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diff --git a/dpd/README.md b/dpd/README.md
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+Digital Predistortion for ODR-DabMod
+====================================
+
+This folder contains work in progress for digital predistortion. It requires:
+
+- USRP B200.
+- Power amplifier.
+- A feedback connection from the power amplifier output, at an appropriate power level for the B200.
+  Usually this is done with a directional coupler.
+- ODR-DabMod with enabled dpd_port, and with a samplerate of 8192000 samples per second.
diff --git a/dpd/show_spectrum.py b/dpd/show_spectrum.py
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+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+#
+# This is an example tool that shows how to connect to ODR-DabMod's dpd TCP server
+# and get samples from there.
+#
+# Since the TX and RX samples are not perfectly aligned, the tool has to align them properly,
+# which is done in two steps: First on sample-level using a correlation, then with subsample
+# accuracy using a FFT approach.
+#
+# It requires SciPy and matplotlib.
+#
+# Copyright (C) 2017 Matthias P. Braendli
+# http://www.opendigitalradio.org
+# Licence: The MIT License, see notice at the end of this file
+
+import sys
+import socket
+import struct
+import numpy as np
+import matplotlib as mp
+import scipy.signal
+
+SIZEOF_SAMPLE = 8 # complex floats
+
+if len(sys.argv) != 3:
+    print("Usage: show_spectrum.py <port> <number of samples to request>")
+    sys.exit(1)
+
+port = int(sys.argv[1])
+num_samps_to_request = int(sys.argv[2])
+
+
+def get_samples(port, num_samps_to_request):
+    """Connect to ODR-DabMod, retrieve TX and RX samples, load
+    into numpy arrays, and return a tuple
+    (tx_timestamp, tx_samples, rx_timestamp, rx_samples)
+    where the timestamps are doubles, and the samples are numpy
+    arrays of complex floats, both having the same size
+    """
+
+    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    s.connect(('localhost', port))
+
+    print("Send version");
+    s.sendall(b"\x01")
+
+    print("Send request for {} samples".format(num_samps_to_request))
+    s.sendall(struct.pack("=I", num_samps_to_request))
+
+    print("Wait for TX metadata")
+    num_samps, tx_second, tx_pps = struct.unpack("=III", s.recv(12))
+    tx_ts = tx_second + tx_pps / 16384000.0
+
+    print("Receiving {} TX samples".format(num_samps))
+    txframe_bytes = s.recv(num_samps * SIZEOF_SAMPLE)
+    txframe = np.fromstring(txframe_bytes, dtype=np.complex64)
+
+    print("Wait for RX metadata")
+    rx_second, rx_pps = struct.unpack("=II", s.recv(8))
+    rx_ts = rx_second + rx_pps / 16384000.0
+
+    print("Receiving {} RX samples".format(num_samps))
+    rxframe_bytes = s.recv(num_samps * SIZEOF_SAMPLE)
+    rxframe = np.fromstring(rxframe_bytes, dtype=np.complex64)
+
+    print("Disconnecting")
+    s.close()
+
+    return (tx_ts, txframe, rx_ts, rxframe)
+
+
+tx_ts, txframe, rx_ts, rxframe = get_samples(port, num_samps_to_request)
+
+print("Received {} & {} frames at {} and {}".format(
+    len(txframe), len(rxframe), tx_ts, rx_ts))
+
+
+# The MIT License (MIT)
+#
+# Copyright (c) 2017 Matthias P. Braendli
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.