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Why to some weak signal digital modes (jt9, jt65, etc.) require knowing a clock time? Does knowing this clock time provide any gain in signal detection ability? (lower required S/N?)

What clock synchronization accuracy to what master clock source is required?

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Knowing when a signal is likely to be present helps the software to decode signals. If signals are of varying lengths, and sent at any time, then it's much harder to decode weak signals that may or not be present.

An analogy is the 'old' RS-232 serial data protocol, which could run in 'synchronous' (clocked) or 'asynchronous' modes: knowing from the clock signal when a byte is going to start and when it is going to finish, allows more data to be sent down the wire because you don't need to add 'start bits' and 'stop bits' to tell the listener when a byte starts and stops.

Having everyone send their signal from hh:mm:00-hh:mm:47 means that the software KNOWS exactly when data will be present. And if it can't find any signals that start and end within that exact timeframe then it is certain that there is nothing decodable sent in that time slot.

Generally, by synchronising the times during which data can be sent, allows the software designers to make assumptions while decoding that VASTLY improve the throughput of data at extremely low signal levels.

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According to the WSJT system requirements, the time must be synchronized within ±1 second.

It's generally the case that the more that is known about a signal, the easier it is to detect.

In the case of WSJT, the software records a 4 kHz or so wide swath, and then must find and decode all the signals in it. It must work this way because the nature of the transmission isn't really amenable to interactive tuning by a human operator like CW or SSB are. Furthermore, being able to simultaneously monitor dozens of transmissions and pick the most exciting is a big appeal of the mode.

WSJT transmissions follow a limited format, and are of a fixed length. To successfully decode them requires identifying the start and the end of the message. The transmissions also occur somewhere within a range of possible frequencies within the receiver passband.

With the start time fixed, the search for signals involves only one dimension: frequency. If signals could start at any time there are now two dimensions, frequency and time. The computational complexity is thus squared.

Of course the time isn't exactly synchronized, so there's still a little time synchronization that needs to happen. However, knowing the start of the message within ±1 second is better than knowing nothing at all about the time.

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