measurement Method for m e a s u r i n g jitter in VDUs STURE ERIKSSON
A method for n m u u r k ~ jitter in visual display units (VDUs) is p ~ . The method is b a s e d ~ t a m fact that jittor is ~ b y a variation in pulse amplitudm w h e n the eloctrcm b m m travorsm in paths which a r e displaced in rulaflon to m c h other. The obtained data dmnonstmto that froquemcy ~ of jitter are of the white noise type. The pulse smpliatdo p r o p e r t t m oa the other hand a p p e a r to provide a promising basis for measuring jitter in VDUa.
Eeywords: j~ter, VDUs, pulse amplitudes, white noise
Several organizations of standardization have formulated the requirement that in order to achieve a good image quality, a visual display unit should be free from perceptible jitter. The term jitter stands for small movements of the picture elements and the requirement implies that the range of the motions should be less than 0.0001 times the viewing distance. This figure corresponds to a visual angle of 20 s of arc and it relects data for motion perception thresholds as shown by Stevens 1. However, no accepted method for the measurement of jitter has been established although equipment for such meashrements is available, e.g. standard equipment for light measurements and signal analysis as well as dedicated systems (the Microspot Device). The present paper presents a method for jitter measurements based on standard laboratory equipment.
cycles/nun), 0 -- the phase angle (in rad), and M ffi mean luminance (M = (Lmx + / . ~ ) / 2 ) . If there were no jitter in the VDU, then luminance would be constant in a given part of the raster. On the other hand, if jitter is present, then the raster would move back and forth and a luminance change would be registered by a microphotometer. These facts constitute the basis for the present method.
If the raster is moved a certain distance, say 8x, a change in luminance will most effectively be registered by a microphotometer with a small measuring area positioned over the inflexion point (m). The luminance variation is dependent on the sLze of the lateral displacement of the raster. This displacement is a function of the phase
The raster in a display with positive polarity (dark symbols on a light background) consists of lines with a luminance distribution which essentially is sinusoidal, see Figure 1. "q
The change of luminance over the VDU surface perpendicular to the raster lines is described by LffiAsin (wx + O) + M
~- } M
/ / /
where L = luminance, A -- the amplitude of the modulation, w ffi 2gf (f = spatial frequency, expressed in POSITION
Unit for Computer Ergonomics, Department of Psychology, Box 1854, S--751 48 Uppsala, Sweden DISPLAYS, OCTOBER 1989
Figure 1. Sinusoidal raster in three different positions reflecting fitter around a mean position
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fflerafgn'mntmt angle (0), and for a given spatial frequency, a given amplitude, and a given mean luminance, the luminance for an optional point (x) can be computed. However, in order to measure jitter, defined in units of position (e.g. mm on the screen), the reverse computation is needed. Hence, jitter (Sx) is defined by the difference in phase angles for the raster in two extreme positions, see Figure 1. This difference is accompanied by a luminance difference (SL) defined by 8L = ,4sin (wx + 0/2) - Asin(wx- 0/2)
If the coordinate system is chosen in such a way that the measurement is made for x = 0, then Equation (2) reduces to
Figure 2. Sample of pulses obtained from the middle of a bright line
Hence, it follows that
0 = 2arcsin(SL/2A)
Once the 0-value has been computed according to Equation (4), the jitter range (Sx), expressed in mm on the screen, is computed according to
8x = 0/(2,tJ)
In order to apply the above model to jitter in VDUs, it should be noted that a fast photodetector is needed to investigate frequency and amplitude characteristics of the jitter. Consequently, one has to measure the pulse which represents the motion of the electron beam over the small measuring area. The amplitude of this pulse (measured in volts) is lawfully related to the luminance value (measured in cd/m2). In the present study, a VDU was used in which pulse amplitude was linearly related to the candela values. The conditions for the application of the model (with luminance terms replaced by intensity terms in volts) are described below.
METHOD In a first experiment, jitter was measured in a VDU using a Pritchard Spectra Microphotometer. The procedure was as follows: 1. In order to position the small measuring area (0.02 mm in diameter) over the inflexion point, Lmaxand Lmin were registered (in candelas per square metre). Then the mean (M) was computed and the microphotometer was moved in small steps until it read M. 2. Then the video output of the photometer was used to measure the pulse amplitudes (in volts). The signal was sent to an Ortec signal analyser equipped with a 208
Figure 3. Sequential peak dewct graph for pulses measured in the middle of a bright line memory module (mod. 4 620 A) and an amplitude histogram analyser (mod. 4 622 A). The analyser provides two modes of data analysis:
Sequential peak detect mode. In this mode, the analyser registers the peaks of successive pulses and displays the amplitudes in volts on the y-axis. The x-axis shows time with an interval between data points equal to the inverse of refresh rate (in the present case the time interval was 14.1 ms). A total of 225 pulses were measured in each registration. First the measuring area was positioned at the middle of a bright line. A sample of the pulse registrations is shown in Figures 2, and the sequential peak graph is shown in Figure 3. DISPLAYS,OCTOBER 1989
Figure 4. Sample of pulses from inflexion point measurements
Figure 6. Results displayed in a peak detect histogram
Figure 6 shows two registrations. The distribution to the right in Figure 6 was obtained with the measuring area over the middle of a bright line. The maxium value (the right flank of this distribution gives Imax, i.e. the pulse amplitude corresponding to the maximum luminance (Lmax)). The left distribution in Figure 6 was obtained when the measuring area was positioned over the inflexion point. This distribution gives two important values, namely the mean pulse amplitude corresponding to the inflexion point (M) and the jitter range expressed in volts.
APPLICATION Figure 5. Sequential peak detect graph for inflexion point measurements
The results show that pulse amplitudes essentially are constant under this condition. Next the measuring area was positioned over the inflexion point. A sample of pulse registration is depicted in Figure 4, and the sequential peak graph is shown in Figure 5. As seen from these figures there is a considerable variation in pulse amplitudes over time. Hence, some jitter is present in this VDU. However, no frequency components are discernible in the registration.
Histogram peak detect mode. In this mode, the analyser displays the data with number of pulses on the y-axis, and pulse amplitudes (measured in volts) on the x-axis. DISPLAYS, OCTOBER 1989
To illustrate the method, the data obtained above were used to derive a measure of jitter in the VDU. Since the photometer delivers the signal expressed in volts, the luminance changes were replaced by intensity values in volts. Equations (4) and (5) were combined to yield a single expression of jitter
8x = arcsin(81/2A)/(xJ)
In Equation (6), (Sx) stands for the total range of jitter (in mm on the VDU screen), 81 is the corresponding intensity change in volts for the two extreme positions, A denotes ampfitude in volts and f is a measure of the spatial frequency of the raster. In order to arrive at an estimation of the jitter in the VDU, the following procedure was used: • The value of 8/is read directly from the peak amplitude histogram when the measuring area is positioned over the inflexion point (mean position), see Figure 6, left distribution. Note that the distribution is symmetrical and esssentially Gaussian. The obtained 81value was 1.2 V. 209
• The value o f / I x (i.e. the pulse height in volts for the bright line) is read from the peak detect amplitude histogram (Figure 6, right distribution). The obtained Imuvalue is 4.6 V. ~ • The value of M is obtained from the peak amplitude histogram (Figure 6, left distribution). M = 2.5 V. • A=I,_-M=2.1V. • The spatial frequency of the raster was measured via a measuring microscope. The f-value was 2.5 cycles/ mitt.
When the above values were inserted into Equation (6), we obtained 8x = a r c ~ ( l.2/4.2)/(z2.5) = 0.04
The ISO-recommendation implies that for a viewing distance of 50 cm, jitter could not exceed 0.05 ram. The present VDU, which thus has a jitter of 0.04 mm~ exhibits no perceptible jitter.
DISCUSSION A new method for measuring jitter in VDUs has been proposed. The method allows an objective determination of both frequency and amplitude characteristics of jitter. Consequently, it now is possible to perform psychophysical experiments in which perceived jitter can be related
to objective properties of VDU jitter. The recommendations mentioned in the introduction is based on data concernin~ motion perception thresholds. However, it is doubtful whether these data apply to the peculiar kind of motion stimulation which jitter presents to the eye (amplitude variation but no discernable frequency charactemtics). Obviously, new experiments are needed in order to f'md out whether VDU jitter thresholds are comparable to traditional motion perception thresholds. In such experiments, it may be necessary to include a correction for a possible nonlinear transformation from luminance measures (candelas) to pulse heights in volts. In the VDU under consideration, the relation essentially is linear over the range of interest, but this relationship has to be verified in the particular VDU under test. ACKNOWV.g~r~GEMENTS
This study has been supported by grants from the Swedish Work Environment Fund. The author wishes to express his thanks to Research Engineer Lars B~ickstr6m, Dr Mikael Franzon and Research Engineer Kurt Vikman for their valuable comments on this paper.
REI~RI~CES 1 Stevem, S Handbook of experimentalpsychology, John Wiley & Sons, Inc., NY, USA (1951)
DISPLAYS, OCTOBER 1989