[RTTY] Test files

[Kok Chen]

On Jan 30, 2009, at 9:15 PM, Rick Ruhl wrote:
> I just use TNC to TNC or TNC to soundcard to do my testing.  You can  
> set the
> level of the KAM XL or KAM 98 to emulate weak signal RTTY using the  
> XMITLVL
> command.

By doing that, you will only be testing the AWGN case in VE3NEA's  
examples

http://www.dxatlas.com/RttyCompare/

Well, it won't actually be exactly the AWGN case if the input noise of  
the modem under test does not have Gaussian statistics (i.e., if the  
noise is predominantly flicker noise or something else).

If you want realistic testing, you will need an HF Channel Simulator  
(like AE4JY's PathSim) in between the two modems.  That will add  
Doppler spreading and Doppler shifting which cause QSB and flutter  
effects in Rayleigh channels (what we roughly know of as "HF  
Propagation").   Most Channel simulators are also capable of adding  
noise that is very close to Gaussian; see figures 4-2 and 4-3 here,  
for example

http://homepage.mac.com/chen/w7ay/cocoaPath/Contents/technical.html

In general, what determines a modem's performance is not whether the  
signal is weak but instead, the performance depends on the Signal-to- 
Noise Ratio, so long as the signal is 10 dB or so above the A/D  
converter noise floor for digital modems -- i.e., when the predominant  
noise is no longer quantization noise from the A/D converter.

(Many people are amazed to copy weak RTTY signals when they should not  
be.  If the noise level is also low, weak signals are easy to copy as  
long as you adjust the sound card correctly.   I.e., you always want  
the noise floor of the sound card to be lower than the noise floor  
coming from the receiver -- as long as nothing is clipping the sound  
card.  Very often, you cannot satisfy both (noise floor and clipping)  
conditions, and that is when we say that the receiving system  
(receiver + sound card + modem) does not have enough dynamic range.   
RTTY copy is dependent on SNR, not on how strong or weak a signal is  
on the S meter.  Again, look at the curves at the VE3NEA site.)

For comparing between modulation modes, textbooks actually use Eb/No  
-- ratio of energy per bit to spectral noise density -- instead of  
SNR, since Eb/No also takes into account the baud rate of the signal  
and the effective bandwidth of the signal.  So it is a fairer test  
between modulation modes.  For testing different modems that use the  
same mode (e.g. FSK), it is perfectly fine to use SNR as the  
criterion, as Alex VE3NEA has done, since energy per bit and  
bandwidths are fixed.  But you will often not see plots that are based  
on SNR, but instead based on Eb/No.  They are equally useful -- there  
is a simple relationship between Eb/No and SNR, as explained here:

http://www.sss-mag.com/ebn0.html

By the way, the figure in this last web page shows why PSK can get  
through with lower power than FSK.  But note that it is for AWGN  
(notice that the shape of the curves are similar to the shape of the  
AWGN curve at VE3NEA's web site).

Once you add enough multipath however, the PSK curve "blows up" and  
PSK no longer prints even when you use enormous amount of power, while  
FSK will produce printable output if you raise the power enough to  
have a good enough SNR.  Again, if you refer to VE3NEA's plot, a quite  
band with no QSB (AWGN) needs a SNR of -8 dB (in a 3 kHz passband) to  
print well. Whereas, when you have flutter (go to his flutter graph)  
you need +7 dB SNR to print well.

VE3NEA uses 3 kHz bandwidth because that appears to be the standard  
among ham HF Channel simulators (the ITU specs appear to prefer the  
use of 4 kHz and 1.24 kHz).  If the signal is narrower than 3 kHz, it  
is easy to derive the SNR with a narrower filer.  If you are using a  
300 Hz filter instead of 3 kHz, you would just add 10 dB to the SNR  
scale in Alex' graphs.  If you use 600 Hz filters, add 7 dB to Alex's  
SNR, etc.

73
Chen, W7AY