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Encoding and Recording Your Surround Sound Mix to Disc
Written by Mike Sokol - © 2005, PRIMEDIA Business Magazines & Media Inc. Reprinted with Permission.
So you've bitten the proverbial bullet and installed a mixer or workstation software application with surround panners, set up a 5.1 monitoring system, and actually recorded some surround music mixes on a Tascam DA-88 or ADAT. (See Introduction to Surround Sound and Setting Up Your Studio for Surround Sound). Now, how do you get anyone else to hear your surround opus? You can't play a DA-88 tape on a consumer system, and dragging a multitrack deck around isn't practical.

You need to encode your six discrete tracks into a format that will play on a consumer home-theater system. The process is not really that complicated, and it's thrilling to hear something you mixed in surround play back properly on a home system. This article describes the final part of the surround-production process.

Codec The Halls
To easily distribute 5.1-channel recordings, you must encode them into a format that can be stored on common media, such as CDs and DVDs, and transmitted over common digital-audio interfaces, such as S/PDIF. In general, that requires the data to be compressed (reduced in size). Two types of 5.1 encoding are often used: Dolby Digital and DTS from Digital Theater Systems. DTS offers 5.1 and 6.1 formats, whereas Dolby Digital can be implemented in any format from mono to 5.1.

Both systems encode the six channels of a discrete surround mix into a single data file that can be transmitted through a standard AES/EBU or S/PDIF interface. Then, on the playback end, the file is decoded back into its original six channels before being sent to the speakers. (The entire encode-decode cycle is often referred to as a codec, which is a contraction of enCOde and DECode.) Think of encoded data as being like instant mashed potatoes. After the water is removed (encoded), the powder is compact to store and transport. Then, the end user just adds water and heats it to a boil (decoding) and voilà … instant mashed potatoes.

You don't have to cook a DVD or CD in boiling water (kids, don't try this at home), but all home-theater receivers do an analogous rehydration process. If you play a DVD with a Dolby Digital or DTS soundtrack, the receiver decodes the digital bitstream from the player's S/PDIF output as the disc plays back.

DVDs have much more storage space (4.7 GB compared with a CD's 640 MB), so they have lots of room for audio. Unfortunately, most of the data space on DVD-Video discs is hogged by the video portion, so the official DVD-Video specification mandates a Dolby Digital audio program at a transfer rate of 56 to 448 Kbps in order to fit as much audio as possible within the limited storage space. DTS files can use transfer rates of 1.5 Mbps or 768 Kbps from a DVD.

DTS and Dolby Digital files can also be burned onto CDs; in fact, DTS-encoded CDs use the same 1.4 Mbps transfer rate as stereo 16-bit, 44.1 kHz CDs, and they can be duplicated by any pressing house that makes standard stereo CDs. In addition, the file structure of a DTS track is the same as a standard CD's, so you can fit the same 74 minutes (or as many as 99 songs) of 5.1-channel audio on a disc. However, instead of containing pulse-code modulation (PCM) digital-audio data, such CDs hold Dolby Digital or DTS data files. Those discs will play in a CD player that has an S/PDIF output connected to a home-theater system with a Dolby Digital or DTS decoder. (All digital home-theater receivers have a Dolby Digital decoder built in, and most also have a DTS decoder.)

Be sure not to play a Dolby Digital or DTS CD from the player's analog audio outputs; the encoded datastream sounds like nothing but noise, which can damage speakers if the level is high enough. The same problem arises if you send the data from the CD player's digital output to a sound system without a decoder, so carefully label any Dolby Digital or DTS CDs accordingly.

Dolby and DTS make hardware encoders, but they may be too expensive for small studios. For example, Dolby's DP569 encoder retails for $5,000, and the DTS CAE-4 encoder costs $7,250. As a result, most small studios use software encoders. Minnetonka Audio manufactures the only available standalone DTS software encoders: SurCode CD Pro ($499), which allows you to encode DTS files for CDs, and SurCode DVD Pro ($1,999), which encodes DTS files for DVD or CD. In addition, Minnetonka makes SurCode Dolby Digital ($999), which encodes Dolby Digital files for DVD or CD. Minnetonka's software is Windows-based and cannot run on a Mac. Users of Pro Tools can use the SmartCode Pro Dolby Digital ($795), DTS ($1,495), and DTS-CD ($495) encoding plug-ins from Kind of Loud, along with that company's excellent Woofie and Tweetie surround-monitoring and panning plug-ins.

A Sense Of Loss
Like all dehydrating-rehydrating processes, encoding entails the loss of a little flavor. Dolby Digital and DTS use lossy encoding methods; as a result, a portion of the original audio information is lost in the compression process. The audio coming out of the decoder is not bit-for-bit identical to what went into the encoder.

Put that way, encoding sounds like a horrific thing to do to your carefully crafted surround mix. However, the final results are quite good and difficult to distinguish from the original tracks except under close scrutiny in a controlled listening environment.

A lossy codec system works by looking for redundant and masked audio information. After deciding what humans can and cannot hear, it keeps the important sonic information and throws away the things that theoretically can't be heard anyway. Thus, you can reduce data files to a tenth of their original size, depending on the codec and the selected data rate.

The new DVD-Audio discs don't use lossy compression; instead, they use a lossless compression scheme called Meridian Lossless Packing (MLP), which was developed by British audio manufacturer Meridian and reduces the size of audio files by approximately half, depending on the material. MLP allows as much as 74 minutes of 6-channel, 24-bit, 96 kHz audio without any loss of information. It's a beautiful thing. This is an excerpt from the following article: Secret Encoder Ring.

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