PACTOR-4 is an adaptive transmission mode that provides higher throughput and improved robustness compared to PACTOR-I, –II and -III. Ten so-called Speed Levels (SL) are specified, which are predefined combinations of modulation type (DBPSK, DSSS-DBPSK, DSSS-DQPSK, BPSK, QPSK, PSK-8, QAM-16, QAM-32) and channel coding (1/3, 1/2, 5/6).

Speed Level 1 uses two DBPSK-modulated carriers, similar to PACTOR-II and PACTOR-III, but the carriers change frequencies within a frame. This is done in order to eliminate single-carrier distortions.

For levels 2 to 10, PACTOR-III uses a single PSK-modulated carrier at 1500 Hz with a bandwidth of about 2400 Hz and a modulation rate of 1800 baud. Levels 2 - 4 use spread-spectrum modulation in order to increase the robustness. Levels 5 - 10 use BPSK to QAM-32, and an equalizer to eliminate the effects of the transmission channel.

Initial link establishment is achieved using the frequency shift-keying (FSK) protocol of PACTOR-I, corresponding to the lowest protocol level. If both calling and called stations are capable of PACTOR-4, automatic switching to this protocol is done.

For decoding, the center frequency should be set to a value within +/- 50 Hz of the received signal. Polarity (NOR or INV) has to be selected manually.

The decoder supports several output modes. Decoded Data shows the application-level data. Raw Frames can be used to display the raw data, either with decompression or as received. In this mode, control signals are displayed as well.

If the "HEX" alphabet is selected, the data is displayed in binary (hex) and text, otherwise as normal text.