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the ultimate speed experience!

PACTOR-III is our new data transmission mode which is superior to PACTOR-II in both speed and robustness.

PACTOR-III is not a new modem or hardware.
Current PTC modems are upgradable to use PACTOR-III via a software update.

PACTOR-III is included in the standard PTC-II Firmware.
You can evaluate PACTOR-III and the other professional firmware features for up to 20 connects.
For a permanent use you have to obtain a license!

To use PACTOR-III both, transmitting and receiving stations, must support PACTOR-III. If you are a mobile station transmitting to a land based station both, mobile and land stations, must be in PACTOR-III mode in order to benefit from the higher data rates PACTOR-III offers.

Link establishment:

The calling modem uses the PACTOR-I FSK connect frame to be compatible with the lowest level. The called modem then answers and the modems negotiate to the highest possible level both modems are capable of. 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 modems highest mode. An example: a user may set MYL to "1" and a PTC will only make a PACTOR-I connection, 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 will behave like a PACTOR-II firmware.

Listen to the PACTOR-III sound:


Use your favorite MP3 player to play the sound sample!

PACTOR-III is included in the standard PTC-II Firmware.
You can evaluate PACTOR-III and the other professional firmware features for up to 20 connects.
For a permanent use you have to obtain a license!


PACTOR-III Protocol Specification:

SLV = "Speedlevel", sub protocol level - adaptation fully automatic.
NTO = Number of tones (sub carriers) used on actual "Speedlevel".
PDR = Physical data rate, raw bit rate (Bit/sec) transferred on the physical protocol layer.
NDR = Net user data rate (without data compression) (Bit/sec). If PMC (automatic online data compression) is activated, multiply with factor 1.9 in case of text files.
CFR = Crestfactor, envelope power ratio, peak / average power (dB). If CFR is 3 dB, a 100 W SSB transmitter generates 50 W mean output power without signal clipping or limitting.

1 2 200 76.8 1.9
2 6 600 247.5 2.6
3 14 1400 588.8 3.1
4 14 2800 1186.1 3.8
5 16 3200 2039.5 5.2
6 18 3600  2722.1 5.7

On an average channel, PACTOR-III is around 3.5 times faster than PACTOR-II. On good channels, the effective throughput ratio between PACTOR-III and PACTOR-II can exceed 5. PACTOR-III achieves slightly higher robustness at the low SNR edge compared to PACTOR-II.

Maximum occupied bandwidth: 2.4 kHz @ -40 dB, audio passband: 400-2600 Hz.

Maximum net throughput with online data compression: ca. 5200 Bit/sec.

Notice that the online data compression provided by the PTC modems is especially useful for applications which do not allow offline (file) compression, e.g. email via TCP/IP, etc. The PTC-II is the only HF modem on the market which offers efficient online data compression.

The PACTOR-III spectrum:

PACTOR-III spectrum

PACTOR-III spectrum at speedlevel 6.

A more detailed PACTOR-III protocol description for download.


The PACTOR-II protocol is essentially based on the Level-I standard, consisting of a synchronous half-duplex ARQ protocol.

New, however, is the ability to choose four different speed steps, so that a greatly improved adaptability is obtained.

The modulation system

The modulation system used for PACTOR-II is based on DPSK (differential phase shift keying) which leads to a very narrow spectrum, virtually independent of the data rate. The robustness of the DPSK modulation qualifies itself noticeably higher at lower information speeds in comparison to FSK.

PACTOR-II spectrum

PACTOR-II spectrum

In order to effect a further step towards robustness, PACTOR-II uses high performance convolutional coding, that is evaluated with a real Viterbi decoder in the data receiver. The high correction capability of the decoder allows not only links with extremely weak or noisy signals, but also, with more normal signals, enables short error bursts, or fadeouts, to be entirely ignored, and a repetition of that packet is not required. This is especially important with PACTOR-II, as the new protocol allows switching to a triple cycle length if there is enough data in the transmit buffer. The relatively long resultant data packet would be very prone to impulse errors from clicks or atmospherics (QRN), if not for the highly effective error correction designed.


Online data compression

As with the Level-I protocol, PACTOR-II uses Huffman coding for text compression on a packet by packet basis. As an alternative, PACTOR-II can also use pseudo Markov coding (PMC) as a compression method. PMC has been developed by SCS, and increases the throughput of plain text by a factor of 1.3 compared to Huffman coding. The PTC-II examines each packet individually to see if it would be faster to send it using Huffman, PMC, or normal ASCII transmission. There are thus no disadvantages incurred by using PMC. As a further selection criterion, the PACTOR-II protocol supports separate German and English coding tables for PMC, as well as a capitals mode for Huffman coding and PMC. There is a total of 6 different compression variations available for use. The PTC-II checks each packet automatically, and then very reliably chooses the best compression method for transmitting the data.

Additionally, PACTOR-II uses "run length coding", so that sequences of repeated characters, e.g. underlining, or columns in graphics, may be transmitted very efficiently. With "run length coding", the system does not transmit each character individually, instead an sample character is sent, followed by the required number of same.

Modulation Scheme Total bit rate (Bit/sec) Code
Net absolute throughput (Bit/sec)
DBPSK 200 1/2 100
DQPSK 400 1/2 200
8-DPSK 600 2/3 400
16-DPSK 800 7/8 700
Total Bit Rate
(without data compression)

Maximum net throughput with online data compression: ca. 1200 Bit/sec.


PACTOR® (Latin: the mediator) is a modern radio teletype mode developed in Germany by Ulrich Strate (DF4KV) and Hans-Peter Helfert (DL6MAA) to improve on inefficient modes such as AMTOR/SITOR and Packet-Radio (AX.25) in weak short wave conditions.

PACTOR® offers a much better error correction system, and a considerably higher data transfer rate, than AMTOR/SITOR. The synchronous transmission format, and the short packet lengths of AMTOR/SITOR, have been retained. These result in a protocol much more resistant to interference than Packet-Radio under poor propagation conditions.

The PACTOR® protocol allows a much higher throughput than AMTOR/SITOR, with the efficient error correction and data transparency of Packet-Radio.

One should not, however, be under the impression that PACTOR® is just a combination of Packet-Radio and AMTOR/SITOR! Although essential parts of both systems have been included, such as data integrity, by using a CRC from Packet-Radio, and the synchronous transmission format and


short block lengths (compared to Packet-Radio) of AMTOR/SITOR, a fully new concept has also been included from the very beginning. For the first time in amateur radio, online data compression is used to markedly increase the effective transmission speed. Also the use of memory ARQ in PACTOR® is a milestone, although it has been known for a long time in the commercial sector.

Previously it has been very difficult, or impossible, to apply this concept in amateur radio. The use of memory ARQ is the main reason that PACTOR® does not loose the link under bad conditions. With memory ARQ, defectively received packets or blocks are not just simply thrown away. They are stored and added to other defective packets, until enough data is collected to reconstruct the original packet, and thus keep the link during operation. The original SCS-PTC uses a real analog memory ARQ, whereby the received AF tone is not simply turned into 0 or 1 data, but intermediate values are also stored. Therefore a more fine-tuned analysis is possible than with so-called "digital memory ARQ".


Do you like to know how PACTOR® sounds on your radio?

Here we provide some typical sounds of the most common modes:

PACTOR-I Unproto (308k)
PACTOR-II Unproto (247k)
AMTOR ARQ (288k)
AMTOR FEC (207k)

Use your favorite MP3 player to play the sound samples!