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.


TN1003 - LecNet Serial and AMX Cable Wiring Diagrams

Serial Cable Wiring Diagram

The serial port on the LecNet device is a minimal RS-232 implementation. The figure shows the wiring diagram to accommodate interconnection with either a 9 or a 25 pin serial port on a PC or other serial device.


Since LecNet devices don’t drive the Host RX line except when addressed, the Host RX line will sometimes be in a “floating” state. The 10k, 1/4W resistor will be needed for a small number of PCs whose serial port hardware requires the Host RX line to be pulled low when the RX line is idle. Symptoms that would indicate the need for the resistor include the inability for the PC to identify LecNet devices when running the Lectrosonics supplied software, or intermittent identification.


AMX Cable Wiring Diagram


AMX Programming Notes

If you are using an AMX system to control your LecNet equipment, you'll want to purchase the Lectrosonics PT3 Protocol Translator. The PT3 connects between the AMX bus and any LecNet equipment. With the PT3, the LecNet equipment looks just like native AMX equipment. The PT3 is the fastest and most productive way to control LecNet devices with an AMX system.


TN1002 - About Automatic Mixers

Feedback Control

An open microphone in a room with a loudspeaker system creates a potential source of feedback as the sound from the speaker system reenters the microphone. As someone speaks, the microphone picks up the sound and

This recirculation cycle can be repeated many times in reflective rooms or where very little margin exists between the necessary loudness level and the maximum possible loudness level. If the sound level in the room is high enough, the recirculation will build up to the point where a loud howling or squealing will occur. This oscillation is referred to as acoustic feedback. As more microphones are added to the system, the problem grows worse. An automatic mixer solves this basic problem by applying a process commonly referred to as NOM attenuation.

NOM stands for: Number of Open Microphones. An automatic mixer differentiates between active and inactive microphones, turns down the inactive ones and then adjusts the the overall sound system level according to how many active microphones remain. The result is that all of the microphones in the room act the same as a single microphone with respect to feedback.

There are many different types of automatic mixers available from different manufacturers. Simple, low cost types offer nothing more than a switch that opens and closes the microphone with voice activity. Better ones use more involved algorithms to differentiate between active and inactive microphones. The best automatic mixers utilize sophisticated algorithms with either analog or digital controls to smoothly turn mics up and down or distribute gain among the channels to apply NOM attenuation in a seamless, inaudible manner.

Improved Sound Quality

An automatic mixer also minimizes recirculated sound and background noise to dramatically improve sound quality in public address, teleconferencing and recording systems.

In a public address system, the sound will recirculate through the microphones and speakers and hit the listener's ears numerous times, spaced a few milliseconds apart, making it very difficult to understand what is being said. Background noise gathered by unused microphones in any type of sound or recording system will also add an unnatural "hollow" character to the sound, reducing intelligibility.

Automatic mixers can also be an important addition to a studio production facility where multiple microphone systems are used to produce talk shows, by reducing background noise and applying NOM attenuation. The lower background noise improves the clarity of the sound and the NOM attenuation automatically limits the overall sound level to avoid overloading the audio signal chain.

Automatic Hands Free Operations

In modern sound systems such as boardrooms, courtrooms, churches and conference centers, multiple microphone systems are often used for sound reinforcement, teleconferencing and recording all at the same time. It is virtually impossible for someone to operate a sound system like this manually and keep up with rapid voice activity. The sound system operator would also have to be in the room with the talkers and microphones to have any hope of keeping up with the activity. Thus, automatic mixers are virtually mandatory in multiple microphone sound systems to provide dependable response to the voice activity and the convenience of unattended hands free operation.


TN1001 - LecNet™ Sound System Design Assistance


Every year, Lectrosonics works with hundreds of consultants, system integrators and design consultants to assist in configuring sound systems to meet the client’s needs using LecNet(tm) products. The growing family of LecNet products and the increasing complexity of modern automatic sound systems in boardrooms, courtrooms, teleconferencing, distance learning facilities, worship centers, etc. requires considerable effort to design systems that are reliable and easy to install. Lectrosonics has developed unique, cost saving solutions with unique products. As an example, the following diagram depicts a three way conferencing system that requires only a minimal number of components, for a considerable cost savings over methods used in the past to provide the same functions.


The most effective way to design and implement a sound system involving LecNet components for a specific client is to contact the factory during the initial stages of design. Please feel free to contact us with any new system you have been challenged to design. We can help. Afterall, we brought "mix minus" sound reinforcement to the installed sound market.

The LecNet products include:

  • Two patented, gain proportional, adaptive level automatic mixers
  • A programmable 12x8 matrix mixer
  • An integrated 16 input automatic mixer/matrix mixer in a 2-space 19" rack mount
  • A patented digital telephone hybrid sound system interface
  • An 8-channel, bridgeable power amplifier in a single 19" rack space
  • A programmable interface for "Video Follow Audio" applications
  • Multi-line bridging for audio/video teleconferencing

To use this service, contact the factory and find out how simple automatic sound systems can be:

Call (800) 821-1121 or (505) 892-4501
FAX (505) 892-6243
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
Mail: 581 Laser Road, Rio Rancho, NM 87124 USA