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Clover 2000


CLOVER-2000 is a faster version of CLOVER (about four times faster) that uses eight tone pulses, each of which is 250 Hz wide, spaced at 250-Hz centers, contained within a 2 kHz bandwidth between 500 and 2,500 Hz. The eight tone pulses are sequential, with only one tone being present at any instant and each tone lasting 2 ms. Each frame consists of eight tone pulses lasting a total of 16 ms, so the base modulation rate of a CLOVER-2000 signal is always 62.5 symbols per second (regardless of the type of modulation being used). Its ITU emission designation is 2K0HJ2DEN or 2K0HJ2BEN and it has a 2:1 voltage (6 dB power) crest factor. CLOVER-2000’s maximum raw data rate is 3,000 bits per second. Allowing for overhead, CLOVER-2000 can deliver error-corrected data over a standard HF SSB radio channel at up to 1,994 bits per second, or 249 characters (8-bit bytes) per second. These are the uncompressed data rates; the maximum throughput is typically doubled for plain text if compression is used.


CLOVER-2000 is similar to the previous version of CLOVER. The transmission protocols and Reed-Solomon error detection and correction algorithm sections in the original CLOVER technical description are therefore not repeated here. The original descriptions of the CLOVER Control Block (CCB) and Error Correction Block (ECB) still apply for CLOVER-2000, except for the higher data rates inherent to CLOVER-2000. Just like CLOVER, all data sent via CLOVER-2000 is encoded as 8-bit data bytes and the error-correction coding and modulation formatting processes are transparent to the data stream — every bit of source data is delivered to the receiving terminal without modification. Control characters and special “escape sequences” are not required or used by CLOVER-2000. Compressed or encrypted data may therefore be sent without the need to insert (and filter) additional control characters and without concern for data integrity.


Five different types of modulation may be used in the ARQ mode - BPSM (Binary Phase Shift Modulation), QPSM (Quadrature PSM), 8PSM (8-level PSM), 8P2A (8PSM + 2-level Amplitude Shift Modulation), and 16P4A (16 PSM plus 4 ASM). The same five types of modulation used in ARQ mode are also available in Broadcast (FEC) mode, with the addition of 2-Channel Diversity BPSM (2DPSM). Each CCB is sent using 2DPSM modulation, 17 byte block size, and 60% bias.


Two different CLOVER-2000 modems are available from HAL Communications, the PCI-4000/2K and the DSP-4100/2K. The PCI-4000/2K is for use inside dedicated desk-top personal computers. The PCI-4000/2K may be installed in any IBM-compatible personal computer that uses an 80386 or faster microprocessor (386, 486, Pentium, etc.) and supports the ISA PC plug-in card bus. The DSP-4100/2K is for connection to lap-top or non-IBM-PC data systems. The DSP-4100/2K is a stand-alone DSP modem that may be used with any computer or data terminal equipment having an RS-232 port.


Most ARQ protocols designed for use with HF radio systems can send data in only one direction at a time. For example, when using Recommendation ITU-R 476 or 625 (SITOR) or PACTOR, one station sends all of its data, ending the transmission with an “OVER” command. The second station may then send its information. Because CLOVER-2000 does not need an “OVER” command, data may flow in either direction at any time. The CLOVER ARQ time frame automatically adjusts to match the data volume to be sent in either or both directions. When first linked, both sides of the ARQ link exchange information using six bytes of the CCB. When one station has a large volume of data buffered and ready to send, ARQ mode automatically shifts to an expanded time frame during which one or more 255-byte data blocks are sent. If the second station also has a large volume of data buffered and ready to send, its half of the ARQ frame is also expanded. Either or both stations will shift back to CCB level when all buffered data has been sent. This feature provides the benefit of full-duplex data transfer but requires use of only simplex frequencies and half-duplex radio equipment. This two-way feature of CLOVER can also provide a back-channel order-wire capability. Communications may be maintained in this “chat” mode at 55 words per minute, which is more than adequate for real-time keyboard-to-keyboard communications.


The effective data throughput rate of CLOVER-2000 can be even higher when binary file transfer mode is used with data compression. The binary file transfer protocol used by HAL Communications operates with a terminal program explained in the HAL E2004 Engineering Document listed under References. Data compression algorithms tend to be context sensitive-compression that works well for one mode (e.g. text), may not work well for other data forms (graphics, etc). The HAL terminal program uses the PK-WARE compression algorithm which has been proven to be a good general-purpose compressor for most computer files and programs. Other algorithms may be much more efficient for some data formats, particularly for compression of graphic image files and digitized voice data. The HAL Communications PCI-4000/2K and DSP-4100/2K modems can be operated with other data compression algorithms in the users’ computers.


HAL Communications: “CLOVER-2000 High Performance Protocol,”

HAL Communications: “Binary File Transfer Protocol,” E2004, Rev. B, HAL Communications Corp., Urbana, IL, December 1997.

HAL Communications: “CLOVER-2000 Waveform & Protocol,” E2007, Rev. C, HAL Communications Corp., Urbana, IL, May 1999.

Wickwire, Ken, Mike Bernock and Bob Levreault, “On-air Measurements of CLOVER II and CLOVER 2000 Throughput,” Proc. 17th ARRL/TAPR Digital Communications Conference, ARRL, Newington, CT, 1998,


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


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