Multichannel audio Part 2
May 5, 2010 9:50 AM, Russell Brown
The last tutorial covered some of the history of multichannel sound and recording 5.1 audio, as well as embedded audio and MADI transport systems for multichannel audio. This tutorial will continue with AES50 and Dolby E. (See Figure 1.)
AES50
AES50 is another standard for transporting multichannel audio around a facility. AES50 uses IP packets and Cat 5 cabling, and each channel is assigned 1Mb/s to assure a very high level of quality. AES50 will handle up to 16 channels of 192Kb/s digital audio or more channels at lower bit rates and is meant as a point-to-point system even thought it uses Ethernet cable and IP packets. Trying to pass it through a network can cause a high degree of latency, so it has not found widespread acceptance or use.
Dolby E
Dolby E is a proprietary system developed by Dolby Labs to overcome many of the issues that engineers face when trying to use any of the aforementioned systems for broadcasting multichannel audio. Dolby E is an encode/decode system for transporting up to eight channels of digital audio over any two-channel digital link of 16-bit depth for 5.1 audio and 20 bits for the full eight channels. Besides the audio channels, it also carries metadata and time code.
Once a program is finished, its surround-sound 5.1 mix is encoded to Dolby E along with metadata about the audio tracks, including dialnorm, dynamic control and downmixing. With a complete broadcast audio chain, this metadata will be passed through to the audio processor and to the home DTV receiver. By encoding every program with Dolby E, each one would have its own audio parameters adjusted in the audio chain whenever it played on-air.
Once encoded, the Dolby E signal will pass through the complete distribution chain from videotape or server to satellite up/down link through the broadcast facility, including the master control switcher, and then be decoded back to its original 5.1 channels on three AES3 outputs, which are then connected to the on-air audio processor.
Dolby E is only meant as a finished product transport system and not for production use. Another advantage to Dolby E is that its data frames are synchronized with the video frame rate, thus making it much easier to keep the two synchronized. Still, there is a one-frame delay in both the encode and decode process, which has to be kept in mind when installing a system.
Many equipment manufacturers, including those of routers and master control switchers, have added Dolby E pass-through capabilities to their equipment to accommodate the signal.
Monitoring/metering
Monitoring 5.1 audio can be problematic within a control room environment, where two speakers and an amplifier will not suffice. Special 5.1-audio monitoring consoles have been developed to address this issue. Being able to solo the front or rear (surround) as well as the center and LFE channels is very important to being able to isolate problems in the audio, as is the ability to perform a two-channel stereo downmix. (See Figure 2.) 5.1 headphones are also available and can come in handy in a noisy control room environment. As the number of audio channels increases, so should the level of automatic monitoring and alarms.
Watching level meters, or, better yet, a graphic display, of the surround-sound environment is the way most engineers monitor 5.1 audio. Several manufacturers make such instruments that display a roughly circular pattern showing that all channels are present with no phase problems; six bar graph meters usually accompany this display. Monitoring in this way is essential when working with 5.1 audio in a TV facility. Check out Broadcast Engineering’s latest on visually monitoring surround sound here.
Downmixing
Normally, all six channels are transmitted, received and played back on the viewer’s surround-sound audio system. But there many are cases where the viewer only has the two stereo speakers to listen to, so what happens then?
All ATSC receivers have the capability to downmix the 5.1 surround sound down to two channels. They do this with two techniques: L/R total and L/R only. For L/R total, the two channels are shown as left (Lt) and right (Rt), and each is derived in this way: Lt = L + -3dB Center + -3dB [Ls + Rs] and Rt = R + -3dB Center + -3dB [Ls + Rs]. The Ls/Rs are also phase-shifted 90 degrees. For L/R only, the two channels are shown as left (Lo) and right (Ro), and each is derived thusly: Lo = L + -3dB Center + [Ls may or may not be added with higher attenuation] and Ro = R + -3dB Center + [Rs may or may not be added with higher attenuation].
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