r/RTLSDR • u/sunnychon • 1d ago

Is the missing line normal?

Gallery image

Gallery image

I just received this image from JMH4 and I noticed that the middle of the H is gone , is it my SDR's fault or the transmitter?

14 Upvotes

95% Upvoted

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4

u/tj21222 1d ago

Op considering the noise that is in this capture, I suspect it could just taken a hit. Got to say good eyes to catch such a small detail. Over all I am pretty confident that your equipment is fine and that the missing line is either caused by atmospheric conditions or as others have said a fault in the chart loaded into the fax machine

2

u/-BruXy- 1d ago
  1. What about your PC? Sometimes a high CPU load may cause it not to process the audio buffer.
  2. They are most probably missing from the original picture or were lost during the scanning process before transmission.
  3. In the case of a faulty transmitter, you will have a few lines with noise.

1

u/sunnychon 1d ago

I am using my owrx plus server , the cpu usage is lower than 50%

1

u/kc2syk K2CR 1d ago

Try reducing your sample rate.

1

u/sunnychon 1d ago

I am using 500KHz

1

u/kc2syk K2CR 1d ago

Sample rate, not observed bandwidth. Sample rate is in MSps (mega samples per second).

1

u/sunnychon 1d ago

sorry about that, it's only 500Ksps

2

u/Tishers Tishers, RF engineer 1d ago

Those sort of image defects are normal in WEFAX messages. I have been monitoring WEFAX images for the last 30 years or so and it depends entirely upon signal conditions.

You can improve the quality of captures if you were to use diversity antennas/ receivers that patch together the missing spots but it never-ever will be perfect.

Diversity reception goes way back and was very common in military RTTY (teletype) communications.

-1

u/Mr_Ironmule 1d ago

It's more likely that someone made a boo-boo designing the identification block for the chart. Just wait till the next SO7 chart is transmitted and see if it's the same. Good luck.