TN1004 - Eliminating PT3 and VI24 Data Noise

tn1019aIn LecNet systems using a PT3, a small burst of noise can sometimes be heard each time the PT3 translates a command. This signal is measureable at -60dBu and is normally not audible. This can be problematic if the PT3 has “monitors” programmed, since the PT3 is then constantly sending commands. The problem can be exacerbated by suboptimal gain structure, where output levels are set very high and input levels are set low.

The correct solution is to increase the gain of the microphone inputs on the mixer and decrease the output levels (if too high for the next device). If changing the gain structure does not yield satisfactory results, it is possible to eliminate the problem completely with modified LecNet expansion cables. LecNet expansion cables carry audio and data next to each other. In cases where the data lines are busy and the output gains are high, the leakage can be heard.

All such problems can be solved by using this three step process.

  1. Modify a LecNet expansion cable by removing pins 1 and 2 from both ends of the cable. Figure 1 shows how to locate pins 1 and 2. The connector has two rows of three pins and one row of two pins. Pins 1 and 2 are both on the two pin row. These are the only pins to carry sensitive audio signals. If they are not brought out into the cable, they will not pick up any noise. Use this cable to connect the PT3 to the first LecNet component in your system. If you have only one LecNet component besides the PT3, you can stop here.
  2. If multiple LecNet components are attached, the situation is trickier, since each segment of expansion cable contains both audio and data signals, and the leakage can occur in any segment. This step removes the data signals from these cable segments. The data can then be rerouted as needed using separate cables. Modify the remaining LecNet expansion cables by removing pins 5 and 6 from both ends. If the PT3 only needs to control the component that it is directly plugged into, you are done.
  3. If the PT3 must be able to control more than one LecNet component, or if the single component that the PT3 must control is in the middle of the LecNet chain and it cannot be moved to an end, you will now need to arrange for the data signals to reach all of the components. The way to do this is to use the 3.5mm RS-232 connectors on the backs of each component. To restore the data connections, connect enough 3-conductor patch cables and Y-adapters so that all the RS-232 connectors are wired to each other.

At this point the data and audio paths have been isolated and the noise problem fixed.