diff --git a/backend/lib/DataAggregationIterator.php b/backend/lib/Interpreter/DataAggregationIterator.php
similarity index 92%
rename from backend/lib/DataAggregationIterator.php
rename to backend/lib/Interpreter/DataAggregationIterator.php
index cb2dac9..635bbe3 100644
--- a/backend/lib/DataAggregationIterator.php
+++ b/backend/lib/Interpreter/DataAggregationIterator.php
@@ -21,7 +21,7 @@
  * along with volkszaehler.org. If not, see <http://www.gnu.org/licenses/>.
  */
 
-namespace Volkszaehler;
+namespace Volkszaehler\Interpreter;
 
 use Doctrine\DBAL;
 
@@ -75,8 +75,8 @@ class DataAggregationIterator extends DataIterator {
 
 		$this->aggregatedKey++;
 		$this->current = $current;
-		$this->current[0] = $tuple[0];
-		$this->current[2] = $this->packageSize;
+		$this->current[0] = $tuple[0];				// the last timestamp of a package
+		$this->current[2] = $this->packageSize;		// how many pulses do we have aggregated? how accurate is our result?
 	}
 
 	/**
diff --git a/backend/lib/DataIterator.php b/backend/lib/Interpreter/DataIterator.php
similarity index 98%
rename from backend/lib/DataIterator.php
rename to backend/lib/Interpreter/DataIterator.php
index f9dbc64..17257dd 100644
--- a/backend/lib/DataIterator.php
+++ b/backend/lib/Interpreter/DataIterator.php
@@ -21,7 +21,7 @@
  * along with volkszaehler.org. If not, see <http://www.gnu.org/licenses/>.
  */
 
-namespace Volkszaehler;
+namespace Volkszaehler\Interpreter;
 
 use Doctrine\DBAL;
 
diff --git a/backend/lib/Interpreter/GroupInterpreter.php b/backend/lib/Interpreter/GroupInterpreter.php
index 3276de6..607a6bf 100644
--- a/backend/lib/Interpreter/GroupInterpreter.php
+++ b/backend/lib/Interpreter/GroupInterpreter.php
@@ -42,7 +42,7 @@ class GroupInterpreter {
 	/**
 	 * Constructor
 	 *
-	 * @param Model\Group $group should only contain channels with the same indicator
+	 * @param Model\Group $group should only contain channels of the same indicator
 	 * @param ORM\EntityManager $em
 	 * @param integer $from timestamp in ms since 1970
 	 * @param integer $to timestamp in ms since 1970
diff --git a/backend/lib/Interpreter/Interpreter.php b/backend/lib/Interpreter/Interpreter.php
index adb6a07..b833d5b 100644
--- a/backend/lib/Interpreter/Interpreter.php
+++ b/backend/lib/Interpreter/Interpreter.php
@@ -98,16 +98,19 @@ abstract class Interpreter implements InterpreterInterface {
 			$sqlFields = ' timestamp, value';
 		}
 
+		// get total row count for grouping
 		$rowCount = $conn->fetchColumn($sqlRowCount, $params, 0);
 
+		// query for data
 		$stmt = $conn->executeQuery('SELECT ' . $sqlFields . $sqlFrom . $sqlWhere . $sqlGroupBy . $sqlOrderBy, $params);
 
+		// return iterators
 		if ($sqlGroupBy || is_null($groupBy)) {		// aggregation by sql or skip it
-			return new Volkszaehler\DataIterator($stmt, $rowCount);
+			return new DataIterator($stmt, $rowCount);
 		}
 		elseif (is_numeric($groupBy) ) {			// aggregation by php
 			$tuples = (int) $groupBy;
-			return new Volkszaehler\DataAggregationIterator($stmt, $rowCount, $tuples);
+			return new DataAggregationIterator($stmt, $rowCount, $tuples);
 		}
 		else {
 			throw new \Exception('invalid groupBy parameter');
diff --git a/backend/lib/Interpreter/MeterInterpreter.php b/backend/lib/Interpreter/MeterInterpreter.php
index a8babf2..afd4cbf 100644
--- a/backend/lib/Interpreter/MeterInterpreter.php
+++ b/backend/lib/Interpreter/MeterInterpreter.php
@@ -105,19 +105,12 @@ class MeterInterpreter extends Interpreter {
 	 */
 	public function getValues($groupBy = NULL) {
 		$pulses = parent::getData($groupBy);
-		$count = $pulses->count();
 
 		$values = array();
 		foreach ($pulses as $pulse) {
 			if (isset($last)) {
-				$delta = $pulse[0] - $last[0];
+				$values[] = $this->raw2differential($last, $pulse);
 				$last = $pulse;
-
-				$values[] = array(
-					(int) ($pulse[0] - $delta / 2),															// timestamp
-					$pulse[1] * (3600000 / (($this->channel->getResolution() / 1000) * $delta)),	// value
-					(isset($pulse[2])) ? $pulse[2] : 1
-				);
 			}
 			else {
 				$last = $pulse;
@@ -126,6 +119,22 @@ class MeterInterpreter extends Interpreter {
 
 		return $values;
 	}
+
+	/**
+	 * Calculates the differential quotient of two consecutive pulses
+	 *
+	 * @param array $last the last pulse
+	 * @param array $next the next pulse
+	 */
+	protected function raw2differential(array $last, array $next) {
+		$delta = $next[0] - $last[0];
+
+		return array(
+			(int) ($next[0] - $delta / 2),												// timestamp
+			$next[1] * (3600000 / (($this->channel->getResolution() / 1000) * $delta)),	// value
+			(isset($next[2])) ? $next[2] : 1
+		);
+	}
 }
 
 ?>
\ No newline at end of file