Documenting Media Art

Assessing Digital Video

Peter Oleksik assessing Boomerang by Nancy Holt and Richard Serra (1974).

Assessing your digital video media is a crucial step on entering your collection. Not only will it help you to understand the technical characteristics and anomalies of the file(s), it will help you create a baseline against which future examinations can be compared, and generally gain a better understanding of each artwork and your collection as a whole.

Video files are assessed and condition checked both visually and aurally with the appropriate tools. This section covers the design requirements of a work station, approaches to running initial technical analysis on your file(s), the visual and aural technical assessment and its documentation.

• visibilityVisual Assessment

  luminance, chrominance, color space and gamma, are the key characteristics to visually asses

  Tools for Visual Assessment

  • Reference monitor (SD vs HD)
  • Oscilloscopes
  • Playback Software
• hearingAural Assessment

  Key characteristics for aural assessment: Levels (db/VU), phase

  Tools for Aural Assessment
  • Meters (digital)
  • Phase Oscilloscope
  • Monitors
  • Waveform visualization
• warningVisual and Aural Artifacts
  • Analogue artifacts
  • Digital artifacts
  • Identifying cause / original – A/V Artifact Atlas

Assessment Workstation

To assess, quality-control and condition report your digital video files, you might want to consider designing a workstation that is customized to your needs and dedicated to these video evaluating activities. Below you will find three examples of workstations that serve various budgets and are aimed at different levels of quality-control. The three tiers outlined are designed to serve both standard definition (SD) and high definition (HD) video systems. If you are working exclusively with either SD or HD material, you could make savings by acquiring just the tools for your specific needs. The following aspects are the 7 key components relevant to all digital video assessment workstations.

• settings_input_compositeConnection Types

  Since the signal connections of your devices will play an important role in the playback of your video files, these will be briefly described together with their commonly associated formats. It is important to note that many analog formats only support 1 signal type and therefore outputting them on a “better” (or more recent) signal type can actually change the authenticity of the signal. Keeping the signal native is has been referred to as transparent migration.

  Composite Video [analog connection, SD]: All the information that composes the signal, luminance, chrominance, blanking and synchronization (CVBS), is merged together into one signal. Composite video is a single connection type (either BNC or RCA). Analog formats that utilize a composite signal output and are recorded using a composite signal include ½” open-reel, ¾” U-matic, laser disc and VHS.

  S-Video or Y/C [analog connection, SD]: As opposed to the composite signal, S-Video separates the luminance (Y) and the chrominance (C) into two different channels. Thereby it is a step up in quality from the composite video. It is transmitted by a distinctive looking, 4-pin connector. Formats that allowed for Y/C include VHS and DVD.

  Component Video [analog connection, SD]: Component video is the best quality connection to transmit analog video signals. The signal is separated into three channels: Y (luminance), Pb (Blue minus L) and Pr (Red minus L). The green is derived using the pythagorean theorem. Component video comes as three adjoined plugs, sometimes color-coded to green (Y), blue (Pb) and red (Pr). This connection should be used on analog Betacam and Betacam SP.

  Serial Digital Interface [SDI, SD or HD]. The SDI signal is a digital bit stream that can be used to transmit analog and digital video signals, and has also been used to by digital formats to record the signal to tape (Digital Betacam). SD-SDI is a signal connection type, using the BNC connection type, that packages the video, aural, and any ancillary data (timecode, closed captioning, etc) into a signal bit stream for transmission. SD-SDI is a 4:2:2 YUV system of video transmission. HD-SDI uses the same principal but splits the signal amongst multiple connections to increase the bandwidth necessary for transmission of High Definition material.

  High Definition Multimedia Interface [HDMI]: HDMI is typically used in the connection of monitors and other devices together in the transmission of high definition material.

• settings_overscanMonitor

  For standard definition video files –for example videos that come from SD tape transfers–, watching the video on a Cathode Ray Tube (CRT) monitor is crucial, as only these monitors will show you both fields of the interlaced content and provide you with an authentic display of the video signal. CRT monitors are no longer manufactured, however they can be found through appropriate web platforms (e.g. eBay) or forums who post when Broadcast studios sell off their equipment. Make sure the video inputs fit your system before you buy it, and calibration is recommended when setting up.

  Viewing interlaced video files on a computer monitor is problematic because these monitors cannot display interlaced footage and will attempt to merge the fields together into discrete frames. Certain errors, e.g. a head clog from a tape digitization, can only affect one field and lead to a complete area of image loss, which you would not see on a computer monitor.

  With the advent of high definition video, the term progressive entered the video world. Progressive means that a video –just like film– contains full discrete frames, as opposed to the two-fields interlaced. Progressive video content is often indicated by a lower case p, for example 720p. These videos can be quality-controlled easily on a computer monitor. A more advanced approach for viewing these kinds of files would be a color-calibrated, LCD studio monitor.

• settingsComputer

  Watching digital videos, especially uncompressed or high data rate, can be challenging for older or regular “desktop”-computers. Your workstation requires a relatively powerful computer, with a minimum of several GB of RAM. Most videos will lag, if you play them directly from external hard drives, depending on your interface, and from servers, depending on your connection; therefore it is good practice to save them locally for assessment requiring sufficient storage space on your computer. Alternatively you could consider an external hard drive or RAID with a fast interface connection. The platform you choose –Mac or PC– depends on the kind of software to be used: Final Cut Pro only works on Macs, whereas Adobe Premiere is available for both platforms. Make sure to choose your computer platform depending on your software and hardware needs and compatibilities.

• inputA/D converter

  An Analog to Digital converter is necessary to process both standard definition material from your computer to a CRT monitor as well as provides connection types for connecting external HD monitors and scopes.

• track_changesScopes

  Monitors, even when calibrated, are inherently subjective in how they represent the visual information. Oscilloscopes provide an objective view of the signal by providing access to the luminance and chrominance electronic information. The two critical scopes for the assessment of both analog and digital video are a waveform monitor and a vectorscope.
  
  Waveform Monitor: The waveform monitor measures the luminance (black and white) information of the video signal. A graticule is used to plot the information on a scale and allows one to see whether the blacks are too low in the signal (which may disrupt sync in an analog system) or the whites are two high (which would allow visual confirmation that the whites are “blown out” or exceed 100 IRE).
  
  Vectorscope: The vectorscope is a 360 degree color wheel that measures the hue and saturation of the chrominance information in the video signal. Known references like fleshtone, blue sky, etc. can be used to discern whether the chrominance information is being represented faithfully.

• perm_device_informationSoftware

  Software is crucial in the assessment of your digital video material. It is important to understand the basics of the software and how it functions to know whether your video is being processed and faithfully represented. Typically, capture and editing software like Blackmagic’s Media Express, Adobe Premiere and Final Cut Pro X can be used to analyze the discrete portions of your video material and allows for an in-depth assessment of its basic characteristics (luminance, chrominance and audio). Free tools such as QCtools also allow for in-depth assessment of your video files and have been developed for the archiving community.

• volume_upAudio

  Quality audio monitors and meters are also necessary in the assessment of video material. A pair of studio grade, near-field speakers is recommended for listening to a piece. In addition, audio scopes using either the decibel (dB) or volume unit (VU) method of measurement are useful in looking at whether the audio is clipped, compressed or otherwise distorted, and for measuring whether the audio is in phase or not.

Set-ups for Assessment Workstations

Initial Technical Analysis

The amount of technical information that you might receive with a digital video artwork will vary depending on the artist.

Assessing Files

• Confirm that the content is as expected.
• Ensure that there are no technical problems with the streams.
• That there is no conflicting information between the streams and the wrapper. For example, a particular combination of stream(s) and wrapper may provide ambiguous instructions for decoding, leaving the possibility that different players might play back the video stream differently. If this is the case it is helpful to clarify with the artist or their representative, if at all possible.
• That this is not an of absence of key information in the streams and the wrapper.
• Confirm that the basic matches content matches what was expected from the artist, for example the codec, the wrapper, audio sample rate and bit¬depth, subsampling, frame rate and the duration.

Assessing Files: How to

• Run MediaInfo on your file(s) to expose the technical metadata for analysis:

MAC Users:

• Open Terminal
• Run MediaInfo on the video files you are about to assess with the command: mediainfo -f [path to input file]

Assessing files: Identifying Technical Problems

Integrity

This step is to confirm that the files integrity is intact and there are no obvious signs that the file is incomplete or corrupted. The first method of checking the integrity of the file is to confirm that the file checksum associated with the file, if supplied with the artist on receipt, validates when transferred or moved to your storage infrastructure. If the checksum does not validate, this is a clear sign that something technically has changed within the file. The second method of checking the file integrity is to analyze the MediaInfo report for clear indicators of technical problems. These can vary, but an example is a file that will not open in any player and the mediainfo report looks like the image below:

When analyzing this report, it becomes clear that the file is incomplete and lacks the video and audio streams necessary for playback as the MediaInfo report finishes without displaying the video or audio stream technical metadata. This makes it clear that this information is not present in the file and is the reason for its inability to be displayed. This is a critical error with the file and will necessitate replacement.

Conflicts in Wrapper versus Streams

Problems might arise in the consistent playback of a video stream with regards to aspect ratio and color interpretation if there is conflicting metadata held in the wrapper and the stream. For example as demonstrated in the “Video” section of this MediaInfo report:

In this example, the 3 fields to look at are:

• color primaries
• transfer_characteristics
• matrix_coefficients

There is a conflict in the subsampling between the “color_primaries” and the “matrix_coefficient” where the file contains decoding information that relates to 2 very different color spaces, BT.709 and BT.601. This conflict would expose itself if played back in Quicktime where the player would default to BT.601, which is the incorrect colorspace.