Video Transcript
Advanced Digital Video (Part 1)
5m 17s
00:07
Welcome everyone to “Introduction to digital video concepts”, as part of our QSC Quantum Training,
00:13
an advanced service and troubleshooting curriculum.
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My name is Patrick Heyn and I’ll be giving you a brief introduction
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to some of our more advanced video topics as they relate to NV Series video end points.
00:25
If this is your first experience with Q-SYS Video,
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we highly recommend that you START with our Q-SYS Video 101 online training.
00:33
This should give you a good baseline on the NV Series’ functionality before we dive into deep-geek stuff.
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Speaking of which, let’s get start with a discussion of video color, shall we?
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RGB or Red, Green and Blue color model is the most commonly used today
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because it mostly closely models the human eye
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and it also best represents the actual pixel data on video display devices.
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The human eye contains rods, that detect the light intensity and cones, that perceive color.
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Each pixel contains a subpixel
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usually an RGB element which are controlled to display a desired color and light intensity.
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The RGB color model can be mathematically mapped and transformed into something called a Color space.
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Color space defines the maximum and all allowable color on a given device.
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The complete subset of colors within a color space is defined as its color gamut.
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Many of these standards you see on the left here, define more than just the mapping function,
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color space and color model, as they can also define dynamic range, resolutions, frame rates, bit depths, etc.
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Speaking of which, the color bit depth is the number of bits that define a single color of a pixel.
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The color precision portion of the video can be defined as bits per pixel or per color.
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With 8, 10 or 12 bit color, it is defined as bits per color not per pixel.
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The complete range is called Full range color and 16 – 235 is called Limited range color.
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Typically SMPTE/CE rates support a Limited color range and IT/VESA rates support a Full color range.
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Now don’t get color space confused with color encoding.
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Color encoding systems are defined as sets of rules that are used to encode RGB data
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into a more easy-to-use form that can be transmitted using less bandwidth.
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Using the YCbCr encoding system the Luma (Y) is the brightness portion of the image,
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sometimes referred to as the black and white or monochromatic portion of the image.
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Chroma (C) is the color portion of the image.
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Transforming an RGB signal into the luma and chrominance components has many useful properties.
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RGB signals carry redundant data,
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one reason being that the luminance is being carried over all three RGB components.
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Transforming to YCbCr is performable via a mathematical transform
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which is 100% recoverable when transforming back to RGB.
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Digital subsampling is frequently used when transmitting and or saving digital data.
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The human visual system is much more sensitive to brightness than it is to color.
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When a video signal is separated into the luma and chrominance components,
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the luma portion can be transmitted via high resolution bandwidth.
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However, the chrominance portion can be subsampled to reduce bandwidth requirements.
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This image shows the chroma was significantly compressed but the luma remains uncompressed.
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The subsampling process is expressed by a 3 or a 4 (4 parts if alpha channel is included).
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These subsampling ratios are used to create a conceptual bitmap J pixels wide and 2 pixels high.
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Ratios use arbitrary variables J:a:b which are:
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J= the sample space for the conceptual bitmap
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a = the number of unique samples in the first row
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b = the number of unique samples in the between the first and second row
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The resultant bitmap is not an actual representation of how many pixels are displayed on a device.
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Instead, this just gives us the representation of how many color samples are transmitted per pixel
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and can be used to calculate the amount of bandwidth reduction.
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For example, 4:4:4 means 4 samples.
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4 Unique samples in the first row and 4 unique samples between the first and second row.
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4:2:2 means you still got 4 samples.
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2 Unique samples in the first row and 2 unique samples between the first and the second row.
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But this does provide a bandwidth reduction by 1/3
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Here are the supported color formats, RGB which is always 4:4:4: and YCbCr 444 and 422.
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And that's pretty much the baseline vocabulary on color.
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Let’s take a quick break now and dive into some more Ancillary video data when you get back
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Advanced Digital Video (Part 1)
5m 17s