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TECH CORNER

Fall 2003 ExtroNews 14.3 19

throughout the active vertical time frame for

the new output rate. The digital video infor-

mation is then converted back to analog and

output as RGB to the display.

The sync signals are sampled and reclocked

as well, but it is the analog portion of the

video that will be used when resizing and po-

sitioning the signal within the scaler.

Now the video has been scaled to what

should be the correct format for the projector,

and what should be the correct size and

position. However, a 1024 x 768 image

that is perfectly sized on one display may

appear too large or too small on another,

or in the wrong place.

If an image appears too wide (Figure 3-1),

the lines of video can be altered to clock the

pixels into a shorter line of active video, to

correct the image size (Figure 3-2). If the im-

age is too narrow, the active video time can

be increased. If the image is shifted too far to

the left or right, the start of the active video

timing can be delayed or started earlier rela-

tive to the H sync, to shift the image to the

right or left, respectively.

A similar process takes place when making

adjustments in vertical size or position. Size is

determined by the number of active lines

output by the scaler, and position by when

those lines start relative to the vertical sync

pulse. Increasing or decreasing the number of

lines that hold the active video information

that was originally sampled changes the

size of the image (Figure 4). Increasing or

decreasing the time between the start of

those lines and the vertical sync pulse moves

the image up or down.

It is important to note that features have

limitations. At some point when increasing

the size of an image, the active video time

frame will reach its maximum, and increasing

the size further will result in cropping the

edges of the image. Similarly, if an image is

shifted too far left or right, up or down, the

edges of the image will fall out of the active

portion of the video and vanish from the

screen, one line or pixel at a time.

A good scaler will allow the image to be

adjusted so the image ﬁlls the raster of the

display, thus getting the full potential out of

the display device. However, the size adjust-

ment may not be ﬁnished; the clock adjust-

ment must be considered.

Clock

Adjusting the clock of a scaler is often done

without much consideration for, well, adjust-

ing the clock. This is not necessarily bad, and

it occurs for a number of reasons. The most

pertinent is that in many scalers the clock ad-

justment is made by adjusting the horizontal

size of an image.

The clock adjustment of a scaler adjusts

the number of samples taken across a line

of active video input to the scaler. Sampling

is an important concept in analog to digital

conversion and is the subject of many theo-

rems, rules, and principals which serve to

demonstrate, amongst other things, that

when sampling a video signal, it is best to

sample at a rate of, at least, one sample per

incoming pixel.

Optimizing Your Image with Auto-Image (continued)

H-Line

of Video

H-Sync

Figure 3-2: Correct image size

1024 pixels with correct horizontal time frame

Figure 3-1: Image too wide

1024 pixels with too wide horizontal time frame

Pixels in

shorter line

Pixels in

too wide a line

continued on page 20

RGBHV

1024 x 768

Y, Pr, Pb

NTSC

Extron ISS 408

Integration

Seamless

Switcher

DVD

A/D

Sampling

D/A

Scaling

Reclocking

Figure 2: Transmission of an NTSC signal.

Figure 3: How a scaler corrects horizontal image size.

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