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Demystifying a black art... by Alan Wheable A t the pure analogue end of the audio world (which these days is very narrow with much of the audio content being captured digitally) it is quite straight forward to check that an audio channel is present, that it sounds right and has the right levels. Likewise if the audio channel has hum or a buzz on it is relatively easy to identify the problem and fix it. In the digital audio world where multiple digital audio channels are being delivered at different bit rates, as PCM or in a variety of compression formats, as mono, stereo or surround sound (in many flavours) for use in HDTV, DVD, Cable, Satellite, internet, mobile phone, etc, it can be nearly impossible to spot where things are going wrong let alone work out how to fix them. The daily use of technologies such as Dolby® E, Dolby® Digital and Dolby® Digital Plus in all areas of broadcast television content delivery requires an understanding of the importance of metadata, the transport mechanism as well as the tools to check them. Digital Audio is a very broad and sophisticated subject area so here I will only focus on the specific challenges around Dolby® E, Dolby® Digital and Dolby® Digital Plus as part of a SDI transport stream. Dolby® formats in common use in media distribution There are a number of Dolby® digital audio standards that transport audio data within the SMPTE 337M-2000 data burst or AES audio and embedded within the SDI data stream. These include: Dolby® E Dolby® Digital Dolby® Digital Plus These standards can be used to transport mono, stereo, 5.1 and 7.1 audio programmes: Dolby® 5.1 uses five channels for normal-range speakers (20 Hz – 20,000 Hz) (right front, centre, left front, surround right and surround left) and one channel (20 Hz – 120 Hz allotted audio) for the subwoofer driven low- frequency effects. Dolby® 7.1 (non broadcast) uses six channels in the primary program (Independent Substream) for a standard 5.1 surround sound mix and then the remaining 2 channels in an ancillary programme (Dependent Substream) to provide the additional down-mix version. Dolby® E Dolby® E is a video frame-based audio encoding and decoding technology developed by Dolby Laboratories that allows up to 8 channels of audio (mono, stereo, 5.1 or 7.1) for a primary programme (Programme 1) and optional ancillary programs. These 8 channels are compressed into a digital stream that can be transferred between compatible devices and stored on a standard stereo pair of audio tracks. Dolby® E is primarily a production format that allows the relatively transparent movement of a finished Dolby Surround Sound audio programme, via SD-SDI, HD-SDI or 3G-SDI, through the production chain until it reaches the point of transmission where it is converted to Dolby® Digital or Dolby® Digital Plus. As Dolby® E is frame-based it allows simple editing such as cuts that do not affect the audio. Dolby® Digital Dolby® Digital (AC-3) is a ‘perceptual audio’ system for digital audio that allows the reduction of data needed to deliver high-quality sound. This system relies on the fact that the human ear will screen out certain levels of sound that are perceived to be noise. The removal of this noise reduces the amount of data needed to deliver the sound to the listener. This system was developed primarily for DTV, DVD and HDTV. Dolby® Digital technology was developed by Dolby LaboratoriesTM to allow up to six channels of sound (mono, stereo or 5.1) in the form of a single ‘program’ that can be delivered at different bit rates. These 6 channels are compressed (lossy) into a digital stream that can be broadcast. Dolby® Digital Plus Dolby® Digital Plus (E-AC-3) is a more advanced version of Dolby® Digital that provides a more efficient encoding algorithm that provides enough bandwidth to support sophisticated multi-programme content combining mono, stereo, 5.1, 7.1 & 13.1 for a 56 | TV-BAY MAGAZINE: ISSUE 79 JULY 2013 TV-BAY079JUL13.indd 56 09/07/2013 16:51