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Digital video is for all practical purposes synonymous with digitized component video. Digital video refers to digital video signals comprised of three separate channels, each carrying a different color component. The components may be either RGB, or luminance and chrominance (color difference) components. Digital video is closely related to Component Analog Video (CAV) which also includes three separate video channels, each carrying a different component. Thus, you can find many of the measurements from CAV in digital video analysis, as well. Digital video analysis also includes some additional measurements not included in CAV.
Digital video formats supported by NI VMS include HDMI, DVI (StudioRGB, sRGB and YCbCr coded), and BT-656. Refer to Digital Hardware for more information about acquiring digital video.
Digital video signals are encountered in broadcasting and studio environments, and in computer and consumer electronics products with e.g. DVI and HDMI interfaces. Digital video does not imply a specific picture resolution or scanning format, but embraces various formats, both SDTV, HDTV, and a number of computer-related formats. In comparison with CAV, digital video has the advantage that it allows interconnections and transfers without loss or impairments.
There are basically two types of digital component video signals:
Common to both component formats is that the sampling depth (sample word length) depends on the physical interface and the signaling standard. You may encounter sampling depths from 8 bits and above. Many modern Blu-ray players use a depth of 10 or 12 bits, also known as deep color. Deep color requires a double acquisition, so you must have a stable signal to acquire deep color. Ordinary compression systems have 8 bit depth, common broadcast interfaces have 8 or 10 bit depth, and DVI has 8 bit depth. HDMI offers various depths depending on the version. With HDMI, a physical output may support a larger sampling depth than that provided by the video material, though post-processing after decoding may increase the pixel word length.
Digital signals have a resolution of 8, 10 or 12 bit per color component and are in NI VMS reported as their quantization value, which means that the following report values are possible:
|8 bit sRGB||8 bit StudioRGB/YCbCr||10 bit sRGB||10 bit StudioRGB/YCbCr||12 bit sRGB||12 bit StudioRGB/YCbCr|
|0 (black)||16 (black)||0 (black)||64 (black)||0 (black)||256 (black)|
|255 (white)||235 (white)||1023 (white)||940 (white)||4095 (white)||3760 (white)|
Digital video signals use one of the three scanning modes defined as:
The DVI specification targets progressive RGB video formats. Refer to Digital Hardware for information about scanning modes specific to the hardware you useThe required color space conversion and the detection of interlace/progressive formats take place in the NI VMS software. The standards configuration file used by NI VMS to define the unique timing parameters of analog video signals is not required for the digital measurements.
There are three categories of video signals often used to group the picture resolution, frame rate, and bandwidth:
The characteristics that distinguish the above categories are listed in the following tables for the most common standards (or systems). Progressive systems 480p and 576p are often grouped in with STDV standards. Here, they are referred to more appropriately as EDTV to indicate that they lie somewhere between the SDTV and HDTV resolutions and rates. HDTV does not imply a specific resolution or scanning format. However, HTDV would commonly suggest a format from 1280x720p and above.
|System Name||Pixels per Active Line||Active Lines per Frame||Sample Rate (MHz)||Frame Rate (Hz)||Luma samples per Total Line||Total Lines per Frame||Category||Scanning Mode|
The frequency in the system name refers to frame rate for progressive, or field rate for interlaced systems. There are many other official standards defined by SMPTE and ITU-R that differ in total frame size, active picture size, and frame rate.
Common compression formats, such as MPEG-2, require a source format in multiples of 16 pixels, vertically and horizontally. Hence, you will not see a compressed 1920x1035 video format, for instance. For example, if you see a discrepancy between the number of active lines in the original source and in the output signal, it may be because a digitized and compressed NTSC signal contains 480 vertical lines, and not all active lines of the original NTSC signal.
Note also that compression systems may encode at a lower horizontal resolution than the scanning format. This is common practice in broadcasting, for instance, as a means to reduce the required bit rate. The decoder may upsample the decoded material to output video in the required format. However, the horizontal response would reflect the horizontal resolution of the encoding. Examples are 640 or 480 encoded horizontal pixels for 720x576 and 720x480 formats, and 1440 encoded horizontal pixels for 1920x1080 formats. When measuring sweeps, for instance, you should be aware that subsampling before compression may limit the bandwidth.
The digital representation of the above scanning formats is typically YCbCr with 4:2:2 chroma sampling structure for studio equipment and for studio interfaces. Digital compression systems for TV broadcasting and for consumer formats like DVD or Blu-Ray subsample the YCbCr chroma further down to 4:2:0. Some digital camcorders have a sampling structure of 4:1:1. A few compression systems for e.g. studio contribution use the 4:2:2 Profile of MPEG-2, or a similar scheme for compression, and carry 4:2:2 chroma information. However, the 4:2:0 sampling structure is predominant for compression systems. For that reason, as the DVI or HDMI interface of your unit under test outputs YCbCr or RGB with 4:4:4, you should expect the effective vertical and/or horizontal chroma resolution to be lower than the resolution supported by 4:4:4 if the content originates from compressed media.
|Tip If you test the output of a DVD player, ensure the lines you choose for your measurements are not too close to another test signal due to the reduced vertical chroma bandwidth.|
The following table provides a guideline of the required clock rates for a series of video formats. The table groups intentionally different frame rates to provide an overview of the pixel rate. The table is not intended as an exhaustive list of video formats, but it covers a range of typical formats you may encounter. The lowest pixel format required by the DVI specification is 640 x 480 @ 60 Hz with a clock timing of 25.175 MHz.
|NI PXIe-1491 Support|
|Format||Applications and standards||Frame rates (Hz)||Pixel rate (MHz)||Max. clock frequency (165 MHz)|
|1920 x 1200||WUXGA||60||154.0||yes|
|1920 x 1080p||SMPTE 274M||50/60/59.94||148.35-148.5||yes|
|1920 x 1080p||SMPTE 274M||24/25/30/23.98/29.97||74.18-74.25||yes|
|1920 x 1080i||SMPTE 274M||25/30/29.97||74.18-74.25||yes|
|1920 x 1035i||SMPTE 240M||29.97/30||74.18-74.25||yes|
|1600 x 1200||UXGA||60||162.0||yes|
|1440 x 1080p||Eg. cameras||50/60/59.94||111.26-111.38||yes|
|1440 x 1080p||Eg. cameras||24/25/30/23.98/29.97||55.63-55.69||yes|
|1280 x 1024||SXGA||60-85||108-157.5||yes|
|1280 x 1080p||Eg. cameras||50/60/59.94||98.9-99||yes|
|1280 x 1080p||Eg. cameras||24/25/30/23.98/29.97||49.45-49.5||yes|
|1280 x 960||SXVGA||60-85||108-148.5||yes|
|1280 x 768||WXGA||75-85||102.25-117.5||yes|
|1280 x 768||WXGA||50-60||65.25-82.23||yes|
|1280 x 720p||SMPTE 296M||24/25/50/60/23.98/ 29.97/59.94||74.18-74.25||yes|
|1152 x 864||Monitors||75-85||108-121.5||yes|
|1152 x 864||Monitors||70||94.5||yes|
|1024 x 768||XGA||60-85||65.0-94.5||yes|
|800 x 600||SVGA||85||56.25||yes|
|800 x 600||SVGA||56-75||36.0-50.0||yes|
|720 x 576i||BT.601||25||13.5||*)|
|720 x 480i||BT.601||29.97||13.5||*)|
|640 x 480||VGA||60-85||25.18-36.0||yes|
*) The lowest pixel clock rate guaranteed by the NI PXIe-1491 is 25 MHz. This clock rate determines the minimum resolution and frame rate supported by the input module. Legacy standard resolution ITU-R BT.601 video material, perhaps originating from PAL/NTSC, is encoded and distributed as interlaced video and has a pixel clock rate of 13.5 MHz which is too low for a DVI-compliant interface. Though not guaranteed, the NI PXIe-1491 may accept lower clock frequencies than 25 MHz, which may allow captures of e.g. digitized legacy video at the native 13.5 MHz clock rate. Also, your unit under test (UUT) may output low-rate material, such as legacy material, at multiple pixel rate for DVI compatibility. In that case, the NI PXIe-1491 will accept legacy material. Furthermore, if your UUT is a play-out device with de-interlacing and/or upscaling circuitry, you are likely able to output the legacy video material as progressive video with a sufficiently high pixel rate.