Integrated solutions for embedded Dolby E and AC-3
Romolo Magarelli and David Strachan, Evertz Microsystems Ltd.
Synopsis Dolby E offers the television broadcaster the ability to carry up to 8 channels of audio, complete with metadata, on a single AES-3 digital audio cable. Many broadcasters are adopting this convenient technology as a way to reduce the number of cables needed to transportmultiple audio channels around a facility, ensure phase coherent treatment of multiple related channels (e.g. 5.1 audio) and bind AC-3 encoded information (metadata) together with the audio. We need to be aware that, if all audio and video paths are not designed carefully, the advantages can be marred by lip sync problems, audio errors, and loss of information. This paper describes the Dolby Eprocess and ways to avoid common pitfalls. We show how to de-embed and decode Dolby E material and to re-embed the audio back onto the SDI or HDSDI video as PCM audio. The metadata is preserved throughout the process and can be reembedded along with the PCM audio. Lastly we describe the process of converting embedded PCM audio back to embedded Dolby E or AC-3.
Receiving Dolby E or Dolby Digitalencoded audio at the TV station We will assume that the signal being received at a broadcast affiliate station, has been correctly encoded using a Dolby E or Dolby Digital (AC-3) encoder. There is a good chance that the broadcaster will need to decode the audio, listen to it, modify it and maybe transport it to other parts of the facility before re-encoding it for onward distribution. Figure 1 showssome of the devices which would be needed to achieve this.
Figure 1 - Discrete devices to handle Dolby E content
Engineers are often content to learn the hard way - I guess because we like a challenge. But it can be time consuming and an expensive exercise for our companies, especially if an installation has to be re-worked to correct errors. The following are examples of the problems thatbroadcasters regularly find, as soon as they start playing with compressed audio. Audio clicks, pops and discontinuities when switching discrete or embedded audio Complete loss of audio Loss of audio channels Loss of audio metadata Lip sync problems between the program video and the audio
Everyone who has watched television has witnessed one or more of these annoying discrepancies. Dealing withcompressed audio is a complex subject and a brief summary of Dolby audio signal structure may help us understand how things can so easily go wrong if we are not careful. Dolby E and Dolby Digital Modern home theatre systems use 5.1 or 7.1 audio channels, to provide a surround sound experience. The 5.1 systems have six channels of audio with Left (L), Right (R), Center (C), Left surround (Ls) andRight surround (Rs) together with a Low Frequency Effects (LFE) channel covering the lower 10% of the audio spectrum. More advanced home theater systems feature eight channels of audio, adding Left Back (Lb) and Right Back (Rb) loud speakers, to provide sounds which completely surround the audience. Dolby Digital (AC-3) is the encoding scheme used by broadcasters to transmit all of the audiochannels to the home. The audio data is carried in the ATSC or DVB encoded bit stream. Dolby E encoding, on the other hand, is used within the TV station. The main difference between the two systems, is that Dolby E is frame locked to the video, allowing clean switching between video channels, whereas AC-3 has no relationship to video. Dolby E is also more robust and can better survive multiplegenerations of transcoding. Both systems compress the audio to accommodate eight channels (Dolby E) or 6 channels (AC-3) of audio into one AES stream.
Figure 2 AES-3/Dolby E Packet Structure
The Dolby E encoder, bundles the audio data into packets corresponding to each video frame. Each frame starts with a burst header (guard band) consisting of a minimum of four zeros. This is followed by a sync...
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