All About Pactor Modems

All About Pactor and Pactor Modems

-by Tim Hasson, W3IJP

Rev: June 15, 2006

Strictly speaking, Pactor is a protocol, or language for transmitting and receiving text and digital information via radio. The interface box that connects to a radio and enables it to speak this special language may be referred to (depending on who you’re speaking with) as a radio modem or sometimes as a terminal node controller (TNC). Radio modems manufactured by the company who invented the Pactor protocol are called PTC’s, an acronym for Pactor Terminal Controller. Most people, myself included, simply refer to these boxes as a “pactor modem” – an apt description that seems to work well in casual conversation.

So Pactor is the protocol, or language, while the terms TNC, radio modem, or pactor modem refer to a piece of equipment.

Here’s a tidbit that will let you demonstrate your superior wit and brilliance at that upcoming social gathering. The word pactor is Latin, and means “one who arranges contracts or mediates, a negotiator”. Other translations define it as “one who arranges a marriage”. Regardless of which definition you subscribe to, in modern English the word would equate to “making a pact”. This clever name is actually a direct reference to the sophisticated methods the Pactor protocol employs to set up and manage a digital radio connection between, say, your boat and a land-based service provider – all the while keeping the connection humming at the highest speed achievable under the circumstances.

Pactor, the protocol, traces it’s roots to the late 1980’s and efforts by a couple of German amateur radio operators to improve digital radio communications. Using the word “amateur” in connection with these guys is quite a piece of understatement. The present-day team includes a combination-degreed Physicist/Chemist who specializes in communications signal theory; an electronics engineer and programmer; and a military electronics designer whose particular specialty is something called “EMC”, or Electro-Magnetic-Compatibility. If that’s not enough, their business manager is a physician who used to specialize in nuclear medicine. Talk about your bunch of rocket scientists!

When these talented radio experimenters introduced the first version of Pactor (Pactor-I from here on, to avoid confusion later) to the rest of the world around 1990, it caused quite a sensation. OK, it wasn’t exactly Playstation 2. But simply put, this new method of transmitting text and data was superior to all other methods available at the time. Chief among these were radioteletype (which dates to WW II and the 1940’s) and SITOR (SImple Text Over Radio, sometimes called Narrow Band Direct Printing, which dates to the 1950’s and is employed to this day by the Coast Guard and similar agencies around the world to receive weather observations and position reports from ships.

What made the upstart Pactor-I so special? Well, it was significantly faster than existing methods, an achievement made possible because it compresses plain text data during transmission. More notably, Pactor-I introduced the ability to correct errors in transmitted data when they occur, something earlier protocols just couldn’t do. But the true breakthrough was something called data transparency – the ability to exchange binary information, such as an executable computer program or a digital photograph with no changes to the underlying data. It’s hard to believe, but 12 years ago this was an outstanding achievement.

The German team formed a company, Specialized Communications Systems (SCS), in Hamburg Germany. When SCS introduced the first PTC (their version of a hardware box) it included the new Pactor-I protocol along with support for all the established “legacy” protocols like radioteletype and SITOR that Pactor-I was out to supersede. SCS went a step further, and also released the specifications for Pactor-I to the world so other “box” manufacturers could incorporate this new mode in their hardware as well.

Why do such a thing? It invited side-by-side comparison! A user could connect one simple interface to his/her radio, and operate the digital modes they’d been using for decades. Without changing connections, this same operator could then switch to the next-generation Pactor-I mode and note the differences. The improvements, not only in speed but in lack of transmission errors, were truly remarkable and they remain so to this day. When you consider that the early adapters of this new technology were fellow radio experimenters (radio amateurs, or “hams”), this approach to introducing a new product becomes a stroke of brilliance.

Never content to rest on their laurels, the group at SCS continued to refine and improve the Pactor protocol. Like it’s distant cousins radioteletype and SITOR, Pactor-I was based on a modulation technique called Frequency Shift Keying, or FSK. Don’t be put off by terms like “modulation technique”, which refer simply to how information is impressed on a radio signal. Improvements in Digital Signal Processing (DSP) chip technology intrigued the SCS engineers, and in 1995 they used a new DSP chip from Motorola as the basis for an improved Pactor protocol, Pactor-II. Pactor-II implements a much more sophisticated modulation scheme called Phase Shift Keying (PSK) which offers even further speed improvements and data robustness over Pactor-I. On average, Pactor-II is about six times faster than Pactor-I.

Technical specifications on the new Pactor-II protocol remained proprietary to SCS, however, and Pactor-II (and it’s subsequent iteration, Pactor-III) are available only in PTC’s manufactured by SCS. But Pactor-II remained “backward-compatible” with Pactor-I, which means that two stations operating both the old/new protocols could still communicate, although only at the slower Pactor-I data rate. Maintaining this kind of backward compatibility seems to be at the core of the SCS engineering/design philosophy, and it’s a practice which bodes well for consumers of SCS products.

Pactor-III is the latest offering from SCS. Released late in 2001, Pactor-III can achieve data througput rates anywhere from three to five times faster than Pactor-II and does a still-better job of maintaining a connection even under harsh radio operating conditions. Because it can achieve faster, more reliable connections Pactor-III is quickly evolving as the defacto standard for commercial HF service providers (MarineNet, and SeaWave Digital are two examples) as well as SailMail and the amateur WinLink system.

Pactor-III is offered as an upgrade option to existing PTC’s. Actually, the protocol is just one part of what SCS calls the “Professional Firmware Upgrade”. The professional firmware includes other features as well, including support for a unique feature called “Free Signal” – but that’s a subject for a future article. In any case, the upgrade is essentially a software update, and can be field-installed in most existing PTC’s by users with average technical skills.

Hardware-wise, SCS offers three versions of the PTC. The PTC-IIpro is the premium version, with support for multiple simultaneous radio connections (a feature really only of interest to amateur radio operators); built-in automatic radio control (given a compatible transceiver) plus GPS input; and a more sophisticated front panel with an LCD display. For high-end amateur radio operators or the person who “wants it all”, this is the way to go.

The PTC-IIex (the “e” is for “economy”) is less costly than the IIpro, but doesn’t sacrifice anything in terms of performance or reliability. The IIex does not include built-in provisions for automatic radio control, but this functionality can usually still be provided by a direct connection between the computer and transceiver. There is no dedicated GPS input, but this can be managed using a special “Y-cable” adapter or, more commonly, configuring the computer to accept GPS input directly. The IIex can only be connected to one radio at a time, a point which is seldom an issue for most cruisers.

The newer IIusb connects to a computer via a USB port, as opposed to a standard serial port connection.

Both the IIex and the IIpro can be equipped with the optional Professional Firmware, with full support for all features including the Pactor-III protocol. The IIusb includes the Professional Firmware.

Often overlooked is the fact that these devices can decode a host of other digital signals, in addition to sending/receiving email via Pactor. With compatible computer software installed, the PTC can decode weatherfax transmissions, NAVTEX, radioteletype, SITOR and it’s amateur cousin AMTOR, Morse Code, and yet others mostly of interest to radio amateurs. They are versatile little boxes.

It’s worth noting that there are other TNC’s, or radio modems available on the market. Frequently mentioned names include Kantronics, Hal Communications and MFJ Enterprises, to list a few. These are all fine products, and many of these suppliers offer support for the original Pactor-I protocol and other digital modes of operation. But the faster Pactor-II (and now Pactor-III) protocols are proprietary to SCS, and available only in their own PTC-family of modems.

As a practical matter, most users will want the speed and reliability afforded by at least Pactor-II (or even better, Pactor-III). Many service providers have daily connect-time limits (for example, 10 minutes/day for SailMail). Connection speed has a direct bearing on the amount of email or weather information that can be exchanged in a limited amount of time. And those travelling in remote areas, where stations are fewer and far between, will want to maximize the opportunity of every available connection.

Finally, we have to note that not all SSB transceivers (marine or amateur) are “up to the task” when it comes to HF digital modes like Pactor. The three biggest factors which come in to play are the radio’s duty cycle, frequency stability and transceiver switching time.

Duty Cycle refers to the radio’s ability to transmit continuously at maximum power, without damage to the equipment. Pactor drives the radio close to this limit, particularly when first calling another station. Not all SSB’s are rated for this 100% duty cycle, and some may need to operate at reduced wattage when operating in digital modes. Still others may require modification, in the form of additional cooling fans or the like.

Frequency Stability is expressed in Hertz, and reflects the radio’s ability to remain solidly on a tuned frequency when transmitting. PTC controllers from SCS can compensate somewhat for minor frequency variations (+/- 80 Hz) between two stations, but it’s better if the radios are in tune in the first place. Users of older equipment, in particular, should allow the equipment to warm up to normal operating temperature before operating to minimize “drift”.

Transceiver Switching Time is measured in milliseconds, and refers to how quickly the radio switches from transmit to receive mode (for example, when you let go of the microphone key). Pactor requires a minimum switching time of 20ms. Older equipment which uses mechanical relays to flip between transmit and receive may not meet this specification. Newer radios perform this switching function electronically, and are most likely compatible.

From Morse Code to Pactor-III, there has been a steady process of continuous improvement in ways to transmit digital information via radio.

Tim Hasson can be reached at (1) 610.287.0703 or via Email to thasson@techyacht.com.