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Re: [digsstv] Transmitting Pictures

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  • jdow
    Garrett, there is a range of signal levels at which the decoding loss from clipping is traded off against higher power over all. For best decoding efficiency
    Message 1 of 5 , May 30, 2009
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      Garrett, there is a range of signal levels at which the decoding loss from
      clipping is traded off against higher power over all. For best decoding
      efficiency you want very low power. But 25% might be a workable compromise
      if you are willing to tolerate a higher than necessary minimum error rate
      when signals are really good. It also depends a GREAT deal on your PA's
      clipping performance. If it's being run class-A it might do well at 50%
      while an AB1 or AB2 PA would perform very poorly at that sort of level.

      (Most PAs are designed for SSB voice service where some clipping is seen
      merely as splatter and you still get through just fine. Some contest rigs
      are designed to minimize that splatter (IMD) for multi-operator setups.
      The former wants the 10% to 15% power setting while the latter may take
      the 50% setting. It is rather fun to watch reducing drive to the PA
      stages of the rig improve received signal to noise ratio (decoded accuracy)
      as a vindication of all that ugly math I studied years ago in comm theory
      classes.)

      {^_^} Joanne, W6MKU
      ----- Original Message -----
      From: "AA0OI" <aa0oi@...>
      Sent: Saturday, 2009/May/30 05:16


      Hi Rick:
      Most havve found that by setting the output power by using the ALC on the
      radio is the way to go.. start sending and reduce the alc setting on the
      radio just to the point where it shows NONE... Power output should show now
      more than 15 to 20 watts... There is nothing set in stone,, on my Yahoo
      ft990 my alc shows about 1/2 and output to the amp shows 50 watts.. but this
      seems to be the exception, and not the rule..
      Just get someone to monitor your transmittion and send a "test tone" to make
      sure your not overdriving..
      Then try 20 to 30 sec of a picture and have receiving station check your snr
      and see if they are getting a
      green line at the bottom and the beginings of a picture..

      Garrett / AA0OI




      ________________________________
      From: rickprice407 <no_reply@yahoogroups.com>
      To: digsstv@yahoogroups.com
      Sent: Friday, May 29, 2009 8:03:58 AM
      Subject: [digsstv] Transmitting Pictures





      I recently got into Digital SSTV and finally was able to receive a few
      pictures using Easypal. Now I'd like to send some as well.

      I'm using an IC-746Pro. I know for PSK-31, RTTY and other didtal modes, I
      have to reduce my power output setting to about 50% of max. Do I need to do
      that as well if I am sending pictures? I know i don't with regular SSTV.

      Any help appreciated.

      Rick Price
      KC4RP
    • AA0OI
      Hi Joanne: I know it does not make any sense, but after testing with my radio and set up and testing this program for better than  years,, I knnow.  we have
      Message 2 of 5 , May 30, 2009
      • 0 Attachment
        Hi Joanne:
        I know it does not make any sense, but after testing with my radio and set up and testing this program for better than  years,, I knnow.  we have test for hour on end  day after day.. Mine is just different ,, we have tried it everyway  ( my radio- for some reason puts out  125 watts and has from day one -on external watt meter and radio meter..This is only one of my radios,,, and the only one that acts this way....
         
        Garrett / AA0OI



        From: jdow <jdow@...>
        To: digsstv@yahoogroups.com
        Sent: Saturday, May 30, 2009 8:10:32 AM
        Subject: Re: [digsstv] Transmitting Pictures

        Garrett, there is a range of signal levels at which the decoding loss from
        clipping is traded off against higher power over all. For best decoding
        efficiency you want very low power. But 25% might be a workable compromise
        if you are willing to tolerate a higher than necessary minimum error rate
        when signals are really good. It also depends a GREAT deal on your PA's
        clipping performance. If it's being run class-A it might do well at 50%
        while an AB1 or AB2 PA would perform very poorly at that sort of level.

        (Most PAs are designed for SSB voice service where some clipping is seen
        merely as splatter and you still get through just fine. Some contest rigs
        are designed to minimize that splatter (IMD) for multi-operator setups.
        The former wants the 10% to 15% power setting while the latter may take
        the 50% setting. It is rather fun to watch reducing drive to the PA
        stages of the rig improve received signal to noise ratio (decoded accuracy)
        as a vindication of all that ugly math I studied years ago in comm theory
        classes.)

        {^_^} Joanne, W6MKU

        ----- Original Message -----
        From: "AA0OI" <aa0oi@yahoo. com>
        Sent: Saturday, 2009/May/30 05:16

        Hi Rick:
        Most havve found that by setting the output power by using the ALC on the
        radio is the way to go.. start sending and reduce the alc setting on the
        radio just to the point where it shows NONE... Power output should show now
        more than 15 to 20 watts... There is nothing set in stone,, on my Yahoo
        ft990 my alc shows about 1/2 and output to the amp shows 50 watts.. but this
        seems to be the exception, and not the rule..
        Just get someone to monitor your transmittion and send a "test tone" to make
        sure your not overdriving. .
        Then try 20 to 30 sec of a picture and have receiving station check your snr
        and see if they are getting a
        green line at the bottom and the beginings of a picture..

        Garrett / AA0OI

        ____________ _________ _________ __
        From: rickprice407 <no_reply@yahoogroup s.com>
        To: digsstv@yahoogroups .com
        Sent: Friday, May 29, 2009 8:03:58 AM
        Subject: [digsstv] Transmitting Pictures

        I recently got into Digital SSTV and finally was able to receive a few
        pictures using Easypal. Now I'd like to send some as well.

        I'm using an IC-746Pro. I know for PSK-31, RTTY and other didtal modes, I
        have to reduce my power output setting to about 50% of max. Do I need to do
        that as well if I am sending pictures? I know i don't with regular SSTV.

        Any help appreciated.

        Rick Price
        KC4RP


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