CW20 - KL4YFD

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:

  • Allows amateurs with CW-only equipment to experience the benefits of soundcard-based digital modes.
  • Has UTF-8 Language support, so hams can communicate in the language with which they are most fluent.
  • The Forward error correction used has an estimated gain of 9-10dB. This FEC gain acts like a download-able amplifier. With this: 1 watt is like 10 watts, and 10 watts is like 100 watts, etc.
  • Simple: The computer keys the transmitter on and off for "1" and "0" using the CW-Key connection. It simply replaces the manual CW key.
  • Using CW mode allows the use of narrow CW filters which drastically increases received signal to noise ratio, and pulls signals from the noise.
Keying the Transmitter:

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.

  • For every 1 to be transmitted, a signal is sent in the form of a 3.2Khz audio tone on the soundcard's right channel. Using an interface circuit given in the Fldigi manual, this tone is converted electrically to a CW Key Down signal.
  • For transmitting a 0, the mode simply sends no signal on the right channel for a CW Key Up signal.

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.

  • 2400Hz --> 500Hz = 6.0dB gain
  • 2400Hz --> 250Hz = 9.5dB gain
  • 2400Hz --> 100Hz = 13.8db gain

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:
 Mode Baud WPM FEC
 CW 2.0 20 ? No
 CW 2.0 FEC 40 ? 1/2 Rate
 CW 2.0 Fast 80 ?No

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)

  • Twenty baud is close to slow CW speed.
  • Fourty baud is almost identical to RTTY speed.
  • Eighty baud is slightly faster than RTTY fast.
CW 2.0 and CW 2.0 Fast:

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 encoder

of 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

interleaver

which 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 decoder

has 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:
  • FEC details:
    • gain: 9.5db+
    • constraint length = 13
    • polynomial 1 octal: 016461
    • polynomial 2 octal: 012767
    • polynomial 1 decimal: 7473
    • polynomial 2 decimal: 5623
  • Interleaver:
    • 2 x 20 self-synchronizing
    • 500 milliseconds
https://sites.google.com/site/kl4yfd/home/cw20