2.1 Information and Information Transmission

|

9

Tab. 2.2: Bandwidth of common signals compared to biosignals.

Signal / application

Approximate bandwidth

Nuclear magnetic resonance spectroscopy

0.1 Hz

Electroencephalogram (EEG)

100 Hz

Electrocardiogram (ECG)

300 Hz

Electromyogram (EMG)

5 kHz

Speech

3.6 kHz

Audio CD

22 kHz

Mobile radio (GSM)

200 kHz

FM broadcast signal

300 kHz

DVB-T

7 MHz

WLAN according to IEEE 802.11 a/b

22 MHz

Front Side Bus in the computer

400–800 MHz

Fibre – Ethernet

20–50 GHz

The bandwidth of the transmission channel, or the signals that are transmitted in

it, depends on the information content to be transmitted. Since information cannot

be transmitted via a single frequency⁶, the information signal is modulated onto so-

called carrier signals. The simplest type of analogue modulation are methods in which

a signal parameter such as the amplitude or frequency or the phase position of the

carrier signal are modulated by the information signal. In the case of amplitude mod-

ulation this is done by multiplying the carrier signal with the information signal (cf.

Figure 2.2).

In todays technology, often digital modulation methods are used in addition to

analogue amplitude and frequency modulation in order to optimally utilise the chan-

nel capacity. Corresponding to the analogue modulation such as amplitude modula-

tion (AM) or frequency modulation (FM), in the digital procedures on the one hand

in pulse amplitude modulation (PAM) the amplitude and on the other hand in pulse

position modulation (PPM) resp. pulse width modulation (PWM), the frequency and

phase position of the digital pulses are modulated with the amplitude values of the

information signal (cf. Figure 2.3 and Listing 2.1).

The problem of every type of modulation, however, is the fundamental widening

of the bandwidth depending on the information to be represented and the possible

(metrologically still just distinguishable) resolution of the transmission path affected

sI (t)

sT (t)

sU (t) = sI (t) ·sT (t)

Fig. 2.2: Amplitude modulator, obtained by multiply-

ing the information signal sI with the carrier signal sT

leading to a transmission signal sU.

6 A signal with a single frequency does not transmit information, since the measurable quantity does

not undergo any change.