MATLAB: Problem with sampling resolution while reading binary data with fread

binarydatafreadresolutionsampling

Hi
I have downloaded an ECG data from "physionet.org", aami3a.dat(the link is below). The site says it is recorded with 12-bit resolution (@720Hz). I used the following code to open and plot the waveform:
Fid=fopen('D:\SomeAddress\aami3a.dat','r');
Data=fread(Fid,'*int16');
fclose(Fid);
plot(Data');
The problem is it works!!! and I don't understand why!! while it is recorded with 12-bit precision, I'm reading that with 16-bit precision. How does 'fread' function handles this?! I mean I thought when I use '*int16' precision, the 'fread' function separates my binary data 16-bit-by-16-bit and treats each separation as a 16-bit integer and then converts that into decimal (but seems I was wrong). I also tried '*uint16' , '*bit16' precisions which show the same results. But when I use '*bit12' or '*ubit12' I receive a malformed shape which is definitely wrong. can anyone give me an explanation for this? Thanks in advance.
ECG Wave: ( https://physionet.org/physiobank/database/aami-ec13/aami3a.dat )

Best Answer

Data that is recorded at 12 bits is, by convention, stored in the 12 most significant bits of a 16 bit word.
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