Reading int16 from a file and writing as int8 to another file

I want to read an I/Q sampled binary file and write it to another file.

I have a binary file consisting of signals stored as complex 16- bit samples at a rate of 25 Msps.

I want to write this file to another file with 8-bit samples and I want to execute that in a loop since sizes of the files are enormous.

But when I try to write to another file, I could not adjust its starting point.

I share my code below, where do I get wrong?

clear all;
clc;
tic;

duration = 20; % seconds to be read
start_time =0;  

sample_rate=25000000;   
point_to_begin=0;   

for c=0:duration
  fid=fopen('/home/read.bin','r'); % open the file
  fseek(fid, point_to_begin+sample_rate*c, 'bof');% position the start
  s_int16=fread(fid,sample_rate,'int16')';% read in Is and Qs
  fclose(fid);
  s_int8=typecast(s_int16,'int8');    %typecast int16 to int8
  fileID = fopen('/home/write.bin','w');
  fwrite(fileID,s_int8,'int8',point_to_begin+sample_rate*c);
  fclose(fileID);
end

toc;

Answer 1

[dpb]

Don't open/close the files inside the loop -- just read/write them sequentially...

...
fidi=fopen('/home/read.bin','r'); % open the file
fido = fopen('/home/write.bin','w');
for c=0:duration
  s_int16=fread(fidi,sample_rate,'int16')';
  s_int8=typecast(s_int16,'int8');
  fwrite(fido,s_int8,'int8');
end
fclose('all')
clear fid*