Issues in surround audio and new technologies to address them


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Issues in surround audio and new technologies to address them. Surround audio and the encoding technologies, such as Dolbys 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 It makes sense to be able to compare what a human would hear monitoring impossible. 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 effective throughout Europe, much attention has been given to loudness. It is critical that the loudness of program content and commercials be properly assessed. The EBU R128 recommendation, and the ITU-1770 standard it is based on, defines the assessment of loudness. These documents specify that measurements be made from beginning to end of a programming segment. Practical broadcast streams interrupt programs with commercial advertisements which themselves must be independently measured. The ideal solution is to maintain two independent memories, one dedicated to programmes and one to commercials. Based on triggers received via hardware connections or by software commands through its network interface the measurements are automatically categorized and compared to limits. Sometimes its 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 report of measured loudness (and other measures if desired) for each item broadcast. Content which falls outside legal or user defined limits is flagged. An under-appreciated fact about the standardized method of measuring loudness is that for some material the results vary with the reproduction format. Although a piece of content might match the -23 (+/-1) LU target specified by R128 when measured in surround, the loudness can fail these limits when reproduced in stereo. Surround content can theoretically be up to 3.7 LU louder or as much as 4.5 LU softer when reproduced in stereo. The Sentinel solution from Hamlet measures all content in both its original format and after down-mixing to stereo. If the loudness differs after down-mixing you are warned so corrective action can be taken. Dialog Balance When broadcasting sporting events live it is common to carry ambient sound (crowd or stadium noise) in the four corner channels and to place the announcer in the centre front. A high level of ambient sound is desirable to convey the excitement of being there. However the levels of ambient sound and voice-over dialog must be balanced to maintain intelligibility at all times. Mix engineers have traditionally used the console level meters to insure that the mix meets relevant delivery specs. Level meters are generally a poor representation of audio loudness and the need to interpret multiple meters puts additional strain on the mix engineer. The patent pending dialog balance display is a measurement of CF channel loudness relative to the total loudness of the remaining program channels, gated based on the presence of signal in the CF channel. It can be used directly or a limit threshold can be defined allowing the engineer to ignore the meter and depend on his ears until problems occur in the mix. This is only available with the Qualis Audio Sentinel from Hamlet. The Sentinel is best summarized as an electronic listener who reports judgments over a network connection. It measures loudness, down-mix compatibility, intelligibility, levels, balance, hum, metadata and other relevant parameters although it can display results graphically in a standard browser window, it doesnt need to. All measurements create numeric results which can automatically be compared to desired performance limits and generate alarms if signals fall outside user selected boundaries. This allows the Sentinel to function equally well as an unattended monitor, reducing personnel requirements, or as an assist to a skilled operator, allowing attention to be put on other tasks besides detecting common audio errors.
Tags: iss071 | surround sound | hamlet | dolby ac-3 | itu-1770 | ebu r128 | N/A
Contributing Author N/A

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