edfwrite failure because of 8 byte limit

I am trying convert some data to an EDF with the following code (per this page edf_link):

fs = 250;
data = data_streams{i};

Undefined variable 'data_streams'.

out_dir = '[redacted]';
out_file = data_names{i};
out_file_full = fullfile(out_dir, strcat(out_file,'.edf'));

hdr = edfheader("EDF+");
hdr.NumDataRecords = 1;
hdr.NumSignals = 1;
hdr.SignalLabels = ["SubCTX"];
hdr.DataRecordDuration = seconds(length(data(:,1))/fs);
hdr.PhysicalDimensions = repelem("mV",1);
hdr.PhysicalMin = min(data);
hdr.PhysicalMax = max(data);
hdr.DigitalMin = -32768;
hdr.DigitalMax =  32767;

edfw = edfwrite(out_file_full,hdr,data);

But run into the following error:

_______

Error using signal.internal.edf.write.validateFieldBytes

Value of "DataRecordDuration" field must be less than or equal to 8 bytes. See edfheader documentation for more information.

Error in edfwrite/createFile (line 1685)

signal.internal.edf.write.validateFieldBytes(value, fieldIndex, ...

Error in edfwrite (line 499)

[obj, fileInfo] = createFile(obj, filename, hdr,...

Error in create_edfs_from_OFF_data (line 45)

edfw = edfwrite(out_file_full,hdr,data);

________

I do not know how to avoid this, because the output for

seconds()

is always 10 bytes.

Example:

tmp = seconds(1);
disp(whos('tmp').bytes)

Any guidance would be very welcomed! Thanks in advance.

[Walter Roberson]

After you execute the statement

hdr.DataRecordDuration = seconds(length(data(:,1))/fs);

please show us the value of hdr.DataRecordDuration

[Clay Smyth]

Thanks for the reply, here is the values of the variable:

[Catherine]

Hi, @Clay Smyth! I was struggling with this same issue and just found a workable solution — hopefully still useful for you :)

My theory is that it's not the size of the output of seconds() that matters, but rather the size of the object that you put into it. I tried seconds(1) and the script ran successfully despite seconds(1) being 10 bytes. However, when I tried it with the actual length of my data in milliseconds (which was a 10 byte double before I turned it into a duration) it failed. That being said, this theory didn't hold for when I converted my data length in milliseconds to seconds, which was also an 8 byte double, and set DataRecordDuration to that.

But anyhoo, this workaround is working with multiple files for me and can maybe work for you too:

%create duration (my data is in milliseconds)
data_duration = seconds(width(sig_data)/256);

%turns into a double
hours = hours(data_duration);

%round hours to 1 after decimal point, whos('hours_rounded') = 8 bytes
hours_rounded = round(hours,2);

%add to header as a duration
header.DataRecordDuration = hours(hours_rounded);

Answer 1

[Christopher McCausland]

Hi all,

I have had a look at the above, from the documentation in edfheader.DataRecordDuration the input should be [a] "Duration of each data record, returned as a duration scalar."

@Catherine's solotion does exactly that, takes a duration value and then returns it back as a double (i.e. a scalar).

@Walter Roberson, Is there anyway to get this flagged with the QA team to make the discription a little more indepth/ add an example? Scaler conversion is required several edfheader inputs and the scalar part is easy to miss!

A similar issue can be found here.

Christopher

[Walter Roberson]

@Christopher McCausland

The original poster had

hdr.DataRecordDuration = seconds(length(data(:,1))/fs);

data(:,1) is a vector, but length() of that is going to be a scalar; fs is a scalar so the result of the division will be a scalar. seconds() of a scalar will be a scalar duration.

The first example at https://www.mathworks.com/help/signal/ref/edfheader.html shows

hdr.DataRecordDuration = seconds(4.22)

which tells us that a duration is expected (or at least is one of the valid options.)

[Christopher McCausland]

Hi Walter,

Just gave this a go in 2023b and the error message has been updated. It's now a lot more direct (which is far less confusing):

Error using edfwrite
Expected dataRecordDuration of header structure to be one of these types:

duration

Instead its type was double.

You are correct, as always @Walter Roberson, thank you for the clarity though!

@Clay Smyth I would also have a look at the orignal specification for .edf, if you want to know why it's limited to 8 bytes.Here is a link.

Christopher