QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

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glovisol
Posts: 233
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Fri Nov 16, 2018 1:20 pm

WEATERFAX AND THE RSP DUO RECEIVER

Meteofax reception, with a state of the art RSP receiver and dedicated software, should be like a piece of cake nowadays....but it is not so and some effort is required, as I learned at my cost.

To the next post, for text amendment purposes.

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glovisol
Posts: 233
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Fri Nov 16, 2018 2:22 pm

WEATHERFAX AND THE RSP DUO RECEIVER


The uploaded picture shows a page of my ship's log for May 30th, 2000 with a meteo chart received from the DEUTSCHER WETTERDIENST in early morning, while anchored in a bay in Northern Sardinia, waiting for a westerly storm. My forecast was:"the High on France shoots wind into the Lion's Gulf (A) while the High on Tunisia (B) sends SW wind to lower Tyrrenian sea. Mistral is coming and SW later: better wait and lay low". Later in the day: "Fantastic sunshine, but NW gale at 37 Knots, this evening down to 12/15 Knots...."

The Meteo chart is amazingly clear, considering that I was receiving it from Hamburg, Germany, on 7880 KHz, with a 14 m long antenna made with the backstay of my sailing boat and two (bottom and top) insulators. The receiver was the immortal Sony ICF-SW77 (born 1988, still in its prime) sending the audio to a PAKRATT RS232 terminal (still alive, but with Alzheimer disease, having lost the memory....pardon the software) which drove (thru Serial Port) a Macintosh laptop Powerbook 180 (hard disk: 100 Mb, born 1993, died 2006) with Macratt software. At the time the trick was to carefully tune the Sony receiver, considering that its smallest step is 1 KHz. I started using this setup in 1994 and it helped me while cruising all over the Mediterranean sea until 2006, when unfortunately the Powerbook 180 passed away. It was extremely useful everywhere, but in the Aegean Sea especially, where weather forecasts by radio were difficult to come by and very difficult to undertstand due to language problems. The Deutscher Wetterdiest must be commended for the extremely good and reliable coverage of all of the Mediterranean Sea.

"With a 135 m Beverage antenna, the RSPduo and a PC with a hundredfold better capabilities" I said to myself, "it is a matter of downloading and selecting a good software and I shall be in business". But the first results were far from gratifying, not to say very disappointing.
Attachments
Ship's log, May 2000.jpg
Ship's log page for May 30th, Northern Sardinia
Ship's log, May 2000.jpg (103.46 KiB) Viewed 1438 times
05.11.18.PNG
Bad example
05.11.18.PNG (342.04 KiB) Viewed 1438 times
13.11.18.PNG
Horrible example
13.11.18.PNG (346.36 KiB) Viewed 1438 times

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glovisol
Posts: 233
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Fri Nov 16, 2018 2:31 pm

WEATHERFAX AND THE RSP DUO RECEIVER


It took some effort to get here! The description of my labours in the following posts.
Attachments
16.11.18 It took time to get here!.png
16.11.18 It took time to get here!.png (361.83 KiB) Viewed 1433 times
16.11.18 and here.png
16.11.18 and here.png (444.44 KiB) Viewed 1433 times

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glovisol
Posts: 233
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Sat Nov 17, 2018 8:59 am

WEATHERFAX AND THE RSP DUO RECEIVER - BASIC INFO


For completeness, some background information is necessary. The up to date list of Meteofax radio stations worldwide, with frequencies, technical data and transmission schedules, is dowloadable here in PDF form:

http://www.nws.noaa.gov/os/marine/rfax.pdf

I still have a hardcopy specimen of April 1995.

Many Weather organisations publish their own detailed schedules. Daily schedule for the Deutscher Wetterdienst is transmitted along with weather information each day at 14:25 to 14:45 GMT and is also available below in PDF format:

https://www.dwd.de/EN/specialusers/ship ... onFile&v=1

Meteofax HF transmission parameters (to be set on the decoding software) are as follows (Deuscher Wetterdienst abbreviated DW):

Frequency and call sign:.................Example DW: DDK3/DDK6: 3885KHz / 7880 KHz / 13882.5 KHz.
Class of emission:.........................Example DW: F1C with +425Hz (white) and -425Hz (black).
Transmission mode reference...........USB or LSB for NORMAL. Example DW: USB/NORMAL.
Transmitted lines per minute (LPM)... 60, 90, 100, 120, 180, 240. Example DW: 120.
5 seconds start tone frequency.........IOC576/300 Hz. IOC288/675 Hz. Example DW: IOC576.
Phasing signal duration...................30 sec. Black pulses on a white line.
5 seconds stop tone frequency..........450 Hz.

In a Meteofax transmission, which is only black and white, the carrier is modulated with two tones, one at +425Hz for white and the other at -425Hz for black off center of this double amount from the nominal frequency. This means that if we look at the emission spectrum we shall see the carrier at -1900 Hz, shifting up 425 Hz and we can receive and decode the signal setting the receiver to USB or to LSB: if we chose the wrong sideband setting (tuning the receiver above the nominal frequency) white will become black and vice-versa, but there is always a software command for "NORMAL" or "REVERSE". Sometimes, with interference present, going to the "wrong" sideband setting can improve reception.

To set up our decoding software, we must make sure we have selected the relevant parameters. For the DW example, LPM=120; IOC576; USB. IOC is the Index of Cooperation and it is almost always set to the value of 576, corresponding to 300 Hz.

Supposing we are listening to DW, transmission starts with a 5" tone at 300 Hz which tells our software to get ready to receive, then we have 30" of phasing signal, then a variable duration time with no tones, then transmission. At the end we have a 5" stop tone at 450 Hz which stops the software and the received chart is stored. After a pause we may have or not have another transmission.

How to set it all up in the next post.
Attachments
181116-115004.PNG
181116-115004.PNG (270.43 KiB) Viewed 1356 times
181116-121104.PNG
181116-121104.PNG (225.56 KiB) Viewed 1356 times

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glovisol
Posts: 233
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Location: Piedmont, Italy

Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Sat Nov 17, 2018 7:49 pm

WEATHERFAX AND THE RSP DUO RECEIVER - SETTING IT UP

AMENDED 18/11/2018 for locking out fractional resampler (see next posts) and for larger bandwith.


Here is the rationale for setting up the receiver frequency. Assuming a shift of +/- 425 Hz, we have a total shift of 850 Hz. To allow for this shift with some margin, the BLACK frequency is set at (850+150)*2= 1900 Hz below the nominal frequency, if we are in USB mode. As an example, let us consider Deutscher Wetterdienst at the nominal frequency of 3855 KHz.

Fnom. = 3,855KHz.....Ftune = 3,855,000 - ((850+150)*2) = 3,855.000 - 1900 =3,853.100 KHz RECEIVER TUNE FREQ.
3,853.100 + 950 = 3,854.050 KHz WHITE FREQ.

The receiver bandpass is from: 1900 - 425 = 1475 Hz to: 1900 + 425 Hz = 2325 Hz this information is useful for setting up the audio filter's bandpass.

Another example with Deutscher Wetterdienst at the nominal frequency of 7880 KHz.

Fnom. = 7,880KHz.....Ftune = 7,880,000 - ((850+150*2) = 7,880.000 - 1900 =7,878.100 KHz RECEIVER TUNE FREQ.
7,878.100 + 950 = 7,879.050 KHz WHITE FREQ.

Receiver bandpass remains the same.

System setup is shown in the figures below. More details, including WFAX software, in the next post.
Attachments
RSP Setup 2. 1.jpg
Figure 1 - RSP general setup for Meteofax reception
RSP Setup 2. 1.jpg (216.67 KiB) Viewed 1213 times
RSP Setup 2.2.jpg
Figure 2 - RSP filter setup for Meteofax reception
RSP Setup 2.2.jpg (98.4 KiB) Viewed 1213 times
RSP Setup 2.3 Sync Period.jpg
Figure 3. Initial Sync period
RSP Setup 2.3 Sync Period.jpg (81.7 KiB) Viewed 1213 times

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Mike2459
Posts: 125
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Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby Mike2459 » Sat Nov 17, 2018 9:39 pm

Glovisol:

Very interesting, you've put a lot of effort into this piece. If you don't mind, I'd like to suggest enabling 'Lockout Fractional Resampler' per Section 11 of the SDRUno User Manual: '11. The I/O Sample Rate difference issue'. Slant correction may also need tweaked after making this change. This will help produce vertically straight images.

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glovisol
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Location: Piedmont, Italy

Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Sat Nov 17, 2018 11:09 pm

Hi Mike 2459,

Thanks for the suggestion: I was not aware of Section 11 of the manual and I shall give it a try tomorrow.

glovisol

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glovisol
Posts: 233
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Sun Nov 18, 2018 8:18 am

Hi Mike 2459,

Enabling the 'Lockout Fractional Resampler' seems to bring about a very significant improvement in image quality and imposes some changes. I still have to do some more tests before completing this thread.

Thanks again,

glovisol

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glovisol
Posts: 233
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Sun Nov 18, 2018 4:42 pm

WEATHERFAX AND THE RSP DUO RECEIVER - TECHNICAL PROBLEMS

The two Weatherfax programs that have been used to test the RSPduo are the SeaTTy and the Fldigi 4.0.18. Figure 4 below shows the two working side by side with equivalent quality. More details on the software later, after we have discussed two very important technical problems.

The first problem is the one solved by the keen eye of Mike2459, who discovered that the vertical lines of my uploaded charts were not straight, but slightly wobbly, as seen in Figures 4 & 5. Furthermore this "wobbliness" also affects resolution and readability of small items, such as written text. The problem is solved as described by Mike, e.g. by disabling the output resampler as follows:

RX Control ->SETT.->OUT->Lock Output Fractional resampler.

The second problem is far more serious in my opinion and will be discussed in the next post.
Attachments
SeaTTY Vs. Fldigi.jpg
Figure 4. SeaTTY vs. Fldigi
SeaTTY Vs. Fldigi.jpg (178.85 KiB) Viewed 1206 times
Wobbly vertical line example.PNG
Figure 5. Wobbly vertical lines example
Wobbly vertical line example.PNG (180.08 KiB) Viewed 1206 times
Straight vertical lines example.jpg
Figure 6. Straight vertical lines
Straight vertical lines example.jpg (86.06 KiB) Viewed 1206 times

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glovisol
Posts: 233
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Location: Piedmont, Italy

Re: QUALITY METEOFAX RECEPTION WITH THE RSP RECEIVER

Postby glovisol » Mon Nov 19, 2018 10:53 am

WEATHERFAX AND THE RSP DUO RECEIVER - TECHNICAL PROBLEMS


The big problem is that I seem unable to trace the charts using THE SAME PC running the SDRuno. If I open SeaTTY or Fldigi in the PC, the recorded Meteofax charts are all smeared up. The high quality charts I have been posting have been obtained by taking the audio from the audio socket of PC#1 and sending it to the audio input of PC#2, where they are decoded by SeaTTY or Fldigi installed therein.

This would appear as a problem of power of PC#1: in fact when attempting to use the Weatherfax in it the Sys indication goes over 50% even though the audio does not start to pop. My laptop PC is a Lenovo/IBM X61 /Intel Core 2 Duo CPU T7300 / @ 2 GHz / installed RAM 3 GB. In this respect Fldigi appears to be heavier than SeaTTY.

It would be interesting to verify that the smearing problem does not occur with a higher capacity PC...Comments are invited. Here below comparison between smeared and non smeared charts taken at the same time.
Attachments
non smear 1.jpg
non smear 1.jpg (152.86 KiB) Viewed 1170 times
smear 1.jpg
smear 1.jpg (159.32 KiB) Viewed 1170 times

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