Generates audio frames from PCM audio data.
        Takes the desired frame duration in milliseconds, the PCM data, and
        the sample rate.
        Yields Frames of the requested duration.

def _frame_generator(self, frame_duration_ms, audio, sample_rate):
        """Generates audio frames from PCM audio data.
        Takes the desired frame duration in milliseconds, the PCM data, and
        the sample rate.
        Yields Frames of the requested duration.
        """
        n = int(sample_rate * (frame_duration_ms / 1000.0) * 2)
        offset = 0
        timestamp = 0.0
        duration = (float(n) / sample_rate) / 2.0
        while offset + n < len(audio):
            yield self.Frame(audio[offset:offset + n], timestamp, duration)
            timestamp += duration
            offset += n