special report audio processing


Recent injections of innovation have given the broadcast industry a firmer grasp on audio processing. Hardware technologies such as digital signal processors (DSPs) have made audio processing dramatically more powerful and efficient compared to earlier techniques. Well defined audio metadata now helps control perceptual loudness disparities across programme content, while real time and file-based loudness processing will accomplish the same task where metadata does not exist or cannot be used. Mike Richardson, director of products at Linear Acoustic, reports.


Processing audio within transport streams:


a new frontier M


ost audio processing is currently performed on discrete audio signals and transported as either a PCM encoded signal


(AES/EBU) or embedded into a digital video signal. These audio signals are either baseband or encoded to reduce bit rate and fit more audio channels in the space an unprocessed stereo pair would occupy. Popular encoding techniques include MPEG 1 Layer 2, Dolby Digital, Dolby Digital Plus, and HE AAC. But progress never sleeps. The drive


to reliably pass more programme content continues to push operators to find methods that combine a much higher number of video and audio signals together. The most common transport stream - MPEG 2 -


56 l ibe l november/december 2011 l www.ibeweb.com


accomplishes this by combining bit rate reduced video and audio signals onto a single physical transport signal. Well- defined data describes which signals are video, audio, or data, and their relationships. Audio processing on transport


Mike Richardson, director of products at Linear Acoustic.


streams brings an entirely new set of problems though. Such processing has to accommodate issues that do not exist in other transport methods, such as very high signal density, (potentially) different encoding formats in the same transport stream, & ensuring table data remains accurate. What are the advantages and


potential pitfalls of processing audio contained in a transport stream? Why process audio at all? In an ideal world, all audio content


would be produced with dialogue (or other anchor element) at a consistent


with proper audio processing. Processors exist currently with a wide range of specific features. Loudness


level, programme dynamics would be appropriate for the content type, and all signals would carry proper metadata to relay this information. All content would also be created uniformly as 5.1, stereo, or some other mode. Suffice it to say that we are not yet at this ideal point. Commercials are still produced to be as ear-catching as possible rather than to blend with the surrounding programme content. There are no guarantees that older content levels will match newer. There are many situations where accurate metadata simply doesn’t exist. And it is not unusual for switches to occur between stereo commercials and 5.1 surround coding throughout a broadcasting day. These issues can all be mitigated


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