Aegisub/aegisub/src/audio_provider_downmix.cpp

// Copyright (c) 2007-2008, Niels Martin Hansen
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// $Id$

/// @file audio_provider_downmix.cpp
/// @brief Intermediate audio provider downmixing the signal to mono
/// @ingroup audio_input
///


//////////////////
// Headers
#include "config.h"

#include "audio_provider_downmix.h"


/// @brief Constructor 
/// @param source 
///
DownmixingAudioProvider::DownmixingAudioProvider(AudioProvider *source) : provider(source) {
	filename = source->GetFilename();
	channels = 1; // target
	src_channels = source->GetChannels();
	num_samples = source->GetNumSamples();
	bytes_per_sample = source->GetBytesPerSample();
	sample_rate = source->GetSampleRate();

	if (!(bytes_per_sample == 1 || bytes_per_sample == 2))
		throw AudioOpenError("Downmixing Audio Provider: Can only downmix 8 and 16 bit audio");
	if (!source->AreSamplesNativeEndian())
		throw AudioOpenError("Downmixing Audio Provider: Source must have machine endian samples");
}

/// @brief Actual work happens here 
/// @param buf   
/// @param start 
/// @param count 
///
void DownmixingAudioProvider::GetAudio(void *buf, int64_t start, int64_t count) const {
	if (count == 0) return;

	// We can do this ourselves
	if (start >= num_samples) {
		if (bytes_per_sample == 1)
			// 8 bit formats are usually unsigned with bias 127
			memset(buf, 127, count);
		else
			// While everything else is signed
			memset(buf, 0, count*bytes_per_sample);

		return;
	}

	// So alloc some temporary memory for this
	// Depending on use, this might be made faster by using
	// a pre-allocced block of memory...?
	char *tmp = new char[count*bytes_per_sample*src_channels];

	try {
		provider->GetAudio(tmp, start, count);
	}
	catch (...) {
		delete tmp;
		throw;
	}

	// Now downmix
	// Just average the samples over the channels (really bad if they're out of phase!)
	// XXX: Assuming here that sample data are in machine endian, an upstream provider should ensure that
	if (bytes_per_sample == 1) {
		uint8_t *src = (uint8_t *)tmp;
		uint8_t *dst = (uint8_t *)buf;

		while (count > 0) {
			int sum = 0;
			for (int c = 0; c < src_channels; c++)
				sum += *(src++);
			*(dst++) = (uint8_t)(sum / src_channels);
			count--;
		}
	}
	else if (bytes_per_sample == 2) {
		int16_t *src = (int16_t *)tmp;
		int16_t *dst = (int16_t *)buf;

		while (count > 0) {
			int sum = 0;
			for (int c = 0; c < src_channels; c++)
				sum += *(src++);
			*(dst++) = (int16_t)(sum / src_channels);
			count--;
		}
	}

	// Done downmixing, free the work buffer
	delete[] tmp;
}