File-based workflows are increasingly used in post production and broadcast, supplanting earlier, less integrated digital workflow architectures. File-based environments can streamline content production, reduce operational expenses, and are critical for the delivery of OTT programming to PCs and mobile devices for multi-screen video services.
Given its importance, quality control (QC) is naturally critical for the successful use of file-based workflows. Historically, QC has relied on direct visual inspection, usually with personnel spot-checking a few minutes of a program’s beginning, middle and end. However, this approach has never truly been sufficient for two reasons. The first is that humans tend to notice just two classes of technical impairments:
- Signal-level-related issues such as video luma and chroma, or audio loudness; and
- Obvious distortions and dropouts manifested in problems such as black sequences, frozen frames, blockiness, loss of audio and audio/video sync discrepancies.
Because of this, there are errors that are hard to recognize even when the program is closely watched.
“Slice order” errors in MPEG transport streams can cause blockiness and other picture distortions.
Scalability is the second reason visual inspection is insufficient; the industry is simply “running out of eyeballs” to check the tens of thousands of files being created for multi-screen services monthly.
It is this tremendous volume of content that must be managed and quality-checked that is driving the industry toward automated, process-spanning QC solutions.
Tektronix’s Cerify is a fully automated system for verifying and checking file-based content prior to transmission or use. It can be deployed on stand-alone Windows PCs or as an enterprisewide system that interfaces with third-party automation or asset management systems.
It performs comprehensive QC of file-based video ingested from multiple sources and encoded at different bit rates, formats and compression standards for SD/HD, VOD and IPTV delivery. In particular, it checks file-based video for syntax issues, baseband problems, encoded content errors and structural issues:
- Syntax testing. Syntax testing is critical, as it ensures the integrity of the encoding of the file structure. Syntax issues can cause catastrophic problems, and without syntax checking, it is even possible that the content won’t play out at all! Incorrect syntax can also put set-top boxes into a continual state of rebooting, or do just the opposite — lock them up and necessitate rebooting. Rigorous syntax testing can verify a compliant stream and save a truck roll.
Syntax errors are particularly damaging in VOD and other file-based services, given the growth of these media libraries. Spot checks simply cannot address the literally tens of thousands of programs that need to be checked, which drives the need for automated QC.
- Baseband checking. Cerify can look at the decoded baseband video and audio to evaluate image and audio performance. Looking more closely at baseband errors, it appears that most are part of the original acquisition, though some may be caused by edits made in the workflow.
Gamut violations are classic baseband errors that must be captured with an instrumented decoder, which can also capture other gamut problems with RGB color components. Black frames, frozen frames, letter boxes and pillar boxes, and dropouts also can be detected from the decoded video content.
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Audio problems such as clipping are also observable in the decoded baseband stream. By looking at the signal level, it is possible to determine whether loudness limits, peak limits, instantaneous peaks and true peak value limits have been exceeded, as well as long-term loudness over the span of the content. With the arrival of government deadlines for audio loudness compliance, Cerify can go beyond audio checks by automatically correcting audio loudness in the audio essence(s). Working with all container types, audio level correction can be applied to the entire essence duration, and can be triggered by non-compliance to EBU R 128 or ITU 1770-2 Audio Loudness recommendations.
- Checking encoded content. Encoders, too, are a potential source of errors. A faulty encoder can produce syntax, similar to an editing application that has flaws of its own. A misconfigured encoder, or one with the bit rate set too low, may over-compress the material.
Encoder problems can appear as “slice order” errors in MPEG transport streams. These errors create block artifacts on the screen and degrade picture quality.
Additionally, interlaced videos are encoded either bottom-field-first or top-field-first and must be played out in the same sequence. A conflict in the field order will cause distracting motion artifacts — almost a zigzag motion when the subject should be moving smoothly across the screen. North American and European SD content have opposite field ordering, which is easy to overlook when transcoding or editing.
- Structural checking. Structural checks are as much a part of QC as are baseband and encoder checks. It is possible for significant (and required) elements to be absent from content that seems syntactically correct. The QC system must examine the container structure, the content and the codec headers, comparing the actual contents with the expected information to identify structural problems.
Checking video and audio codec headers may reveal unexpected essence formats and encoding with respect to profile and level, GOP structure, frame and sample rates, picture size and aspect ratio, interlaced or progressive scan, color depth, and color sampling. Ultimately, structural checks are to ensure that the content has the right format characteristics set up in the proper order.
Cerify addresses these issues and can be integrated with automation and asset management systems, as shown in Figure 1, thus feeding them with data using the CeriTalk API required to automatically decide on next workflow steps or drawing attention to assets that need expert review.
Figure 1. The transcode system converts content into formats compatible with diverse receiving platforms; the QC system checks the content quality before transmission and notifies the media asset management system.
The XML-based test templates can be exchanged between Cerify systems, and applied as the definition of the required test standards between suppliers and broadcasters to establish SLAs and reduce costly churn.
Automated QC processes save time and resources and are always able to devote their undivided attention to the content.
In addition, automated QC is more thorough and objective than visual inspection. It is consistent and reproducible, and able to isolate errors that are encoded deep within the files. Problems with syntax errors or encoding parameters and mismatches with structural metadata are not normally perceptible to the human eye at the point of inspection. Nevertheless, they can cause serious problems later in the workflow.
Automated processes supplement human QC and can help sort out the most critical errors. This maximizes labor efficiency by focusing QC technicians on problems requiring urgent attention.
—Richard Duvall is technical marketing manager for Tektronix.