diff --git a/config.py.example b/config.py.example
index 87eaa6f..8757916 100644
--- a/config.py.example
+++ b/config.py.example
@@ -6,3 +6,4 @@ class Config:
             ''.join(random.choices(string.ascii_letters, k=20))
     DATA_DIR   = os.environ.get('DATA_DIR') or '/home/orange/devel/prolefeeder/tmp'
     MAX_LENGTH = os.environ.get('MAX_LENGTH') or 180
+    KBITS      = os.environ.get('KBITS') or 128
diff --git a/generate-test-data.py b/generate-test-data.py
index 559ef5f..63aed88 100644
--- a/generate-test-data.py
+++ b/generate-test-data.py
@@ -1,14 +1,14 @@
 from datetime import datetime, timedelta
 import subprocess
 
-KBITS = 128
+from config import Config
 
 
 def generate_test_file(fn, delta):
     subprocess.call(['ffmpeg',
                      '-f', 'lavfi',
                      '-i', 'sine=frequency=1000:duration=%d' % delta.total_seconds(),
-                     '-b:a', '%dk' % KBITS,
+                     '-b:a', '%dk' % Config.KBITS,
                      fn])
 
 
diff --git a/prolefeeder.py b/prolefeeder.py
index 1b415ff..547f7bc 100644
--- a/prolefeeder.py
+++ b/prolefeeder.py
@@ -6,6 +6,10 @@ from wtforms import DateTimeField, DecimalField, SubmitField
 from wtforms.validators import DataRequired, NumberRange
 
 from datetime import datetime, timedelta
+import os
+import random
+import re
+import subprocess
 
 from config import Config
 
@@ -47,7 +51,53 @@ def download_file(filename):
 
 
 def prepare_download(form):
-    return 'test.mp3'
+    """Prepare a download given the user's request form"""
+
+    def start_time_for_source_fn(fn):
+        return datetime.strptime(fn, 'qfhi-%Y%m%d-%H%M.mp3')
+
+    def length_for_source_fn(fn):
+        size = os.stat(os.path.join(app.config['DATA_DIR'], fn)).st_size
+        length = timedelta(minutes=size / (1000*app.config['KBITS']/8) / 60)
+        return length
+
+    # Get a sorted list of all source files with start time and length
+    sources = []
+    for fn in os.listdir(app.config['DATA_DIR']):
+        try:
+            start_time = start_time_for_source_fn(fn)
+            length = length_for_source_fn(fn)
+            sources.append({'fn': fn, 'start_time': start_time, 'length': length})
+        except ValueError:
+            pass
+    sources = sorted(sources, key=lambda s: s['start_time'])
+
+    # Only interested in the source files from the start file
+    start_index = None
+    for i, source in enumerate(sources):
+        if source['start_time'] <= form.start_time.data < source['start_time'] + source['length']:
+            start_index = i
+    sources = sources[start_index:]
+
+    # Seek into the first file
+    ss = (form.start_time.data - sources[0]['start_time']).total_seconds()
+
+    # Let ffmpeg do the rest of the job
+    # XXX Necessary to limit the concat files?
+    output_filename = '{}_{}.mp3'.format(form.start_time.data, form.length.data)
+
+    c = ['ffmpeg', '-y']
+    c += ['-ss', str(ss)]
+    c += ['-i', 'concat:' + '|'.join([os.path.join(app.config['DATA_DIR'],
+        source['fn']) for source in sources])]
+    c += ['-codec', 'copy']
+    c += ['-t', str(form.length.data * 60)]
+    c += [os.path.join(app.config['DATA_DIR'], output_filename)]
+
+    app.logger.debug(' '.join(c))
+    subprocess.call(c)
+
+    return output_filename
 
 
 if __name__ == '__main__':