Consumers have come to expect surround sound from their entertainment sources such as movies and now television shows. Toward this end, the ATSC standard incorporated multichannel sound into the DTV specification. How to record, mix, playout and monitor multichannel sound is the focus of this article.
Recording 5.1 audio
Out in the field, many videographers just record two-channel stereo sound and sometimes record extra channels to catch the ambient sound. Then back in post, they mix a 5.1 surround output for the final cut.
There are several 5.1 microphones that will supply you with six channels of sound, but most of today's camcorders only come with two to four audio channels. To really record six channels of audio, you would need a professional multichannel audio recorder with SMPTE time code locked to your camcorder.
5.1 audio signals
Professionally, it's called 5.1 audio, and it consists of six separate audio channels — Left (L), Right (R), Center (C), Left surround (Ls), Right surround (Rs) and the Low Frequency Effects (LFE) channel. (See Figure 1.) These six audio channels are used for DVD, cable, satellite and broadcast DTV audio.
One of the problems with using three separate AES3 cables to transport 5.1 audio is keeping them all synchronized together and with the video, as well as routing all three around the plant. Because AES3's data frames are not synchronized with video, it's easy enough for one to slip out of sync, let alone three. Today, there are better options, including Multichannel Audio Digital Interface (MADI), Dolby E and embedded audio within SDI.
SDI video can handle up to 16 channels of digital audio or eight AES3 audio feeds. Thus, 5.1 audio can easily be accommodated by embedding it within the associated SDI video. For SD-SDI, the standard is SMPTE 272M. For HD-SDI, it is SMPTE 299M. The audio data is transmitted during the horizontal blanking period that is not used.
MADI is capable of transporting up to 64 separate digital audio channels over a single 75V coax cable or fiber-optic cable. MADI is basically a high-speed AES3 transport. Where AES3 will handle just two channels compared with MADI's 64 channels, its official designation is AES10-2003. While MADI has been mostly found in recording studios to connect mixing consoles to multitrack recorders, it has now found use in television production studios and broadcast facilities.
AES50 is another standard for transporting multichannel audio around a facility. It uses IP packets and Cat 5 cabling. Each channel is assigned 1Mb to assure a very high level of quality. AES50 will handle up to 16 channels of 192Kb digital audio or more channels at lower bit rates.
It's meant as a point-to-point system even though it uses Ethernet cable and IP packets. Trying to pass it through a network can cause a high degree of latency. AES50 has not found widespread acceptance or use.
Dolby E is a proprietary system developed by Dolby Labs. It was designed to overcome many of the issues that engineers face when trying to use any of the above 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.
Monitoring 5.1 audio can be problematic within a control room environment, and 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. So is the ability to perform a downmix to two-channel stereo. Because many viewers are only listening in stereo audio, it's important to monitor it.
Most of the time, being able to look at a level meter or, better yet, a graphic display of the surround-sound environment is the way most engineers will monitor 5.1 audio. Several manufactures make such instruments that display a roughly circular pattern showing that all channels are present and that there are no phase problems. This display is usually accompanied by six bar graph meters. This type of monitoring is essential when trying to monitor 5.1 audio in a television facility.
Converting to two channels
Normally, all six channels are transmitted, received and played back on the viewer's surround-sound audio system. But, there are many cases where viewers have only the two stereo speakers to listen to. What happens then? All ATSC receivers have the capability of downmixing the 5.1 surround sound down to two channels. There are two ways they can do this: The first is called L/R Total, and the other is L/R Only. (See the “Downmixing 5.1 audio” sidebar.)
Converting to six channels
If most of your programming is in 5.1 surround sound, then switching to spots and other programming that is in stereo-only may not be what you want, jarring your viewers when switching between the two. In that case, you can upmix — take a stereo audio feed and convert it to 5.1 surround. There are several processors on the market that will do this for you. These can be added to your audio chain with some master control switchers capable of being programmed to switch in the 5.1 converter as needed for stereo-only sources.
Even though 5.1 audio has been around for several years, many stations have not embraced adding it to local productions. To do so means changing audio boards as well as control room monitoring and even signal routing. But with proper planning and installing the right equipment, it does not have to be as big of a problem as it first appears.
Russell Brown is chief engineer at KMTP-TV in San Francisco and writer of Broadcast Engineering's “Transition to Digital” e-newsletter.
Downmixing 5.1 audio
There are two types of downmixes that can be performed on 5.1 audio: L/R total downmix and L/R only downmix.
L/R total downmix:
Lt = L + [-3dB center] + -3dB [Ls + Rs]
Rt = R + [-3dB center] + -3dB [Ls + Rs]
L/R only downmix:
Lo = L + [-3dB center] plus
[Ls maybe be added at a higher attenuation]
Ro = R + [-3dB center] plus
[Rs may be added at a higher attenuation]
* Ls = left surround; Rs = right surround