Robert Oostenveld r.oostenveld at FCDONDERS.RU.NL
Mon Mar 27 21:52:16 CEST 2006

Hi James.

On 27 Mar 2006, at 21:14, James Kilner wrote:
> I am trying to understand how one forms synthetic gradiometers from
> the MEG data. I spoke to CTF about this sometime ago and they told
> me that the raw data is recorded with whatever gradiometer settings
> you choose but I wanted to check this. Do you use the coefs (as
> defined in ctf_read_meg4) in your latest release? Do you already
> have functions to change the order of the gradiometers?

No, we are still using our own read_ctf_res4 without the coefs. I
know that the raw data in the meg4 file can be stored in different
formats, i.e. with or without the coefs being applied. Furthermore,
there is also the option in CTF to only store the flag in the
processing params file, whereby DataEditor remembers it the next time
that you open the dataset (but the data in teh meg4 itself is not
touched). The coefs can always be undone, which means that there
should be a flag in the res4 header specifying whether the coefs have
been applied or not.

I am working on a redesign of the reading functions, which will also
clear up the low-level functions: currently the mix between
read_ctf_res4 and ctf_read_res4 is kind of vague. Furthermore, I have
some synthetic gradient code that I would not mind sharing with you,
but it requires testing and tighter integration with FT. I already
want to add support for the synthetic gradients for quite some time
now, but am lacking the time to implement it myself (and the MEG site
in Nijmegen is relatively noise-free). I can point you to the right
spots in the FT code, conceptually it is not too complex.

best regards,

PS I just replied to your download mail before I noticed this mail in
my inbox. I moved this mail over to the discussion list, since I
think that it is also relevant for the other CTF users.

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