Practical audio solutions
Jun 1, 2010 12:00 PM, By Guy Marquis
Lip-sync errors, 5.1 continuity and variable loudness levels can be addressed with the latest technology.
Figure 1. A digital fingerprinting probe allows content comparison across two points.
Select figure to enlarge.
When you ask station engineers about the technical issues they face on a day-to-day basis, it seems that the same bunch of audio problems keep cropping up. The top three problems will typically be lip-sync errors, maintaining the continuity of 5.1 and stereo audio and excessively variable loudness levels. The good news is that these annoying and recurrent audio issues can now be convincingly tamed using a mix of highly practical, new technologies that are readily deployed.
Measuring and addressing lip-sync issues
Lip-sync issues are common in broadcast and have their roots in the different processing time required for video and audio content. This difference is even more pronounced with the move to HD and 3Gb/s. Although video equipment is designed to manage the different video and audio delays, lip-sync problems can emerge down the playout chain as signals pass through various devices from different vendors.
Traditionally, it has been difficult to trace the emergence of lip-sync errors during TV playout, and subsequently at the set-top box, while a channel is on-air. Digital fingerprinting, however, now offers an elegant solution to identify, measure and trace lip-sync errors. The technology is based on a comparison of the video between a reference source without any lip-sync problems and other points in the playout chain, where lip-sync problems may emerge due to processing delays. For example, lip-sync testing points may be established at an incoming feed, after branding and closed-captioning/VBI insertion, at the exit of master control and when checking off-air feeds.
Figure 2. Shown here is an example of end-to-end lip-sync monitoring using digital fingerprinting.
Typically, the process is performed using a probing module, which analyzes signals at both points using a nonintrusive fingerprint generator engine. This operates on a field-by-field basis to generate a number that is unique to the video or audio content for that field. With this numeric data, the probe can then make sure that the content is the same at the source and destination. This allows the system to check for content mismatches, such as video and track swaps, as well as pure lip-sync errors. A probe can check all 16 audio channels and report any lip-sync errors within plus or minus 1ms. Every channel will get its own fingerprint to allow measurement of any phase shift between the audio channels. (See Figure 1.)
A key advantage of this digital fingerprinting technology is that it allows content comparison across different video and audio formats. For example, it can compare an HD 1080i or 720p signal with audio from a broadcast facility’s master control room to a signal received in the home using an SD set-top box. (See Figure 2.)
These lip-sync measurements can be monitored over IP, using a WAN or LAN, with an SNMP-based facility monitoring system, and any errors can be immediately flagged for remedial action. This would typically involve the operator making a delay adjustment directly from the facility monitoring desktop interface using the channel’s signal processor. This highly flexible, end-to-end lip-sync monitoring process can be used for multiple TV channels across multiple sites and also is well-suited for applications such as TV network affiliate monitoring. (See Figure 3.)
Digital fingerprinting is still in the early roll-out stage, and it is currently based on proprietary solutions; however, SMPTE has taken note of the considerable potential of the technology and is investigating the possibility of producing a SMPTE standard for the fingerprint signal and the methods of metadata carriage, with the review being performed by the SMPTE TC-22TV-01 AHG Lip Sync Committee.
Maintaining the continuity of 5.1 and stereo
Traditionally, delivering both 5.1 and stereo programming simultaneously has proven problematic. A typical viewer complaint is inconsistent 5.1 delivery to the surround speakers, which is often caused by ineffective upmixing when moving from a 5.1 to 2.0 signal. This can happen when broadcasters are playing out a mix of newer 5.1 content and legacy 2.0 content. While this type of problem may not be a full-on broadcast emergency, it’s certainly not the high-quality acoustic experience broadcasters strive for.
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