Stereo and Surround Formats

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In the audio post-production world, you must be prepared to mix down your project to just about any format, from mono through 7.1 Surround. One project will often be mixed down to several formats at the same time. Specialized tools are needed to perform these down-mixing routines. Dolby systems require that you encode a special signal when mixing down surround tracks. In today's ever-changing consumer world, you never know how your original 5.1 mix is going to be heard . It might end up coming out of a mono television speaker, audiophile home theater system, or anything in between. Being able to preview all these possibilities is an important part of your mixdown procedure.

Encoding and Decoding with Dolby Technologies

Dolby Laboratories is the leader in multi-channel, sound-encoding techniques. They have pioneered methods for bringing multi-channel surround sound into theaters and into the home. You should be familiar with the encoding processes that certain Dolby formats require.

Dolby Stereo and Pro Logic

The original Dolby surround format is known in theaters as Dolby Stereo and in the home as Dolby Pro Logic. This format encodes a center and surround channel within a stereo signal. The stereo signal can then be recorded onto 35mm optical film, VHS tape, DVD, or even broadcast over stereo television without losing any of this encoded material. The stereo signal is then decoded into left, center, right, and surround (LCRS) channels when played back through a proper Dolby Stereo cinema processor or Dolby Pro Logic AV system. In addition to manufacturing over 70,000 Dolby Stereo capable cinema processors, Dolby has licensed this technology to nearly every manufacturer of consumer playback equipment, making it the most widely used encoding technology for surround sound.

The encoding process requires four discrete inputs that are processed into a resulting stereo output file. The center channel information is simply added equally to both the left and right channels of the stereo output at the appropriate level. The surround channel is band -limited and encoded as a +90 degree phase signal in the stereo mix. This is called phase-matrixed encoding . The decoder can then take any completely out-of-phase signal within that band and send it to the surround channel. Similarly, any signals that are equally present in both the left and right speakers will be redirected to the center channel. This is an oversimplification of the process, but it will suffice for the purposes of this book.

This encoding-decoding process can provide surprising results in certain situations. Stereo synthesizers are notorious for creating false surround images through the decoder because of the radical phase manipulation used to create these sounds. In order to be sure of what your mix will sound like once it's been through the encoding and decoding process, you must mix while listening through an encoder and decoder. Dolby provides hardware units that will encode and then decode the signal for you, allowing you to hear the results of the process. For TDM users, there are the Dolby Surround Tools plug-ins, which will encode and decode Dolby Surround signals right inside Pro Tools.

As is often the case these days, the Dolby Pro Logic mix will be derived from a discrete 5.1 master. The process is called downmixing , whereby the extra channels in a 5.1 system are blended together in certain ways to create an LCRS or Dolby Pro Logic Master. Downmixing will be discussed later in this chapter.

5.1 Surround and Dolby Digital

5.1 Surround formats have a left, center, right, left surround, and right surround channels plus a LFE channel, making a total of six. These channels can be encoded using the Dolby Digital format for a theatrical presentation and DVD home-theater presentation.

This encoding scheme differs from that of Dolby Surround (Pro Logic) in that each channel is discrete and there is no matrixing of the surround channels into a stereo-encoded signal. It is a digital process rather than analog. This digital encoding can occur in the studio with the proper equipment. There are certain parameters affecting levels that can be encoded into the metadata that the mix engineer should have control over.

If encoding the Dolby Digital signal is not possible, the audio studio can provide a 6-channel discrete mix, with time code, to the film transfer house or DVD-authoring studio for encoding. Being present at the encoding stage is advisable for quality control purposes. It might also be necessary to prepare a Dolby Stereo or Pro Logic downmix of the discrete channels for compatibility with older systems. This downmix will also be compatible with simple stereo and even mono broadcast chains. It is always a good idea to preview your mix in all of these formats.

NOTE

MULTI-CHANNEL PROCESSING AND FORMAT CONFUSION

Take care to differentiate between the different Dolby formats and bass management systems. If these get confused , improperly formatted mixes will be the result. Dolby Stereo and Pro Logic must be encoded during mixdown. Dolby Digital signals will not be encoded during mixdown, but will be encoded from an all discrete, 6-channel master when the optical film print is created or in the DVD authoring suite. Bass management systems are only used for playback calibration and monitoring, not for processing the actual audio recording.

Downmixing for Other Formats

As described above, downmixing is the process of taking multi-channel surround sound mixes and recombining the channels for compatibility with systems having fewer channels. 5.1 surround mixes will often need to be downmixed to Dolby Surround or Pro Logic formats, regular stereo, and even mono in some broadcast applications.

Dolby Digital decoders have the ability to perform downmixing during the decoding process. Using the same preprogrammed algorithm as found in a Dolby Surround encoder, center and surround channels will be recombined with the left and right signals to create a stereo signal encoded in Dolby Surround. The Dolby Stereo signal is also compatible with stereo and mono playback systems. So creating one downmix provides compatibility with every type of playback system at once.

This automatic downmixing involves some dynamic processing to avoid overloading the stereo channels caused by combining the other various channels. The results of the automatic process might not be desirable in every situation. The Dolby Digital encoder offers a wide degree of flexibility to the encoding engineer in how such dynamic range processing is applied to the signal by the decoder. Often, these powerful tools are preset and ignored. Creating downmix files in the studio is an alternative that allows predictable results with more control. Plug-ins, such as the Waves Surround Toolkit, have been developed to facilitate this process easily. But unless the delivery system offers both the 5.1 and the 2-channel soundtracks , the user may not have access to the handmade downmix. In that case, it is worthwhile taking time to best exploit the internal downmix options as much as possible.

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