Mixing for surround

Mixing in a surround environment creates an added dimension, so less time is spent using effects to create a sense of depth.

Over the past couple of years, the DVD has been infiltrating more and more homes. VHS tapes are becoming ancient archives, and families are enjoying higher-quality video and multichannel digital audio. Satellite and HDTV broadcast are delivering multichannel audio as well. What does this mean for the post-production studio's sound mixers?

More speakers for starters — a lot more. The preferred multichannel format is 5.1 surround, which is being delivered by two popular companies in all the hi-fi stereo shops in town. The first is Dolby Digital. Dolby Digital delivers 5.1 discrete surround sound channels via a new method of digital audio coding developed by Dolby Laboratories, called Dolby AC-3. Although this method was originally developed to carry 5.1 channels to film, because of its flexibility in handling multiple bit rates and channels, it has become a standard for some of the following formats: DVD, laser disc, digital cable, satellite TV and even some video games.

Not to be overshadowed by Dolby Digital, DTS also delivers 5.1 surround. DTS can be found on more than 200 DVD titles, players and audio receivers. DTS also is the only format to carry seven discrete audio channels via DTS-ES; an extra channel is added to rear. Dolby Digital also has a format that delivers an extra channel to the rear — Dolby Digital-EX. Dolby Digital's EX format is not discrete as it uses matrix encoding to hold information for an additional rear channel. The biggest difference between these formats is their bit rate or data compression. DTS delivers at a 4:1 ratio while Dolby Digital delivers at 12:1 ratio. Why go into such detail about these two formats? When mixing in surround, it's very important to know the specifications and limitations of the format to which you are mixing.

Let's start with the speakers and their relationship to the mixer/sound designer. Because the two 5.1 formats mentioned earlier are meant to handle full frequency and dynamic range in all channels, including the surrounds (except for the .1 or “boom” channel), it's recommended that all five speakers match. All of the speakers should have the same timbre characteristics, creating a smooth frequency response between them. The speakers, including the center speaker, should be placed equidistant to the mixing engineer's position. Front right and left speakers should be at 30-degree angles to the center, while the two rear speakers should be at 110 degrees from the front speakers. With those in place, no matter which way you turn — 90 degrees to the right or left, or 180 degrees to the rear — you should be able to reproduce a satisfactory stereo image between any set of speakers.

Now that the speakers are in place, it's time to calibrate. At a record level of -20dbfs using bandwidth-limited pink noise, set all five speakers to 85db SPL. A standard Radio Shack SPL meter set to C-weighted and slow response is ideal for calibrating speakers. Once you've calibrated those speakers, turn them off and begin to calibrate the .1 channel or subwoofer speaker. Adjust the subwoofer 6dB lower than the other five speakers. The frequency range audible to humans is 20Hz to 20Khz and is divided into 10 octaves. A subwoofer with a crossover frequency of 80Hz will handle the two lower octaves of 20- to 40Hz and 40- to 80Hz, so the subwoofer's total output should be lower than that of the other five speakers. Remember, it is important to use a test signal that contains the full spectrum of audio frequencies.

Now let's turn our attention to mixing and the importance of bass management. All surround-sound, hi-fi receivers have a bass management circuit built into them. That circuit takes all the frequencies below 80Hz out of the five main speakers and mixes them into the subwoofer, providing more use out of the subwoofer than the occasional “boom” effect. This gives the speaker system a greater frequency response down to 20- to 25Hz. So, because the vast majority of consumers have this bass management circuit in their system, if you are not bass managing while mixing, then you are not hearing what the people at home are hearing. This could result in unwanted rumble that you can't hear while mixing.

Mixing in a surround environment creates an added dimension, so less time is spent using effects such as reverbs and delays to create a sense of depth, although those effects can be used to greater enhance the performance. With multiple speakers creating a sense of depth, you can even make a mono source sound big and dimensional by panning it across the multiple speakers. If you listen to many surround mixes of music, you might find that the center channel is not used as much as you would think. The importance is not as great as it is in mixing to picture. However, that center channel can offer a more uniform mix no matter where you sit in the room. Mixing stereo fields between front right and center speakers, and doing the same on front left and center speakers do this. That method creates a more uniform and balanced image on either side of the “sweet spot” — the center listening position located within equal distance to all speakers.

Stereo compatibility is just as important because the majority of the viewing audience still can only listen to a stereo or, even worse, a mono mix. Many encoders and DVD discs will automatically downmix, but the results are unpredictable and many times unsatisfactory at best. Always go between surround, stereo and even mono when mixing to make sure the mix translates the way in which you intended.

Unlike the days of Quadraphonic surround, today's surround formats are here to stay, so it's up to the sound designers and mixers to deliver a creative listening experience.

Rob Fritts is an audio mixer for Henninger Digital Audio.