Thursday, July 21, 2011
Posted by The Ultimate Posted in 12:06 PM
For these engineers, understanding the anatomy of traditional digital video signals is the first step in analyzing 3D video content. Digital video signals can be broken into three main parts: the vertical blanking region, the horizontal blanking region contains the vertical sync pulse that signifies a new frame, an the horizontal blanking region contains a sync pulse that signifies a new line within the frame. The Vendor-Specific Info Frame specifies the frame rate and resolution of the video signal, which defines the timing of the horizontal and vertical sync pulses.
For example, a Full High-Definition (HD) 1080p/60 Hz video stream has a vertical blanking pulse every 16.67 ms (60 Hz) to signify a new frame and a horizontal blanking pulse every 14.8 µs to signify a new horizontal line.
Within the 14.8 µs horizontal line duration there are 2,200 pixels, which include the horizontal blanking period and the active picture. This requires a pixel clock of 148.5 MHz (1/14.8 µs horizontal line duration*2,200 pixels) to generate the video frame composed of the three regions.
The active picture region is the location of the pixels that a user would see on the display. The pixel color value is stored in three channels that are typically RGB or YUV (YPbPr) encoded. The color levels can range from 8 to 12 bit per channel, meaning that each pixel has a 24 to 36 bit color value. Understanding how content is stored and the various timing aspects of a video signal provides the baseline knowledge for taking measurements to test the quality of the content.
Three Types of Measurements for Every Video Test Application
The three types of measurements used to evaluate video signals are timing, level, and linearity. Measuring the timing parameters of the horizontal and vertical blanking intervals ensures that a device meets industry standards. The most essential parameters of the signal to evaluate are the horizontal sync amplitude and width, vertical sync amplitude and width, start and end of active video, and the horizontal line time. Additionally, a channel delay measurement tests whether the three channels are synchronized and displaying their content on the correct pixels. A delay of 7 ns (1/148.5 MHz) between the channels could mean the difference between a pixel containing the correct values or not.
The second set of measurements tests the active picture region for image quality and ensures content is packaged correctly. The color bar test pattern is the most common technique for evaluating the content levels of the active picture region because it spans the color spectrum, so engineers can test each channel at minimum and maximum values.
The final set of measurements relates to noise and linearity. Channel and inter channel noise measurements will identify if the system introduces any noise. Even minor alterations to the video content can cause an image to look distorted.
The best way to test for channel noise is to use a ramp pattern. Measuring noise on a ramp pattern that increases from zero to full scale will identify any noise on the channel. For inter channel noise, the same measurement can be made on a simultaneous ramp of all channel. Linearity measurements are more complex and will characterize the video quality. The channel linearity measurement uses a ramp pattern to test for a linear slope with minimal bit errors on the least-significant bit (LSB) of each channel. If nonlinearities exist, there will not be smooth transitions from black to red, green, or blue for each signal.
To view a webcast series on testing multimedia device, visit National Instrument, input code nsi0206.