the Radio Spectrum
SIGNAL MODULATION SYSTEMS. Commonly
talk AM stations and FM, and are often confused this with the broadcasting bands on medium wave and VHF respectively. AM and FM refer to the type of modulation used by stations in these bands and not the band itself. A DXers to explore different bands looking for different types of broadcasters (broadcasting, utility, amateur radio, etc..) Experiencing different types of modulation (AM, FM, SSB, CW, RTTY, etc.) That your receiver must be able of demodulating if you want to hear.
What is modulation?
In a radio transmitter generates a radio frequency signal that is emitted through the antenna and picked up by a receiver. However, this signal would just meaningless noise. To broadcast information through radio, the message (eg an audio signal, voice or music) must be "mixed" with the radio signal (now called "carrier" for carrying the signal information to receiver ), ie the signal is modulated by the transmitter.
Various modulation systems, which can be divided into 2 groups: the audio transmission systems (voice): AM, FM, SSB, and the systems 'voiceless': CW (Morse), RTTY (RTTY) that serve for transmission of text, images, etc.
voice modes:
AM - Amplitude Modulation
is the oldest way of voice and the standard used between the radio stations on long wave, medium and short. As its name suggests this method uses amplitude modulation wave to "carry" audio. As shown, the signal generated by the transmitter (carrier) is mixed with the audio you want issued by varying the amplitude of the carrier waves (vertical axis of the graph) while the stroke rate remains constant (horizontal axis).
FM - Frequency Modulated
mode is used for VHF stations, Channels TV and lots of "transceivers" portable (walkie-talkie "," handy ", wireless). Modular FM is to vary the frequency of the carrier to "rhythm" of information (audio), which means that an FM signal, the amplitude and phase of the signal frequency remains constant and changes depending on changes amplitude and frequency of the signal to be transmitted (audio) as shown in the figure below shows the FM signal equivalent to the previous example. Notice how the stroke rate varies (horizontal axis) while the amplitude of the wave is always the same (vertical axis).
SSB - single sideband (SSB-Single Side Band)
AM In a sign there are 3 elements: the carrier signal and 2 "side bands" that contain the information (audio) so it is also known as BLD - Banda Lateral Double. "
SSB modulation is the carrier suppression and one of the sidebands thus transmitting only one sideband containing all the information (see figure). Once captured the SSB signal at the receiver, it reinserted carrier to demodulate the audio signal and transform it again.
The advantage of this system on the AM is its lower bandwidth requirements, since a signal 2 AM carries BLU sidebands and only one, for example, a signal that requires 10kHz wide AM SSB need of plus or minus 5 kHz. Moreover, it does not require carrier, all the transmission power can be applied to a single sideband, which in fact this system the most popular among amateurs (who have licenses that limit the transmission power of their equipment) and shortwave utility services.
should be clear that there are variants of this mode of transmission as the removal bands:
- USB-Banda Lateral Superior : when the carrier is suppressed and the lower sideband.
- Lower Side Band LSB-: when the carrier is suppressed and the upper sideband. Banda Lateral
- with suppressed carrier : when only the carrier is suppressed. MODES
VOICELESS :
CW - Continuous Wave
The "CW" is the transmission system used for broadcast in Morse code. This is the issue of the unmodulated radio signal (carrier) which is issued and continuously interrupted by the operator forming the string of "dots and dashes" of Morse code.
Morse code is still used extensively by amateur, coastal stations aviation, diplomatic and military.
| The Morse Code Alphabet | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| A | · - | G | - · | N | - · | T | - | 0 | --- - | 7 - | |
| B | - · · · | H | · · · · | Ñ | ---- | U | ··- | 1 | ·---- | 8 | ---·· |
| C | -·-· | I | ·· | O | --- | V | ···- | 2 | ··--- | 9 | ----· |
| CH | ---- | J | ·--- | P | ·--· | W | ·-- | 3 | ···-- | , | --··-- |
| D | -·· | K | -·- | Q | --·- | X | -··- | 4 | ····- | . | ·-·-·- |
| E | · | L | ·-·· | R | · - · | And | - · - | 5 | ..... | + - | |
| F | · · - · | M | - | S | · · · | Z - · · | 6 | - | ? - | ||
RTTY - Radio
The other method the transmission of texts called RTTY (RTTY-Radioteletype). Most RTTY transmissions heard on shortwave RTTY using a format called Baudot. Baudot format represents each character with a series of 5 bits. Each bit 5 is a MARK (1) or a space (or). Your radio should receive the dial tone and space. The distance between the two frequencies is called "shifts" (changes). The "shifts" ordinary shortwave include 170, 425 and 850 Hz are hundreds of regular seasons (Baudot) RTTY that can be heard, even with a modest receiver. The major types include: News agencies stations, meteorological, military, diplomatic, scientific research, commercial and maritime. Hams also use various modes of tickers.
special modes RTTY ...
Most RTTY stations broadcast in standard format "Baudot" above, but there are many other formats (protocols). One of the less popular ASCII format is where each character is sent as a series of 7 bits. Most equipment can decode RTTY ASCII. Another most common protocol is called TOR (Telex-Over-Radio). This is often referred to as SITOR A / B, ARQ, FEC or AMTOR. This mode is largely used by maritime users as well to become popular among diplomats and amateur radio services. This mode allows the sender and receiver of a communication enjoy almost free of errors. There are other more exotic modes such as ARQ-M2, ARQ-E/E3, FEC-A FEC-S, SWED-ARQ, ARQ-S, Piccolo and Frequency Division Multiplex (VFT).
Note: For reception of RTTY is necessary to have a good communications receiver and a RTTY decoder. These can be both expensive equipment "dedicated" as computer interfaces at low cost. For more information on these would find in our section on "equipment DX
for an extension of this type of transmission seen in the section Utility DX page on DX RTTY and Fax
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