond to the active line and 280 remain for blanking and TRS. The colour difference sampling rate is one half that of luma at 37.125 MHz and 960 cycles correspond to the active line. As there are two colour difference signals, when multiplexed together the symbol rate will be 74.25 + 37.125 + 37.125 = 148.5 MHz. The standard erroneously calls this the interface sampling rate, which is not a sampling rate at all, but a word rate or symbol rate.

image from book
Figure 7.9: In HD interfaces it is the data rate that is standardized, not the sampling rate. At (a) an 1125/30 picture requires a luma sampling rate of 74.25 MHz to have 1920 square pixels per active line. The data rate of the chroma is the same, thus the interface symbol rate is 148.5 MHz. At (b) with 25 Hz pictures, the symbol rate does not change. Instead the blanking area is extended so the data rate is maintained by sending more blanking. At (c) an extension of this process allows 24 Hz material to be sent.

Thus the parallel interface has a clock rate of 148.5 MHz. When ten-bit symbols are serialized, the bit rate becomes 1.485 GHz, the bit rate of serial HD. If the option of adhering to the picture rate reduction of 0.1% is taken, all of the above frequencies fall by that amount.

If the frame rate is reduced to 25 Hz, as in (b), the line rate falls to 1125 — 25 = 28 125 Hz and the luma sampling rate falls to 2200 — 28 125 = 61.875 MHz. The interface symbol rate does not change, but remains at 148.5 MHz. In order to carry 50 Hz pictures, time compression is used. At 28 125 lines per second, there will be 2640 cycles of 74.25 MHz, the luma interface rate, per line, rather than the 2200 cycles obtained at 60 Hz. Thus the line still contains 1920 active luma samples, but for transmission, the number of blanking/TRS cycles has been increased to 720.

Although the luma is sampled at 61.875 MHz, for transmission luma samples are placed in a buffer and read out at 74.25 MHz. This means that the active line is sent in less than an active line period.

Figure 7.9(c) shows that a similar approach is taken with 24Hz material in which the number of blanking cycles is further increased.

The 1.485 GHz rate is adequate for interlaced video and for progressively scanned film, in which the frame rate is only 24 or 25 Hz. However, for progressively scanned video, the frame rate may be as high as 60 Hz and this would require the bit rate to be doubled . However, it is becoming increasingly known that because progressive scan eliminates interlace artefacts , it does not need twice the data rate of interlaced systems to give the same perceived quality. Resolution falls dramatically in the presence of even quite slow motion in interlaced video, whereas in progressively scanned video it does not 2 . Thus on real moving pictures, progressively scanned systems with relatively modest static resolution give better performance because that resolution is maintained. Consequently the 50 and 60 Hz 1920 — 1080 progressive standards are quite unnecessary for television purposes.

SMPTE 296M describes the 720P standard 3 that gives the best results of all of the television industry standards, although not as good as the progressive standard developed by the US military which has a higher frame rate.

720P uses frames containing 750 lines of which 30 correspond to the vertical interval. Note that as interlace is not used, the number of lines per frame does not need to be odd. 720P has square pixels and so must have 720 — 16/9 = 1280 pixels per line. The production aperture is thus 1280 — 720 pixels. A clean aperture is not defined.

The 1280 — 720 frame can be repeated at 60, 50, 30, 25 and 24Hz. The same interface symbol rate as 274M is used, so clearly this must also be a common multiple of 24, 25, 30, 50 and 60 times 750 Hz.

Figure 7.10(a) shows that 720/60 has a line rate of 45 kHz and has 1650 sample periods per line, corresponding to a luma sampling rate of 74.25 MHz. The colour difference sampling rate is half of that, but as there are two colour difference signals, the overall symbol rate becomes 148.5 MHz and so this format transmits in real time. As the frame rate goes down, the number of interface symbol periods per line will rise, but the number of pixels remains constant at 1280. As a result the number of blanked symbols rises, as does the degree of time compression of the transmission of each line. This is shown in the remainder of Figure 7.10.

image from book
Figure 7.10: In progressively scanned images, the level of artefacts is much lower than interlaced systems allowing the quality to be maintained with fewer picture lines. At (a) the structure of 720P/60 is shown to have the same data rate as 1080I/30. Lower frame rates are supported by blanking extension as shown in (b).


Digital Interface Handbook
Digital Interface Handbook, Third Edition
ISBN: 0240519094
EAN: 2147483647
Year: 2004
Pages: 120

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