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Wireless Gain Structure

Optimizing Audio Quality in Lectrosonics Wireless Microphone Systems

The amplification of any type of signal is commonly referred to as applying Gain. The term Gain Structure refers to the relative levels of gain applied in a series of gain stages in a signal chain.

In an audio chain, the maximum S/N (signal-to-noise) ratio is achieved when gain is applied at the beginning of the signal chain, and subsequent stages are at unity gain (no increase or decrease). With regard to a wireless microphone system, the gain should be applied at the transmitter input and every component thereafter should be set for unity gain. The idea is to minimize the number of gain stages being used because every time gain is applied to the signal, noise is also increased.

The problem with many wireless microphone systems is that the transmitters have little or no limiting in the input stage and gain adjustment precision is limited. With these limitations, the gain must be set low enough to avoid distortion when a loud peak in the audio takes place. This lower gain setting compromises the overall S/N ratio of the wireless system.

Lectrosonics wireless microphone systems offer you a unique design that lets you apply the optimum gain at the transmitter input for the average audio level, with a DSP-controlled limiter to protect against distortion during loud peaks. When properly set up, there is only one adjustment needed from one shot to the next. If the microphone, mic placement or talent has changed, the gain is adjusted at the transmitter and everything else stays the same. This is made possible by:

Wide range, high resolution input gain adjustment in the transmitters

This is the most critical adjustment on any wireless microphone system. It determines the maximum S/N ratio and lowest distortion the system can deliver. A wireless transmitter must be able to adapt to the enormous range of voice levels from various people, and a similar range of variation in sensitivity and output levels from microphones. A precise match to the signal level coming from the microphone will provide optimum modulation and headroom to minimize noise and distortion.

Lectrosonics transmitters provide a range of well over 40 dB of input gain adjustment in 1 dB steps. Incoming signal level is indicated by two dual-color LEDs to indicate the audio level at four different thresholds. This design makes accurate gain adjustment quick and simple. Gain can be set high enough to ensure full modulation, but not so high that the limiter cannot cleanly handle loud peaks in the audio.

Wide range, DSP controlled input limiting in the transmitters

The input limiter on a wireless transmitter is an essential element in the signal processing chain to handle the wide dynamic range of voice and environmental sounds. The audio level at the microphone can range from a whisper to an explosion, which is well beyond the dynamic range that an A/D converter can handle, so high quality analog limiting is mandatory in a professional quality wireless transmitter.

Lectrosonics transmitters employ a wide range, DSP-controlled, dual-envelope limiter to add intelligent control of dynamics in the audio with a clean limiting range of more than 30 dB for excellent overload protection.

GainStructure Wireless-1

The dual envelope makes the limiter acoustically transparent while maintaining low distortion. It can be thought of as two limiters in series – a fast attack and release limiter followed by a slow attack and release limiter. The limiter recovers quickly from brief transients, with no audible side effects, and also recovers slowly from sustained high levels to keep audio distortion low while preserving short-term dynamics. The onset of limiting action is indicated by dual-color LEDs on the transmitter control panel to enable accurate gain adjustment.

Line-level audio output with attenuation in the receivers

Lectrosonics receivers are designed to provide line-level audio output with an attenuator to reduce the output to microphone level. What this means is that there is no difference in the S/N ratio of the wireless system regardless of where the receiver output level is set. This ensures that an ideal level match can be achieved for the camera or mixer that is being fed by the receiver without affecting the S/N ratio.

Calibration with a Camera-mounted Wireless Receiver

Ideal gain staging is achieved with a few simple steps:

The purpose of this setup procedure is to use the limiter in the transmitter to achieve the maximum S/N ratio in the wireless system, with little or no risk of overloading the audio input of the camera. The audio output of the receiver will not increase beyond the point where the transmitter is in limiting. The action of the dual envelope limiter is not audible unless it is driven beyond its 30dB range, and even then it is very well behaved. Most lavaliere microphones will overload before the input preamp on the transmitter.

1) Adjust transmitter input gain

The sensitivity of microphones varies by type and model, and the sound level entering the microphone also varies significantly with different distances from the talker’s mouth. Begin by placing the microphone in the position where it will typically be used. Speak into the microphone at the anticipated voice level and adjust the input gain on the transmitter so that the -20 LED starts to flicker when you speak. Leave the transmitter gain set at this level as a starting point.

FrontPanel-FullModulation

Generate a constant noise source such as FM interstation noise from a radio, or pink noise from a smart phone or iPad using a downloadable application. Place the microphone close to the noise source. Adjust the volume of the noise source and move the microphone toward and away from it until the -20 LED begins to light up. This sets the audio level at the onset of limiting.

OnsetOfLimiting

2) Set the camera’s audio input level control to a mid-position

The input gain control on the camera should be set at a mid-position within its adjustment range. In most cases, a line-level feed to the camera is best since it bypasses a microphone preamp and its gain stage that could add some noise to the audio in the camera.

3) Adjust receiver output to drive the camera input just below maximum

Turn the output level of the receiver all the way down. Then connect the receiver audio output to the camera. While the noise source is running, gradually turn up the output level on the receiver until the camera audio input level meter is just below maximum (to allow for a bit of headroom). This will calibrate your levels so that the camera level meter will indicate full level at the same time the audio begins to hit the limiter in the transmitter. The audio output level of the receiver will not increase beyond the point at which the limiter in the transmitter begins to operate. The output level control in the receiver is an attenuator, so there is no difference in S/N ratio across its entire range of adjustment.

CameraMeter

With the gain staging set up in this manner, the only adjustment that needs to be made from one production scene to the next is the input gain on the transmitter. Over time, you may want to go through this procedure again to make minor adjustments to the setup. For example, if you consistently find that you end up with the transmitter gain near either maximum or minimum when setting up for production, it would make sense to go through the procedure again, starting with the noise source either louder or quieter as needed to compensate. That way you will typically end up closer to the middle of the gain adjustment range on the transmitter during normal use. There is a difference of only a few dB in the S/N ratio of the wireless system across the entire range of the gain control in the transmitters.

Some video cameras have limiters on the audio input and some do not. It doesn’t really matter if the wireless system and camera record level are set up following this procedure, because the limiter in the wireless transmitter will not allow the audio output to increase above full modulation. In other words, the transmitter limiter will protect the camera from input overload. This, of course, requires that the transmitter input gain is high enough to allow the limiter to begin working, which is the primary purpose of this setup procedure.

Different microphones have different sensitivity levels, so their output levels will vary. Go through this procedure for each microphone that you will be using and make notes (or even put labels on the equipment) to save time during setup. You may end up tweaking these first settings a bit with experience gathered over time, but this will give you an excellent starting point.

Calibration Using an Intermediate Mixer

BagSystem

If you use an intermediate mixer between the wireless receiver and the camera, gain staging must be applied in the same manner as using a receiver on the camera to achieve unity gain through the entire audio chain. A wireless “bag system” is the most common audio equipment setup used for field production of documentaries and outdoor reality shows, which provides wireless links from talent mics to mixer, and from mixer to camera. Properly set up, a bag system is a mobile, multi-channel wireless system, mixing station and backup audio recorder for single and multi-camera production. A bag system also presents a somewhat complex audio gain structure.

Correct input gain adjustment at the talent’s transmitter is critical to achieve the best overall S/N ratio. Noise or distortion that occurs in the transmitter cannot be removed later in the chain. The talent microphone is also the point where the wildest variations in sound levels occur.

Starting at the talent transmitters and ending at the camera audio input, the following step-by-step procedure will optimize the S/N ratio and minimize distortion. Once the following setup procedure has been completed, there is only one adjustment needed from one shot to the next. If the microphone, mic placement or talent has changed, tweak the input gain adjustment on the talent transmitters to make certain you have full modulation on the transmitter audio level LEDs.

1) Adjust gain on the talent transmitters for full modulation

The sensitivity of microphones varies by type and model, and the sound level entering the microphone also varies significantly with different distances from the speaker’s mouth. Begin by placing the microphone in the position where it will typically be used. Speak into the microphone at a normal voice level and adjust the input gain on the transmitter so that the -20 LED starts to flicker when you speak. Leave the transmitter gain set at this level as a starting point.

Generate a constant noise source such as FM interstation noise from a radio, or pink noise from a smart phone or iPad using a downloadable application. Place the microphone close to the noise source. Adjust the volume of the noise source and move the microphone toward and away from it until the -20 LED begins to light up. This sets the audio level at the onset of limiting.

FullModulation

2) Set input pots on the mixer to a mid-position

The input gain control on the mixer should be set at a mid-position within its adjustment range. In most cases, a line-level feed to the mixer is best since it bypasses a microphone preamp and its gain stage that could add some noise to the audio.

3) Adjust output levels on the talent receivers for the correct mixer input levels

Adjust the output level of the receiver so the level meter on the mixer is slightly below 0VU with the steady state noise. This will calibrate your mixer input level meter with the input level LEDs on the transmitter. Leave the talent receiver output controls in this position.
NOTE:  The output control on the receiver is strictly an attenuator. It  has no direct effect on the S/N ratio of the wireless system. If the camera or recorder is being fed at line-level, it is good practice to set the talent receivers for line-level output to avoid unnecessary gain in the mixer.

4) Adjust input gain on the bag transmitters

With the noise still running, adjust the input of the transmitters being fed from the mixer for full modulation (-20 LED just starts to come on). The output master on the mixer (if available) should be set to the normal level for the mic or line level normally used. Lectrosonics transmitters can be fed mic or line-level signals by using the appropriate cable and wiring.

5) Set camera audio input level control to a mid-position

The input gain control on the camera should be set at a mid-position within its adjustment range. In most cases, a line-level feed to the camera is best since it bypasses a microphone preamp and its gain stage that could add some noise to the audio in the camera.

6) Adjust receiver outputs for correct input level on the camera

While the noise is still running, adjust the output level of the camera mounted receiver/s so the camera’s level meter indicates just below maximum.

CameraMeter-MAX

Setup Notes: