Samsung BX2240-LED

Specifications

Brand:	Samsung
Model:	SyncMaster BX2240LED
Size:	21.5"
Resolution:	1920x1080
Panel type:	TN
Max. refresh rate:	60/75 Hz
LightBoost:	no
Backlight type:	LED (white, edge)
LED driver:	OZ9998GN (o2micro)
TachistoMode:	(yes) no
Price (approx.):	USD 170
Monitor release date:	2010-09
This review's date:	2012-12
Remarks:	Devices manufactured after 10/2011 use an LED driver which is not suitable for tachistoscope applications.

External backlight control

Devices manufactured before 10/2011 (or even earlier) used the LED driver MSL3165 (Msilica, meanwhile Atmel), which was extremely well suited for tachistoscope applications. Thereafter, Samsung switched to the LED driver OZ9998GN (o2micro), which falls asleep when the backlight is switched off for longer than 80 ms and performs miserably when waking up (see below).

In Sperdin et al. (2013) it was mentioned that the driver falls asleep already after 40 ms of inactivity, which is wrong (measurements have been mixed up those of the BenQ XL2410T). The difference between 40 ms and 80 ms is crucial though as 80 ms is long enough for working around the problem by using wake-up pulses. Meanwhile, better monitors are available for the tachistoscope setup anyway. Even though these monitors might require a wake-up pulse workaround as well, their wake-up timing is not as crucial because the LED drivers fall asleep more slowly and wake-up more rapidly.

Since the publication by Sperdin et al. (2013) is based on the earlier version of this monitor, the backlight modification is described here for the sake of completeness.

LED switching characteristics

For more info, see figures and tables in Sperdin et al. (2013).

gap=80ms

gap=90ms

PWM input (blue, low-active, i.e., low="LED on") and luminance output (red) for a 10ms pulse which was preceded by either a 80ms or a 90ms gap during which the LED backlight was switched off. Obviously, with a 90 ms gap the LED driver falls asleep and it takes the driver about 8 ms to fully wake up. There is also a small delay (0.5 ms) of the first wake-up luminance peak with respect to the input signal. This delay is not present in the 80 ms gap measurement. The measurements have been taken with a Thorlabs PDA36A photo diode whose gain was set to 50dB (BW=100kHz). The traces were recorded with a Picoscope 4224 oscilloscope at 10MS/s.

Making the PWM signal available

Modification as described in Sperdin et al. (2013), i.e., using a HCT00 as a multiplexer which allows to switch on-the-fly between internal PWM mode or external backlight control mode. The legs of the IC socket were bended to the side and the socket was glued on to the board. The PWM signal is carried by the red wire (pin 1 of the connector) and is normally routed to the right side by a blank wire (second white line above the connector), which has been disrupted for the modification. The power supply for the IC was taken from the other side (black and red wire) and the wires (Signal ground, Enable, and External PWM) were connected to the external 9-pin connector.

Monitor's backside after the modification. The added vertical 9-pin connector gives access to the control signals. The control button bar was removed from its original place so as to have better access to these buttons when the monitor is placed in the tachistoscope setup.

Monitor's interior (after modification).

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• Sperdin HF, Repnow M, Herzog MH, Landis T (2013). An LCD tachistoscope with submillisecond precision. Behavior Research Methods. doi:10.3758/s13428-012-0311-0   (a copy can be requested here)