Phil, > It would be valuable to the community to be able to recognize the presence of > Doppler spread by some visual or > aural means.
There are times when the effect can be detected by sight and sound. Even a moderate disturbance in the polar ionosphere can cause enough Doppler spread to make HF voice communications sound hollow or as if the signal was emanating from under water. The same applies to CW and digital mode signals. Visually, narrow band modes like PSK31 will appear dispersed (spread out) compared to others in the waterfall. The individual tones that make up wide-band signals (MFSK) will also appear diffuse. Other visual indications of Doppler induced frequency dispersions caused by signals that propagate over the poles, would be the chaotic movement of the PSK31 tuning indicator. Although there are times when Doppler spread is too small to be detected, it can still cause throughput problems with certain modes. The amount of throughput loss depends on the severity of the Doppler spread and how resistant the mode is to that type of distortion. Narrow modes like PSK10, for example, are especially sensitive to small frequency spreads (1Hz) and can suffer throughput loss on a relatively quiet mid-latitude path. The reflecting surface of the ionosphere is somewhat similar to the surface of a pool of water where waves are traveling in all directions. As sunlight hits the pools surface, the motion of the waves cause the light to scatter at random. With RF waves, the motion of the ionospheric irregularities cause random shifts in frequency creating wavelets around the main signal (see attached: Normal vs. Doppler spread). Doppler spread is more pronounced in the active regions of the auroral and equatorial zones than it is in the mid-latitudes. Its common to see an increase in the effect on long distance contacts since the signal will eventually end up near those regions. How much it effects digital mode throughput depends on the state of the ionosphere and the mode in use. As for PSK63F, it combats this type of distortion much better than standard PSK modes. Tony -K2MO ----- Original Message ----- From: Phil Williams To: digitalradio@yahoogroups.com Sent: Tuesday, January 05, 2010 8:49 AM Subject: Re: [digitalradio] IZ8BLY's PSK63F Demonstrating a suite of digital coding methods are vulnerable to Doppler spread does not tell the whole story. What does the signal look like on the a spectrogram when subjected to Doppler spread? Yes, you have incomplete or scrambled text, but then the root cause of that could be anything. It would be valuable to the community to be able to recognize the presence of Doppler spread by some visual or aural means. Armed with this information then one begins to make choices of other modes that would be less vulnerable to the effects of Doppler spread. philw de ka1gmn On Tue, Jan 5, 2010 at 2:13 AM, Tony <d...@optonline.net> wrote: Phil, > What about PSKFEC31 under the same test scenarios? Have a look: Path Simulation: High Latitude (Moderate) Path Delay: 3ms, Doppler spread 10Hz Pangram Text: Quick Brown Fox PSK31FEC t e tio E ttaeH loo etee- e e e ˆyaooe n o ao t aeepvede n neete ueeeu .tna0 o een it=pctidr a ieae t e tio E ttaeH loo etee- e e etˆyaooe on oe ne 6etnuEenoel o·b geogtee PSK63F the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog Tony -K2MO ----- Original Message ----- From: Phil Williams To: digitalradio@yahoogroups.com Sent: Monday, January 04, 2010 5:16 AM Subject: Re: [digitalradio] IZ8BLY's PSK63F Very interesting. What about PSKFEC31 under the same test scenarios? Certainly, there would be more a in throughput, but that is a matter of some liberal use of CW shorthand. philw de ka1gmn On Mon, Jan 4, 2010 at 2:48 AM, Tony <d...@optonline.net> wrote: All, Recent path simulation tests indicate that Nino Porcino's PSK63F offers better performance over PSK31 and PSK63 in a couple of areas. The most significant improvement is it's ability to endure Doppler spread found on paths that cross the polar ionosphere. Both PSK31 and PSK63 fail miserably in this area; see high-lat test samples below. Path Simulation: High Latitude (Moderate) Path Delay: 3ms, Doppler spread 10Hz Pangram Text: Quick Brown Fox PSK63F -- the quick brown fox jumps over the lazy dog PSK63 -- mev roe tt#dtorl|f- bn ô mp e o ihe Fzy dg PSK31 -- nls oSer Òe naAeta qlipM h nV o T rn agâ o RTTY -- TH QACKH492, FOJUMP OR THTLAZY G Sensitivity-wise, it's quite a bit more sensitivity than PSK63, but only marginally better than PSK31. Although it's speed is about 25% faster than PSK31, it's about 40% slower than PSK63. Average wmp rate seems to be 63 wpm for PSK63F. Lowest S/N (sensitivity) PSK63F -12db PSK63 -7db PSK31 -11db RTTY -5db Additional path tests indicate that PSK31 and PSK63F perform about the same under moderate mid-latitude conditions (CCIR fading channel). Tests show that PSK31 and PSK63F will outperform PSK63 when signals are weak under quiet conditions since they both have greater sensitivity. It would be interesting to hear from our HF digital friends up north who experience the distorting effects of the polar ionosphere on a regular basis; this is where the PSK63F mode can be put to the test. Available software: Nino Porcino's Stream -- http://xoomer.virgilio.it/aporcino/ Patrick Lindeckers Multipsk -- http://f6cte.free.fr/index_anglais.htm (thanks for including PSK63F Patrick) Tony, K2MO
