GENLOCK - Theory GENLOCK (General Lock).

<b>GENLOCK</b> - Theory GENLOCK (General Lock).

$ THEOREM I genlock $
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   Genlock - reduction of generated lock (make a seizure)
way to synchronize the device that displays the image of
external reference sinhroili video. In the world
Amiga computers, the term genlock understand
specialized device to capture, synchronize, and
overlay computer graphics on the Amiga computer issued
external video signal. For a more complete understanding of the 
principles Work genlock, we present the basic provisions of 
television. As known image in the analog video consists of 625 
to PAL / SECAM (525 for NTSC) lines and consists of 2 fields of 
312.5 (262.5) lines. 1 second passed 25 (30) or 50 frames

(60) fields. Band occupied by the video signal is 6 MHz.
For the color use the principle of addition of basic
colors: red (R), green (G) and blue (B). A similar method
used in monitors and graphics cards. However, the use of 3
Component component to transfer color TV signal
presents significant limitations. The main of them is
broad band occupied by such a video signal and the 
incompatibility with conventional B & W TV. To narrow occupied 
bandwidth and to achieve compatibility with BW TV signals, used

Some features of the image of men
eye. It was found that the human eye is more sensitive to
brightness component image than to its color. (This
wonder who is familiar with the anatomy of the human eye know
that the number of cones, sensitive to brightness, is much
greater than the number of sticks that are sensitive to color
component of the image).


   Using this feature of human vision, was
developed a system of transferring the color chroma
image. Color TV signal consists of luminance (Y) and two
chroma (RY and BY) components. Plain Black & White TV Receiver
perceives only the Y component and transmits the image in
black and white form. In color TV receiver used
special schemes of summation: the three components (Y, RY and
BY) restores the original components of red (R), blue
(B) and green (G) colors, which are submitted to the relevant
CRT electron guns.


   To narrow band of frequencies occupied by the color TV signal
RY and BY signals are transmitted using the frequency band of 2
times already. Bandwidth chroma signals with narrow
Using Filters and applied modulation schemes that allow
transmit two chroma signal in a common area
spectrum. Color difference components are transmitted, so that 
their spectrum is located in the high-frequency signal spectrum

brightness.


   At the expense of additional tweaks to the subcarrier 
frequency, Chroma-modulated signals (PAL or NTSC only)

can even achieve that brightness information is transmitted
in one of multiple narrow spectral regions, and information 
about color - in others. In the end it turns out that the 
spectral width color video when broadcasting is not wider than 
the black and white. 


   The narrowing of the band reduces the resolving power of
color 2 times, but given the peculiarities of the eye, almost
unnoticed. In a digital TV made a partition of one TV frame on
pixels, which is most convenient for further digital processing.
Each TV frame is divided into 768 pixels horizontally and 576
points along the vertical signals PAL / SECAM (640 x 480 for 
NTSC) the standard TV ratio of width to height of the image

equal to 4:3. It should be noted that the High-Definition TV
(HD or HDTV) adopted 16:9, closer to the size of
movie screen. Thus, we have two half frame (field)
resolution of 384x288 (320x240 for NTSC) at a frequency of 50 
(60) fields or 25 (30) frames per second.



   It is believed that full information on the perception of 
brightness and color to each point of the image to transmit at 
least 256 levels or 8-bit encryption. Each point is colored

Video must be encoded 24 bit word (8 bits
X 3 components) or on computer terminology 24
bits per pixel (24bpp). For the film uses the bit
64 bits per pixel.


   Video controller in the Amiga is constructed in such a way 
that it well dock with the above video formats.

Each pixel is encoded with 24 bits, and each pixel
corresponds to one bit - genlock, "saying," where does
the current pixel: from the Register of the background (B0) or 
from any another. The format of the displayed image: 736H566 
(overscan) with interlaced TV (interlace) scans

images (two fields 736H283 pixel).


   In the Amiga computers have a special video connector, 
through the which analog signals R, G and B, as well as sync,

served on the external display device (TV, monitor, genlock
etc.). In addition, this connector signal is present PXLSW
(Pixel switch), is associated with a bit of genlock, and 
contact for XCLK supply external clock frequency (28 MHz) 
synchronized with video. Contact XCLKEN thus should be shorted 
to common.



   In addition to output the mixed clock CSYNC, on
This connector contains two separate signal synchronization
HSYNC and VSYNC (used to feed the "normal" mode for
synchronize the image on the monitor), which in genlock mode
become inputs. They are fed sync allocated
from the video output to synchronize the computer
image.


   At the initial stage of load control computer program
Amiga (kickstart) detects the presence of vertical sync on
contact HSYNC and, if they exist, the computer is put
genlock. Input video signal is decoded before the signals R, G, 
B and served on high-speed switch, which just

control signal PXLSW. The second input switch,
naturally fed RGB signals from a computer Amiga. So
way to synchronize and the imposition of a computer
graphics on the video. Output Signal Switchers
fed to an encoder of the image on which to better
color stability may send a signal to the color subcarrier
video connectors Computer Amiga - C1 (3,58 MHz). Generator of 
foreign Clock (pixel) of the frequency is synchronized with 
lowercase sync pulse provided from the video separatorm

clock, with the help of a phase-locked loop
Frequency (PLL).


   The figure does not show some elements of the switching and
management, as well as the formation of visual effects.
Typical visual effects:


   o alpha channel (the "transparency" of computer images
     line)

   o chroma-key, the effect at which the video area

     certain color is replaced by a computer image.

     Very popular with blue-box keying - Prokeivanie the blue

     color.

   o luma-key, the effect at which the video area

     certain brightness is replaced by a computerized image

   o invert ("keyhole")

   o Color correction video

   o negative

   o Mono

   o various "blind" (wipes)

   o Burn (fader), etc.


   It should be noted that for some of the effects of the scheme
genlock can get much more complicated, leading to a marked
more expensive device.


   There is another way of blending computer
image. The schematic for the timing of output
Graphics remains the same. Differences - in
constructing a path of processing the video signal. Computer
image is first encoded, and then fed to
switcher, a second input video signal is fed.
For the quality of synchronization and the overlay
necessary to satisfy the following conditions:


   o synchrony at the level of training

   o synchronous row-level

   o Provide the same phase of the color subcarrier (in 
composition      tnom PAL)


   o the same level of "black"


   The advantage of this way - the absence of decoding and
video coding and, consequently, better quality.
Drawback - the inability to implement a simple visual effects 
with video (eg, color correction, alpha channel, chrome-Kay and

etc.). Of course, you can combine both methods to eliminate
"Weak" sides, but the device gets even more complicated.


   Genlock can be implemented in practice by
analog or digital signal processing. Digital gives
undeniable advantages. For example, since the output
decoded signal is represented in digital form, can be
"Write" it in a special frame buffer, which was built on
the principle of FIFO (first-in - first out) and allows
asynchronous write and read information from it. With
controlling microprocessor can automate
parameter control and video switching. Encode
You can do this on any other system other than the input
signal. In this case, it becomes possible to implement the 
following Device:



   o synchronous switch

   o Digital Transcoder

   o corrector time-distortion device, allowing the value
     significantly improve the quality of the video

   o video effects generator

   o Generator logos and essential savers, not depending on to
     mainframe


   In addition, the digital circuitry significantly reduces the 
number of components, increasing reliability, increasing the 
stability and simplifies the device configuration. The truth 
has to pay complexity of development and appreciation of such 
devices, translating development of the plane with a soldering 
iron into the plane writing and debugging programs for 
specialized microprocessors and LSI.



   All of the above devices can be operated as a front
panel, and from your computer. Also require some elements of
indication of operating modes and state regulations. Logically, 
these functions entrusted to a single-chip microprocessor 
control, which provides a general algorithm of the device.



   (C) AMIGA SERVICE 1999


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