Analog Signal Details: Video Test Reference Architecture

Analog video testing can be broken into two main parts: signal acquisition and video quality measurements. You must first acquire and extract the desired information from the Composite, S-Video, or Component video signal that you are testing. Once you have acquired the analog video data, you must develop software algorithms to analyze the video signal quality, ensuring the product is sending the correct brightness and color information to be displayed on a TV or other device.

Analog Video Signals

A composite video signal is a signal in which all the components required to generate a video signal are embedded in a single signal. The three main components that together form a composite signal are as follows:

• The luma signal (or luminance)—Contains the intensity (brightness or darkness) information of the video image
• The chroma signal—Contains the color information of the video image
• The synchronization signal—Controls the scanning of the signal on a display such as the TV screen

These signals change with different composite video signal standards.  These signals are the final signals that allow your television to display the image.  There are several parameters that go into these signals

Parameters of interest:

• # of lines/frame:
• line frequency:
• line duration:
• active horizontal duration:
• # active pixels/line:

You can see several examples of current Composite video standards in Table 1

 

Format	Country	Mode	Signal Name	Frame Rate (frame/sec)	Vertical Line Resolution	Line Rate (lines/sec)	Image Size (WxH) pixels
NTSC	US, Japan	Mono	RS-170	30	525	15,750	640x480
		Color	NTSC Color	29.97	525	15,734
PAL	Europe (except France)	Mono	CCIR	25	405	10,125	768x576
		Color	PAL Color	25	625	15,625
SECAM	France, Eastern Europe	Mono		25	819	20,475	N/A
		Color		25	625	15,625

Table 1: Composite Video Signal Standards

 

Let's look at some calculations we can make, in this case for an RS-170 signal (See Table 1):

• Pixel clock (PCLK) frequency (the frequency at which each pixel arrives at the frame grabber):
  640 pixels/line / 52.66 e-6 sec/line = 12.15 e6 pixels/sec (12.15 MHz)
• Total line length in pixels of active video + timing information (referred to as HCOUNT):
  63.556 e-6 sec * 12.15 e6 pixels/sec = 772 pixels/line
• Frame rate:
  15.734 e3 lines/sec / 525 lines/frame = 30 frames/sec

Looking at this information we can see to account for every pixel of the signal we need to acquire at a rate of at least 12.15 MHz.  We will look further into what this means for setting up the video test system.

 

Acquisition System

The two main parts of a video acquisition system are a PXI controller to run the software and provide the data management and permanent storage for a signal being acquired by a digitizer. The NI PXIe-5122 100 MS/s, 14-bit digitizer includes features such as video triggering, input termination, and DC restoration to help with the testing of video signals. It also uses the PXIe bus to allow streaming video data to high speed RAID storage devices. The digitizer also has large on board memory options to allow adequate capture of video signals depending on the required sample rate and image resolution. A higher-resolution image has more pixels, which requires larger on board memory. Figure 1 shows flow of data in an image acquisition.

[+] Enlarge Image

Figure 1: Analog Acquisition Data Flow

Analog Video Test Hardware

There are two main hardware components required for the video data acquisition.  The PXIe-5122 allows us to digitize each incoming pixel at the desired sample rate.  The maximum sample rate is 100 MS/s, which is sufficient for both standard definition Composite video, as well as HD Component video.

A PXI chassis provides the necessary bus connections to pass video data between the digitizer and the embedded controller. The preferred chassis is the PXIe-1062Q for a small system, while for higher channel count or combined analog and digital video applications a PXIe-1065 chassis provides more PXI and PXIe slots.  Figure 2 shows a PXIe-5122 and PXIe-8130 installed in a PXIe-1062Q chassis.  This system uses only one of the seven available instrument slots, which could be filled with multiple digitizers for larger channel count systems or other instruments to have a mixed signal test system. It is also possible instead of the embedded controller to use a MXI Express connection to control the system using an existing desktop or laptop computer.

Figure 2: Analog Acquisition Ready PXIe System

Acquisition Software  

In order for the video data to be acquired, the embedded controller must send the instructions to the digitizer to configure the acquisition and then to fetch the data from the onboard memory.  This interaction is handled through the NI-SCOPE driver software.

NI-SCOPE provides the ability to configure NI digitizers entirely in software.  It allows for multi-threaded control of multiple sessions.  This means that for large channel count systems the NI-SCOPE driver will take advantage of multi core and hyper threading technologies to keep the processes running in parallel, maximizing large channel count test performance.

NI-SCOPE is usable in a variety of application development environments.  It is easy to customize the acquisition for multiple channels. The link at the bottom of the page links to an example program, which demonstrates how to acquire two channels at a rate appropriate for an NTSC signal. 

Data Analysis

Once analog video data has been acquired, software algorithms must be developed to analyze the video signal quality, ensuring the signal contains correct brightness and color information to be displayed on a TV or other device.   The example at the end of this tutorial demonstrates how to perform measurements on a single frame of video data. It shows how to perform a basic luminescence test for a color bar image.  Additional algorithms could be developed for other tests including chrominance, multiburst, K-factor, or other measurements. 

The VideoMASTER analog video analysis suite from microLEX Systems is a complete video test solution built on the PXI platform. It uses the NI-5122 100 MS/s, 14-bit digitizer for acquisition of Composite, S-Video or Component Video signals. Once you have acquired the data, you can use the configuration-based environment to easily select test steps from a comprehensive library of measurements, or you can create custom measurements for your signal. While you configure measurement steps in LabVIEW, you can create a test sequence using the NI TestStand test executive to manage all of the measurements for the test unit. You can also add quantitative limits to each step to provide pass/fail criteria. In addition, NI TestStand gives you easy access to test data and reporting.

Additional Links

Video Test Reference Application (All Video Test example code)

Video Test Portal

Digital Video Test Details

Analog Video Test Example Code

Reader Comments | Submit a comment »