| |
AE5ME > ARES 29.03.16 03:57l 21 Lines 4118 Bytes #999 (0) @ WW
BID : 6MGTY0LYN2FN
Read: GUEST
Subj: Digital Emergency Communications Series by AD5XJ (
Path: IZ3LSV<IV3SCP<IW0QNL<JH4XSY<JE7YGF<7M3TJZ<HG8LXL<N0KFQ<AE5ME
Sent: 160329/0209Z 34477@AE5ME.#NEOK.OK.USA.NOAM BPQ1.4.64
The waterfall, time domain, and spectrum signatures of the transmitted signal look like this:
The two lighter colored bands in the display are the binary tones of RTTY that have been detected by FLDIGI.
We can also look at the spectrum of the RTTY signal that generated the waterfall display and that would display in FLDIGI as shown below right.
Understand that the waterfall is a spectrum display, one line at a time over a very long time domain. The spectrum display is a display of the instantaneous frequency domain for a limited time. The difference is subtle, but distinctive.
In this spectrum display, the two distictive tones of RTTY modulation are shown as a rise in the signal at the frequency of the digital signal. The baud rate is fast enough to see both tones in one sweep of the detected signal.
The faster RTTY modes will look similar except the peaks will be separated by more space, indicating a greater difference in the tonal frequencies.
The image below it is the appearance of a detected AFSK signal when displayed in the time domain.
Many software programs display both the spectrum and the waterfall displays. It is a useful and somewhat complicated way of showing the same information - amplitude vs. frequency.
You can see from the spectral display that RTTY occupies little spectrum and power is concentrated in only two frequencies. Efficiency of the transmitted signal is quite high although the duty cycle is 100%. Be sure your rig is capable of full-time transmit if you intend to have long QSOs over RTTY.
RTTY is almost as popular as PSK31. As a result, there are several RTTY contests and DX events each year that may be heard in the CW portion of each band.
You must operate your transceiver in the USB mode for the FLDIGI RTTY signal to be the correct polarity (transmitting the correct tones for each 0 or 1 data signal). If your transceiver is set to LSB then use the FLDIGI "Rev" button to reverse the sense of the mark and space signals.
You must maintain transmitter LINEARITY in the AUDIO AMPLIFIERs. Do not think that you can improve performance by over driving the audio input. A good operating procedure for most transceivers is the set the audio level to the level for which there is just barely a hint of ALC. Then reduce the input to 80% of that power level. Over driving an AFSK signal is as disastrous as over driving a PSK signal. In fact, this is true for all digital modes. Transmit audio level should be held to the minimum needed to communicate, never increase the audio signal where the ALC is obviously limiting the signal.
This is an actual on air signal that was being over driven (but not on purpose):
The presence of harmonic distortion is a dead giveaway.
It is possible to use FLDIGI to generate the keying waveform for use with an FSK type of transmitter. See Pseudo FSK for a description of how this can be accomplished.
Thor
Thor is a new forward error correcting, incremental frequency shift keyed, communications mode. It was developed specifically to meet the needs of ARQ transfers in the HF spectrum. It is particularly well suited under conditions of atmospheric static noise. Thor borrows from two current modem technologies, MFSK and DominoEX.
Thor emits a distinctive double rising tone sequence at the beginning of each transmission. It is used to flush the receive decoder and also provides a visual and audible clue to its' being used.
The modem code for Thor uses a wide band multiple frequency detector that can lock on and detect the incoming signal even when badly mis-tuned. Frequency domain oversampling is used to allow proper tone detection without the need for AFC. The AFC control does not alter the decoder in any way.
The FLDIGI implementation of the Thor modem includes the ability to send and receive images and avatars. The default avatar is the "Tux" logo. Sending, receiving and saving avatars is discussed in the avatar section of the FLDIGI documentation. Other images of very small size may also be sent in addition to the Tux avatar. Transmission lengths dramatically increase with data that is bit intensive.
Read previous mail | Read next mail
| |
