Extra Class question of the day: Digital signals and communications modes

Morse Code is arguably the original digital mode. Morse code is a digital code consists of elements having unequal length. (E8C01) One advantage of using Morse Code is that it is very narrow bandwidth. The bandwidth necessary for a 13-WPM international Morse code transmission is approximately 52 Hz. (E8C05)

The next oldest digital mode is radioteletype, or RTTY. RTTY uses a five-bit code called Baudot. Most modern digital devices these days use ASCII, which is a 7-bit or 8-bit code. Some of the differences between the Baudot digital code and ASCII are that Baudot uses five data bits per character, ASCII uses seven or eight; Baudot uses two characters as shift codes, ASCII has no shift code. (E8C02) One advantage of using the ASCII code for data communications is that it is possible to transmit both upper and lower case text. (E8C03)

The reason that some ASCII transmissions have only seven bits, while others use eight bits is that the eighth bit is a parity bit. The advantage of including a parity bit with an ASCII character stream is that some types of errors can be detected. (E8C12)

The bandwidth needed for ASCII digital transmissions increases as the data rate increases. The bandwidth necessary for a 170-hertz shift, 300-baud ASCII transmission is 0.5 kHz. (E8C06) The bandwidth necessary for a 4800-Hz frequency shift, 9600-baud ASCII FM transmission is 15.36 kHz. (E8C07)

PSK has become a very popular digital mode. One reason for this is that it occupies a very narrow bandwidth – only 31 Hz. One technique used to minimize the bandwidth requirements of a PSK31 signal is the use of sinusoidal data pulses. (E8C04)

An up-and-coming digital mode is JT-65, named after its inventor, Nobel Prize winner and amateur radio operator, Joe Taylor, K1JT. It uses 65 different tones spread over a bandwidth of 175 Hz. One advantage of using JT-65 coding is the ability to decode signals which have a very low signal to noise ratio. (E8C13)

Spread-spectrum communication is a wide-bandwidth communications system in which the transmitted carrier frequency varies according to some predetermined sequence. (E8C08) Direct sequence is a spread-spectrum communications technique uses a high speed binary bit stream to shift the phase of an RF carrier. (E8C11) Frequency hopping is a spread-spectrum communications technique alters the center frequency of a conventional carrier many times per second in accordance with a pseudo-random list of channels. (E8C10) Spread-spectrum techniques causes a digital signal to appear as wide-band noise to a conventional receiver. (E8C09)

Extra Class question of the day: operating HF digital modes; error correction

Perhaps the most popular digital mode these days is PSK31. PSK stands for “phase shift keying.” One of its main advantages is that it had a very narrow bandwidth—only 31 Hz. In fact, PSK31 is the digital communications mode that has the narrowest bandwidth. (E2E10)

One of the ways is achieves this narrow bandwidth is that uses variable length coding. That is to say, characters have different numbers of bits, depending on how frequently they appear in normal text. PSK31 is an HF digital mode that uses variable-length coding for bandwidth efficiency. (E2E09)

Another type of modulation commonly used on the HF bands is frequency-shift keying, or FSK. RTTY, for example uses FSK modulation. FSK is a type of modulation that is common for data emissions below 30 MHz. (E2E01) One type of FSK modulation is MFSK16. The typical bandwidth of a properly modulated MFSK16 signal is 316 Hz. (E2E07)

Amateur transceivers use two different methods to modulate a signal using FSK: direct FSK and audio FSK. The difference between direct FSK and audio FSK is that direct FSK applies the data signal to the transmitter VFO. (E2E11) When using audio FSK, audio, typically from a computer sound card, is used to shift the frequency of the transmitted signal.

To tune an FSK signal, one often uses a crossed-ellipse display. You have properly tuned a signal when one of the ellipses is as vertical as possible, and the other is as horizontal as possible. When one of the ellipses in an FSK crossed-ellipse display suddenly disappears, selective fading has occurred. (E2E04)

PACTOR is one digital mode that uses FSK. It also uses the ARQ protocol to detect errors. Because of this, PACTOR is an HF digital mode that can be used to transfer binary files. (E2E08) How does ARQ accomplish error correction? If errors are detected, a retransmission is requested. (E2E05)

Another way to detect and correct errors in a data transmission is forward error correction. The letters FEC mean Forward Error Correction when talking about digital operation. (E2E02) Forward Error Correction is implemented by transmitting extra data that may be used to detect and correct transmission errors. (E2E03)

No matter what type of modulation you use, data transmission over an HF radio link is very slow. 300 baud is the most common data rate used for HF packet communications. (E2E06) In fact, due to bandwidth limitations, 300 baud is the maximum data rate.

Many of the digital modes were designed to allow keyboard-to-keyboard operation. That is to say, that operators can type messages back and forth to one another, almost as if they were having a conversation using SSB. Winlink, however, does not support keyboard-to-keyboard operation. (E2E12)

Extra Class question of the day: VHF and UHF digital modes; APRS

One of the most commonly understood concepts in digital communications is the baud. A baud is not equal to a bit per second, except for very simple systems. Rather, the definition of baud is the number of data symbols transmitted per second. (E2D02) A data symbol may represent multiple bits.

The baud rate is a measure of how fast a digital communications system can transmit data. Under clear communications conditions, 300-baud packet is the digital communication mode that has the fastest data throughput. (E2D09)

In the past ten years or so, we’ve had an explosion of digital modes become available. JT65 is one example. JT65 is a digital mode especially useful for EME communications. (E2D03) JT65 improves EME communications because it can decode signals many dB below the noise floor using FEC.(E2D12) FSK441 is a digital mode especially designed for use for meteor scatter signals. (E2D01)

One of the most popular digital modes is the Automatic Packet Reporting System, or APRS. AX.25 is the digital protocol used by APRS. AX.25 is more commonly known as packet radio. (E2D07) Unnumbered Information is the type of packet frame used to transmit APRS beacon data. (E2D08)

APRS stations can be used to help support a public service communications activity. An APRS station with a GPS unit can automatically transmit information to show a mobile station’s position during the event. (E2D10) Latitude and longitude are used by the APRS network to communicate your location. (E2D11) 144.39 MHz is a commonly used 2-meter APRS frequency. (E2D06)

Amateurs that enjoy satellite communications also use digital modes. For example, store-and-forward is a technique normally used by low Earth orbiting digital satellites to relay messages around the world. (E2D05) The purpose of digital store-and-forward functions on an Amateur Radio satellite is to store digital messages in the satellite for later download by other stations. (E2D04)

TAPR announces Digital Communications Conference details

Where: Atlanta, GA Sheraton Gateway Hotel Atlanta Airport 1900 Sullivan Road Atlanta, GA 30337

When: September 21 – 23, 2012

Website: www.tapr.org/dcc

Technical / Introductory Sessions Schedule http://www.tapr.org/pdf/DCC_2012_Schedule.pdf

Technical Sessions Friday – Saturday: Introductory Sessions

Saturday Night Banquet Speaker & Topic: http://www.tapr.org/dcc#banquet
DCC Saturday Night Banquet Speaker will be Bdale Garbee, KB0G talking about the “Sharing the Joy of Making.

Sunday Morning Seminar Speaker & Topic: http://www.tapr.org/dcc#seminar
DCC Sunday Morning Seminar will be a hands-on tutorial using Gnuradio  to design and implement software defined radios on your laptop presented by Tom Rondeau, KB3UKZ, the leader of the Gnuradio project.

Extra Class question of the day: modulation methods; modulation index and deviation ratio; pulse modulation; frequency and time division multiplexing

In FM modulation, the two primary parameters of interest are deviation ratio and modulation index. Deviation ratio is the ratio of the maximum carrier frequency deviation to the highest audio modulating frequency. (E8B09) The deviation ratio of an FM-phone signal having a maximum frequency swing of plus-or-minus 5 kHz when the maximum modulation frequency is 3 kHz is 1.67. (E8B05)The deviation ratio of an FM-phone signal having a maximum frequency swing of plus or minus 7.5 kHz when the maximum modulation frequency is 3.5 kHz is 2.14. (E8B06)

The term for the ratio between the frequency deviation of an RF carrier wave, and the modulating frequency of its corresponding FM-phone signal is modulation index. (E8B01) The modulation index is equal to the ratio of the frequency deviation to the modulating frequency. The modulation index of a phase-modulated emission does not depend on the RF carrier frequency. (E8B02)

The modulation index of an FM-phone signal having a maximum frequency deviation of 3000 Hz either side of the carrier frequency, when the modulating frequency is 1000 Hz is 3. (E8B03) The modulation index of an FM-phone signal having a maximum carrier deviation of plus or minus 6 kHz when modulated with a 2-kHz modulating frequency is 3. (E8B04)

Some amateur radio communications are pulse-width modulated. That is to say that the information being sent is proportional to the time the carrier is on. When using a pulse-width modulation system, the transmitter’s peak power greater than its average power because the signal duty cycle is less than 100%. (E8B07)

Some signals are pulse-position modulated. That is to say, what is significant is when the pulse occurs. The time at which each pulse occurs is the parameter that the modulating signal varies in a pulse-position modulation system. (E8B08)

Frequency division multiplexing is one method that can be used to combine several separate analog information streams into a single analog radio frequency signal. (E8B10) When a system uses frequency division multiplexing, two or more information streams are merged into a “baseband,” which then modulates the transmitter. (E8B11)

When a system uses digital time division multiplexing, two or more signals are arranged to share discrete time slots of a data transmission. (E8B12)

Military thinking about “wideband” sideband for datacomm?

Bob Brewin, editor at large, for  Nextgov,  a website that cover government IT, speculates about the use of wideband SSB to achieve higher data rates than is currently possible on HF. His column notes:

The Air Force High Frequency Global Communications System Program Office at Tinker Air Force Base is looking for some folks to help tweak single sideband so it can function as a relatively wide band over the air data transmission system as well as handle voice over IP. Single sideband will never have the throughput of a fiber optic connection. On the other hand, there’s not a lot of fiber drops in the sky or a forward operating base in Afghanistan, so a 120 kbps connection from a radio looks real good.

This certainly sounds like a project for some enterprising hams. And, who knows, this  new mode might even make its way into the ham radio bands someday.

Papers Requested for ARRL/TAPR Digital Communications Conference

Amateurs are invited to submit technical papers for presentation at the 31st Annual ARRL and TAPR Digital Communications Conference to be held September 21-23, 2012 at the Sheraton Gateway Airport Hotel in Atlanta, Georgia.

These papers will also be published in the Conference Proceedings (you do not need to attend the conference to have your paper included in the Proceedings).

The submission deadline is July 31. Please send papers to: Maty Weinberg, ARRL, 225 Main St, Newington, CT 06111. Or you can make your submission via e-mail to: maty@arrl.org.

For more information about the conference, see the Tucson Amateur Packet Radio www.tapr.org/dcc, or call 972- 671-8277.

TAPR Conference Videos Online

From the ARRL Letter, 1/12/12:

Amateur Radio Video News (ARVN) has released high-definition videos of all the talks presented at the 2011 ARRL/TAPR Digital Communications Conference (DCC), held September 16-18. The programs are now available online on the ARVN website.

The DCC is a three-day conference on Amateur Radio digital technology. Among the video presentations are 18 seminars, the Saturday Banquet and the welcome introduction by TAPR Chairman Steve Bible, N7HPR. According to ARVN Producer Gary Pearce, KN4AQ, most of the talks are fairly technical, although there are four separate sessions that cover “Intro to” topics. “All of the talks — except the ‘Intro to’ talks — were shot in three-camera high-definition, with wireless mics for ‘close-up’ audio of the presenter, as well as the question-and-answer period,” he explained. “The ‘Intro to’ talks were shot with a single, standard-definition camera.

This year, Pearce decided to release the programs on the web instead of the DVDs that have been produced in previous years (although the DVDs will be available later). “I wanted to make the programs available more quickly and easily to a worldwide audience,” he said. “The web has become an easy, high-quality distribution medium.”


Yaesu thinks the future of ham radio is digital

A Digital Communications GuideYaesu thinks the future of ham radio is digital, and of course, that amateurs should adopt its digital mode (C4FM) over Icom’s (D-STAR). At least that’s what they say in their latest publication, A Digital Communications Guide for Amateur Radio Operators.

This publication claims several advantages for digital communications techniques, including:

  • reduced bandwidth,
  • digital data transfer,
  • better performance,
  • immunity to interference, and
  • product and system cost reduction.

It talks about some of the theory behind digital communications, explaining in relatively simple terms how the various modulation techniques work. Of course, it slams D-STAR:

Now, this method [GMSK] is considered old fashioned and no longer used by LMR [land mobile radio]. Currently, GMSK is still being used by D-STAR.

One problem I have with this publication is its implicit assumption that digital is better than analog, and that if we want to be “progressive” amateurs, we should all adopt digital communications techniques. I’m not all that convinced, and to its credit, Yaesu does concede that “analog FM can show an advantage over digital radio in some areas.”

I haven’t compared prices, but if the D-STAR radios are any indication, the prices of Yaesu’s digital radios are bound to be more expensive than the analog radios. I just don’t see that the added functionality is worth the extra cost.

What do you think? Do you think D-STAR or Yaesu’s C4FM will gain widespread acceptance anytime soon? Do you currently own a digital radio? If not, what would convince you to buy a digital radio?