SETI Encounter 2001/Dutil Message


David Woolley (david@djwhome.demon.co.uk)
Wed, 8 Sep 1999 00:16:09 +0100 (BST)


Having just caught up with the Encounter 2001 message (there's
certainly less technical information on their site than
SETI@Home's), the following thoughts and questions struck me
after a quick look at the maths section and the overall structure.

Why is the Mersenne prime introduced before power-of and subtract
operators are defined (although they might be used to define those
operators, the next question may be relevant)?

Why is power-of not represented by an operator character - the notation
seems to be based on human typographic conventions, not on any fundamental
principle (the only advantage is that the operator precedence is rather
clearer in the Mersenne prime)?

Why is the mathematical message at the start of the transmission (my guess
is commercial reasons, although the fast code for the greetings might also
make the receiver think it not worth decoding)? I would want at least
one copy at the end to allow time to bring the most sensitive receivers to
bear and to optimise them for the signal.

I've established that there are three signalling states, 0, 1 and pause,
but how are these used to represent clock and data information, e.g. is
1 0 1 sent as P1P0P1P... with each of equal length? Where are runs of
Ps used?

What pulse shape is used?

What is the frequency stability?

Are the borders shown in the GIFs actually transmitted (I would hope so,
as they provide synchronisation patterns)?

Is there any clock run-in and training sequence? I guess that, if you
have the whole sampled signal to post-process, you don't really need a
clock run-in, but training is less clear; depending on the nature of
the propagations distortions and how they vary with time, it might pay
off to include training sequences at beginning and end (you can play the
data backwards, or interpolate) and ideally at intermediate points).

(Training sequences are predictable patterns which start simple and
then build up in complexity, so that you can calibrate the channel with
more and more accuracy.)



This archive was generated by hypermail 2.0b3 on Sun Oct 10 1999 - 15:46:34 PDT