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Maintaining Audio Quality by Philip Humphries Product Specialist Hamlet Issues in surround audio and new technologies to address them. Surround audio and the encoding technologies, such as Dolby’s AC-3 used to transmit it, have introduced new issues in maintaining audio quality. At a time when skilled audio personnel are getting scarce and budgets are shrinking new technologies hold the key to delivering top quality audio to viewers. Down-mix compatibility Although DTV brought surround sound with picture, the vast majority of viewers listen in stereo or mono. Compatibility of the surround signal with stereo and mono reproduction is crucial. However, other commercial solutions that detect down-mix compatibility problems require interpretation of a complex visual display. Essentially a multidimensional version of the Lissajous pattern long used to assess stereo-to-mono compatibility, their interpretation requires a skilled operator. Such displays cannot be distilled down to a pass/fail result, making unattended monitoring impossible. It makes sense to be able to compare what a human would hear when listening to the surround program with what would be heard listening to a down-mixed version. Spectral differences between the two are computed, allowing simple alarm thresholds to be defined. The user simply selects how many dB of cancellation is acceptable when the surround mix is converted to stereo. The detection algorithms provide duration thresholds as well, insuring that transient problems are ignored and only sustained problems generate errors. Loudness With legal requirements instituted or becoming Figure 1 Downmix Compatibility Assessment effective throughout Europe, much attention report of measured loudness (and has been given to loudness. It other measures if desired) for each is critical that the loudness of item broadcast. Content which falls program content and commercials outside legal or user defined limits is be properly assessed. The EBU flagged. R128 recommendation, and the An under-appreciated fact about ITU-1770 standard it is based the standardized method of on, defines the assessment of measuring loudness is that for some loudness. These documents specify material the results vary with the that measurements be made from reproduction format. Although a beginning to end of a programming piece of content might match the segment. Practical broadcast -23 (+/-1) LU target specified by streams interrupt programs with R128 when measured in surround, commercial advertisements which the loudness can fail these limits themselves must be independently when reproduced in stereo. measured. Surround content can theoretically Real-time monitoring and logging be up to 3.7 LU louder or as much requires independent assessment as 4.5 LU softer when reproduced of program material and of the in stereo. The Sentinel solution commercials which separate or from Hamlet measures all content interrupt it as illustrated in the figure in both its original format and after below. down-mixing to stereo. If the loudness differs after down-mixing The ideal solution is to maintain you are warned so corrective action two independent memories, one can be taken. dedicated to programmes and one to commercials. Based on triggers received via hardware connections or by software commands through its When broadcasting sporting events network interface the measurements live it is common to carry ambient are automatically categorized and sound (crowd or stadium noise) compared to limits. Dialog Balance Sometimes it’s inconvenient to issue this information in real-time from the play-out server. For these applications it is possible to obtain software which merges the log data with as- run logs, automatically creating an itemized Figure 2 Audio dual loudness measurement architecture 56 | TV-BAY MAGAZINE TV-BAY071NOV12.indd 56 06/11/2012 18:10