Clover is one of my favorite high frequency (HF) modes. It was developed in the early 90's and was brought out as a proprietary digital mode in the early 1990's by Hal Communications. There are currently 3 variations of the mode in current use, Clover II, Clover 2000, and Clover 2500. A good history and technical discussion can still be found at Hal's website http://www.halcomm.com I won't try to rehash all of the ins and outs of the mode, but will instead give a fairly brief overview of the mode.
Clover II was the first commercially available iteration. It consisted of 4-tones sent sequentially in a 500 Hz pass band. Each tone shifted phase in order to pass binary characters. The number of phases increased with better band conditions until it reached 16 phase shifts. In addition, in the highest modulation schemes, amplitude modulation was added to pass more data. All of this was done at the low symbol rate of 31 Hz. Clover II was targeted at both the amateur and commercial interests. Unfortunately the cost of the mode was too much for the amateur community and only caught on with some, primarily US based, commercial interests. Data rates for Clover II range from 125 bps to 750 bps and was available new until recently in several modems. The last Hal unit to carry Clover II was the DSP4100
In 1999 Clover 2000 was introduced. The basic modulation was the same, but the number of tones was increased to 8 and the bandwidth spread to 2 KHz. Additionally the symbol rate was doubled for more throughput. Hal decided that this mode was a bit of overkill for the amateur community and marketed it almost exclusively to commercial interests. The modems remain expensive, but the increase in throughput was impressive at 500 to 3,000 bps.
In 2011, Clover 2500 was introduced in the late model DSP4100/2K and DSP4200 modems. The symbol rate was increased even more to 71.125 baud which lead to an increase of bandwidth to 2.5 KHz. This shows the practical limit of Clover in standard SSB transceivers and seems to be marketed in response to a rival's introduction of a new mode at approximately the same time. The data rate for Clover 2500 is 625 to 3750 bps.
While there are published reports about the technical specification of Clover on the internet, there are few, if any on-air reports available except those found in relatively old equipment reviews of clover II.
I have fairly extensive experience with Clover II over the last 10 years or so, primarily with my DXP-38 modem. This was the last modem marketed to amateurs with Clover in it. It featured a tuning indicator which was omitted on commercial modems which primarily operate on fixed frequency channels and should not require manual tuning.
Clover II operation requires precise tuning, to within 20 Hz for link establishment. The modem does not have the ability to compensate for frequency drift, but with statistics passed between the modems on a regular basis, the radio can be slowly (no more than 10 Hz at a time) zeroed in to each other. Amateurs using the mode tend to have a set frequency to meet on (14.065.5 LSB dial is the most common) and fine tuning for rig errors can be done once the link is established.
Once the link is established, Clover is pretty much on auto pilot. All the operator has to do is type or send files. The mode works on automatic overs after certain amounts of data is passed. The length of these blocks between overs is determined by the amount of data to be sent and can last several seconds. Overhead and a small amount of keyboard data are sent via short blocks with a very robust waveform while data blocks are longer and will be sent at the highest speed possible as determined by conditions on the other end of the link.
I have also experimented with Clover 2000 several times. This mode is much more difficult to operate than Clover II, especially with lower power systems such as amateurs often use, and me especially. The same amount of power is now spread over 2 KHz and 8 tones instead of 4 tones in 500 Hz. This means that there is a lower average power on the other end of the link requiring more ERP and better conditions. Clover 2000 is also set up to run with fewer error retries and is very prone to link failure if struck by fading. Still, when the conditions are right, Clover 2000 will move data very quickly, even at it's more robust waveforms.
Clover is unique in current modes in that statistics for both sides of the link are readily displayed. Waveform , amount of online error correction used, S/N ratio, frequency offset, throughput, and phase dispersion are all shown to the operator. The most important to judge the link condition are S/N and PHS (phase dispersion). The former shows the relative strength of the signal and higher numbers are desirable. The second shows how much phase distortion is being caused as the waveform is propagated. lower numbers are better in this case. Since Clover relies on being able to determine phase shift, higher amounts of dispersion require fewer shifts per tone, and that will mean less throughput. SN around 30 and PHS in the teens indicate a good link and will allow speedy transfer of data. PHS in the 40's and S/N in the teens will expect low throughput. Any worse than those numbers, you can expect link failure or at least numerous error signals.
For Keyboard chats, using Clover 2000 is both frustrating and wasteful of spectrum. The enhanced signal conditions that are required coupled with the relatively slow rate typing and reading make the mode difficult to use. Save this mode for passing traffic.
Clover II is a very nice keyboard mode due to it's speed and semi-duplex nature. Also the link status panel is of interest to radio enthusiasts. It can also serve as a relatively fast file transfer mode provided the file isn't very large.
There is a yahoo group dedicated to Clover operating with a small but dedicated following. So if you see a Hal modem online that has the mode for cheap, have a go at it. If you think that clover alone isn't worth even the modest used prices of these modems, you can still use them as great RTTY or Pactor I modems.