On 10/08/17 13:21, Bill Somerville wrote:
> On 10/08/2017 00:22, Richard Lamont wrote:
>> In a perfect station, exactly how soon after the beginning of the
>> 15-second (or whatever) window should the Tx start to radiate RF? Is a
>> nominal amount of delay baked into the protocol(s) to allow for T/R
>> switching, the default 0.2s before audio etc?
> it depends on the mode. Fast modes start as soon as PTT is asserted and
> "Tx Delay" has expired. Slow modes apart from FT8 start audio at the
> later of 1s or PTT asserted plus "Tx Delay". FT8 audio starts after the
> later of 0.5s or PTT asserted plus "Tx Delay".
> With the slow modes the start time does not impact the timing of the
> symbols transmitted, so a late start does not increase the DT seen at a
> receiving station, it truncates the front symbols of the message (sort
> of analogous to stopping early).

Hi Bill and thanks for the reply.

>From the above, if I've understood correctly, for FT8:

If T=start of any 15-sec UTC period,

Then FT8 audio 'starts' at T+0.5 (but audio output to TX may be muted,
not delayed, until PTT assertion + Tx Delay);

Tx duration = 79/6.25 = 12.64s;

So FT8 audio always ends at T+0.5+Tx duration = T+13.14s;

The first 7 channel symbols are sync symbols (repeated in the middle and
at the end) and the payload begins with the 8th symbol, so we can afford
to have up to 1.12 seconds muted at the beginning without compromising
the payload;

If I've got this right, then to get all 3x7 sync symbols we need audio
at T+0.5, and to get all 58 payload symbols we need audio by T+1.62.

So the answer to my original question is 0.5s for FT8, and 1.0s for the
other slow modes.

Richard G4DYA

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