PA Processing Part 1

I’ve meant to write about our PA processing for the past few months but an experience last week during Easter rehearsals has moved this up the topics list. We have a BSS Soundweb London BLU-80 that functions as our primary system controller. The London is tremendously powerful, allowing the user to create a completely customized signal chain that is only limited by the available processing power. In our configuration, it takes separate stereo music and speech L/R mixes from the Digidesign D-Show Profile and handles all system equalization, delay, crossover, and limiting to 12 output channels:

1. KF650 Left Hi
2. KF650 Left Mid
3. KF650 Left Low
4. KF650 Right Hi
5. KF650 Right Mid
6. KF650 Right Low
7. KF300 Left Outside
8. KF300 Left Inside
9. KF300 Right Inside
10. KF300 Right Outside
11. KF260 Downfills
12. Balcony Delay

Here is the routing design in London Architect for how the BLU-80 is configured. Each box on the diagram below represents a processing unit. From left to right, yellow boxes represent physical inputs or outputs from the box, orange represents delay, purple is equalization, gray is a high pass filter, green is a matrix mixer, blue is gain control, and green is limiting.

The signal flow through the box is as follows:

• Signal enters and is separated onto two paths – one for music and one for speech. Each goes through a delay that aligns the master mix to the subwoofers (12 ms) and then into a master “house” parametric EQ. This is the artistic curve that adjusts the frequency response of the entire system. There are different curves optimized for music & speech.
• After passing through the artistic parametric EQ, each of the two mixes are split into a full range version and a high passed version that then hits a matrix mixer which routes to the various individual speaker chains. With music, for example, a full range mix is sent to the main KF650s, but a high passed version is sent to the downfill speakers and center cluster in order to reduce 250 hz buildup in the room. The matrix mixer really acts more as a router – its all or nothing for each speaker channel, either full range or high passed.
• From there, the KF650s hit another parametric that allows for some sculpting of the 650s as a whole. The output from this parametric hits the three-way crossover for the boxes. These crossover points are factory specified. From there, each driver’s output hits a dedicated parametric with factory specified settings, driver gain controls, and then driver limiting.
• For the KF300s, KF260s, or Balcony feeds, the output from the matrix hits a dedicated parametric for box sculpting, then a parametric for factory specified box settings, gain and limiting, then channel delay to align with the KF650s.

While you read the signal chain in the London from left to right, the process of equalizing and aligning the system basically works from right to left. The first step is to enter all of the factory box settings, gain balance between low/mid/hi drivers, and align the delay times between the various boxes. From there, we use EASERA to start with the KF650s, scope them, and make adjustments until the response is as desired. Then add the KF300s one at a time, scoping them individually, then adding each one into the 650s and making the necessary adjustments. Continue the process until all of the boxes are active and frequency response is fairly flat. At this point the artistic master parametric EQ comes into play to sculpt the overall response to fit the mix and the room