VPixx manufactures three models of CRT replacement monitors: The VIEWPixx, the VIEWPixx /3D, and the VIEWPixx /EEG. If you are wondering about the differences between these models, and which one is best suited to your research goals, this guide is for you.
We will start with an introduction to some of the general features of our CRT replacement monitors, including the display characteristics, temporal response profile, and data I/O and synchronization features. We will end with a side-by-side comparison of the three displays and a brief summary of their strengths and intended use.
This guide serves as a broad overview of features and functionality. For full technical specifications of the three displays, click on the images below to view their detailed product pages and datasheets.
Technical questions can also be directed to our team of staff scientists ([email protected]), who are happy to provide one-on-one consultation about your research needs.
Why replace a CRT monitor?
Cathode ray tube (CRT) screens are recognizable by their heavy, boxy shape. They are largely considered outdated, and have been replaced by high resolution liquid crystal displays (LCDs). In fact, it is increasingly difficult to find a modern graphics card that supports the analog video protocol required to drive a CRT display. Simply put, the world of displays has gone digital.
So, why are vision scientists concerned about recreating a CRT monitor?
The answer has to a lot to do with the limitations of early generation LCDs, and the consequences these limitations have for presentation of highly-controlled visual stimuli.
CRT monitors are comprised of a layer of phosphors, organic material which glows briefly when struck by an electron beam from a gun inside the monitor. The beam rapidly scans down the display, from left to right, top to bottom, in a pattern known as a raster. At the end of each row, the beam briefly stops and skips back to the start of the next row; this is known as a horizontal blanking period. When the gun reaches the bottom of the screen, there is a longer, vertical blanking period while the gun jumps back to the top of the display to start the next frame.
In an analog video signal, the cue for the gun to jump back to the top of the display is called a vertical sync pulse. This pulse, which is also included in digital video communication protocols, effectively marks the beginning of the next frame of the video signal. We will return to the vsync signal in the next section, where we talk about video synchronization.
Commercial LCD panels do not illuminate in a raster pattern. Instead, they have a grid of pixels which are all simultaneously illuminated by a backlight. The light shines through a filter that blocks all light that is not horizontally polarized. The light then passes through a liquid crystal layer, where molecules rotate the polarization of the light between 0-90 degrees. The degree to which a given molecule rotates the light depends on its structure. When a charge is passed through the molecules, their structure changes and the light can pass through with more or less polarization.
After the light passes through the LCD layer, a red-green-blue filter limits the output spectrum to a specific colour of light. A second filter allows only vertically polarized light to pass to the display, meaning any light that was not altered by the liquid crystal layer (e.g., still horizontally polarized) does not pass through, and the pixel will be fully dark.
Every time we change the value of an individual pixel on the display, we must change the structure of the molecules responsible for that pixel’s RGB intensity. Depending on the display, it can take several milliseconds for the molecules in the panel to fully stabilize in a new position. The time it takes for a single pixel to stabilize is referred to as the pixel response time.
Typically, an LCD backlight is on continuously, meaning the display shows the entire change in pixel luminance while the molecules change configuration. This ramp can last around 5 milliseconds on a typical LCD transitioning from black to white, and is visible in photodiode recordings like the one shown below.
A very slow pixel response means that pixel transitions trail behind the speed of the video signal. This leads to images bleeding across several frames, an artifact known as ghosting:
Another consequence of a slow pixel response time is motion blur. Especially for high contrast, fast-moving objects, pixel stabilization lags behind the movement speed. As a result, the edges of the moving object are smeared:
These display artifacts are not ideal for vision scientists looking for tightly controlled stimuli, particularly high contrast, moving stimuli.
CRT scanning patterns don’t produce the same kind of ghosting and blurring artifacts, which is why historically, CRTs have been the monitor of choice for vision science. However, CRT displays have their own limitations, notably their size, lack of availability and graphics support, and limited refresh rates (typically less than 75 Hz).
In response to the need for a modern display with crisp CRT-like features, VPixx has developed a high resolution, 120 Hz LCD display with a scanning backlight that mimics the raster pattern of a CRT monitor.
With a scanning backlight, individual pixels are not illuminated until the molecules in the liquid crystal layer are almost fully stabilized. The perceptual result is cleaner frame transitions, with minimal ghosting and reduced motion blur.
Consider the pixel transition on a VIEWPixx /3D, as measured by a photodiode. The image on the left below shows the pixel’s illumination as it transitions from black to white to black, without the scanning backlight enabled. The rise time to peak illumination is 5 ms, with a fall time of 1 ms. The image on the right shows the illumination of a pixel with the backlight enabled (black-white-black-white-black). Here, rise and fall time are both 1 ms.
The VIEWPixx /EEG has the scanning backlight feature enabled by default. On the VIEWPixx and VIEWPixx /3D this feature can be enabled and disabled via software tools.
We’ve already covered the basics of how an LCD works, but not all LCDs are made equal. In this section, we’ll talk about different LCD technologies, with an emphasis on their consequences for the overall display.
Backlight technology: fluorescent vs. white and RGB LEDs
The light source used in an LCD’s backlight has major consequences for the display’s colour gamut, as well as the uniformity of the display’s luminance and colour.
Older, consumer LCD panels typically use cold-cathode fluorescent light (CCFL) as a backlight source. While inexpensive, this light source tends to produce a relatively narrow colour gamut; perceptually, colours appear somewhat washed out. CCFL panels also have relatively poor display luminance and colour uniformity (<80%), and are prone to hotspotting.
With the rise of reliable light-emitting diodes (LEDs), many high-end LCDs have begun using LED backlights in order to take advantage of their wider colour gamut, better contrast, and increased energy efficiency. LED backlights may be white, or a mixture of RGB LEDs which produce a broad-spectrum white.
Generally speaking, RGB LEDs produce a wider colour gamut compared to white LEDs. Both types of backlight can be calibrated to achieve uniform display luminance. RGB LED displays can also be factory calibrated to an industry-standard white point, ensuring colour uniformity across the display.
The VIEWPixx /EEG uses a white LED backlight, while the VIEWPixx and VIEWPixx /3D use RGD LEDs. All three displays have uniform luminance >95%. The VIEWPixx /EEG has a colour uniformity of approximately 90%. The VIEWPixx and VIEWPixx /3D are factory calibrated to a D65 industry standard white point, and have >95% colour uniformity across the panel.
Panel technology: Twisted Nematic vs. In-Plane Switching
The structure and behaviour of molecules in the liquid crystal layer of an LCD also have consequences for display performance. Specifically, the layout of the molecules impacts their colour fidelity, contrast and viewing angle— the range of angles at which the display can be viewed without compromising colour or luminance.
The molecule behaviour also determines the pixel response time, which we have seen has consequences for crisp transitions and motion blurring.
Two major types of LCD panel technologies are twisted nematic (TN) panels and in-plane switching (IPS).
TN panels have relatively fast pixel response times (~5 ms for black -> white -> black). This means TN panels that are capable of keeping pace with the rapid movement of a VIEWPixx scanning backlight. Both the VIEWPixx /3D and VIEWPixx /EEG have TN panels.
Unfortunately, TN panels have a fairly small viewing angle, meaning the colour and luminance of a pixel can change dramatically when the display is viewed eccentrically. By contrast, IPS uses a structure of parallel molecules in the LCD layer to maintain richer colours over a wider range of viewing angles.
The downside is that compared to TN panels, IPS panels are slow (~7 ms pixel response time). This stabilization time can lag behind the scanning backlight, which creates ghosting, so we don’t recommend using an IPS display with the scanning backlight enabled. The VIEWPixx uses an IPS panel and is thus better suited to presenting rich, static colour images.
Bit depth
Digital video signals convert colour information into binary steps. A display’s bit depth refers to the number of distinct colour planes that can be transmitted and displayed on the screen. The higher the bit depth, the more planes that can be used and the finer the gradient between colours or grayscale values shown in an image.
A typical screen uses 8-bit RGB colour, where full white is denoted by an RGB colour triplet of [255, 255, 255]. This means users can adjust between black and white in 256 discrete steps (0-255). By contrast, a 10-bit colour divides the same colour space into 1024 steps, and 12 bits yields 4096 steps.
All three CRT replacement monitors use 8-bit colour by default. The VIEWPixx /3D and VIEWPixx both have special colour modes which allow users to show images at higher bit depths. The VIEWPixx /3D is capable of up to 10 bits, while the IPS panel technology of the VIEWPixx allows up to 12 bits.
Data I/O and synchronization
Our CRT replacement monitors are more than just a screen. All three displays have built in hardware for data synchronization. In this section, we will cover some of these features in detail.
Zero image processing and deterministic video signal
All three CRT replacement monitors receive Dual-Link DVI video from your computer’s graphics card.
Remember the vertical sync pulse from our description of CRT raster scans? This signal exists in digital DVI video protocols for LCDs as well. When a full frame of video data is received by your monitor, a vertical sync pulse immediately follows. At 120 Hz, our monitors do not hold frames in a buffer; instead they are passed immediately to the display. This means that the time between when this pulse is received and when the screen illuminates is fully deterministic. That is, using the vsync pulse we can determine with microsecond precision when an image actually appears on your display.
When the scanning backlight is enabled. The time between the vertical sync pulse and the stabilization of the top left pixel of the display is exactly 6 milliseconds.
Our hardware can be programmed such that the vertical sync pulse also acts as a triggering event for outgoing signals. On the VIEWPixx /EEG, this triggering is driven exclusively by Pixel Mode.
The VIEWPixx and VIEWPixx /3D can be configured to send custom triggers on digital, analog or audio channels. The vertical sync pulse can also be marked on the onboard system clock and used to relate incoming data (e.g., from a button box or recording system) to the onset of visual stimuli.
I/O Connections: Full vs. Lite systems
The VIEWPixx and VIEWPixx /3D come with a full onboard I/O hub similar to the DATAPixx2. The Lite version of these two displays offers digital input and output on two DB25 ports. There is also a VESA standard 3D port for use with an infrared emitter and active 3D shutter glasses, although we only recommend using the VIEWPixx /3D for 3D presentation, as the TN screen is better suited to rapid transitions and minimal crosstalk.
The Full version of the VIEWPixx and VIEWPixx /3D includes all of the features of the Lite system, and enables analog I/O on a third DB25 port (4 channels digital to analog, 16 channels analog to digital). The Full system also includes audio in, microphone in and audio out on three 25 mm jacks.
The VIEWPixx /EEG has digital out capabilities on a single DB25 connector. This connector drives digital output exclusively via Pixel Mode.
Left: VIEWPixx and VIEWPixx /3D bottom view.
Right: VIEWPixx /EEG bottom view.
Console monitor output
The VIEWPixx and VIEWPixx /3D have a secondary Dual-Link DVI output, capable of sending a copy of the video signal to an optional second console monitor. This monitor can be used by the experimenter to observe participant progress during data collection.
Importantly, video signal duplication happens after the video has left your computer’s graphics card, meaning you are not compromising display timing by placing extra demand on your graphics engine to extend the test display.
MATLAB/Psychtoolbox and Python support
The synchronization and data acquisition features of the VIEWPixx and VIEWPixx /3D can be accessed in MATLAB/Psychtoolbox and your Python IDE of choice, using our VPixx Software Tools.
For a more detailed explanation of the VIEWPixx and VIEWPixx /3D’s synchronization and data acquisition features, please check out our Introduction to Registers and Schedules.
VIEWPixx | VIEWPixx /3D | VIEWPixx /EEG | Consumer LCD | |
|---|---|---|---|---|
General | ||||
Ideal for | Rich colour images and fine gradients, static displays | Dynamic, high-contrast moving stimuli; 3D stimuli | Dynamic, high-contrast moving stimuli | – |
Resolution | 1920 x 1200 @120 Hz | 1920 x 1080 @120 Hz | 1920 x 1080 @120 Hz | various |
Pixel response time* | 1-7 ms | 1 ms (with scanning backlight) | 1 ms | variable, typically >7 ms |
Scanning backlight | Optional; not recommended | Optional; recommended | Default operation | None |
LCD Technology | ||||
Backlight technology | RGB LED | RGB LED | White LED | White LED or CCFL |
Panel technology | IPS | TN | TN | Typically TN |
Bit depth | 8 bits, up to 12 bits in custom video mode | 8 bits, or 10 bits in custom video mode | 8 bits | 8 bits |
Colour gamut | Wide | Wide | Medium | Medium – Narrow |
Display colour uniformity | 95% | 95% | 90% | <80% typical |
Display luminance uniformity | 95% | 95% | 95% | <80% typical |
Factory white point D65 calibrated | ||||
Data I/O and Synchronization | ||||
Digital I/O | Digital out only | |||
Analog I/O | ** | ** | ||
Audio I/O | ** | ** | ||
Pixel Mode | ||||
MATLAB/Python software support | ||||
*Based on a transition from full black to full white. Some commercial LCD manufacturers report response times for 10% – 90% gray, and may or may not average the rise and fall time in their estimates
**Full version only
Summary
In this guide we covered some of the rationale for creating CRT replacement monitors, and in particular, recreating the scanning illumination pattern that originated with CRT raster scans. The VIEWPixx, VIEWPixx /3D and VIEWPixx /EEG CRT replacement monitors leverage different LCD technologies in order to emphasize different strengths.
The VIEWPixx is optimized for rich colour, fine control over display luminance, and wide viewing angles. Its relatively slower pixel response time means it is best suited to static image presentation.
The VIEWPixx /3D has a faster pixel response time, and is ideal for dynamic and high contrast 2D and 3D stimuli with minimal motion blurring.
Similar to the VIEWPixx /3D, the VIEWPixx /EEG provides crisp transitions with minimal blur and is ideal for high contrast or moving displays.
The VIEWPixx and VIEWPixx /3D also have an onboard data acquisition system and synchronization hub for full experimental control, while the VIEWPixx /EEG has an out-of-the-box digital output triggering system based on Pixel Mode.
Still have questions about what monitor is best for your research, or interested in getting a quote? Reach out to us at [email protected] Our team of trained scientists are happy to provide you with more details and consult with you about what system is best for your needs.
Further reading
Elze, T. (2010). Achieving precise display timing in visual neuroscience experiments. Journal of neuroscience methods, 191(2), 171-179.
Elze, T., & Tanner, T. G. (2012). Temporal properties of liquid crystal displays: Implications for vision science experiments. PloS one, 7(9), e44048.
Cite this guide
Fraser, L. (2020, June 17). Choosing the Right VIEWPixx. Retrieved [Month, Day, Year], from https://vpixx.com/vocal/choosing-the-right-viewpixx/
