Digital

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digital representation (red) of an analog signal (grey)

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Digital

Property of representing values as discrete numbers rather than a continuous spectrum.

A digital system uses discrete (discontinuous) values, usually but not always symbolized numerically (hence called "digital") to represent information for input, processing, transmission, storage, etc. By contrast, non-digital (or analog) systems use a continuous range of values to represent information.

Although digital representations are discrete, the information represented can be either discrete, such as numbers, letters or icons, or continuous, such as sounds, images, and other measurements of continuous systems.

Modulation

Amfm3-en-de.gif

In telecommunications, modulation is the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch.

Normally a high-frequency sinusoid waveform is used as carrier signal.

The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal.

Analog modulation AM · SSB · FM · PM · SM

Digital modulation OOK · FSK · ASK · PSK · QAM MSK · CPM · PPM · TCM · OFDM

Spread spectrum FHSS · DSSS

Modes

The four fundamental digital modulation techniques: 

   * In the case of PSK, a finite number of phases are used.
   * In the case of FSK, a finite number of frequencies are used.
   * In the case of ASK, a finite number of amplitudes are used.
   * In the case of QAM, a finite number of at least two phases, and at least two amplitudes are used.

Digital modulation List of common digital modulation techniques 

   * Phase-shift keying (PSK):
         o Binary PSK (BPSK), using M=2 symbols
         o Quadrature PSK (QPSK), using M=4 symbols
         o 8PSK, using M=8 symbols
         o 16PSK, usign M=16 symbols
         o Differential PSK (DPSK)
         o Differential QPSK (DQPSK)
         o Offset QPSK (OQPSK)
         o π/4–QPSK
   * Frequency-shift keying (FSK):
         o Audio frequency-shift keying (AFSK)
         o Multi-frequency shift keying (M-ary FSK or MFSK)
         o Dual-tone multi-frequency (DTMF)
         o Continuous-phase frequency-shift keying (CPFSK)
   * Amplitude-shift keying (ASK)
   * On-off keying (OOK), the most common ASK form
         o M-ary vestigial sideband modulation, for example 8VSB
   * Quadrature amplitude modulation (QAM) - a combination of PSK and ASK:
         o Polar modulation like QAM a combination of PSK and ASK.[citation needed]
   * Continuous phase modulation (CPM) methods:
         o Minimum-shift keying (MSK)
         o Gaussian minimum-shift keying (GMSK)
   * Orthogonal frequency division multiplexing (OFDM) modulation:
         o discrete multitone (DMT) - including adaptive modulation and bit-loading.
   * Wavelet modulation
   * Trellis coded modulation (TCM), also known as trellis modulation


Amtor

An FSK mode that is hardly used by radio amateurs in the 21st Century. While a robust mode, it only has 5 bits (as did its predecessor RTTY) and can not transfer extended ASCII or any binary data. With a set operating rate of 100 baud, it does not effectively compete with the speed and error correction of more modern ARQ modes like Pactor. The non-ARQ version of this mode is known as FEC, and known as SITOR-B by the Marine Information services.

Clover

Domino

G-TOR

(Golay -TOR) An FSK mode that offers a fast transfer rate compared to Pactor. It incorporates a data inter-leaving system that assists in minimizing the effects of atmospheric noise and has the ability to fix garbled data. G-TOR tries to perform all transmissions at 300 baud but drops to 200 baud if difficulties are encountered and finally to 100 baud. (The protocol that brought back those good photos of Saturn and Jupiter from the Voyager space shots was devised by M.Golay and now adapted for ham radio use.) GTOR is a proprietary mode developed by Kantronics. Because it is only available with Kantronics multi-mode TNCs, it has never gained in popularity and is rarely used by radio amateurs.

Hellscreiber

A method of sending and receiving text using facsimile technology. This mode has been around along time. It was actually developed by Germany prior to World War II! The recent use of PC sound cards as DSP units has increased the interest in Hellschreiber and many programs now support this new...well I mean, old mode. The single-tone version (Feld-Hell) is the method of choice for HF operation. It is an on-off keyed system with 122.5 dots/second, or about a 35 WPM text rate, with a narrow bandwidth (about 75 Hz). Text characters are "painted" on the screen, as apposed to being decoded and printed. Thus, many different fonts can be used for this mode including some basic graphic characters. A new "designer" flavor of this mode called PSK HELL has some advantage for weak signal conditions. As with other "fuzzy modes" it has the advantage of using the "human processor" for error correction; making it the best overall mode for live HF keyboard communications. Feld-Hell also has the advantage of having a low duty cycle meaning your transmitter will run much cooler with this mode.

Packet

(300 baud) is a FSK mode that is an adaption of the very popular Packet radio used on VHF (1200 baud) FM amateur radio. Although the HF version of Packet Radio has a much reduced bandwidth due to the noise levels associated with HF operation, it maintains the same protocols and ability to "node" many stations on one frequency. Even with the reduced bandwidth (300 baud rate), this mode is unreliable for general HF ham communications and is mainly used to pass routine traffic and data between areas where VHF repeaters maybe lacking. HF and VHF Packet has recently enjoyed a resurgence in popularity since it is the protocol used by APRS - Automatic Position Reporting System mostly on 2 meter VHF and 30 meter HF.

Pactor

An FSK mode and is a standard on modern Multi-Mode TNCs. It is designed with a combination of packet and Amtor Techniques. Although this mode is also fading in use, it is the most popular ARQ digital mode on amateur HF today and primarily used by amateurs for sending and receiving email over the radio. This mode is a major advancement over AMTOR, with its 200 baud operating rate, Huffman compression technique and true binary data transfer capability.

Pactor-II

A robust and powerful PSK mode which operates well under varying conditions. It uses strong logic, automatic frequency tracking; it is DSP based and as much as 8 times faster then Pactor. Both PACTOR and PACTOR-2 use the same protocol handshake, making the modes compatible. As with the original Pactor, it is rarely used by radio amateurs since the development of the new PC based sound card modes. Also, like GTOR, it is a proprietary mode owned by SCS and only available with their line of multi-mode TNC controllers.

PSK31 - QPSK

The first new digital mode to find popularity on HF bands in many years. It combines the advantages of a simple variable length text code with a narrow bandwidth phase-shift keying (PSK) signal using DSP techniques. This mode is designed for "real time" keyboard operation and at a 31 baud rate is only fast enough to keep up with the typical amateur typist. PSK31 enjoys great popularity on the HF bands today and is presently the standard for live keyboard communications. Most of the ASCII characters are supported. A second version having four (quad) phase shifts (QPSK) is available that provides Forward Error Correction (FEC) at the cost of reduced Signal to Noise ratio. Since PSK31 was one of the first new digital sound card modes to be developed and introduced, there are numerous programs available that support this mode - most of the programs available as "freeware".

Sub-bands

BAND FREQ MHz Best time

RTTY

"Radio Teletype" is a FSK mode that has been in use longer than any other digital mode (except for morse code). RTTY is a very simple technique which uses a five-bit code to represent all the letters of the alphabet, the numbers, some punctuation and some control characters. At 45 baud (typically) each bit is 1/45.45 seconds long, or 22 ms and corresponds to a typing speed of 60 WPM. There is no error correction provided in RTTY; noise and interference can have a seriously detrimental effect. Despite it's relative disadvantages, RTTY is still popular with many radio amateurs. This mode has now been implemented with commonly available PC sound card software.

What Sideband?

RTTY has a unique tradition of not following the conventional "USB/LSB" split at 10Mhz. On HF - ALL RTTY is on Lower Sideband (LSB) - regardless of the band.

Other than being the current convention, the only reason I've found for this was this comment, posted to a contesting usenet group by Bill Henry - Dec, 2001

The rule is LSMFT - "Low Space Means Fine Teletype". This slogan used to be printed on the editorial page of the RTTY Journal. The rule comes from using relays and later diodes to switch an extra capacitor across the VFO tank circuit in the 1950's. The normal "rest" condition - RTTY Mark - is with the extra capacitor out of the circuit. This corresponds to keeping the same VFO dial calibration as

A similarly 'themed' posting by Rye, K9LCJ in Sept 2007 to the sourceboost.com discussion board yields:

If you remember the old Lucky Strike motto, LSMFT, "Lucky Strike Means Fine Tobacco", we could remember TTY loop levels by "Low Space Means Fine Teletype". The high mark kept the selector magnets engaged. ---but then that was long ago and far away so most don't remember it. Kinda like ELI the ICE man......although I understand that's still true.

If you've some spare time, I'd suggest a google search on those phrases - there's a lot of history, some way cool theory and even a band out there!

Sub-bands

The HF RTTY sub-band boundaries are defined and limited by your license. Below are approximations of "normal" operating RTTY sub-bands world-wide. When operating RTTY, be sure respect the band guards around the IBP/NCDXA beacon frequencies noted below.


MFSK16

An advancement to the THROB mode and encodes 16 tones. The PC sound card for DSP uses Fast Fourier Transform technology to decode the ASCII characters, and Constant Phase Frequency Shift Keying to send the coded signal. Continuous Forward Error Correction (FEC) sends all data twice with an interleaving technique to reduce errors from impulse noise and static crashes. A new improved Varicode is used to increase the efficiency of sending extended ASCII characters, making it possible to transfer short data files between stations under fair to good conditions. The relatively wide bandwidth (316 Hz) for this mode allows faster baud rates (typing is about 42 WPM) and greater immunity to multi path phase shift. A second version called MFSK8 is available with a lower baud rate (8) but greater reliability for DXing when polar phase shift is a major problem. Both versions are available in a nice freeware Windows program created by IZ8BLY.

MT63

A new DSP based mode for sending keyboard text over paths that experience fading and interference from other signals. It is accomplished by a complex scheme to encode text in a matrix of 64 tones over time and frequency. This overkill method provides a "cushion" of error correction at the receiving end while still providing a 100 WPM rate. The wide bandwidth (1Khz for the standard method) makes this mode less desirable on crowded ham bands such as 20 meters. A fast PC (166 Mhz or faster) is needed to use all functions of this mode. MT63 is not commonly used by amateurs because of its large bandwidth requirement and the difficulty in tuning in an MT63 transmission.

Throb

Yet another new DSP sound card mode that attempts to use Fast Fourier Transform technology (as used by waterfall displays). THROB is actually based on tone pairs with several characters represented by single tones. It is defined as a "2 of 8 +1 tone" system, or more simply put, it is based on the decode of tone pairs from a palette of 9 tones. The THROB program is an attempt to push DSP into the area where other methods fail because of sensitivity or propagation difficulties and at the same time work at a reasonable speed. The text speed is slower than other modes but the author (G3PPT) has been improving his MFSK (Multiple Frequency Shift Keying) program. Check his web site for the latest developments.

where?

File:Conop.pdf

BAND FREQ MHz Best time
160ML 1.838 Night-time
80ML 3.580 Night-time
40ML 7.035 DX Night-time
40ML 7.070 US Night-time
30MU 10.140 DX Daylight hours
30MU 10.142 US Daylight hours
20MU 14.070 Dawn til dusk, sometimes later
17MU 18.100 Mid-morning til mid-afternoon
15MU 21.070 Daylight when bands open
15MU 21.080 Daylight when bands open
12MU 24.920 Daylight when bands open
10MU 28.120 Daylight when bands open
6MU 50.290 Daylight when bands open
2MU 144.150 Mostly Local & moonbounce
1.25MU 222.070 Local only 223.55
70cmU 432.200 Local only 434.55
33cmU 909.000 Local only 1243

http://w7hd.dynalias.com/pskfreqs.html

LSB 160, 80, 40
USB on rest

40 Meters
7030 QRP CW calling
7040 European RTTY call
7035 to 7040 - Digital outside US
7070 up - Digital inside US

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