# sound waves

Sound is converted into electricity by a telephone and then transmitted as an analog signal.

These waves have 3 fundamental characteristics:

1. Amplitude, meaning the height (intensity) of the wave
2. Frequency, which is the number of waves that pass in a single second and is measured in Hertz (cycles/second) (wavelength, the length of the wave from crest to crest, is related to frequency.).
3. Phase is a third characteristic that describes the point in the wave’s cycle at which a wave begins and is measured in degrees. (For example, changing a wave’s cycle from crest to trough corresponds to a 180 degree phase shift). # Wave Modulation – analog and digital

This article touches upon wave modulation techniques used in physical layer that allows one or multiple low bandwidth signals to use or share a high frequency bandwidth. Note that post only covered the lowest layer of OSI model ie physical layer and not the data link layer which actually provides access technologies liek FDMA , TDMA , CDMA etc.

## Amalog Modulation

Modulating a wave means changing one or more of its fundamental characteristics to encode information.

The general function for a sinusoid is

or

#### f(t) = A sin(omega t + phi)

we can see that this has 3 parameters that can be altered .

• A – called the magnitude, or amplitude of the sinusoid.
• omega – known as the frequency
• phi – known as the phase angle.

Therefore there are three basic ways to modulate a carrier wave ( which is the modulated unaltered wave carrying some information) :

### 1. Amplitude Modulation

The amplitude of the carrier signal is modulated (changed) in proportion to the message signal while the frequency and phase are kept constant.

### 2. Frequency Modulation

The frequency of the carrier signal is modulated (changed) in proportion to the message signal while the amplitude and phase are kept constant.

### 3. Phase Modulation

This encodes information as variations in the instantaneous phase of a carrier wave.It changes the phase angle of wave in direct proportion to the message signal.

The above discussed techniques are only applicable for analog modulation . For digital modulation got the next blog : Wave Modulation – digital.

### Need for Modulation

1. Transmitting medium is acts like a bandpass filter

The transmitting channel ( air / wire o/ fibre ) is like a bandpass filter ie both the lowest frequency components and the highest frequency components are cut off beyond a range with transmission only being practical over some intermediate frequency range . In such a case our orignal wave is unusuable unless we shift our frequency up wthout changing the information contained . Thus modulation is the only solution for transmission.

2. Limits the size of Antenna from huge to small

A baseband (low frequency) signal has a long wavelength thus an anteena of the size of about 1/10 of wavelength is required to listen to it .This results in really large antenna sizes. However since modulation shifts the baseband signal up to a much higher frequency, which has much smaller wavelengths it allows the use of a much smaller antenna.

## Digital Modulation

Information can be sent from A to B as an electromagnetic signal, in either an analog or digital form. The difference between the two is that :

Data is typically sent as a packet that contains one or more bytes.
The “time to send” Ts= bits in packet / bits sent per packet .
The propagation delay or Tp = distance in metre / velocity in metre per second .

The most fundamental digital modulation techniques are based on keying:

#### PSK (phase-shift keying):

a finite number of phases are used.

#### FSK (frequency-shift keying):

a finite number of frequencies are used.

a finite number of amplitudes are used.