To refer to fields as odd and even invites confusion. Use first field and second field instead. Some people refer to scanning first the odd lines then the even; however, scan lines in interlaced video were historically numbered in temporal order, not spatial order: Scan lines are not numbered as if they were rows in the frame’s image matrix. Confusion on this point among computer engineers – and confusion regarding top and bottom fields – has led to lots of improperly encoded video where the top and bottom offsets are wrong.

Figure 8.5 Interlaced format represents a complete picture – the frame – from two fields, each containing half of the total number of image rows. The second field is delayed by half the frame time from the first. This example shows 10 image rows. In analog scanning, interlace is effected by having an odd number of total scan lines (e.,g., 525, 625, or 1125).

power across each pixel – the pixel’s spot profile, or more technically, point spread function (PSF). If the spot profile is such that there is a significant gap between the intensity distributions of adjacent image rows (scan lines), then image structure will be visible to viewers closer than a certain distance. The gap between scan lines is a function of image row (scan-line) pitch and spot profile. Spot size was historically characterized by spot diameter at 50% power. For a given image row pitch, a smaller spot size will force viewers to be more distant from the display if scan lines are to be rendered invisible.

Interlaced format

Interlacing is a scheme which – for given viewing distance, flicker sensitivity, and data rate – offered some increase in static spatial resolution over progressive scanning in historical CRT displays, which exhibited flicker. The full height of the image is scanned leaving gaps in the vertical direction. Then, 1⁄50 or 1⁄60 s later, the full image height is scanned again, but offset vertically so as to fill in the gaps. A frame thereby comprises two fields, denoted first and second. The scanning mechanism is depicted in Figure 8.5. Historically, the same scanning standard was used across an entire television system, so interlace was used not only for display but for the whole chain, including acquisition, recording, processing, distribution, and transmission.

Noninterlaced (progressive) scanning is universal in desktop computers and in computing; also, progressive scanning has been introduced for digital television and

Figure 8.6 Twitter would result if this scene were scanned at the indicated line pitch by

a camera without vertical filtering, then displayed using interlace on a short duty cycle display such as a CRT.

HD. However, the interlace technique remains universal in SD, and is widely used in broadcast HD. Interlace-to- progressive (I-P) conversion, also called deinterlacing, is an unfortunate but necessary by-product of interlaced scanning.

CRTs are now obsolete. The dominant display technologies now used for video – LCD and plasma panels – have relatively long duty cycles, and they don’t flicker. The raison d’être for interlace has vanished. Nonetheless, interlace remains in wide use.

Twitter

The flicker susceptibility of vision stems from a widearea effect: In a display such as a CRT that flashes, as long as the complete height of the picture is flashed sufficiently rapidly to overcome flicker, small-scale picture detail, such as that in the alternate lines, can be transmitted at a lower rate. With progressive scanning, scan-line visibility limits the reduction of spot size. With interlaced scanning, this constraint is relaxed by a factor of two. However, interlace introduced a new constraint, that of twitter.

If an image has vertical detail at a scale comparable to the image row pitch – for example, if the fine pattern of horizontal line pairs in Figure 8.6 is scanned – then interlaced display causes the content of the first and the second fields to differ markedly. At usual field rates – 50 or 60 Hz – this causes twitter, a small-scale phenomenon that is perceived as a scintillation, or an extremely rapid up-and-down motion. If such image information occupies a large area, then flicker is perceived instead of twitter. Twitter is sometimes called interline flicker; however, flicker is by definition a wide-area effect, so interline flicker is a poor term.

Twitter is produced not only from degenerate images such as the fine black-and-white lines of Figure 8.6, but also from high-contrast vertical detail in ordinary images. High-quality video cameras include optical spatial lowpass filters to attenuate vertical detail that would otherwise be liable to produce twitter. When computer-generated imagery (CGI) is interlaced, vertical detail must be filtered in order to avoid flicker. Signal processing to accomplish this is called a twitter filter.

Figure 8.7 Horizontal and vertical drive pulses historically effected interlace in analog scanning. 0V denotes the start of each field. The halfline offset of the second 0V causes interlace. Here, 576i scanning is shown.

HD

1

VD

0

FIRST FIELD

V (FRAME)

…

SECOND FIELD 0V

625

Details are presented in Chapter 2,

Analog SD sync, genlock, and interface, in Composite NTSC and PAL: Legacy Video Systems, and in the first edition of the present book.

We’ll take up resolution in interlaced systems on Interlace revisited, on page 105.

Interlace in analog systems

In analog video, interlace was historically achieved by scanning vertically at a constant rate, typically 50 or 60 Hz, and scanning horizontally at an odd multiple of

half that rate. In SD in North America and Japan, the field rate is 59.94 Hz; the line rate (fH) is 525⁄2 (2621⁄2)

times that rate. In Asia, Australia, and Europe, the field rate is 50 Hz; the line rate is 625⁄2 (3121⁄2) times that

rate.

Figure 8.7 shows the horizontal drive (HD) and vertical drive (VD) pulse signals that were once distributed in the studio to cause interlaced scanning in analog equipment. These signals have been superseded by a combined sync (or composite sync) signal; vertical scanning is triggered by broad pulses having total duration of 21⁄2 or 3 lines. Sync is usually imposed onto the video signal, to avoid separate distribution circuits. Analog sync is coded at a level “blacker than black.”

Interlace and progressive

For a given viewing distance, sharpness is improved as spot size becomes smaller. However, if spot size is reduced beyond a certain point, depending upon the spot profile of the display, either scan lines or pixels will become visible, or aliasing will intrude. In principle, improvements in bandwidth or spot profile reduce potential viewing distance, enabling a wider picture angle. However, because consumers form expectations about viewing distance, we assume a constant viewing distance and say that resolution is improved instead.

A rough conceptual comparison of progressive and interlaced scanning is presented in Figure 8.8 at the top of the facing page. At first glance, an interlaced system offers twice the number of pixels – loosely, twice the

Progressive

Interlaced

rowImage 0

1 2 3

rowImage 0

1 2 1

2 numberLine field)(first

0 1 2

0 1 2 3

3

4 numberLine

(secondfield)