Introduction

Sennheiser is the leading supplier of state-of-the-art systems for wireless sound transmission.  With the Mikroport wireless transmitter and receiver, entertainment and broadcast professionals can communicate sound loud and clear.

Technical

The principle of a Mikroport system can be illustrated with the signal path, which starts with the source such as a human voice or instrument and travels to the listener. 

When directed into the microphone, the source's sound is converted into an electrical signal, which is called the audio-frequency signal (AF signal).  The AF signal contains frequencies within the range of 20-20,000 Hz, exactly like the audible sound.

The Mikroport transmitter converts this information into a signal suitable for radio transmission.  It is then radiated into the room by a transmitting antenna.

A receiving antenna captures the signal and conducts it to the Mikroport receiver.

The signal between transmitter and receiver is called the radio-frequency signal (RF signal) because it contains frequencies in the radio-frequency part of the electromagnetic spectrum.

The receiver converts the RF signal back into an AF signal and presents it on its output socket, where it is available for further processing by, for example, a mixing console.

Transmission quality

The signal path described for Mikroport systems also applies to cordless telephones or CB radios.  However, these communications applications have a distinctly different requirement for the quality of the sound transmission.

In the case of telephones, the intelligibility of the spoken word is sufficient.  When using Mikroport technology, the requirement is much higher: music and voice must be transmitted in studio quality.

The frequency response of a telephone is approximately 300-3,400 Hz, which is just about adequate for speech transmission only.  However, Mikroport can clearly send out the sounds of both the lowest notes of a bass singer and the highest audible notes of a triangle.  The hand-held SKM 5000 transmitter, in combination with the EM 3031 receiver, transmits frequencies from 80-20,000 Hz. 

But it is not only the frequency response of the Mikroport system that should meet these high standards.  The distortion and noise characteristics must also meet the most stringent requirements.  The following information shows how these high standards are met.

Modulation

The standard rule of radio-microphone technology is that an audio signal should be converted in such a way as to produce a RF signal suitable for radio transmission.

Modulation is the process of impressing AF signal information onto an RF signal.  The signal is then located in frequency ranges which are much more suitable for radio transmission.

Since you can modulate at different frequencies in the RF band, the transmission path via electro-magnetic (radio) waves can be used many times, using a different frequency each time.  This makes it possible to transmit a large number of AF signals at different RF frequencies.

Frequency modulation

There are various modulation methods.  The practice which is most suitable for high quality sound is called frequency modulation (FM).
This is used for the hi-fi stereo transmissions that one hears on the radio.

The following explanation of frequency modulation represents the most complex part in understanding Mikroport technology.  Once you have understood this, you can solve many problems in radio-microphone technology.
 
FM operation
Let us look at the relationship between the AF signal and the RF signal in FM:

In the left-hand illustration, no AF signal is present.  In practice, this means that no one is speaking into the microphone there is silence.

At the RF end, this state is interpreted as follows: the RF signal consists of oscillations having a fixed frequency.  This frequency is called the carrier frequency.

To be able to represent the relationship between RF signal and AF signal more easily, the system of coordinates of the AF signal is rotated by 90°.  In the right-hand illustration, an AF signal is now present.

Greater amplitude or volume of the signal now results in a greater deflection of the graph to the right and to the left.  A higher frequency (pitch) appears as compression of the oscillation.

RF oscillation is shown in the diagrams below, which illustrate the amplitude of the RF signal versus frequency.
If the line is longer, the amplitude of the RF signal or the field strength of the RF signal is greater.  The change in frequency is demonstrated by whether the line moves to the right or to the left.

Now, let's return to our initial situation, in which no AF signal is present.  In this scenario, a signal is generated at only one frequency the carrier frequency at the RF end.

What happens if an AF signal is present when a singer sings into the microphone The RF signal is now no longer at any one fixed frequency but moves rapidly to and fro between frequencies which are slightly above and below the carrier frequency.

The frequency of the RF signal dips to lower frequencies and rises to higher ones just as the diaphragm of a loudspeaker is deflected in both directions from the center position.

At a higher frequency of the AF signal, the movement of the carrier frequency is faster.  The RF signal quickly moves back and forth in the range around the carrier frequency.

If someone projects sound into the microphone with a greater volume, the amplitude of the AF signal becomes higher.

As a result, the maximum deflection from the carrier frequency increases in the RF signal.  The RF signal now reaches lower frequencies on one side and higher frequencies on the other side.

The magnitude of the deflection of the RF signal towards higher and lower frequencies is called deviation.

Inter-modulation products

The previous explanation of FM relates to a model.  It is useful for understanding the most important relationships between AF signals and RF signals.

Unfortunately, transmitters and receivers do not stick to this model.  At the transmitter end, RF signals are produced that go beyond the deviation as shown above.  Due to the principle used, an infinite number of sidebands with decreasing amplitude are produced.

At the transmitter end, selective filters ensure that RF signals with high spectral purity are radiated.

At the receiver, the RF signal is converted to a fixed intermediate frequency by means of an oscillator.  The nonlinear characteristic of this conversion, which is a consequence of the FM system, leads to different combinations at new frequencies. These are called inter-modulation products.

IM products at a greater distance are effectively eliminated by highly selective filters in the receiver.

Proper frequency selection of multi-channel systems is vital to avoid interference caused by IM products coinciding with wanted frequencies. 

Wideband vs. narrowband transmission

The concept of deviation is important for understanding the distinction between wideband and narrowband transmission.

In wideband transmission, the maximum deviation from the carrier frequency is 56 kHz, whereas it is only 10 kHz in narrow-band transmission.

A limiter circuit sets an upper limit for the highest possible AF signal.  The limiter ensures that the peak deviation is not exceeded.  A transgression would cause the signal to interfere with nearby transmission frequencies and must also be avoided for radio-communication reasons.

Wideband transmission allows greater frequency response, and also delivers better sound quality at the end of the transmission link.

However, a wider band also takes up more space on the frequency scale.  Given a frequency band of the same width, it is therefore possible to transmit many more audio signals in narrow-band technology than in wideband technology.

Narrowband transmission is most adequate for high-quality voice transmission for basic communication purposes.  However, if studio quality is required for speaking or singing, wideband transmission must be used.

Narrowband systems are used in Mikroport technology for tour-guide systems in such places as museums, visitor attractions and factories.
In this situation, a hand-held transmitter sends out a signal to mobile receivers, which are provided for each visitor.  The speaker uses the hand-held SKM 1030 transmitter and the visitors can hear him or her via the HDE 1030 stethoset receivers.

With the exception of some special broadcasting transmitters, all other Mikroport systems operate in wideband.