Method GetAudioPTSLocation

src/FFmpegReader.cpp:2624–2682

Calculate Starting video frame and sample # for an audio PTS

// Calculate Starting video frame and sample # for an audio PTS
AudioLocation FFmpegReader::GetAudioPTSLocation(int64_t pts) {
	const double audio_timebase_value =
		(info.audio_timebase.num > 0 && info.audio_timebase.den > 0)
			? info.audio_timebase.ToDouble()
			: (1.0 / 48000.0);
	const double fps_value = (info.fps.num > 0 && info.fps.den > 0) ? info.fps.ToDouble() : 30.0;

	// Get the audio packet start time (in seconds)
	double audio_seconds = (double(pts) * audio_timebase_value) + pts_offset_seconds;

	// Divide by the video timebase, to get the video frame number (frame # is decimal at this point)
	double frame = (audio_seconds * fps_value) + 1;

	// Frame # as a whole number (no more decimals)
	int64_t whole_frame = int64_t(frame);

	// Remove the whole number, and only get the decimal of the frame
	double sample_start_percentage = frame - double(whole_frame);

	// Get Samples per frame
	int samples_per_frame = Frame::GetSamplesPerFrame(whole_frame, info.fps, info.sample_rate, info.channels);

	// Calculate the sample # to start on
	int sample_start = round(double(samples_per_frame) * sample_start_percentage);

	// Protect against broken (i.e. negative) timestamps
	if (whole_frame < 1)
		whole_frame = 1;
	if (sample_start < 0)
		sample_start = 0;

	// Prepare final audio packet location
	AudioLocation location = {whole_frame, sample_start};

	// Compare to previous audio packet (and fix small gaps due to varying PTS timestamps)
	if (previous_packet_location.frame != -1) {
		if (location.is_near(previous_packet_location, samples_per_frame, samples_per_frame)) {
			int64_t orig_frame = location.frame;
			int orig_start = location.sample_start;

			// Update sample start, to prevent gaps in audio
			location.sample_start = previous_packet_location.sample_start;
			location.frame = previous_packet_location.frame;

			// Debug output
			ZmqLogger::Instance()->AppendDebugMethod("FFmpegReader::GetAudioPTSLocation (Audio Gap Detected)", "Source Frame", orig_frame, "Source Audio Sample", orig_start, "Target Frame", location.frame, "Target Audio Sample", location.sample_start, "pts", pts);

		} else {
			// Debug output
			ZmqLogger::Instance()->AppendDebugMethod("FFmpegReader::GetAudioPTSLocation (Audio Gap Ignored - too big)", "Previous location frame", previous_packet_location.frame, "Target Frame", location.frame, "Target Audio Sample", location.sample_start, "pts", pts);
		}
	}

	// Set previous location
	previous_packet_location = location;

	// Return the associated video frame and starting sample #
	return location;