wsjt-devel

[wsjt-devel] FT8 and frequency drift .

[David A. <da...@al...>]

Dear developers,

I am wondering how we can decode  FT8 message send by a frequency drift
transmitter .
Do we know from what level of frequency drift a message become impossible to
decode ? 
Do you have any idea how the minimum SNR decoding level is impacted by a
frequency drift?

Regards,
David F4HTQ.

Re: [wsjt-devel] FT8 and frequency drift .

[Bill S. <g4...@cl...>]

On 06/04/2018 14:08, David Alloza wrote:
> Dear developers,
>
> I am wondering how we can decode  FT8 message send by a frequency drift
> transmitter .
> Do we know from what level of frequency drift a message become impossible to
> decode ?
> Do you have any idea how the minimum SNR decoding level is impacted by a
> frequency drift?
>
> Regards,
> David F4HTQ.

Hi David,

modes used for EME and other VHF and up propagation paths necessarily 
have to deal with changing frequency. I don't believe FT8 has explicit 
compensation. Given that, so long as the drift is no more than half the  
tone spacing over a transmission I doubt there is much impact. So that 
would be a drift rate of less than 12.5Hz/minute (50Hz ÷ 8 ÷ 2 = 3.125Hz 
× 4 per minute), above that degradation will be rapid.

73
Bill
G4WJS.

Re: [wsjt-devel] FT8 and frequency drift .

[Daniel E. <kn...@gm...>]

On Fri, Apr 6, 2018 at 3:58 PM, Bill Somerville <g4...@cl...>
wrote:

> On 06/04/2018 14:08, David Alloza wrote:
>
>> Dear developers,
>>
>> I am wondering how we can decode  FT8 message send by a frequency drift
>> transmitter .
>>
>
If the project is to dig out a drifting undecodable signal perhaps it would
be easier to take a recording and process it in some audio software and try
the decoder until it hits ?
Purely academic in this case, but perhaps helping a fellow ham.

Re: [wsjt-devel] FT8 and frequency drift .

[Joe T. <jo...@pr...>]

David --

On 4/6/2018 9:08 AM, David Alloza wrote:

> I am wondering how we can decode  FT8 message send by a frequency drift
> transmitter .

> Do we know from what level of frequency drift a message become impossible to
> decode ?

> Do you have any idea how the minimum SNR decoding level is impacted by a
> frequency drift?

You don't say what has motivated your questions.  Bill already provided 
basic answers corresponding to the "stock" FT8 decoder. There will be 
little sensitivity loss for frequency drifts up to ~3 Hz over a 12.6 s 
transmission.  Things will degrade significantly if the drift is much 
larger.

With sufficient motivation, it would not be very difficult to add "AFC" 
capability to a specialized FT8 decoder.  WSJT-X already includes such 
features for JT65 and JT9.

	-- Joe, K1JT

Re: [wsjt-devel] FT8 and frequency drift .

[David A. <da...@al...>]

Hi Joe,
This question was pure technical curiosity, I do not ask any improvement.
Bill's answer was very understandable.
Some OMs use TCXOs based tranceiver, and some do not. May it be necessary to
indicate in the documentation of WSJT-X the level of maximum drift which
does not degrade the transmission?
Thank you for your answer,
Have a good day,
David, F4HTQ.

-----Message d'origine-----
De : Joe Taylor [mailto:jo...@pr...] 
Envoyé : vendredi 6 avril 2018 16:56
À : WSJT software development <wsj...@li...>
Objet : Re: [wsjt-devel] FT8 and frequency drift .

David --

On 4/6/2018 9:08 AM, David Alloza wrote:

> I am wondering how we can decode  FT8 message send by a frequency 
> drift transmitter .

> Do we know from what level of frequency drift a message become 
> impossible to decode ?

> Do you have any idea how the minimum SNR decoding level is impacted by 
> a frequency drift?

You don't say what has motivated your questions.  Bill already provided
basic answers corresponding to the "stock" FT8 decoder. There will be little
sensitivity loss for frequency drifts up to ~3 Hz over a 12.6 s
transmission.  Things will degrade significantly if the drift is much
larger.

With sufficient motivation, it would not be very difficult to add "AFC" 
capability to a specialized FT8 decoder.  WSJT-X already includes such
features for JT65 and JT9.

	-- Joe, K1JT

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