Signal Path

Theory is no use without practice, but a little theory helps to cast some light on the otherwise invisible phenomena of infra-red technology.  Let's start with the signal path in an infra-red audio transmission system.  

It all begins with the signal source. This can be a unit on the interpreter's desk, a conference system or any other audio system.  They all produce an electrical signal that contains the audio information.  

This signal is fed to the modulator via audio cables.  The modulator then prepares the audio signal for the infra-red (IR) transmission.

This processed electrical signal is fed to the radiator via a special cable.  The radiator diodes produce the infra-red light and radiate into the room.

Within this room the light signal can be received by any amount of receivers.  The receiver converts the light signal back into an electrical signal, and the receiver's headphones return it to an audio signal. 

Frequencies

The signal path of an infra-red audio transmission involves several frequency ranges. 

	The audio information to be transmitted is an audio frequency (AF) signal that has frequencies from 20-20,000 Hz once it reaches the modulator.
	The modulator produces a radio frequency (RF) signal, which is used as a 'carrier' for the information contained in the AF signal.  The frequency of the RF signal is between 55 kHz, which is the lowest narrowband channel, and 2.8 MHz, which is the highest wideband channel.  An RF signal is necessary for reliable transmission in multi-channel systems.
	The information is then radiated as infra-red light.  The frequency for the type of infra-red diodes used in Sennheiser radiators is 3.5 x 10^14 Hz.

Modulation

The process of putting AF signal information onto an RF signal is called modulation. 

The specific properties of IR diodes determine the type of modulation used.  IR diodes can only radiate light of one color, so they only radiate one frequency. However, by applying different voltages to the diode, the intensity ' or amplitude ' of its light can change. 

If direct amplitude modulation were used, a bass sound of, say, 100 Hz would make the IR diode light up 100 times per second.  This type of modulation has two major disadvantages. If several AF signals are mixed for transmission, a receiver will not be able to 'reconstruct' the individual signals but will instead take all of the AF signals.  Secondly, fluorescent lights could cause interference, which disturbs the audio signal.

This is why Sennheiser IR technology uses a combination of amplitude and frequency modulation.  Early in the process, the AF signal is transferred to RF bands, or channels, and different AF signals are adjusted to different channels.  These signals are mixed and control the intensity of the IR diodes.  The infra-red signal is then demodulated by the receiver.  If several AF signals have been transmitted, the receiver demodulates the signal on the channel to which it has been switched.

Wideband/Narrowband

The RF signal of a channel consists of a carrier frequency and frequencies that are slightly above and below the carrier.  The band of these higher and lower frequencies is called deviation. Rule of thumb: the greater the deviation of the RF signal, the better the sound quality at the end of the transmission path.  However, the greater the deviation, the smaller the number of channels that can occupy a given frequency band.

Wideband channels allow stereo transmissions with hi-fi quality and an audio frequency response of 20 - 20,000 Hz.  Two new channels can now be used for wideband transmissions, up from just two approved wideband channels (carrier frequencies 95 and 250 kHz). The higher carrier frequencies, 2.3 MHz and 2.8 MHz, are more resistant to interference, which guarantees extremely reliable transmission.

Audio frequencies of 50-8,000 Hz can be transmitted on the 32 standardized narrowband channels and are suitable for high-quality speech transmission.  Frequency response, however, is not as comparable to that of wideband channels, but speech transmissions are sent out without a problem.  

It's important to have a larger number of channels, as is required in interpreting applications. 

Operating Principle of a Modulator

A typical modulator operates as follows: first, it amplifies the incoming AF signal so that it can be processed with the least possible noise.  Devices with an integrated radiator have a control loop which automatically sets the modulation level, such as the volume of the AF signal.  For all other modulators, the modulation level must be adjusted manually.

A limiter makes sure that the peak deviation is not exceeded during the modulation process.  If an RF carrier frequency goes beyond the peak deviation in a multi-channel transmission, this will lead to the disturbance of nearby channels.

The modulation circuit is at the heart of the modulator.  Here, the information contained in the AF signal is 'transferred' onto an RF signal.  The individual steps of this modulation process are shown on the left. 

First, the radiator only emits un-modulated IR light with constant amplitude, which is produced by a DC control voltage.

The modulator replaces this DC voltage with an AC voltage, which produces a carrier frequency. The amplitude of the infra-red light will vary in accordance with the frequency of the modulating AC signal.

The amplitude of the IR light is modulated by the carrier frequency, and the carrier itself is frequency-modulated by the AF signal. Then we have what is known as combined amplitude/frequency modulation.  

The illustration at the bottom left shows how the carrier is modulated by the audio frequency voltage.  The values for the modulated signal refer to channel 2 wideband transmission with a carrier frequency of 250 kHz.

When no AF voltage is present, the modulated signal corresponds to the carrier frequency.  When the AF voltage is positive, the modulated carrier has a higher frequency.  Its frequency becomes lower when the AF voltage is negative.

This gives the two basic correlations of modulation: the greater the amplitude of the AF signal (volume), the greater is the deviation (the shift away from the carrier frequency).  The greater the frequency of the AF signal (pitch), the faster the carrier frequency is deflected. 

The harmonic filter of the modulator filters out interfering harmonics of the RF signal.  In multi-channel systems, this avoids interference with higher channels.

An RF amplifier and an RF output stage increase the RF signal level so that the signal is powerful enough to control the radiators.

RF Cable Path

The modulator transmits its output signal through co-axial cables with a characteristic impedance of 50 ohm, such as RG 58 cables.  The total cable length in an RF chain can amount to several hundred meters without risking deterioration in transmission quality.  

The maximum frequency used in IR transmission is in the lower radio frequency range, at 2.8 MHz.  Attenuation is then considerably lower than with radio-microphone systems, which operate at higher frequencies.  

In a conductor, waves transmit at . nite velocity.  The resulting difference in transmission time can lead to cancellations between two radiators.  When you're working with high frequencies and long cable lengths, signals may cancel each other out if both radiators are broadcasting the IR information to one receiver. You should therefore avoid installations in which the cable length between two radiators is a multiple of the direct distance between two neighboring radiators.  In multi-channel systems, the cable length between two radiators with overlapping coverage areas should be less than 30 m.

Operating Principle of a Radiator

The signal from the modulator is fed to the RF input of the radiator.  A transformer separates the signal from the other devices of the system.  The SZI 1029 radiator is fitted with a voltage controlled amplifier (VCA), which automatically compensates for level losses caused by such things as extremely long cables.  The IR output stage controls the transmitting diodes, which then radiate the IR signal.