PACTOR-III is a software upgrade for existing PACTOR-II modems that provides a new data transmission mode for improved speed and robustness. PACTOR-III is not a new modem or hardware device. Most current PACTOR-II modems are upgradeable to use PACTOR-III via a software update since PACTOR-II firmware accommodates the new PACTOR-III software. Both the transmitting and receiving stations must support PACTOR-III for end-to-end communications using this mode.

PACTOR-III's maximum uncompressed speed is 2722 bps. Using online compression, up to 5.2 kbps is achievable. This requires an audio passband from 400 Hz to 2600 Hz (for PACTOR-III speedlevel 6). The occupied bandwidth at the --40 dB points is 2.4 kHz (from 300 Hz to 2700 Hz).

On an average channel, PACTOR-III is over three times faster than PACTOR-II. On good channels, the effective throughput ratio between PACTOR-III and PACTOR-II can exceed five. PACTOR-III is slightly more robust than PACTOR-II at their lower SNR edges.

The ITU emission designator for PACTOR-III is 2K20J2D. Because PACTOR-III builds on PACTOR-II, most specifications like frame length and frame structure are adopted from PACTOR-II. The only significant difference is PACTOR III's multi-tone waveform which uses up to 18 carriers while PACTOR-II uses only two carriers. PACTOR-III's carriers are located in a 120 Hz grid and modulated with 100 symbols per second DBPSK or DQPSK. Channel coding is also adopted from PACTOR-II's Punctured Convolutional Coding. 

Link Establishment

The calling modem uses the PACTOR-I FSK connect frame for compatibility. When the called modem answers, the modems negotiate to the highest level of which both modems are capable. If one modem is only capable of PACTOR-II, then the 500 Hz PACTOR-II mode is used for the session. With the MYLevel command a user may limit a modem's highest mode. For example, a user may set MYL to "1" and only a PACTOR-I connection will be made, set to "2" and PACTOR-I and II connections are available, set to "3" and PACTOR-I through III connections are enabled. The default MYL is set to "2" with the current firmware and with PACTOR-III firmware it will be set to "3". If a user is only allowed to occupy a 500 Hz channel, MYL can be set to "2" and the modem stay in its PACTOR-II mode.

The PACTOR-III Protocol Specification is available on the Website.

The possible sub-protocol levels used in PACTOR-III are called speedlevels. The carrier localization of the speedlevels is:

The modulation type and code rate are:

A free trial version of PACTOR-III is downloadable, but it operates for only 20 connections so a license must be obtained for permanent use.

ACKNOWLEDGEMENT
This technical description was prepared by Steven L Karty, N5SK.

PACTOR-III specifications are available from the International Telecommunication Union Publications Sales in Recommendation ITU-R 1798, "Characteristics of HF radio equipment for the exchange of digital data and electronic mail in the maritime mobile service," starting on page 40 of the document.