Satellite communication analog and digital signals
Satellite Communication 2011-2012Various sources of Information &Signals, their Characteristics and Application Batch 1 Ajay Kumar N (1RV08TE002) Jagadeesh B (1RV08TE023)
InformationInformation in its most restricted technicalsense is a message (utterance or expression)or collection of messages that consists ofsymbols in an ordered sequence, or it is themeaning that can be interpreted from such amessage or collection of messages.
• Vast term• Covers everything that we see around us• Human Body is continuously processing information
Information Sources in SatComPrimary• Speech• Audio• Video• Data
Audio• Ranges upto 20 Khz (sensitivity of Human Ear)
Video• Ranges upto 5 Mhz• Must usually be synced with Audio
Signals A signal is a changing value of electric voltage or current through a transmission medium.Types• Periodic : Periodic signals repeat themselves after a certain period of time -- after they have cycled through one period, following periods dont contain any new information.• Aperiodic : Aperiodic signals dont repeat themselves, and therefore can contain information.
SignalsClassification• Analog-Continuous in nature• Digital-Discrete in nature
Analog Signals• Analog signals are electrical replicas of the original signals such as audio and video.• Baseband signals are those signals which occupy the lowest, or base, band of frequencies, in the frequency spectrum used by the telecommunications network. A baseband signal may consist of one or more information signals. For example, a number of analog telephony signals may be combined into one baseband signal by the process known as frequency-division multiplexing (FDM).• Other common types of baseband signals are the multiplexed video and audio signals which originate in the TV studio. In forming the multiplexed baseband signals, the information signals are modulated onto subcarriers. This modulation step must be distinguished from the modulation process, which places the multiplexed signal onto the microwave carrier for transmission to the satellite.
The Telephone Channel• Natural speech, including that of female and male voices, covers a frequency range of about 80 to 8000 Hz.• The range of 300 to 3400 Hz is accepted internationally as the standard for “telephone quality” speech, and this is termed the speech baseband.
• Noise, which covers a very wide frequency spectrum, is reduced by reducing the band width.• Reducing the bandwidth also allows more telephone channels to be carried over a given type of circuit
DSBSC Telephony• When the telephone signal is multiplied in the time domain with a sinusoidal carrier of frequency fc, a new spectrum results, in which the original baseband appears on either side of the carrier frequency.• The band of frequencies below the carrier is referred to as the lower sideband and the band above the carrier as the upper sideband
• Only the sidebands, and not the carrier, appear in the spectrum• To avoid distortion, the carrier frequency must be greater than the highest frequency in the baseband.
Single-Sideband Telephony• All the information in the original telephone signal is contained in either of the two sidebands, and therefore, it is necessary to transmit only one of these.• For the lower sideband, the frequencies have been inverted, the highest baseband frequency being translated to the lowest transmission frequency and the lowest baseband frequency to the highest transmission frequency.
• Companded single sideband (CSSB) refers to a technique in which the speech signal levels are compressed before transmission as a single sideband, and at the receiver they are expanded again back to their original levels• The expander decreases attenuation when a speech signal is present and increases attenuation when it is absent, thereby reducing idle noise.
FDM Telephony• FDM provides a way of keeping a number of individual telephone signals separate while transmitting them simultaneously over a common transmission link circuit.• Each telephone baseband signal is modulated onto a separate subcarrier, and all the upper or all the lower sidebands are combined to form the frequency-multiplexed signal• Each voice channel occupies the range 300 to 3400 Hz, and each is modulated onto its own subcarrier.• The subcarrier frequency separation is 4 kHz, allowing for the basic voice bandwidth of 3.1 kHz plus an adequate guardband for filtering.
CCITT Modulation Plan• Group-consists of 12 channels• Supergroup-consists of 5 Groups• Basic mastergroup-consists of 5 Supergroups• Super mastergroup-consists of 3 Basic mastergroups
Color Television• The baseband signal for television is a composite of the visual information signals and synchronization signals• The visual information is transmitted as three signal components, denoted as the Y, I, and Q signals• Y signal is luminance• I and Q signals are chrominance components• Chrominance signal bandwidth is less than that of the luminance signal since eye can resolve intensity better than colors
Frequency Spectra for Luminance and Chrominance Signals One line Waveform for Color TV Signal
AnalogAdvantages• Accurate• Simple• Less BandwidthDisadvantages• Noise
Digital Signals• Analog signals can be converted into digital signals for transmission.• Digital signals may also originate in the form of computer and other data.• In general, a digital signal is a coded version of the original data or analog signal
Applications• Audio signal processing• Audio compression• Digital image processing• Video compression• Speech processing• Speech recognition• RADAR• SONAR• Biomedicine
DigitalAdvantages• Less expensive• More reliable• Easy to manipulate• Flexible• Security• Digitized information can be transported through a noisy channel without degradation
Disadvantages• Complex Circuitry• Sampling Error• More Bandwidth• Synchronization
Conclusion• Overall , digital signals are better than analog signals because of various advantages• Application wise , digital signal processing has a wider gamut of uses compared to analog signal processing