Digital audio has emerged because of its usefulness in the recording, manipulation, mass-production, and distribution of sound. Modern distribution of music across the Internet via on-line stores depends on digital recording and digital compression algorithms. Distribution of audio as data files rather than as physical objects has significantly reduced the cost of distribution.

An example of 4-bit pulse code modulation showing quantization and sampling of a signal (red).

A flow of audio from sound waves through a microphone to an analog voltage, A-D converter, computer, D-A converter, analog voltage, speaker and finally as sound waves again.

Sample rate is the number of samples of audio carried per second.

44.1 kHz is the sampling rate of audio CDs giving a 20 kHz maximum frequency. 20 kHz is the highest frequency generally audible by humans, so making 44.1 kHz the logical choice for most audio material. High quality tape decks using metal tape, and medium quality LP equipment can reproduce 20 kHz (higher for top quality LP equipment, though some of this is harmonic distortion inherent in the medium).

Reduced Bandwidth Recording
Audio may be recorded at below 20kHz bandwidth for a few reasons:

• To reduce file size
• To reduce CPU usage

Audio Bitarte
MP3 Format:
1. 32 kbit/s – MW (AM) quality
2. 64 kbit/s – Group Performance – Low Quality
3. 96 kbit/s – FM quality
4. 100–160 kbit/s – Standard Bitrate quality; difference can sometimes be obvious
5. 192 kbits/s is the highest level supported by most MP3 encoders when ripping from a Compact Disc.
6. 224–320 kbit/s – VBR to highest MP3 quality. Group Performance – High Quality

Other Audio Formats:
1. 800 bit/s – minimum necessary for recognizable speech
2. 2.15 kbit/s – minimum bitrate available through the open-source Speex codec
3. 8 kbit/s – telephone quality (using speech codecs)
4. 32-500 kbit/s – lossy audio as used in Ogg Vorbis
5. 256 kbit/s – Digital Audio Broadcasting (DAB) MP2 bit rate required to achieve a high quality signal
6. 400 kbit/s-1,411kbit/s – lossless audio as used in formats such as Free Lossless Audio Codec, WavPack or Monkey’s Audio to compress CD audio
7. 1,411.2 kbit/s – Linear PCM sound format of Compact Disc Digital Audio
8. 5,644.8 kbit/s – DSD sound format of Super Audio CD

Ref:

http://en.wikipedia.org/wiki/Digital_audio

http://wiki.audacityteam.org/index.php?title=Sample_Rates

http://www.videoconverterfactory.com/glossary/audio-bitrate.html

Microwaves are short-wavelength, high-frequency signals that occupy the electromagnetic spectrum 1,000 MHz (1 GHz) to 1,000 GHz (1 terahertz). This is just above the radio frequency range and just below the infrared range.

Broadcasters use microwave links to send programs from the studio to the transmitter location, which might be miles away. Wireless Internet service providers use microwave links to provide their clients with high-speed Internet access without the need for cable connections.

Microwave systems are used to relay multiplex signals from point to point. A simplex relay system provides one-way communications and consists of a transmitting terminal, a certain number of repeaters.

One of the reasons microwave links are so adaptable is that they are broadband. That means they can move large amounts of information at high speeds. Another important quality of microwave links is that they require no equipment or facilities between the two terminal points, so installing a microwave link is often faster and less costly than a cable connection. Finally, they can be used almost anywhere, as long as the distance to be spanned is within the operating range of the equipment and there is clear path (that is, no solid obstacles) between the locations. Microwaves are also able to penetrate rain, fog, and snow, which means bad weather doesn’t disrupt transmission.

A simple one-way microwave link includes four major elements: a transmitter, a receiver, transmission lines, and antennas. These basic components exist in every radio communications system, including cellular telephones, two-way radios, wireless networks, and commercial broadcasting. But the technology used in microwave links differs markedly from that used at the lower frequencies (longer wavelengths) in the radio spectrum. Techniques and components that work well at low frequencies are not useable at the higher frequencies (shorter wavelengths) used in microwave links. For example, ordinary wires and cables function poorly as conductors of microwave signals. On the other hand, microwave frequencies allow engineers to take advantage of certain principles that are impractical to apply at lower frequencies.

In a microwave link the transmitter produces a microwave signal that carries the information to be communicated. That information—the input—can be anything capable of being sent by electronic means, such as a telephone call, television or radio programs, text, moving or still images, web pages, or a combination of those media.

The transmitter has two fundamental jobs: generating microwave energy at the required frequency and power level, and modulating it with the input signal so that it conveys meaningful information. Modulation is accomplished by varying some characteristic of the energy in response to the transmitter’s input.

The second integral part of a microwave link is a transmission line. This line carries the signal from the transmitter to the antenna and, at the receiving end of the link, from the antenna to the receiver. At microwave frequencies, those media excessively weaken the signal. In their place, engineers use coaxial cables and, especially, hollow pipes called waveguides.

The third part of the microwave system is the antennas. On the transmitting end, the antenna emits the microwave signal from the transmission line into free space. “Free space” is the electrical engineer’s term for the emptiness or void between the transmitting and receiving antennas. At the receiver site, an antenna pointed toward the transmitting station collects the signal energy and feeds it into the transmission line for processing by the receiver.

Antennas used in microwave links are highly directional, which means they tightly focus the transmitted energy, and receive energy mainly from one specific direction. This contrasts with antennas used in many other communications systems, such as broadcasting. By directing the transmitter’s energy where it’s needed—toward the receiver—and by concentrating the received signal, this characteristic of microwave antennas allows communication over long distances using small amounts of power.

Between the link’s antennas lies another vital element of the microwave link—the path taken by the signal through the earth’s atmosphere. A clear path is critical to the microwave link’s success. Since microwaves travel in essentially straight lines, man-made obstacles (including possible future construction) that might block the signal must either be overcome by tall antenna structures or avoided altogether. Natural obstacles also exist. Flat terrain can create undesirable reflections, precipitation can absorb or scatter some of the microwave energy, and the emergence of foliage in the spring can weaken a marginally strong signal, which had been adequate when the trees were bare in the winter. Engineers must take all the existing and potential problems into account when designing a microwave link.

At the end of the link is the final component, the receiver. Here, information from the microwave signal is extracted and made available in its original form. To accomplish this, the receiver must demodulate the signal to separate the information from the microwave energy that carries it. The receiver must be capable of detecting very small amounts of microwave energy, because the signal loses much of its strength on its journey.

Regulatory and Licensing

Each country has a varying requirement for the licensing of microwave radio links. In most cases this license only addresses the transmitter, but in the same instance, it offers regulatory protection to any interference that may affect the microwave receiver.

Ref:

http://www.ieeeghn.org/wiki/index.php/Microwave_Link_Networks

http://www.itblogs.in/communication/technology/microwave-communication-an-introduction/

http://www.trangosys.com/products/point-to-point-wireless-backhaul/encryption-ipsec-tunnel-application-diagram.shtml

http://www.nec.co.jp/press/en/0810/0201.html

Ref: http://www.asiasat.com/asiasat/contentView.php?section=76&lang=0

The feedhorn is covered with a window of polystyrene fiberglass to prevent moisture and dirt from entering the open end of the waveguide. If a dry-air pressure exist in the waveguide system, ceramic materials or quartz glass is used as covering material.

IF is Intermediate Frequency

LO is local oscillating frequency

The relationship between all these parameters are  RF + IF = LO or IF = RF – LO
Ref: http://www.polyphasemicrowave.com/datasheets/IRM0511B.pdf

The satellite signal at 12000 mhz, the local oscillator beats out a constant 10750 mhz. This generates an intermediate frequency of 1250, which is more easily conducted down the coax to the receiver for demodulation. Modulator applies desired modulation to RF or IF carrier wave and Demodulator extracts the original modulation from the  RF or IF.

RF loop: This command is similar to IF loop but does not connect IF out to IF in. You need to make the loop physically yourself, e.g. at the far end of the IF cable network.  This is a good test for the connectors on the IF cabling.

There are two internal loopback commands.

I/O loop: This loops the data interface in both directions. Data sent into the terrestrial interfaces comes back out.  Data received from the satellite is send back to the satellite. Good for diagnosing if the problem in on the terrestrial side of the modem or satellite side related. If done at a remote site the hub can verify that everything is OK right up to the modem terrestiral interface. Note that the hub may need to reduce bit rate so that the return carrier may be accommodated in the satellite transponder.

Digital loop: This command loops back the modem after framing (and Reed Soloman coding) but before main FEC coding. This would be useful if you suspected an internal modem fault.

Additional loop tests not involving modem commands:

Loop test translator:  If you have a “loop test translator” this enables you to downconvert a sample of the BUC output back to a receive frequency, rather like an artificial satellite. e.g. 14-14.5 GHz in and 10.7-11.2 GHz out (or 950-1450 MHz out).  The 14GHz to L band versions and they are very useful for testing BUC/HPA outputs if you don’t have a 14GHz spectrum analyser. They function like an LNB with LO=13.05 GHz LO frequency.

Ref: Radyne LTT6400/A03 C-Band Loop Test Translator

Satellite loop back: If your uplink and downlink are in the same beam then you can monitor your transmit carrier with a receive modem. Many VSAT hubs will have a test remote VSAT co-located for such test purposes.

Ref: http://www.satsig.net/cgi-bin/yabb/YaBB.pl?board=any1;action=display;num=1271640292

Testing modem A with Loopback(Near) from Modem B

Testing modem A with the Satellite

Testing modem A without the Satellite

In operating condition, both Loopback are disable.

Ref: https://hiseasnet.ucsd.edu/wiki/display/hsnops/Comtech+Satellite+Modems

Here is an example of analogy for normal or inverted spectrum.

You are using a windows computer with a serial port
You use a straight-through serial cable with a female 9-pin gender bender
You unplug the data connector (25 pin) when doing the update or it may not work.

Performing a flash upgrade erases the non-volatile RAM, which is where the
modem’s configuration is stored. Users shall re-enter the desired configuration
parameters.

The upgrade is performed without opening the unit, by connecting the modem to the serial port of a computer and executing a flash uploader utility program. The cable to connect the PC to the modem is the same as is used for normal EIA-232 remote control, and comprises three wires connected between two 9-pin ‘D’ type female connectors. Ensure this cable is connected and working properly before proceeding with a
flash update.

Vertical Polarization

sv1bsx.50webs.com/antenna-pol/polarization.html

Vertical polarisation is when the electric field is vertical.  Usually Vertical polarization is for KU-band. To set nominal Vertical receive polarisation the broad faces of the LNB waveguide must be on top and underneath.

www.satsig.net/polangle.htm

Horizontal Polarization

sv1bsx.50webs.com/antenna-pol/polarization.html

Horizontal polarization is when the electric field is horizontal.  Usually Horizontal polarization is for KU-band. References to ‘vertical’ and ‘horizontal’ are frequent since terrestrial antennas are normally vertical or horizontal relative to the ground. LNB rectangular waveguide input oriented for horizontal polarisation starting position.

www.satsig.net/polangle.htm

Circular Polarization
A circular polarized antenna will be able to send or receive both vertical and horizental polarized signals. The cp(circular polarization) wave when reflected by obstacle will not interfere with the original one. Because after reflection LHCP will change to RHCP, and there are orthogonal.

The only advantage with using circular polarization is that with circular polarization Farady rotation will not affect the link. Circular polarization is very popular technique for satellite communication.

A practical advantage is however that the system is much easier to setup as you dont have to worry about adjusting the polarization. The feed assembly is also simpler as there is no need for motors or moving parts.

Usually circular left/right polarization is for C-Band

Left Hand Circular Polarization (LHCP)

sv1bsx.50webs.com/antenna-pol/polarization.html

Right Hand Circular Polarization (RHCP)

sv1bsx.50webs.com/antenna-pol/polarization.html

Bandwidth
In electronic communication, bandwidth is the width of the range (or band) of frequencies that an electronic signal uses on a given transmission medium. In this usage, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component. Since the frequency of a signal is measured in hertz (the number of cycles of change per second), a given bandwidth is the difference in hertz between the highest frequency the signal uses and the lowest frequency it uses.
In signal processing and control theory the bandwidth is the frequency at which the closed-loop system gain drops to −3 dB.In basic electric circuit theory when studying Band-pass and Band-reject filters the bandwidth represents the distance between the two points in the frequency domain where the the signal is 1/Sqrt(2) of the maximum signal strength.
Ref: http://physicsarchives.com/index.php/courses/905

Data Rate
Data Rate or data transfer rate(In computer networks) – the amount of data that can be carried from one point to another in a given time period (usually a second). This kind of Data Rate is usually expressed in bits (of data) per second (bps). Occasionally, it’s expressed as bytes per second (Bps).

The Relation between Bit Rate and Bandwidth

Bit Rate = Bandwidth * log2(1+SNR)  where  SNR = signal to noise ratio in dB

RS-232 defines three types of connections: electrical, functional, and mechanical. The RS-232 interface is ideal for the data-transmission range of 0–20 kbps/50 ft. (15.2 m). It employs unbalanced signaling and is usually used with DB25 connectors to interconnect DTEs (computers, controllers, etc.) and DCEs (modems, converters, etc.). Serial data exits through an RS-232 port via the Transmit Data (TD) lead and arrives at the destination device’s RS-232 port through its Receive Data (RD) lead. RS-232 is compatible with these standards: ITU V.24, V.28; ISO IS2110.

V.35
V.35 is the international standard termed ‘‘Data Transmission at 48 kbps Using 60–108 KHz Group-Band Circuits.’’ It is typically used for DTEs or DCEs that interface to a high-speed digital carrier such as AT&T® Dataphone® Digital Service (DDS).

V.35 was originally developed by the CCITT (now the ITU) and is considered obsolete. However, it is still a popular interface for higher speed data signals.V.35 is a balanced interface that senses current flow, versus voltage levels. A Negative Mark (1) is employed; Output A is negative with respect to Output B for a Mark (1). The characteristic impedance is 100 ohms. Voltage levels employed are +/- 1.1 VDC (differential A to B measurement). Driver offset from Signal Ground may be 0 +/- .2 VDC. The Receiver offset from Signal Ground may be as much as 0 +/- .6 VDC. The Receiver sensitivity is +/- 10 millivolts maximum.

A word of caution about V.35. It has been obsolete since 1988. It originally specified an electrical data interface capable of operation at 48 KHz at 50 feet. Although V.35 equipment today commonly operates at T1 speeds (1.544 MBPS), there have been cases of incompatibility between devices at the higher bit rates.

The V.35 interface is implemented in a square-shell (“Winchester”) connector with 34 pins.

RS-422
RS-422 supersedes RS-449 and complements RS-232. Based on a 25-pin connection, it works in conjunction with either electrical interface RS-422 (balanced electrical circuits) or RS-423 (unbalanced electrical circuits).

RS-422 is a balanced electrical interface capable of operation of T1 speeds or greater. It is most commonly implemented using a D-type connector with 37 pins. The actual connector is specified in the RS-449 specification.

A binary 1 (Mark) is represented by the A output being more negative than the B output. A binary 0 (Space) is represented by the A output being more positive than the B output. The receiver can detect transitions with voltage levels of 200 mVDC. A typical impedance of 100 ohms is used and the receiver can have a Signal Ground potential difference of up to +/- 7V and continue to operate properly.

RS-422 defines a balanced interface with no accompanying physical connector. Manufacturers who adhere to this standard use many different connectors, including screw terminals, DB9, DB25 with nonstandard pinning, DB25 following RS-530, and DB37 following RS-449. RS-422 is commonly used in point-to-point communications conducted with a dual-state driver.

RS-530
RS-530 defines the mechanical/electrical interfaces between DTEs and DCEs that transmit serial binary data, sync or async, at rates from 20 kbps to 2 Mbps. (Maximum distance depends on the electrical interface.) RS-530 takes advantage of higher data rates with the same mechanical connector used for RS-232. Though RS-530 and RS-232 are not compatible,  It is compatible with ITU V.10, ITU V.11, X.26, MIL STD 188-114, RS-422, and RS-423.

The RS-530 specification is basically the RS-422 and RS-423 interfaces implemented on a 25-pin D-shell connector, as opposed to the 37-pin D-shell specified in RS-449.

Either balanced (RS-422) or unbalanced (RS-423) modes are supported. The interface can support Asynchronous or Synchronous transmission rates from 20 KBPS to 2 MBPS.

Ref:

http://telecom.tbi.net/data-if.html

http://www.cpcstech.com/serial-data-transmission-information.htm