QSK operation (full break-in)

QSK is a three-letter code group which is one of the Morse code three-letter Q-code code groups that comprise the venerable International Q-code for radiotelegraph operators established in the first decade of the 1900's. The three letter Q-code code group QSK literally means "I can hear you between my signals; you may break in on my transmission." Because of this, full break-in operation is often referred to as QSK operation. QSK or full break-in operation is one of several protocols and techniques of turning over control of a half-duplex Morse code radiotelegraph communications channel.

Full break-in or QSK operation,^[1]^[2]^[3] is a hardware supported Morse code communications channel turn over communications protocol, a so-called duplexing protocol, that facilitates a style of two-way communications on traditional half-duplex radiotelegraph channels using Morse code that closely simulates full-duplex channel operation. Turning over a half-duplex communications channel is the change in communications protocol transmission status that occurs when a transmitting station releases transmitting control of a half-duplex communications channel turning it over to another station.

Radiotelegraph stations generally comprise: a transmitter, a receiver, and one or more antennas. On half-duplex radiotelegraph channels, which are used one way at a time the: receiver, transmitter, and antennas are usually interconnected by so-called transmit/receive or (T/R) switches which effectively connect and disconnect the receiver and transmitter from the antenna or antennas as required by the radiotelegraph channel protocol in use.

With full break-in or QSK operation the analog hardware antenna change over transmit/receive (T/R) switches are controlled automatically in real time directly by the actions of the Morse code telegraph key with which the operator is originating the Morse code information. With radiotelegraph communications QSK operation enables a fluid style of half-duplex channel control based upon the ability of a receiving operator to interrupt a sending operator in mid-character in a manner very similar to normal human voice conversations which allow mid-syllable interruptions.^[4]

Signals and symbols

In Morse code the basic signals are dots and dashes often verbalized as "dits" and "dahs". According to the rules of Morse code. The dot duration is the basic unit of time measurement. At a 20 word per minute (wpm) rate the dot duration is approximately 50 milliseconds (0.05 seconds); at faster speeds the dot duration is commensurately shorter. The reciprocal of the dot duration is sometimes known as the dotting rate. The length of the dash signal is three dot durations. These Morse code dot and dash signals are always separated from each other by a silent period of at least one dot duration. Thus, the three basic elements of which all Morse code information is comprised are, the two signals: dot and dash, and the silent period; the time extent of all three of these elements being measured in terms of the dot duration.

In contrast to the short signals and adjacent dot duration silent periods, Morse code symbols are unique and are of longer duration than signals. Morse code symbols represent alpha-numeric, punctuation and prosign characters. Each such long duration symbol is comprised of a unique sequence of two or more of the three basic elements of Morse code.

Morse code symbols are separated from each other by at least a three dot duration silent period. Each unique symbol represents either: (i) a unique alpha-numeric character (e.g. letters A-Z and numerals 0-9), (ii) a unique punctuation character, or (iii) a unique procedural symbol. Words or code groups are comprised of a sequence of symbols. Each word or code group is separated from other words or code groups by a silent period of at least five dot durations. The Morse code silent periods of: one dot duration between Morse code signals, three dot durations between symbols, and five dot durations between words or code groups provide operators with: mid-symbol, mid-word and mid-sentence, opportunities to interrupt or break-in on transmitting operators. These silent periods between signals, symbols and words also provide the sending operator with opportunities to listen for interruptions by receiving stations.

Mid-symbol interruption and listening capability provided by QSK operation during the ubiquitous short Morse code silent periods on half-duplex radiotelegraph channels creates the illusion of apparent continuous two-way contact making radiotelegraph operators feel as if they are sharing a full duplex channel.

Signal level range considerations

Enormous signal level ranges must be accommodated by radio transceiver equipment. Transmitter output power for amateur radio stations might typically be 100 Watts (+50dBm) or more, while received power at radio receiver antenna input terminals might typically be as low as -130dBm, which encompasses a total power handling range of up to 180dBm (e.g. a ratio of 1 to 1 followed by 18 zeros!).

Radiotelegraph stations may use either a single antenna for both transmit and receive or separate transmit and receive antennas. When operating on the same or nearby radio frequencies using the same or nearby antennas as used by the associated transmitter, the typical radio receiver would be close enough to the transmitting antenna that the range of signal levels the receiver would be required to withstand without damage from the powerful signals of its associated transmitter would be problematic at best and impossible at worst. As of this writing there has apparently been no receiver technology yet developed that can operate with full sensitivity over such a huge range of received signal levels whilst also safely withstanding the high power levels presented by the associated nearby transmitter.

The low level analog front end (AFE) amplifier circuitry of receivers sensitive enough to detect signals at -130dBm levels and below are invariably extremely sensitive to high power levels. Typically without the protection and isolation provided by T/R switches the typical receiver AFE would be overwhelmed or destroyed by the normal transmitter power levels in the +50dBm or more range. Consequently receiver AFE antenna input terminals must be protected or isolated from high power transmitter outputs. With QSK operation this receiver protection is provided by well designed robust analog hardware T/R switches placed between the AFE circuitry and the radio antenna.

The end result of extreme receiver AFE sensitivity to high power levels is that, for all practical purposes, signal reception is impossible during periods when the associated transmitter is actually transmitting signals. Consequently radiotelegraph operators cannot hear interruptions from remote receiving stations during normal signal transmission periods when the full transmitter power is applied to the antenna.

To protect receiver circuitry, radiotelegraph channels on nearby frequencies and antennas must operate in so-called half-duplex mode wherein the stations at either end alternate between transmitting and receiving (because e.g. simultaneous transmit and receive is simply not possible). To support two way conversations on half duplex channels, analog radio frequency hardware antenna switches must be provided at each station location to connect and disconnect the transmitters and receivers from their antennas whenever the channel transmission control is turned over from one station to the other. These hardware antenna switches are called transmit/receive (T/R) switches.

The four requirements: (i) to prevent receiver desensitization during transmit periods, (ii) to prevent damage to receiver AFE input circuitry during transmit periods, (iii) to enable transmitting stations to listen between signals and, (iv) to provide efficient, fluid and fluent two way communications on half-duplex radiotelegraph channels, were the prime motivations and considerations which drove the development of Morse code radiotelegraph channel full break-in QSK technologies.

QSK transmit/receive (T/R) switch operation

QSK operation is a technique where very fast T/R switches are controlled automatically and directly by the actions of the telegraph key upon which the sending operator is creating the Morse code signals. In QSK operation the T/R switches are capable of automatically and rapidly switching the radio antenna or antennas between the transmitter and receiver during the short (dot duration) silent periods between Morse code signals. Such fast robust analog radio frequency T/R switches automatically controlled by the operator's telegraph key generally have stringent (timing, reliability, and power handling) specifications and are quite expensive.

QSK hardware switch equipped sending stations can be interrupted mid-symbol (mid-character) by receiving stations during the dot duration silent periods between the sending station's signals. A properly QSK equipped station can be interrupted in the middle of sending most single Morse code alpha-numeric characters, punctuation symbols and prosign symbols which requires, generally speaking, a shorter time period than a single syllable of spoken language. Receiving operators may thus break-in on QSK operators mid-symbol at any time; the only exceptions being the single signal Morse code symbols namely: single dot, for letter E, and single dash, for letter T. QSK operation enables a fluid conversational style of Morse code communication based upon the ability of a receiving operator to interrupt a QSK equipped sender mid-character creating the illusion of a conversational style similar to normal human voice conversations.

Although simple in concept, the detailed design and reliable operation of T/R switches automatically controlled by an associated telegraph key requires not only great attention to the functional details of the timing and control mechanisms between the telegraph key and the T/R switch apparatus but also of the robust design of the T/R hardware switch components themselves.

Semi break-in operation

Semi break-in is a technique used by stations where slow (T/R) antenna switches are controlled indirectly by the telegraph key which lack the faster switching of full break-in stations. Semi break-in hardware T/R switches are not required to switch as fast or to have the same long term reliability as their more expensive full break-in counterparts. Instead of using the telegraph key to directly control antenna switching, semi break-in radio transceiver equipment typically uses the telegraph key to control T/R switches indirectly, but still automatically, by passing the telegraph key information (usually in the form of a keyed audio tone) through a radio transceiver's Voice-operated switch or VOX circuitry.

In this technique, VOX circuitry is used to activate the T/R switches. Voice-operated switch (VOX) circuitry is designed to be normally activated by human voice audio picked up by the transceiver microphone during voice communications in order to effect antenna change over at a typical human voice syllabic period. VOX circuitry usually has a front panel adjustable delay that can be used to control the length of time it takes for T/R switches to operate but generally the delay adjustment range is limited to that of human voice syllables and, although automatic, is generally not fast enough to act in the short periods between Morse code dots and dashes. Receiving stations thus cannot break-in or interrupt semi break-in or VOX controlled Morse code stations in mid-symbol or mid-word, during Morse code operation because the semi break-in sending station simply cannot hear in the short durations between the Morse code signals and words or code groups.

Receiving stations wishing to break-in on semi break-in stations must wait for the longer silent periods between the sending station's sentences and words before attempting to interrupt or break-in. At worst, receiving stations must wait until semi break-in stations explicitly turn over the channel to the receiving station by sending a break prosign. Unlike full break-in operation, semi break-in operation is not fast enough to provide a fluid Morse code conversational capability approximating that of normal human voice conversation.

Although not as fluid and efficient as full break-in, semi break-in or VOX controlled break-in is a better Morse code channel turn over technique than pure manual break-in operation as described in the following paragraph.

Manual break-in operation

Manual break-in is a rudimentary Morse code radio station set up where antenna change over (T/R) switches are not controlled by the telegraph key. Instead antenna change over is accomplished manually by mechanical switches separate from the telegraph key on which the operator sends the Morse code. With such a simple manual turn over system there is no possibility of the sending operator listening between signals or symbols and therefore no possibility for the receiving operator to interrupt the sending operator. Instead the receiving operator must wait until a transmitting operator has indicated the end of transmission by means of a turn over prosign and has manually changed the antenna over from transmitter to receiver. Such manual break-in operation leads to a very slow and stilted style of Morse code conversations.

QSK protocols

QSK operation comprises a hardware switch technology and protocol wherein participating Morse code stations are equipped with very fast analog radio frequency switches connecting the transmitter, receiver and antenna. This fast analog hardware switching capability enables a receiving station to interrupt or break-in on a transmitting station in mid-symbol (mid-character), a process known as full break-in. The ability to hear between transmitted signals conferred by fast radio frequency hardware switching only requires Morse code operators to make use of simple communications protocols to manage the channel turn over process. The typical QSK protocol technique is quite simple to learn and to master.

Preliminary protocol

Since not all Morse code radio stations are equipped for QSK operation, sending stations equipped for QSK operation will send the three letter group QSK (e.g. the operator will assert QSK) during an initial Morse code transmission to alert receiving stations that the sending station has the ability to listen between signals and that the receiving station can interrupt, or break-in, on the sending station at will. Conversely a station may query another Morse code station's QSK capability by sending the QSK signal followed by a question mark. The query QSK? asks if the receiving station has full break-in capability. If a receiving station is equipped for QSK operation the receiving operator will respond to the query QSK? with the assertion QSK indicating that the station has QSK capability. Subsequently the two stations can then utilize the fluid and efficient Morse code conversational QSK protocols outlined in the following paragraphs.

In practice, many skilled operators do not necessarily use the preliminary QSK assertion and query protocol, instead they attempt to interrupt a sending station by tapping their telegraph key to check out what happens. If the sending station pauses when interrupted then the two stations immediately start using the following protocols.

Interrupt protocol

Interruptions or break-ins are initiated by receiving stations momentarily depressing their telegraph key while the sending station is actively sending Morse code, thus generating a short interrupting signal which is heard by the sending station between its own signals. In practice usually only a single dot is required to initiate a break-in.

Turn over protocol

Upon hearing the break-in signal between the dots and dashes being sent, the interrupted station stops sending immediately and either: (a) just pauses momentarily or, (b) sends a single letter K (go ahead) prosign and pauses momentarily, thus turning over the channel to the interrupter, and subsequently listens for the other station during the momentary pause. Highly skilled fluent telegraphists seldom bother to send the K prosign when interrupted instead simply letting the interrupter take over the channel during the pause.

Ongoing channel control protocol

The interrupting station recognizing the momentary sending pause by the sender immediately begins sending its own information to the interrupted station. Meanwhile the interrupting station continues listening between its own transmitted signals in case of interruption in the reverse direction by the original sender.

These simple full break-in channel turn over protocols literally mimic the conversational style in which people interrupt each other mid-syllable during normal voice conversations. Full break-in QSK T/R switch hardware together with use of the simple QSK protocols enables a fast, efficient, fluid conversational style of Morse code communication.

QSK T/R switch timing, reliability and power requirements

Full break-in hardware capability requires fast, robust, high power, analog, radio frequency (RF) transmit/receive (T/R) switches capable of operating in sub-millisecond response times over long periods of continuous operation while handling the high radio frequency power of the transmitter.

Switching times

For example, when sending Morse code at a 20 word per minute rate the typical dot signal duration is a mere 50 milliseconds. To enable good quality QSK operation the switching hardware must switch the radio antenna from receiver to transmitter in much less than one tenth of the dot duration. At 20 word per minute code speed this means that T/R switching times must be in the range of 1 to 1/2 millisecond or below. Even smaller sub-millisecond times are required with higher speed Morse code transmissions.

Long time reliability

The dotting rate of Morse code is the reciprocal of the dot duration, e.g. at twenty words per minute based upon the standard word PARIS with a dot duration of 50 milliseconds, the dotting rate is twenty times per second (20 = 1.0/0.05). The dotting rate is even faster for higher speed Morse code. For long time reliability when operating QSK the hardware RF transmit/receive (T/R) switches must be robust enough to open and close at high dotting rates over thousands of hours of operation.

Power handling

T/R switches must operate reliably over many thousands of hours while handling radio transmitter power levels of hundreds of Watts. Such robust high power analog radio frequency high speed switches are not inexpensive.

T/R switch technologies

Switching hardware technologies that can handle the radio frequency currents of high power transmitters and also switch quietly at these high Morse code rates over long periods of time is rare difficult to design and quite expensive to manufacture. Mechanical switches or relays are the most problematic and least reliable and must be protected from arcs (sparking) usually by operating in a vacuum enclosure. Not all radio transceiver equipment provides the costly high speed analog transmit/receive (T/R) radio frequency switching hardware support necessary for QSK full break-in operation. Generally full break-in is available only on the most expensive radio transceivers. Radiotelegraphers who aspire to QSK operation must ensure that their radio equipment includes the hardware capability for radio frequency antenna switching that operates rapidly enough to allow listening between signals at the appropriate Morse code sending speeds.

Examples of radio frequency analog hardware switch technologies are: high voltage vacuum relays^[5] or high power semiconductor PIN diode switches. In recent times, as PIN diode power handling capabilities have been improved by the semiconductor industry, PIN diodes have largely supplanted vacuum relays in the QSK switch function because the absence of moving parts in PIN diode semiconductor devices results in higher reliability and longer lifetimes. ^[6]^[7] Some high-end manufactured radio transceiver equipment contains integrated or factory installed QSK switching hardware while in other cases external QSK switching hardware or commercial switching products may be added to existing non-QSK capable equipment.^[8]

QSK Receiver AGC recovery timing requirements

Not all radio receivers are amenable to QSK operation.

Adding fast robust T/R switching externally to a transmitter/receiver combination (transceiver) will not necessarily result in good QSK operation. Adding such fast switching externally to a transceiver may create transients within receiver circuitry that makes signal copy: very noisy at best, and difficult, or impossible at worst.

Apart from the requirement for fast robust T/R switches, the main factor affecting good QSK operation is the ability for the radio receiver to recover its sensitivity whilst operating quietly (without popping noises) during and after the fast transient signals created by the fast T/R switch operation. Many receivers have automatic gain control (AGC) circuits with time constants that take many milliseconds to recover their sensitivity and volume level after a transient is presented to their antenna input port. Without modifications or AGC circuit re-design such receivers are not suitable for QSK operation. In cases of slow responding AGC circuitry operators may accept the loss of the benefit of AGC functionality instead choosing to turn their receiver AGC function off while operationg their receivers using manual gain control during QSK operation.

Morse code operators aspiring to the convenience and conversational fluency of Morse code QSK operation who plan to add external QSK T/R switches to their existing or planned radio transceiver setups should ensure that their receiver AGC circuitry has recovery times commensurate with the T/R switching transients to be expected and that the AGC circuits can operate quickly in the sub-millisecond range without creating noisy pops and static at the receiver audio output (speaker or headphones).

Expensive high end radio transceiver equipment that has been designed and manufactured with integrated QSK capability will generally meet such fast AGC recovery time requirements. Receiver recovery times may however be a potential issue for QSK operators who plan to add external QSK switching to an existing radio equipment set up.

References

↑ Silver, N0AX, H. Ward, Editor (2013). The ARRL Handbook For Radio Communications 2014 (91 ed.). Newington, CT: American Radio Relay League, Inc. pp. 13–9. ISBN 978-1-62595-000-0.
↑ Biddulph, G8DPS, Editor, Dick (1995). Radio Communication Handbook (6 ed.). Potters Bar, Herts: Radio Society of Great Britain. pp. 7–28. ISBN 1 872309 24 0.
↑ Sheller, KN8Z, J. R. (July 1985). "What Does QSK Really Mean?". QST (7): 31.
↑ Shafer, W4AX, David P. (Feb 1979). "Why QSK?". QST (2): 53.
↑ Technology, Jennings. "Vacuum RF Switching". Jennings Technology.
↑ Garland, W8ZR, James C. "Add Full Break-In QSK Keying to your Linear Amplifier" (PDF). w8zr.net. w8zr. Retrieved 8 March 2016.
↑ Ameritron. "QSK-5 Manual" (PDF). Ameritron. Retrieved 9 March 2016.
↑ Hansen, VE7CA, Markus. "Perfecting a QSK System" (PDF). ARRL. Retrieved 16 March 2016.

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