diff --git a/spleeter/commands/separate.py b/spleeter/commands/separate.py
index 2fac8aa..158190c 100644
--- a/spleeter/commands/separate.py
+++ b/spleeter/commands/separate.py
@@ -11,8 +11,6 @@
         -i /path/to/audio1.wav /path/to/audio2.mp3
 """
 
-import tensorflow as tf
-
 from ..audio.adapter import get_audio_adapter
 from ..separator import Separator
 
@@ -21,11 +19,6 @@ __author__ = 'Deezer Research'
 __license__ = 'MIT License'
 
 
-def get_backend(backend):
-    if backend == "auto":
-        return "tensorflow" if tf.test.is_gpu_available() else "librosa"
-    return backend
-
 
 def entrypoint(arguments, params):
     """ Command entrypoint.
@@ -38,7 +31,7 @@ def entrypoint(arguments, params):
     separator = Separator(
         arguments.configuration,
         MWF=arguments.MWF,
-        stft_backend=get_backend(arguments.stft_backend))
+        stft_backend=arguments.stft_backend)
     for filename in arguments.inputs:
         separator.separate_to_file(
             filename,
diff --git a/spleeter/model/__init__.py b/spleeter/model/__init__.py
index 4c3e200..b99a9dd 100644
--- a/spleeter/model/__init__.py
+++ b/spleeter/model/__init__.py
@@ -320,6 +320,12 @@ class EstimatorSpecBuilder(object):
             self._build_masks()
         return self._masks
 
+    @property
+    def masked_stfts(self):
+        if not hasattr(self, "_masked_stfts"):
+            self._build_masked_stfts()
+        return self._masked_stfts
+
     def _inverse_stft(self, stft, time_crop=None):
         """ Inverse and reshape the given STFT
 
@@ -397,6 +403,10 @@ class EstimatorSpecBuilder(object):
         return tf.concat([mask, extension], axis=2)
 
     def _build_masks(self):
+        """
+        Compute masks from the output spectrograms of the model.
+        :return:
+        """
         output_dict = self.model_outputs
         stft_feature = self.stft_feature
         separation_exponent = self._params['separation_exponent']
@@ -425,12 +435,12 @@ class EstimatorSpecBuilder(object):
             out[instrument] = instrument_mask
         self._masks = out
 
-    def _build_masked_stft(self):
+    def _build_masked_stfts(self):
         input_stft = self.stft_feature
         out = {}
         for instrument, mask in self.masks.items():
             out[instrument] = tf.cast(mask, dtype=tf.complex64) * input_stft
-        return out
+        self._masked_stfts = out
 
     def _build_manual_output_waveform(self, masked_stft):
         """ Perform ratio mask separation
@@ -461,11 +471,10 @@ class EstimatorSpecBuilder(object):
         return output_waveform
 
     def _build_outputs(self):
-        masked_stft = self._build_masked_stft()
         if self.include_stft_computations():
-            self._outputs = self._build_output_waveform(masked_stft)
+            self._outputs = self._build_output_waveform(self.masked_stfts)
         else:
-            self._outputs = masked_stft
+            self._outputs = self.masked_stfts
 
         if 'audio_id' in self._features:
             self._outputs['audio_id'] = self._features['audio_id']
diff --git a/spleeter/separator.py b/spleeter/separator.py
index 73a1a89..668b59d 100644
--- a/spleeter/separator.py
+++ b/spleeter/separator.py
@@ -39,6 +39,14 @@ __license__ = 'MIT License'
 logger = logging.getLogger("spleeter")
 
 
+
+def get_backend(backend):
+    assert backend in ["auto", "tensorflow", "librosa"]
+    if backend == "auto":
+        return "tensorflow" if tf.test.is_gpu_available() else "librosa"
+    return backend
+
+
 class Separator(object):
     """ A wrapper class for performing separation. """
 
@@ -48,13 +56,14 @@ class Separator(object):
         :param params_descriptor: Descriptor for TF params to be used.
         :param MWF: (Optional) True if MWF should be used, False otherwise.
         """
+
         self._params = load_configuration(params_descriptor)
         self._sample_rate = self._params['sample_rate']
         self._MWF = MWF
         self._predictor = None
         self._pool = Pool() if multiprocess else None
         self._tasks = []
-        self._params["stft_backend"] = stft_backend
+        self._params["stft_backend"] = get_backend(stft_backend)
 
     def _get_predictor(self):
         """ Lazy loading access method for internal predictor instance.