		  The Dolby Reader Alignment Screen
		  --- ----- ------ --------- ------
The DRAS system is intended to be used as a tool by theater technicians for 
aligning and troubleshooting a Dolby Digital Reader (Cat. No. 699, Cat. No 700, 
or Basement Reader).  Measurements made by the DRAS system can be extremely 
useful in making adjustments to the reader.  This section of the manual will 
describe the information that the DRAS displays and will give advice as to 
how this information can be used to improve the reader alignment.

The Dolby Reader Alignment Screen, is displayed while the film is being analyzed 
in real time.


		  Error History
		  ----- -------
The Error History graph shows the Error rating for each second of film  The 
error rate numbers on the  vertical axis of the graph are arranged with the 
highest quality at the bottom and the lowest quality at the top.  This makes 
low quality ratings stand out.  One second sections of film with correctable 
block errors are seen as cyan bars, and unacceptable film sections stand out 
as tall red bars on the graph.  This window can be used to identify a region 
of the film where problems are occurring.  It can also be used to track 
improvements in reader alignment as they are being made.


		  Histogram Plot
		  --------- ----
The histogram plot is useful for determining focus, and the signal to noise 
ratio of the CCD video signal.  Ideally the digital section of the film would 
have perfectly clear (100% light transmission) pixels and perfectly opaque (0% 
light transmission) pixels to represent the 1's and 0's of the digital 
soundtrack.  But in reality, a Guassian-like distribution is seen for both the 
dark areas and the light areas.  The histogram shows these two distributions of 
levels across the light transmission range.  On the horizontal axis the light 
range is displayed, with the dark area on the left and the clear area on the 
right.  On the vertical axis the relative quantities of pixels are shown for 
a given light level.  

Special algorithms have been developed in the DFSP to deal with the small number
of pixels that fall in the "gray" area.  These algorithms determine a threshold 
value for the boundary between the clear and dark pixels.  Since the circuitry 
at the input to the DSFP has an AGC, absolute video amplitude cannot be 
determined without using an oscilloscope on the video test point.  However, if 
the histogram is degraded, with broad peaks, this can indicate insufficient 
video amplitude.  In addition, bad focus, incorrect magnification, and badly 
aligned azimuth will cause broadening of the peaks.

		  Histogram
		  ---------

The histogram also includes two lines that show the relative position of two of 
the factors used to determine the dark to clear boundary.  The shorter of the 
two lines shows the position of the determined threshold.  The taller line 
shows the halfway point between the peak values on the histogram.  As the 
distribution between light pixels and dark pixels becomes less symmetrical, the 
distance between the two lines increases, indicating asymmetrical fill-in or 
poor digital crossmod.  Some asymmetry in the histogram is expected and is 
almost always caused by the film, rather than a reader alignment problem.


		  Contour Plot
		  ------- ----

The contour plot shows an indication of the envelope of the data portion of the 
video display.  Crossweb nonuniformity of the illumination will show itself 
mainly as a variation of the top line.  While this gives an indication of light 
uniformity, it is better to use the oscilloscope when checking this parameter.

		  Corners Found 
		  ------- -----

Registration patterns are placed in the four corners of every Dolby Digital 
block.  They are used by the Digital Processor to locate block position.  The 
Corners Found window indicates the percentage of registration patterns that 
have been located by the Digital Processor.  On good film prints this number 
is typically between 97 and 99.  Loss of corners found can be caused by 
incorrect lateral alignment, incorrect magnification, bad focus, incorrect 
azimuth adjustment, or vertical jitter.  In general, corners must be found 
reliably before the system can function.


		  Length (feet)
		  ------ ------

Film length is shown in feet.  This window is updated every foot.


		  Reversion Count
		  --------- -----
The Digital Processor has been designed so that a burst of uncorrected errors 
will cause the Processor to switch from playing the digital soundtrack to 
playing the analog sound track on the film.  The Reversion Count window 
displays a running total of the instances where the processor has reverted from 
digital to analog playback.  A reversion will occur when there are many block 
errors close together.


		  Block Errors
		  ----- ------

The DFSP circuitry employs a multitude of ways to clean up data and correct 
errors from the data stream.  When the data on the film is not of good quality 
the data received by the DFSP may not be correctable. The Block Errors box 
displays a running count of those instances.  Block errors can be caused by 
vertical jitter, bad lateral position, or a variety of other factors.


		  Film Block Position Graphic
		  ---- ----- -------- -------

A graphic representation of the block position is displayed in the large box 
centered between the other quality windows.  This display is updated in real 
time and can be used to trouble shoot problems that show themselves as azimuth 
variations and flutter frequencies.


		  Error Rating
		  ----- ------
The Digital Processor employs an error correction circuit which can recover 
data lost due to a variety of conditions.  The Error Rating box indicates how 
hard the error correction circuit is working to produce perfectly correct data.
These results should correspond fairly closely with the LED display on the DA20.  
Low numbers indicate low error rate, higher numbers indicate higher error rate, 
mid range is typical.  Block errors will cause an F to be displayed.  Misaligned 
focus, azimuth, bad vertical jitter, and incorrect magnification will directly 
effect error rate, and cause a higher number.  A lateral position misalignment 
will not cause the error rating to rise, but will cause block errors, especially 
on films where the lateral position is not perfect.  In some cases, bad vertical 
jitter will also not effect the error rating, but will cause block errors.


		  Focus
		  -----

The Focus window indicates the sharpness of the transitions from dark 
(digital 1's) to clear (digital 0's) areas on the film.  This measurement is 
derived from the edge sharpening programs in the Digital Processor. Higher 
numbers represent good focus, and lower numbers indicate bad focus.  As usual, 
the number that is displayed depends highly on the quality of the film.  This 
window should be used to verify that the best focus is achieved by peaking the 
number rather than attempting to get some absolute good number.


		  Lateral Position
		  ------- --------

The edge of Dolby Digital block should be located laterally (crossweb or 
horizontally) 86 mils (2.19 mm) from the edge of the film.  The number reported 
in the Lateral Position window is the offset distance from ideal position.  
This number is reported in mils (0.001 inch).  A positive number indicates an 
offset toward the closest edge of the film.  A negative number indicates an 
offset toward the picture area of the film.  -1.5 mils to +1.5 mils from the 
calibrated film number (using the Cat. No. 530) are good  readings; from -3.0 
to -1.5 and 1.5 to 3.0 from the calibrated film number are acceptable; readings 
in excess of 3.0 or below -3.0 from the calibrated number are unacceptable.  
This number is typically very stable (under 1 mil of movement) over a few 
seconds but may drift over longer periods.


		  Azimuth
		  -------

The number reported in the Azimuth window is the average azimuth over a one 
second interval.  This number is reported in pixels.  One pixel azimuth 
represents about 0.3 degrees.  The azimuth should be less than 3.  There may 
be some variation in azimuth from one film to another, but generally most of 
the azimuth error will be caused by the reader.


		  Track Width
		  ----- -----
The number reported in the Track Width window shows the percentage of the 
magnification.  If the magnification is adjusted properly, the number should 
read 100%.  If the magnification is too low, it will read lower, and if the 
magnification is too high, it will read higher.  The track width should be set 
to 100%, with a range of 97% to 103%.  On Cat. No. 700s and 699s, track width 
is set at the factory and is not adjustable.


		  Vertical Jitter
		  -------- ------

The number reported in the Vertical Jitter window is the peak distance of the 
block from its nominal position between two adjacent sprocket holes over a one 
second interval.  The number is reported in mils.  Observing the number over a 
few seconds gives the user an indication of any flutter in the reader.  
Typically the variation over a few seconds is seen to be about 1.0 mils. 
