CHAPTER 15

Ensuring appropriate signal levels across all video equipment, adjusting video cameras and displays, and checking audio/video synchronization are all part of the video verification process.

Chapter 10 covers video signal types, transport systems, and installing video components. In this chapter you will learn about factors that affect the quality of the video signal from source to display and how to address some of the concerns presented by digital video and networked audiovisual (AV) systems. You will have to test and verify several items, and an efficient way to keep track is to utilize the Audiovisual Performance Verification Checklist.

Video Verification

As you learned in Chapter 11, the ANSI/AVIXA 10:2013 Standard for Audiovisual Systems Performance Verification provides a framework and supporting processes for determining elements of an audiovisual system that need verification. For video, you will use the Video System performance verification items and the Audio/Video System performance verification items, which are listed in full detail at the end of this chapter. You may find some of these verification processes applicable to an installation you are undertaking, or you may find some processes that can form the basis of your own verification procedures.

At this point, it is a good idea to review Chapter 10 and master basics such as digital video signal types, bandwidth considerations, and high-definition video formats.

Digital video and networked AV require knowledge of extended display identification data (EDID), digital rights management (DRM), and high-bandwidth digital content protection (HDCP).

Introduction to EDID

Extended display identification data was originally developed for use between analog displays and computer video devices, but has since made its way into Digital Video Interface (DVI), High-Definition Multimedia Interface (HDMI), and DisplayPort. EDID has since been further extended in capabilities to become Enhanced EDID (E-EDID), and more recently by DisplayID.

Displays and video sources need to negotiate their highest common resolutions so they can display the image at the best available quality. EDID is a method for source and display (sometimes called sink) devices to communicate this information, eliminating the need to configure the system manually. An EDID data exchange is the process where a sink device describes its capabilities such as native resolution, color space information, and audio type (mono or stereo) to a source device.

Each generation and version of EDID consists of a standard data structure defined by the Video Electronics Standard Association (VESA). Without the acknowledged handshake between display and source devices, the video system could provide unreliable or suboptimal video. Building an EDID strategy will be vital to your success as an AV installer.

EDID Packets

During the handshake process, the sink (display) device sends packets of EDID data to the source. The data packets carry the following information:

How EDID Works

Figure 15-1 shows how the EDID negotiation works, and the steps are detailed after the figure. The process begins with the activation of a hot plug link. The hot plug signal is an always-on, 5-volt line from the video source device that triggers the EDID negotiation process when a sink device is connected.

Figure 15-1 EDID process between source and sink devices

The EDID sequence works as follows:

EDID Table

An EDID table is a list of video resolutions and frame rates supported by a sink/display device. The DVI EDID data structure defines data in a 128-byte table; EDID for HDMI connections uses 256 bytes followed by additional 128-byte blocks, while DisplayID may include multiple 256-byte blocks of capability data. Table 15-1 shows some of the display capabilities of a specific monitor.

Table 15-1 Capabilities of a Specific Display Screen Extracted from Its EDID Table

As an AV professional, what should you do when a user attempts to connect their legacy device to a new system? After all, your job is to make that device work and look as good as possible with the new system you have installed. Managing EDID will help you accomplish this goal.

EDID Troubleshooting

You may encounter disruptions in the EDID conversation. The best troubleshooting method is prevention. Compiling and maintaining an EDID data table for all devices in an installation can avoid many EDID problems. This table will help you track all of the expected resolutions and aspect ratios for every input and output.

If you need to troubleshoot a problem that you think may be an EDID issue, follow the usual fault locating principles:

EDID Tools

There are a range of tools you can use to discover and troubleshoot EDID problems:

Resolution Issues

If a computer cannot read the EDID from the sink, it may default to its standard resolution. If the user subsequently attempts to manually set the system resolution to match the display, some graphics cards may enforce the default lower resolution and create a misaligned (size, aspect, centering) output without actually changing the video resolution.

If a computer is connected to multiple displays but can read the EDID from only one display, it may send an output that is mismatched to the other displays.

Some devices in the signal path between the source and the sink, such as switchers, matrix switchers, distribution amplifiers, video signal processors, and signal extender systems, have factory-set default resolutions. If the equipment has such settings, you will need to configure the device to pass EDID information from the sink to the source and vice versa. This may involve setting the EDID to match preset information about the capabilities of a sink device.

Note that if you make the presets something the source does not recognize, you may not get any image, or the image will have a very low resolution. For source components such as Blu-ray Discs (BD), be aware that some BD players will send a low-resolution 480p output that is compatible with many, but not all, older display devices.

No Handshake, No Picture

Many sources fail to output video if the handshake fails, but computer devices typically will send an output at a default lower resolution to ensure the user can still work with their computer. If this is the case, you may still see a picture from the PC source, but it will be of a lower-than-optimal resolution.

Some source devices will not output a video signal unless the display’s EDID data gives confirmation that it can properly display the signal. If there is no signal output from the source, the problem could be that the display’s EDID data is not being transferred to the source. If the hot plug detection pin cannot detect another device, the initiator of the conversation will interpret the sink as disconnected and cease the EDID communication. Potential problems with hot plug detection can involve the source not being able to supply sufficient voltage due to voltage drop in a long cable run, a bad (resistive) connection, or a digital video component such as a switcher or splitter intercepting the hot plug detect line.

Switching Sources

When switching sources, the changeover can be very slow, and there can be total picture loss during the switching process. This may be related to non-synchronous (crash) switching between sources and the delay required to resynch the frame (vertical sync) signal on the new source, or it may be due to a delay in EDID negotiations with the new source.

When EDID sources are not receiving hot plug signals, they generally conclude that the sink device is disconnected. Some low-quality direct switchers disconnect and reconnect signals when switching between devices. If an EDID connection is broken, when it is reconnected, the negotiation begins anew.

Managing EDID Solutions

Some approaches for dealing with EDID problems include the following:

Building an EDID Strategy

The goal of an EDID strategy is to allow a display to present the signal at its native resolution without scaling. If the system has displays of different resolutions or aspect ratios, without a proper EDID strategy, the resolution and aspect of the output will be unreliable and may vary when switching between sources.

All display devices in a video system should ideally have the same aspect ratio, which will prevent many EDID problems. Many source devices, including most computer systems, have a default output with a 16:9 aspect ratio, so projectors and other display components should match this. In projects where there are mismatches between source and sink capabilities, EDID processing management is required to retain consistent results during source and display switching. If there is an EDID strategy in place, your client’s AV system should work for a long time.

Display devices in fixed installations (particularly projectors) are generally not upgraded as frequently as the computers, which means that those display devices may quickly become outdated.

To ensure that the system you are installing is EDID compliant, use a device that includes an EDID emulator as a sink. It can be set to a specific aspect ratio and native resolution so that the source outputs a consistent aspect ratio and resolution. You can then set the EDID for each connected sink device.

Digital Rights Management

Your customers may want to stream content that they did not create, such as content from video streaming services, music streaming services, or materials from a local media library. Unlicensed distribution of content can violate copyright laws.

Make sure your customers are aware of potential licensing issues related to the content they want to play. It may be necessary to negotiate a bulk license with a content service provider.

If you fail to obtain the proper licenses to play content, you are not just risking the legal repercussions of copyright infringement; you may be risking the system’s ability to function at all. Publishers and copyright owners use DRM technologies to control access to and usage of digital data or hardware. DRM protocols within devices determine whether content can be allowed to enter a piece of equipment. Actual legal enforcement of DRM policies varies by country. It is also potentially illegal to circumvent DRM encryption.

High-Bandwidth Digital Content Protection

HDCP is a form of encryption developed by Intel to control digital audio and video content. If the content source requires HDCP, then all devices that want to receive that content must support it.

HDCP is merely a way of authorizing playback. The actual AV signals are carried on other wires in the cable. HDCP is used authorize the transmission of encrypted or nonencrypted content across a wide range of sources of digital content.

For example, there is a pause when you power up a Blu-ray player while the AV system is verifying that every device in the system is HDCP compliant. The Blu-ray player first checks whether the disc content is HDCP compliant. If so, the player proceeds to conduct a series of handshake exchanges to verify that all other devices in the display chain are also HDCP compliant before making the content available for display.

HDCP Interfaces

HDCP 2.x is capable of working over the following interfaces:

Some Apple devices may block content if HDCP is not present, even if the content is not HDCP-protected.

How HDCP Works

HDCP’s authentication process determines whether all devices in the display system have been licensed to send, receive, or pass HDCP content. No content will be shared until this entire process is completed. If there is a failure at any point in the process, the whole process has to restart.

The authentication steps are

When a switcher, splitter, or repeater is placed between a source and sink/display device to route signals, the inserted device’s input becomes a sink/display, and its output becomes a new source and continues the video signal chain.

HDCP Device Authentication and Key Exchange

The authentication process is designed so the source verifies that the sink is authorized to receive HDCP-protected content. Each device must go through this process.

HDCP Locality Check

The locality check verifies that the source and receiver are in the same locality, for example, in a nearby meeting room, auditorium, or classroom, and not a couple of cities away.

HDCP Session Key Exchange

Once the source and receiver have passed the device authentication and locality check, the process moves on to the individual session using the following steps:

Once the session commences, the HDCP devices must reauthenticate periodically as the content is transferred. Several system renewability messages (SRMs) must be exchanged. If during SRM exchange it is discovered that the system has been compromised, the source will stop sending content.

HDCP and Switchers

All of the HDCP processes explained so far have assumed that the AV system you are installing has a single video source for a single video sink/display. In these systems, the number of keys and how they are exchanged will be handled between the two devices. However, when multiple video sources, displays, processors, and a switcher are added, HDCP key management becomes more complex.

Some switchers maintain key exchange and encryption sessions continuously, so the communication will not need to be restarted. These switchers can act as a source or sink to pass the protected and encrypted HDCP data to its destination. You will need to verify that the switcher can handle HDCP authentication from the manufacturer’s documentation.

HDCP Authentication with Repeaters

An HDCP repeater is a device that can receive HDCP signals and transmit them on to another device, such as a switcher, display device, processor, or distribution amplifier. In a system with repeaters, as shown in Figure 15-2, the HDCP authentication process occurs after the locality check and device authentication have taken place between all the devices in the system. The session key exchange has not begun.

Figure 15-2 Authentication processes in a system with an HDCP repeater

The HDCP authentication process in a system with a repeater follows these steps:

HDCP Device Limits

HDCP 2.x supports up to 32 connected devices for each transmitter and a maximum of four repeater levels for each transmitter. Figure 15-3 illustrates an example of connection topology for HDCP devices.

Figure 15-3 Depth of two repeater levels and device count of six in HDCP 2.x topology

Although HDCP 2.x supports up to 32 connected devices, in practice, the number of sink/display devices allowable from a single HDCP-protected media source is typically much more limited and based upon the number of keys allowed by the source.

HDCP Troubleshooting

When HDCP authentication fails to initiate correctly, there can be a range of symptoms that do not directly implicate HDCP failure as their cause. These may include an image appearing for a few seconds at the start of a session and then disappearing, a green screen, white noise, or a blank screen, any of which could be caused by a broken cable, a bad connector, or dozens of other problems.

An HDMI signal analyzer or signal generator/analyzer is one of the few tools that will let you track and analyze the HDCP negotiation process, providing you insight into where the negotiations have gone wrong and a clue as to how you might remedy that issue. If such an analyzer is not available, then the signal flow troubleshooting strategy is likely to be the most productive. Start at the source end of the signal path with a known working display device and follow the signal path, testing at each point until you find the fault.

Verifying the Video Signal Path

When verifying a video system, you first should check the wiring and cabling of the video equipment.

Once all connections have been completed, verify that the wiring methods and cable pathways are as specified in the system design. This is typically a visual inspection process because you will need to compare the technical drawings, such as a signal flow drawing, against what you discover in the installation.

As with audio system wiring, during the inspection process, you need to look for the following:

As you work through this, record any errors or discrepancies for immediate repair. Permanent changes that occurred during installation should be documented and submitted to the AV system engineer or system designer for updating the as-built documentation.

Signal Extenders

Due to the resistive and reactive properties of all cables, long cable runs can cause significant signal attenuation. The high-frequency components in video signals limit the effective length of DVI, HDMI, and DisplayPort cable runs to a theoretical maximum of 5 meters (16 feet), which is often not sufficient for a professional AV system. One of the solutions to this problem is the use of an extender technology, which not only reaches farther than a direct video connection but is often cheaper than a long run of quality digital video cable.

Passive Extenders

Passive extenders (see Figure 15-4) generally use balun technology to interface between the digital video cable and one or two twisted-pair network cables such as Cat 6+ to extend the video signal over distances up to about 50 meters (165 feet). Short for balanced-to-unbalanced, a balun is a transformer used to connect between a balanced circuit and an unbalanced circuit. In an extender application, one balun is required at the point of connection between the video signal and the twisted-pair network cable, and a second balun is required at the far end of the twisted-pair extension cable to match the video signal to the receiving device.

Figure 15-4 A pair of HDMI baluns

Some extension systems use a single twisted-pair network cable, while others use two cables. As baluns are less than 100 percent efficient and there is signal attenuation in the extension cables, the specified distance limitations of extension systems must be observed. It is important that the extender devices at each end of the extension system are of completely compatible make and model, as there is no standard for pinouts between manufacturers and product ranges.

Active Extenders

Active extenders contain processing circuitry to boost and regenerate the video signals before transmission down the extension line and to regenerate them again at the receiving end. This enables active extenders to operate over greater distances than passive extenders. These extenders may use one or two twisted-pair network cables (Cat 6+) or one or two optical fiber cables as their extension medium. HDMI and DisplayPort signals may easily be extended for distances up to about 100 meters (330 feet) with wired extensions and up to 30km (19 miles) with fiber-optic extensions.

Active extenders require power to drive the processing electronics in both the transmitting and receiving devices. Some wire-connected systems send power down the cable(s) from the transmitter to the receiver, while others, including fiber-based systems, require a power supply at both ends. As with passive extenders, it is important that the transmitters and receivers are of completely compatible make and model, as there is no standard for pinouts and signal levels between manufacturers and product ranges. HDBaseT is a widely used active extender technology, with a full ecosystem of HDMI extension and distribution devices available.

It is also a common practice in event, large venue, and broadcast applications to use standard video conversion devices to convert HDMI or DisplayPort to XX-SDI, run the XX-SDI over coax or fiber to the destination, and then convert the XX-SDI back to HDMI or DisplayPort for display. This is a useful solution for locally originating content, but replay of copy-protected material can be problematic, as XX-SDI does not support HDCP.

Network Extenders

As discussed in Chapter 12, a range of AV over IP technologies can be used to extend video signal delivery to any point on a TCP/IP network.

Verifying Video Sources

In Chapter 10, you learned about the installation of video components, including video cameras and projectors. After the installation, you will need to fine-tune camera adjustments and complete projector calibration.

The location, intensity, and shadow quality of light sources in the picture area play a major part in the quality of the images picked up by cameras. When low-contrast lighting is used, images often look washed out and flat, while appropriately designed lighting will create images with contrast, dimension, and good exposure. The design of the lighting for video camera image capture is an integral part of any AV system design, and checking alignment, light levels, coverage, color temperature, color rendering, and contrast ratios should form part of the verification of the installation. The system design documents should include details of all lighting alignment parameters. It is quite common for a lighting designer, gaffer, director of photography, or lighting technician to be brought in for final alignments and level setting before system handover on projects with many luminaires and complex requirements.

Camera Adjustments

Clients tend to use cameras for videoconferences and to record and stream activities in their facilities. Similar to other AV devices, cameras should be set up for the environment in which they operate. Sometimes, cameras are connected and powered on, and that is the extent of the setup. However, a professional AV installer should make some standard adjustments to a camera to ensure that the images produced are of adequate quality.

Focus

The focus adjustment allows the camera to deliver a sharp, clearly visible image of the subject. To set the focus:

Back-focus

Lenses must stay in focus as they are zoomed from wide-angle shots to narrow fields of view. Adjusting the back-focus on the lens will keep zoomed-out images focused. To adjust back-focus:

Repeat this process until the lens stays in focus throughout its zoom range. If possible, lock the setting in place.

Pan, Tilt, Zoom, and Focus Presets

In system designs that include preset scenes or setups for cameras with remote robotic pan/tilt/zoom (PTZ) capabilities, each preset should be set up as specified in the design and the preset labeled with the name, such as “wide shot,” “presenter closeup,” “white board,” or “full stage,” as indicated in the design documents. Fixed-position cameras with remote zoom and focus preset capabilities should be set up in a similar way.

Some more advanced robotic camera systems may include capabilities for remote dolly/pedestal position, camera elevation, and live moves. These systems will generally be set up in collaboration with the end user during system training prior to sign-off.

Iris Settings (Exposure)

The iris in the camera’s lens controls how much light passes through the lens to reach the camera’s imaging device(s). Some adjustment to the iris will usually be required to correctly set the exposure of the image. If light levels change in the area covered by the camera, the aperture (opening) of the iris will require adjustment to maintain the correct exposure of the image. This adjustment is generally found on the body of the camera, as shown in Figure 15-6.

Figure 15-6 Typical controls on a professional AV camera. Courtesy of Panasonic Corporation.

The automatic iris adjustment found on many AV cameras adjusts the aperture of the iris based on the output of the camera’s image pickup device(s). Auto-iris responses may be based on the light falling on the center of the pickup (center weighted) or on the average of the light across the entire pickup area. Some camera’s auto-iris systems can be switched between several exposure modes. Most lenses will also have a manually operated iris mode, which allows an operator, either local or remote, to set the lens aperture for the desired exposure.

There is a strong interaction in the exposure of an image between the level of light on the subject of the image and the level of light on the background behind the subject (often a wall):

If the system design includes adequate lighting control, you will be able to expose a camera with an auto iris using the following simple procedure.

With the background lighting at its minimum level, set the subject lighting (including fill lighting and backlighting) at close to maximum intensity, and allow the auto iris to expose the camera. Now, adjust the level of the background lighting to make the background visible, but not dominant, in the image. If there is no simple dimming control, it is possible to adjust the intensity of the background lighting by switching some of it off or on or by inserting neutral density filters/meshes or diffusion material in the luminaires illuminating the background.

The balance between subject and background lighting for a good exposure will vary substantially depending on the color and brightness of the subject material. If the subject is a person, the reflectivity of their skin will greatly affect the balance. You will most likely have to adjust the lighting to account for significant changes between darker and lighter skin tones of subjects. If the lighting control system has preset level memory, it is advisable to set up a number of lighting states to accommodate the range of skin tones that may be present in the subject area.

Backlight Adjustment

Some cameras have a backlight adjustment mode, which surprisingly has nothing to do with the lighting focused on the back and edges of a subject to give separation from the background. The backlight adjustment is a compensation control for when a subject is seen against a bright background, such as a daylit window, which usually tricks the auto-iris system into underexposing the less-lit subject.

When activated, the backlight adjustment attempts to compensate for the over-bright background by increasing the aperture of the camera above the auto-iris setting to bring the subject up to a better exposure. This inevitably overexposes the background in the process of solving the subject’s exposure. Use of the backlight adjustment is, at best, a temporary measure to be used under duress. The solution to the problem is to improve the balance of light in the image by either reducing the light in the background (such as closing the curtain or blind over the window) or increasing the light level on the subject. Light-reducing window tinting can be used as a permanent solution to allow a subject to be seen against a brightly daylit exterior.

Automatic Gain Control

In situations where insufficient light is available to properly expose an image, it is possible to lift the brightness of the image by increasing the output of the camera’s imaging device(s) via the video gain control functions. As with any amplification process, increasing gain also increases the noise in the signal, which results in random sparkles in the video output. Engaging the auto gain control (AGC), if available, will usually result in images that are of sufficient brightness, but will often be quite noisy. The best solution is to make sure that the subject is in an adequately lit area, which may be as simple as moving the subject into an existing pool of light or swinging a couple of lights around to cover the subject area.

Shutter Adjustments

The camera’s shutter system controls the amount of light that the imaging system must process. In some cameras this is achieved by varying the size or speed of an actual rotating mechanical shutter that sits in the optical train before the imaging device(s). In other cameras the shutter control is a setting that varies the time between successive readings of the data from the imaging device(s). If a lot of light is entering the camera from a very bright scene, an automatic shutter system will reduce the amount of time the shutter is open, which may cause any fast movements in the scene to appear stuttered or jerky. The best solution to jerkiness caused by the shutter is to use some means other than the shutter to reduce the exposure of the imaging device(s). These solutions include reducing image gain, inserting a neutral density filter, or possibly closing down the iris.

White Balancing

Many cameras have an automatic white balance function. This circuit looks for bright objects in the image, assumes that they are white, and adjusts the color levels in the camera’s output to produce a white signal for those bright objects. It is therefore critical that all automatic white balancing is performed using a matte white reference object, such as a piece of white cardstock or paper, illuminated by the same lighting as the subject. If the automatic white balance feature is off, manual white balancing is required. This is often performed using a grayscale chart to fine-tune the color balance across the camera’s full dynamic range.

To automatically white-balance a video camera:

An automatic white balance is the best attempt the camera’s electronics can make under limited conditions, so it is unwise to expect that two cameras capturing images of the same object will completely match, even if they are of the same model and have the same lens. Where color matching between multiple cameras is important, expert human intervention is usually required.

Some cameras have a number of preset color balance settings that may be selected to accommodate known color temperature conditions.

Verifying Display Components

Displays should be set up and adjusted to produce the best image based on the environment in which they are to be viewed. The viewing environment can have a major effect on the quality of the displayed image. Viewing a movie in the theater, watching a presentation in a boardroom, viewing the IMAG on a live event, and viewing digital signage in bright daylight are very different viewing experiences.

Display setup requires knowledge of signal generators and the purposes of common test patterns. A knowledgeable technician can make the necessary adjustments based on the viewing environment.

Identify Display Parameters

The first step is to identify the parameters of the display device, which could be a projector, an LED screen, or a flat-panel display. Next, determine the input signal types that will be used on the display. The signal will probably be HDMI, SDI, or DisplayPort, although DVI and RGBHV may still occasionally be used in existing systems.

Determine the aspect ratio of the displayed image. The most common ratio is 16:9, although other ratios may be used in cinematic; video-wall; and multi-panel, multi-module, or multi-projector systems. This information for each display device may be in the owner’s manual, or it can be calculated by dividing the width by the height of the displayed image. As a guide, a ratio of width to height of 1.77:1 is equivalent to 16:9.

It is possible to display an image that was designed for a legacy 4:3 display on a 16:9 display, but depending on the display system settings, the image will be either horizontally stretched to fit, or have vertical black bars on either side of the image. Similarly, images intended for wide screen (2.39:1) cinematic display will usually display with horizontal black bars at top and bottom. It is usually preferrable to set up the display to show the entire image unstretched with black bars than with the image either cropped or distorted by stretching. If these settings are not defined in the system design documents, you should consult with the client or end user.

Set Contrast

The test pattern shown in Figure 15-7 is used to set contrast. The white rectangle in the center is super white (step 255), or the brightest item on the screen. The two very black rectangles beside it are super black (step 0), and the black background that surrounds the white and blacker rectangles is normal video black (step 16). The goal is to strike a balance between all the shades in the image. Make adjustments until all the bars are clearly distinct and visible. Ensure the white bars look white and the black bars look black. If the display’s grayscale transfer function (gamma) is set correctly, the bars should appear in even, gradual steps.

Figure 15-7 Test pattern to set contrast

If the black sections of the image are too dark, increase the brightness control. If the image sections are too bright, increase the contrast. It may take several adjustments to balance the image and ensure that all variations of white and gray can be clearly seen.

The procedure for adjusting brightness and contrast is as follows:

Once complete, the gray bars should gradually increase/decrease in value in a linear fashion. Repeat these steps until all bars appear in even, gradual steps.

Set Chroma Level

Color bars are test patterns that provide a standard image for color alignment of displays. They can be used to set a display’s chroma level (color saturation) and, where present, hue shift. Hue shift control is generally only found on systems that display images in the US NTSC format, which is prone to color phase errors.

Adjust the display for accurate monochrome brightness and contrast before color adjustments.

A commonly used test pattern is the HD Society of Motion Picture and Television Engineers (SMPTE) version. Alternatively, you may come across the Association of Radio Industries and Businesses (ARIB) version. You can use these interchangeably, and the process for using them is almost identical.

Check Appendix D for the link to the AVIXA video library, which includes a video explaining the process of setting the contrast, brightness, and chroma.

Image System Contrast

Image quality can be assessed using criteria such as contrast, luminance, color rendition, resolution, video motion rendition, image uniformity, and even how glossy a screen is. However, contrast remains the fundamental metric to determine image quality. Taking the viewing environment into consideration, the difference between system black and the brightest possible image is the system contrast ratio.

AVIXA developed the ANSI standard ANSI/AVIXA 3M-2011, Projected Image System Contrast Ratio, which set out how to measure contrast ratios and what contrast ratios are suitable for different viewing requirements. That standard has since been updated and revised and will shortly be replaced by ANSI/AVIXA V201.01:202X Image System Contrast Ratio (ISCR), a standard that extends beyond projection to cover system contrast for viewing all images, including both projection and direct-viewed displays. The complete standards are available from the Standards section of the AVIXA website at www.avixa.org, but here is a brief summary of the four viewing requirement categories and their required minimum contrast ratios:

The standards include a simple measurement procedure to verify that the system conforms to the desired contrast ratio of the viewing task. First, you must identify the five measurement locations, as shown in Figure 15-8. The five locations are to be recorded on a viewing area plan.

Figure 15-8 Contrast ratio should be measured from five locations within the viewing area

After you have identified the viewing locations, measure the contrast ratio of the system using a 16-zone black-and-white checkerboard (intra-frame) pattern, as shown in Figure 15-9. You will also need a luminance meter or spot photometer with up-to-date calibration.

Figure 15-9 Checkerboard pattern used to measure contrast ratio

The procedure for verifying image contrast ratio is as follows:

If the contrast ratio meets or exceeds the minimum laid out in the ISCR standard at all five viewing locations, then the system conforms to the standard. If the contrast ratio of one but no more than four measurement (viewing) locations falls below the required ratio for the identified viewing category by no more than 10 percent, the system partially conforms. If the contrast ratio at any one of the measured locations falls below the identified viewing category by more than 10 percent, the system fails to conform to the ISCR standard.

Audio/Video Sync

In many AV systems, the audio and video signals may take significantly different paths to get from the source to the point at which the user will experience them, such as in a seat within a venue, at a remote location, or distribution of recorded program material. The audio and video signals may also undergo very different numbers of processing steps, each with their own inherent latency.

In systems where associated video and audio signals may be transported or processed separately and are then subsequently combined for transmission or display, one of those signals could be delayed more than the other, creating synchronization errors sometimes known as lip-sync errors. These errors are typically corrected by applying delay to the least delayed signal so that it aligns with the most delayed signal. For a good user experience, the signals should arrive in synchronization at the point at which the user will experience them, regardless of whether that point is local or remote.

A “blip-and-flash” test is a common method for measuring the synchronization of audio and video signals. A test signal consisting of one full frame of white video, accompanied by an audio tone of the same time length, is required. An alternative signal may be an image changing from full-frame white to full-frame black at regular intervals, with each change accompanied by a brief click or tone burst. In the absence of an appropriate signal generator, this can be produced by a computer running a slideshow. The signal at the point of reception can then be analyzed using a dual-channel digital storage oscilloscope to record the timings at which the audio and video signals are received.

If the sync alignment is not being measured, then a subjective reception test using the same test signal may be used, in addition to replaying video material that clearly depicts a person, such as a news presenter, speaking directly to camera. The subjective effect of any delay can then be evaluated for a pass/fail assessment.

Tests should be undertaken at the inputs of recording devices, at the inputs to transmission devices such as videoconferencing codecs and broadcast circuits, and locally within a presentation space at both near-screen and farthest-viewer positions.

Verifying Audio/Video Sync

After the AV system has been installed and all recording devices, presentation equipment, and link equipment/circuits are operational, the procedure for measuring the time alignment of the audio and video signals is as follows:

Alternative subjective tests may be undertaken using source video with no synchronization error, where a person is seen speaking. Validate subjective synchronization delay at locations listed earlier.

Correcting Audio/Video Sync Errors

Downstream video processing can cause delay. Some displays have built-in video processors, which, when used in conjunction with audio processors, scalers, and other processors, will cause delay in the signal.

Unfortunately, there is no clear way to predict how much, if any, delay will occur in an AV system. The latency introduced depends on the digital signal processors within the system, and the specific amount varies by manufacturer and device type. In some cases, you may be able to depend on the HDMI lip-sync feature, which will automatically compensate for delays in the video signal. In these cases, EDID can be used as a tool for correcting the sync issues. HDMI uses EDID to communicate delay information to upstream devices. During negotiation it will measure the delay introduced and send that information to the sink for automatic correction. This may be sufficient for systems with a single display, but with systems that involve multiple displays, when EDID detects different devices with different delay values, it is unlikely to be able to derive a single delay correction that will work for all sink devices.

In these cases, the integrator can manually introduce delay into an audio distribution system. Delay is a function typically found within digital signal processors (DSPs), switchers, or as a stand-alone, dedicated device.

Delay allows you to extract the audio at the beginning of the signal chain and send it right to the infrastructure. If you are responsible for selecting DSPs for a system, you should consider including a DSP that has the capacity to compensate for delay. Then, if you do encounter this problem, you can compensate for it by using a blip-and-flash test reel and adjusting the delay until the signals are back in sync.

Video Verification Checklists

The Video System Performance Verification Items and the Audio/Video System Performance Verification Items from the ANSI/AVIXA 10:2013 Audio Systems Performance Verification standard are listed here in Figure 15-10 and Figure 15-11, followed by a description of each list item. You may find some of these verification processes applicable to an installation you are undertaking, or you may find some processes that can form the basis of your own verification procedures. Verifying the performance of video systems according to established standards will help you assure overall system performance.

Figure 15-10 Video performance verification items from the ANSI/AVIXA 10:2013 Standard for Audiovisual Systems Performance Verification

Figure 15-11 Audio/video performance verification items from the ANSI/AVIXA 10:2013 Standard for Audiovisual Systems Performance Verification

Chapter Review

Verification that video components and the overall AV system perform as per specification is an important deliverable in an installation project.

Upon completion of this chapter, you should be able to do the following:

After you ensure that the AV system you have installed works as expected, you are ready to perform system closeout, which you will learn about in Chapter 16.

Review Questions

The following questions are based on the content covered in this chapter and are intended to help reinforce the knowledge you have assimilated. These questions are similar to the questions presented on the CTS-I exam. See Appendix E for more information on how to access the free online sample questions.

Answers