8-PSK
he FEC modes do not all use the same Viterbi polynomials to achieve the forward error correction. That is why the WPM rates for the FEC modes are not multiples of the base 125 baud. The WPM rates are only an indication of relative rates. The actual transfer rate is highly dependent on the data content. This is always true for modes which use a VARICODE.
The 8psk signal is similar to both bpsk and qpsk, but with 8 possible phase states instead of the 2 and 4 associated with bpsk, qpsk. The format of the signal does not lend itself easily to a conventional AFC. Instead, the modes should be used with RsID enabled. The RsID signal will both determine the mode and the mode center frequency (to the nearest 2.6 Hz). A finer resolution of the mode center frequency can be made using the optional pilot carrier. This pilot carrier is placed at the frequency
f0 - samplerate / symbollen, f0 is modem center frequency
Decoding errors are reduced as the tracking point nears the actual transmit center frequency. The loss of signal power is more than offset by the decoder improvement. Field testing has shown that the pilot tone needs to be at -40 dB or greater (less negative).
Pilot tone detection does require a bit more cpu power. The pilot tone is detected using a sliding fast Fourier transform, sfft, which computes the frequency of the pilot to approximately 1 Hz resolution. The sfft only evaluates the signal at 11 discrete frequencies, so it is necessary that either the RsID, or manual tuning is used for the initial signal acquisition. The detector is set to provide lock when the pilot s/n is 2:1 or better. The signal tracking point is then adjusted to place the pilot tone at the correct frequency location. For example, if the RsID put the tracking point at 1502 Hz, the pilot would then adjust for 1500 (if that is the correct tracking frequency). The adjustment is made once per second. Unlike AFC, which is continuous, the pilot adjustment is discrete and occurs in 1 Hz steps. If the pilot s/n is less than 2:1 then no adjustment is made to the tracking point. The pilot tone is transmitted during the 8psk preamble as well as during the data transmission. You should see the tracking point adjust once at the beginning of the transmission and then stay fixed.
With these modes, a very linear transmitter is required. Over-driven operation results in excessive bandwidth, poorer reception and difficult tuning. Overdrive usually occurs by having the audio signal much too large. These are very sensitive modes and usually very little power is required. QRP operation of 80, 40, 30 and 20 meters can provide nearly 100% copy over multi-hop paths. In many instances PSK can provide better decoding than CW.
Setting up for a good clean on air signal that will receive the accolades of your QSO partners is easy. Follow the instructions on using the Tune button and you will have a clean on signal.
Good reception of PSK signals requires that the demodulator be phase locked to the incoming signal. Fldigi has both a fast acquire / slow tracking AFC system. Place the red bandwidth bar (see above) so that it overlies the desired signal and then press the left mouse button. The signal should quickly lock on a decoding should commence immediately. It is almost impossible to visually tell whether a BPSK or QPSK signal is being received. Under very high s/n you might be able to hear the difference, but that is even difficult for most operators. If you are not able to decode a signal that looks like a BPSK and the bandwidth of the signal matches the baud rate then it might be a QPSK signal. Just change mode a try reacquiring the signal.
