Measuring video quality
Apr 1, 2009 12:00 PM, By Bruce Devlin and Tim Harris
Quality control is a vital part of the ingest process.
QC as a part of ingest
When creating a broadcaster's business processes, the QC process will often be considered a separate step in the workflow. This is mainly because QC is often done by a separate operator, after the tape-to-file conversion has occurred. Operationally, incorporating the QC process alongside ingest allows an operator to be responsible for all of the steps involved in bringing content into the business. It also allows the operator to address many errors before the tape is even taken out of the VTR, thereby improving operational efficiency. Considering ingest and QC operations as one business process simplifies the business workflow and makes it resilient to changes in the operational aspects of ingest and QC tasks. This improvement can be used to reduce the overall cost of the ingest process to the business.
Figure 3. The actions taken by the operator given the output of a set of QC tests
Click to enlarge
When choosing what tests need to be undertaken as a part of the ingest process, there is a large selection available. It is worth taking into account that when the tests fail, the operator should be able to act on the results. For example, when a test that checks for clipped audio fails, the operator can address the error by reducing the audio gain on the VTR and retrying the ingest. A test for letterboxing in the video could have the operator modify the ingest parameters or send the content to an editor for letterbox removal. However, there are many tests with no obvious solution; these are not a useful part of the ingest process. One example might be MPEG transport stream structural checks which, while important for transmission, may not be useful checks to undertake during ingest, when the content is being repackaged before transmission. (See Figure 3.)
The QC process for ingest should be designed to detect errors that can be introduced by the incorrect operation or configuration of equipment, either at the ingest point or upstream of it, not errors in the design of the equipment. One advantage of integrating QC with transcoding and ingest is that any design errors in upstream equipment (e.g. poor metadata structures or low-level MPEG issues) can be identified, logged and often corrected as part of the integrated QC process.
As human error plays a role in errors introduced into any content, increasing the level of automation involved not only decreases running costs, but also improves the reliability and quality of the output of the workflow. Introducing integrated, automated QC means that operators can inspect and review content by exception, rather than being required to view every frame. For a highly automated workflow to be achievable, the inputs have to meet a tight specification, so QC during ingest can enable a highly automated media factory.
However, automated QC cannot detect all of the important errors in an asset (such as lip sync errors). This makes integrating a human operator with the automated tools an important part of any solution.
In closing
QC is an important part of the ingest process, and a tightly integrated solution allows a business to rely on its ingested content as a valuable revenue-generating asset. In order to make the process cost-effective, broadcasters must carefully choose the tests required to prevent paying for equipment with unnecessary features. However, if broadcasters take the right steps toward treating ingest and QC tasks as one business process, their business will be better equipped to cope with the industry's migration from tape to file delivery.
Bruce Devlin is CTO and Tim Harris is systems design engineer at AmberFin.
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