Re: LF: 500 kHz WSJT JT4A

[Andy Talbot <andy.g4jnt@googlemail.com>]

JT4 is based on the old 11025Hz sampling, which is badly supported by modern soundcards unlike WSPR which is based on 12kHz sampling.   This has lead many an op to get poor decoding performance on otherwise strong signals

 

WSJT does have a self calibration routine built in, and shows you the correction needed.  If the sampling rate is in error, the correction factors come up highlighted in red, and you need to enter these values into the setup screen.

 

Most modern cards implement 11025Hz as 11100Hz, which at the end of a 48 second transmission interval results in 0.33 seconds error.  This is a substantial offset - more than one symbol at the 4.375Hz rate -  and usually fails.

 

The source coding details are the same as JT65, so the manual for that, where it describes valid message formats, also applies to JT4.   You can encode two callsigns plus a four digit locator in the message structure, or other compressed message formats, but to get the full 6 digit locator as we need for microwaves pluss callsign forces  plain text mode.
Andy
www.g4jnt.com

2010/1/19 Gary - G4WGT <g4wgt@tiscali.co.uk>

Ko & Andy,

Thanks Ko for further decodes.

Andy, I thought there may be some restriction/limit. I will look at the pdf for more info, thanks.

I also believe someone is trying to reply but I am not decoding for some reason.

Cheers,

Gary - G4WGT.

2010/1/19 Andy Talbot <andy.g4jnt@googlemail.com>

In its plain text mode which you have used, it can cope with just 13 characters.   See http://www.g4jnt.com/Coding/JT4_Coding_Process.pdf   and the WSJT documantation

Andy
www.g4jnt.com

2010/1/19 Gary - G4WGT <g4wgt@tiscali.co.uk>

Hi Ko,

Thanks for the decode, not bad a -10 db at one point.

There must be a character limit on the Tx text box (Tx1) I have used as there is more than you have decoded. I will try placing the same text into the Tx6 box & see if it sends more or all.

73

Gary - G4WGT.

2010/1/19 Ko Versteeg <nl9222@xs4all.nl>

Gary - G4WGT wrote:

Hi All,

For a while tonight I am transmitting a beacon on 503.600 kHz.

The mode of operation is WSJT JT4A, transmitting on even minutes.

Only using 15 watts RF & ERP 100 mW.

If anyone would like to try & decode the message, post the results on LF & I will confirm correct or not as the case may be.

Sorry, no prizes :-((



73

Gary - G4WGT.

Hi Gary
Just received:

200200  6  -11  0.1 -477 31 *      TEST G4WGT IO             1   0
200300  0  -33  5.1 -451 42
200400 22  -10  0.1 -477 24 *      TEST G4WGT IO             1   0

73 Ko, NL9222 - JO22ke