public function getSampleBlock($blockNum)
    {
        // check preconditions
        if (!$this->_dataSize_valid) {
            $this->setDataSize();  // implicit setSize(), setActualSize(), setNumBlocks()
        }

        $offset = $blockNum * $this->_blockAlign;
        if ($offset + $this->_blockAlign > $this->_dataSize || $offset < 0) {
            return null;
        }


        // read data
        return substr($this->_samples, $offset, $this->_blockAlign);
    }