Maximizing the audio system performance

[Closetalk Support]

Update 2014-Feb-07:
Although the information below is still valid it is recommended to download the new Quick Start Manual instead which contains not only audio programming but also programming the complete system with system testing using Close Talk Install. It can be downloaded from:

http://www.close-talk.com/Download/Manu ... tGuide.pdf


Introduction
In any wireless audio system, maximizing the audio chain is especially important. Usually the wireless transmission parts in the chain are the weakest links and this article will describe how to get the best audio performance from Close Talk Conference System. The article will refer to the audio block diagram that can be found in Section 6 of the system Central Unit User Manual that can be downloaded from http://www.close-talk.com/Download/Manu ... %20108.pdf.

From the Delegate Unit to the Central Unit
As Section 6 shows in the Central Unit User Manual; the microphone audio from a Delegate Unit (DU) is transmitted wirelessly two times, first from the Delegate Unit to the Central Unit (CU) and then back to the other DU:s from the CU. The microphone transmission level in the DU is bandpass filtered and fixed to a level that is set for optimal signal to noise performance, a "normal" speaking voice level at about 10cm/4" from the microphone will appear at the CU Line and Tele output ports at circa nominal 0dBu level. A limiter in the DU will prevent severe overdriving of the transmitter.

In the Central Unit
When the audio signals are received by the CU they are filtered and then mixed to a single combined mono signal that is output on the Line and Tele audio output ports (see figure 6.5 in the CU manual). The Tele output level is fixed but the Line output level can be adjusted as needed, making it possible to connect it directly to a power amplifier.

The main intention for the Tele In&Out port is for it to be used with a telephone hybrid or other communication device that has it's own signal level control and conversion capabilities. The Line In&Out port is intended for general usage such as connecting the system to an external PA or inputting external audio signals into the conference system. Since the Tele In port has a cross-coupling function over to the Line Out port it is also very suitable for connecting to a recording device. The Line In also has a cross-coupling function (see the Line To Tele Volume parameter) to enable incoming audio to be sent out to a telephone hybrid. Both the Tele and Line ports balanced XLR and unbalanced 1/4" tele connectors. Both the Tele and Line In ports are hard-knee-limiter-protected to prevent overdriving of the transmitter.

Connecting the audio to an external PA
The audio signal received from the DU to the CU is frequency limited to about 200Hz - 8kHz (-3dB) but is otherwise flat which means that to achieve maximum audio "quality" (subjective) when connecting the audio to an external PA; a three band parametric or regular graphical EQ is recommended between the CU and the PA system. A very "nice" EQ setting is to at the first filter add about 2-3 dB at about 200Hz with shelving response, add a small amplification of maybe 1-1.5dB at about 5+-1kHz and then dampen everything above 9-10kHz as hard as possible.

Sending the audio signal to the Delegate Unit
The audio received from the DU is passed through a cross-coupling function with variable gain called "Delegate Volume" (see the CU manual).

Important!
Although this parameter is called "Delegate Volume" it should NOT be used to regulate the actual Delegate Unit speaker audio levels! If the audio coming from the DU:s are to be sent back out to the other DU:s it should be left at the default setting. If the audio coming from the DU:s is not to be sent back to the DU:s, this parameter should be set to 0.

After passing the "Delegate Volume" cross-coupling it passes though a simple two band shelving equalizer. Use this built-in EQ to adjust the tone of the DU speaker audio only, it will not affect the signals on the Tele and Line ouputs.

After the EQ it passes an Insert connector function. Use the Insert connector to connect any external audio processing such as an EQ or a feedback destroyer. It is then mixed with the incoming Tele and Line In audio before finally being transmitted out to the system Delegate Units.

Regulating the Delegate Unit speaker and headphones levels
The DU contains a volume know that is used to regulate the speaker and headphones levels. The operating span of this volume knob can be controlled via the CU "Speaker Volume" and "Headphones Volume". These parameters has two settings, the minimum low and maximum high level range of the volume knob. In that way the system can freely limit the amount of audio level change a user is allowed to do. For example, setting the "Speaker Volume" setting to "27 27", the volume knob is effectively disabled and the DU speaker audio level is fixed. A very common setting for these two parameters is "27 27" for "Speaker Volume" and "16 28" for "Headphones Volume". With this setting the DU speaker level is fixed and the headphones level can be adjusted to suit the user using the volume knob. The DU will sense if a headphone is inserted and turn off the speaker.

Maximizing the system signal to noise
Since the audio levels from the DU to the CU is fixed there is no settings need there. If the audio is to be sent out via the Line Out port, preferrably leave the "Line Out" parameter at the default setting for unity (0dBu) operation.

If the; from the DU incoming audio, is to be sent back out to the other DU:s, leave the "Delegate Volume" parameter at the default setting! This is very important to optimize the levels for the transmitter. Finally, adding +2-4dB to the "EQ Low" parameter will give some "warmth" to the DU speaker sound.

At the DU end, adjust the "Speaker Volume" parameter to the desired system sound level. The DU speaker is able to output a surprisingly high audio level; it is usually limited by the installation room feedback characteristics rather than overdriving. NOTE! Always use the "Speaker Volume" parameter to adjust the final DU speaker levels, do not use the "Delegate Volume" parameter. Please note that in a DU with an active microphone the speaker audio is turned off!

If an external PA is also used for the system audio it can be a good effect to keep some audio signal in the buil-in DU speaker anyway, creating a "near field" audio for the Delegate Unit user to increase intelligibility BUT always test the system for feedback prevention, with all numbers of active microphones. In case of feedback with both Delegate Unit and external PA audio, first reduce the Delegate Unit speaker level.

Maximizing feedback prevention
Any experienced audio engineer knows that feedback can be a nightmare and it is far beyond the scope of this article to teach general feedback prevention techniques. Close Talk Conference System does not have an active built-in feedback destroyer but the CU Insert connector can be used to connect any such third party unit.

The CU does however have a "simple" audio level compensation function that works by reducing the total system audio level depending on the number of active microphones via the "Microphone Compensation" parameter. This is a very effective way of controlling feedback and works fine for many situations.

To use this function, tune the system audio level to a desired state using only one active microphone. When the system is tuned, turn on two active microphones. If this results in feedback, add -2dB to the "2:" setting until the feedback stops. When the system is stable, turn on three microphones and redo the setting, this time on the "3:" setting. The "Microphone Compensation" parameter level compensation is also active on the Line Out port, making it possible to use with an external PA.