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Bass Management in Pro Tools

11/13/2007

To hear what your Pro Tools surround mix will sound like through a bass-managed playback system, you simply need to split the surround output into two parts: the subwoofer signal and the filtered surround channels. Download the Bass Management.ptf Pro Tools session to see how to do this. (Be sure to check out the I/O setup.) To hear it in action, import a bounced 5.1 mix into the Source Audio track. Once you grasp the procedure, you can simply import all tracks (except Source Audio) into your own session and assign I/O accordingly.

The source tracks (Source Audio in this example) are assigned simultaneously to two 5.1 buses called Satellite and Subwoofer. To add the second output assignment, hold the Ctrl key (Start on Windows) as you assign the second bus to track outputs. A plus (+) sign will appear next to the displayed output to indicate additional assignments.

The Satellite bus returns to a 5.1 aux input called Satellites. On this aux is a multi-mono EQ. All channels except the LFE are highpass-filtered at 80 Hz. The LFE is highpass-filtered at 20 kHz to effectively mute it—there are probably slicker ways of muting this channel, but not using just the EQ III. The Satellites aux is then assigned to the Main Surround output path, carrying the bass-managed signal for the five main speakers. Five down, one to go.

The Subwoofer bus takes a little more work, because the six channels must be summed together. A master fader is assigned to the Subwoofer bus so that a multimono EQ can lowpass filter all six channels at 80 Hz. Each channel of the Subwoofer bus is then assigned to its own mono aux input, all of which are assigned to the LFE output sub-path. This sums all six channels together. Because of this summing, the aux inputs for the five main channels are lowered by three decibels.

Because some systems use other cutoffs for bass management, there is another EQ on each bus. These use a cutoff of 120 Hz, except for the LFE channel on the Subwoofer Master, as LFE would still be filtered at 80 Hz in production. Bypass the first pair of EQs and un-bypass the second to hear the difference. I''ve also included a Signal Generator with pink noise on the Source Audio track in case you want to hear the effect with a test signal. Turn your monitors down before enabling the Signal Generator so you don''t get blasted.

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