CW 2.0 is the first amateur digital mode designed to use the HF transceiver in CW mode. In many ways, this mode is designed to help amateurs worldwide better access digital communications on the HF bands:
CW 2.0 is different from most soundcard-based digital modes because it manually keys the CW key with the transceiver in CW mode. No audio is mixed into the microphone or data-port. To physically key the CW port using the soundcard, an Fldigi feature called " right channel PTT" is used.
CW 2.0 is received with the transceiver in CW mode. This narrows the bandwidth of the receive filter dramatically from the default SSB bandwidth of 2400Hz. By narrowing the receiver bandwidth, noise in decreased and SNR is increased. Below are a few examples of the benefits , compared to 2.4Khz SSB.
This receive-end gain helps to dig even weak signal from below the noise, increasing the chances of a successful contact or DX. CW 2.0 consists of three variations:
The baudrates of these modes were chosen to be close to those HF transmitters were already designed for. Also, 50 and 60 baud are avoided (50/60Hz hum issues)
Characters to be transmitted are varicoded using the IZ8BLY / MFSK alphabet.The transmitter is then "keyed down" (transmit) for each 1 and "keyed up" (no-transmit) for each 0. The design of these 2 modes was kept intentionally very-simple in order to make adding the mode to other software packages relatively easy. CW 2.0 is intended more for casual keyboard-to-keyboard communication while CW 2.0 Fast is intended more for contesting. As with the non-FEC CW 2.0 modes, characters to be transmitted are varicoded using the IZ8BLY / MFSK alphabet. These bits are then passed to a convolutional encoderof constraint length 13, which applies the Forward Error Correction algorithm. This reduces the bitrate by half. After FEC coding, the bits are passed through an interleaverwhich mixes the bits to be transmitted over a time-window of 500 milliseconds. This length has been proven to be sufficient to handle most HF noise and short fades. The bits from the interleaver are then transmitted like the non FEC modes: "key down" for 1 and "key up" for 0. For added gain on the receive side, the soft-decision viterbi decoderhas a traceback of 156, which is twelve times the constraint length: 13. Engineering books state that 8 constraint lengths is sufficient, and that longer tracebacks simple waste CPU cycles without adding gain. The caveat however is that 8 constraint lengths is only sufficient when starting from a known (good) state. When starting from a random/arbitrary (noise) state it takes 1.5 times longer. So 8 * 1.5 = 12. Technical Details:
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