Digital Video Signals

NI Video Measurement Suite Help

Edition Date: October 2012

Part Number: 373389B-01

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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.

Formats

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:

RGB:

  • R = red, G = green, B = blue,
  • This is the format of the capture device, like a camera or a telecine, and the format of the display device, like a CRT or an LCD display
  • The sampling structure is 4:4:4 with equal bandwidth for R, G and B
  • This is the format that DVI and HDMI were designed for
  • The RGB format is used for computer graphics
  • Players and set-top boxes may also output RGB, however, in many cases converted from YCbCr

YCbCr:

  • Y = digitized luminance (brightness)
  • Cb = digitized color difference, covering B-Y
  • Cr = digitized color difference, covering R-Y
  • The notation YCbCr refers to digital coded components, whereas the notations YPbPr and YUV are used in connection with analog video
  • For natural scenes, the bandwidth of the color difference signals can be greatly reduced without significant visual impairment
  • The chroma sampling structure of available YCbCr formats normally provides less bandwidth for chrominance than for luminance
  • Examples of common chroma sampling structures are 4:2:2 and 4:2:0, though other structures exist
  • Digital compression systems, such as MPEG-2, target YCbCr source material, not RGB
  • Digital compression systems may exploit YCbCr to take different encoding decisions for luminance and chrominance
  • For the above reasons, YCbCr is widely used for broadcasting and consumer play-out formats
  • Material encoded and distributed as YCbCr may be output as RGB
  • Material with 4:2:2 or 4:2:0 sampling structure is output with 4:4:4 structure on DVI/HDMI interfaces

Sampling Depths

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.

Resolutions Reported as NI VMS Values

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)

Scanning Modes

Digital video signals use one of the three scanning modes defined as:

  • P     Progressive scan, progressive transport (also referred to as 1:1) - used for computer graphics, and for some EDTV/HDTV formats
  • PsF  Progressive scan, segmented frame transport (not very common)
  • I     Interlaced scan, interlaced transport (also referred to as 2:1) - used for legacy video (ITU-R BT.601), and for some HDTV formats

The DVI specification targets progressive RGB video formats. Refer to Digital Hardware for information about scanning modes specific to the hardware you use

The 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.

TV Standards

There are three categories of video signals often used to group the picture resolution, frame rate, and bandwidth:

  • SDTV (Standard Definition TV)
  • EDTV (Enhanced Definition TV)
  • HDTV (High Definition TV)

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
720x480i 59.94 720 480 13.50 29.97 858 525 SD i
720x480i 60 720 480 13.51 30 858 525 SD i
720x576i 50 720 576 13.50 25 864 625 SD i
720x480p 59.94 720 480 27.00 59.94 858 525 ED p
720x480p 60 720 480 27.03 60 858 525 ED p
720x576p 50 720 576 27.00 50 864 625 ED p
1280x720p 23.98 1280 720 74.18 24 4125 750 HD p
1280x720p 24 1280 720 74.25 24 4125 750 HD p
1280x720p 25 1280 720 74.25 25 3960 750 HD p
1280x720p 29.97 1280 720 74.18 29.97 3300 750 HD p
1280x720p 30 1280 720 74.25 30 3300 750 HD p
1280x720p 50 1280 720 74.25 50 1980 750 HD p
1280x720p 59.94 1280 720 74.18 59.94 1650 750 HD p
1280x720p 60 1280 720 74.25 60 1650 750 HD p
1920x1035i 59.94 1920 1035 74.18 29.97 2200 1125 HD i
1920x1035i 60 1920 1035 74.25 30 2200 1125 HD i
1920x1080p 23.98 1920 1080 74.18 23.98 2750 1125 HD p
1920x1080psF 23.98 1920 1080 74.18 23.98 2750 1125 HD psF
1920x1080p 24 1920 1080 74.25 24 2750 1125 HD p
1920x1080psF 24 1920 1080 74.25 24 2750 1125 HD psF
1920x1080p 25 1920 1080 74.25 25 2640 1125 HD p
1920x1080psF 25 1920 1080 74.25 25 2640 1125 HD psF
1920x1080p 29.97 1920 1080 74.18 29.97 2200 1125 HD p
1920x1080psF 29.97 1920 1080 74.18 29.97 2200 1125 HD psF
1920x1080p 30 1920 1080 74.25 30 2200 1125 HD p
1920x1080psF 30 1920 1080 74.25 30 2200 1125 HD psF
1920x1080i 50 1920 1080 74.25 25 2640 1125 HD i
1920x1080p 50 1920 1080 148.5 50 2640 1125 HD p
1920x1080i 59.94 1920 1080 74.18 29.97 2200 1125 HD i
1920x1080p 59.94 1920 1080 148.35 59.94 2200 1125 HD p
1920x1080i 60 1920 1080 74.25 30 2200 1125 HD i
1920x1080p 60 1920 1080 148.5 60 2200 1125 HD p
1920x1152i 50 1920 1152 74.00 25 2304 1250 HD i

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.

Compression and NI VMS Measurements

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 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.

Pixel Clock and Video Formats

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.

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