[georgia]

A brief History

We’ve been mixing in 5.1 and surround for a year or two now. Mostly in someone else’s studio. After a rather long period of research, soul search, and for whatever reason, search me, we decided to enter the 21st century. Our studio, Samurai Music went over to the “other” side. We are now a 5.1 Surround Sound Facility… Of course the first thing we did, err..well, actually the second thing we did, the first being, the emptying of our checking account…., was to order a solid and capable 5.1 system.
Since we already owned a Protools 24|Mixplus system we selected Kind-of-Loud’s Smart Pan Pro, the AXIS surround panner and, of course, my favorite… Genelec Monitors and Subwoofers.
So lets break this down a bit. Genelec 1030A, 5 of them, plus a single 1092A (about the size of a big trash compactor). Note: we also have 1031A’s but I personally prefer the 1030A’s for surround sound mixing.
Protools 5.0, Smart Pan Pro, Tweetie, Woofie, and Realverb 5.1. Axis Hardware/Software Panner.
After pulling everything out of boxes and installing our newly beloved 5.1 system, installing the software, mucking about with a new DVD player and generally adding another 2 miles of cable to the studio…. We sat back and listened… it just sounded so.. well, lets put it this way… I never want to listen to stereo again…ever. Wow. Until you’ve heard a good 5.1 system you haven’t lived, and I’m not talking about the ones in most movie theaters (aka: blown tweeters, ripped subs, channels out, levels wrong etc etc), I’m talking about a well tuned 5.1 system in a good listening room. Breathtaking. Anyway, after a couple of days of listing, watching DVDs and Surround CDs, we looked at the system, shrugged, and opened the hood.
Most of what we found made complete sense. The one thing that rather baffled some of our staff was bass management. After digging through manuals, the Internet, books, etc, I came up with the following simplified review of 5.1 bass management.. Since all this research was sitting here, I figured, why not share it. So for you surround sound dummies, like me, here’s my attempt to de-mystify 5.1 bass management.

5.1 Bass Management – an overview

Bass management revolves around the use of Sub-frequencies from the 5 channels (Left, Right, Center, Left Surround (Ls), and Right Surround (Rs)), as well as the use of the Low Frequency Effects Channel (LFE) Together this set of audio data represents the .1 channel in the 5.1. Low frequencies, nominally around 80 to 100 Hz are filtered from the 5 main channels, routed and summed to the sub-woofer. It’s just like a normal stereo system with the additional of a sub. The difference is in the addition of LFE. The LFE channel offers a unique way to add additional “oomph” to any mix, either in music or with effects such as the standard Hollywood explosion, tornado, sinking ship, train wreck, etc. Simply add a bit of low frequency via the LFE at just the right moment and bingo the room shakes like the A-train in a New York Subway. Managing the added low frequency content is made relatively easy with the addition of the LFE channel. The LFE channel provides an independent bass path that can be utilized at will without affecting, or being affected by, the normal bass coming from the 5 main channels. With the ability to utilize both the traditional subwoofer signal and the addition of an LFE audio path, 5.1 bass management plays an important, although sometimes overlooked, role in 5.1 mixing.
We selected the Kind-of-Loud system in order to utilize its bass management abilities, as well as, its natural ability to easily manipulate a sound field in an array of ways. Additionally, our new Genelec sub-woofer provides its own rudimentary bass management. With the internal Genelec capabilities, and the addition of our Smart Pan Pro, Woofie, and Tweetie plug-ins we can manipulate the bass in more ways than we though possible. In fact we’ve found a number of ways to completely decimate a two thousand dollar sub and our neighbor’s goodwill in less that the time it takes to generate a 20Hz 120db sine wave…

Genelec Monitoring System’s Bass Management

Lets start with the Genelec speaker system. Our system includes 5 matched Genelec full range monitors and 1 sub-woofer. The sub woofer provides 4 inputs and 3 outputs. I/O (Input /Output) 1 is the Left Front channel (L), I/O 2 is the Center Channel (C), I/O 3 in the Right Front Channel (R), and Input 4 is the sub input (S). The reason for this is that Genelec has included a low frequency filter that has a center frequency at 85Hz as part of the subwoofer system. All signals from the L, C, and R can be routed through the subwoofer. When the L, C, and R speakers are routed through the sub all signals 85 Hz and below are routed to the subwoofer. Additional low frequency signals, like LFE, can be independently routed to the sub via the SUB input. If you wish to bypass the internal filter, Genelec kindly provides a simple 1/4” jack and a wiring diagram for a bypass/ mute switch. I can hear my system without a sub, or with the flick of a switch, provide a justifiable homicide plea for my downstairs neighbor.

Bass Management Over Simplified

On top of the Genelec’s bass management, Kind-of-Loud’s software provides even more bass management capabilities. Ok, we need to deal with low frequencies from general audio tracks and specific special effects. Let’s take a look at the basic block diagram. A single audio signal, lets say a typical Hollywood explosion, is placed on an audio track. We route the track to the 5 main channels via a 5.1 panning system, or simply routing on any 8-buss console. In this case we’ll use the Smart Pan Pro and its associated Software.
The input signal is routed to the panning software plug-in where we can send the basic explosion audio to any, or all of our 5 main channels (L, R, C, Ls, Rs). Additionally, via the LFE output we can bus the same signal to our LFE channel.
So far we have full range audio heading for up to 5 independent channels:
 The L and R channels are routed to the Smart Pan Pro (SPP) LR outputs
 The C Channel to the SPP CS outputs
 The Ls and Rs channels to the SPP LsRs outputs
You might be thinking I forgot the Sub channel. I didn’t, it gets sorted out later down the audio pipeline.
Finally, via the LFE gain control, we can send the same signal via an independent route to the subwoofer.
Let’s follow each signal path separately.
L / R signal path
The full range audio is routed to the bass management, where it is filtered and the low frequency sent to the Subwoofer, while the remaining audio is sent on to the L and R front speakers. The low frequency audio is then sent to the bass management module where independent gain adjustment can be made to the overall bass levels. Following the input gain, the low frequency audio is sent to the Bass Extension on to the Bass Redirection.
Center / Sub signal path
The full range Center channel audio is routed to the bass management, where it is filtered and the low frequency sent to the Subwoofer, while the remaining audio is sent on to the Center speaker. Again, like the L and R signal path, the low frequency audio is then sent to the bass management module where independent gain adjustment can be made to the overall bass levels. Following the input gain, the low frequency audio is again, sent to the Bass Extension on to Bass Redirection.


Ls / Rs signal path
The full range audio signals of the Left surround channel and the Right Surround Channel are routed to the bass management, where the low frequency filtered signal sent to the Subwoofer, while the remaining audio is sent on to the Ls and Rs (rear) channels. Again, like the L and R signal path, the low frequency audio is then sent to the bass management module where independent gain adjustment can be made to the overall bass levels. Following the input gain, the low frequency audio is again, sent to the Bass Extension on to Bass Redirection.
Bass Extension
Bass Extension allows the low frequencies to be sent, not only to the sub, but back to all 5 main channels (L,R,C,Ls,Rs) channels. This allows for the bass to emanate from all 5 speakers (L,R,C, Ls, and Rs). The Bass extension can be disabled. In this case the low frequencies are routed to the Bass Redirection module only.
Bass Redirection
Bass Redirection provides Mutes for all 5-channel low frequency signals. Un-muted low frequency signals are then summed and sent to the subwoofer via the Sub input.
LFE Channel
Finally, the Low Frequency Effects channel provides a completely independent path to the subwoofer from our initial explosion. The LFE send (in this case the LFE gain on the SPP Plug-in) routes the LFE signal directly the bass management where its’ gain can be separately adjusted, muted and routed to the subwoofer. In this manner you can decide exactly when, what and how much low frequency is utilized for exactly what purpose.
Between the monitor system’s bass management and the software bass management we have the ability to add as much or as little bass as necessary to meet the needs of the moment. But, a word of caution… bass frequencies can reach the sub from four specific ways in this system.
1. LFE signal
2. Bass Redirection
3. Bass Extension
4. Genelec’s internal filter
I should take a minute to mention a couple of issues regarding phase relationships with the Sub/LFE channel. Make sure you align your subwoofer correctly, since the sub and your 5 other speakers are now rather distant from one another. Remember, low frequencies are omni-directional so you have a reasonable amount of “wiggle room” for sub placement. If you have a subwoofer system similar to Genelec make sure that you either bypass the filter in the Genelec or mute the bass re-direction from the software. If you don’t you will run the risk of sending low frequencies to the sub from different sources, potentially out of phase.

In closing, It is entirely possible to significantly test the structural integrity of not only the subwoofer, but of your studio and your ears. Be careful, it’s amazing what a bunch-o-in-phase-bass signals can accomplish. It’s also amazing what a well-placed Low Frequency Effect can accomplish as well.
So, Get ready to Rumble!

cheers
geo