public function Master_Update($masterId, $envelope, $targeting, $scheduling) {
        $ret = false;

        $data = $this->_package_master_elements($envelope, $targeting, $scheduling);
        if ($data !== false) {
            $ret = $this->_call_api('put', "{$this->_url}/masters/$masterId", $data);
        }

        return $ret;
    }