音频处理基本概念及音频重采样
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音频处理基本概念及音频重采样
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音頻處理基本概念及音頻重采樣
目錄
1. 重點(diǎn)問(wèn)題
2. 重采樣
1. 什么是重采樣
2. 為什么要重采樣
3. 可調(diào)節(jié)的參數(shù)
3. 對(duì)應(yīng)參數(shù)解析
1. 采樣率
2. 采樣格式及量化精度（位寬）
3. 分?（plane）和打包（packed）
4. 聲道分布（channel_layout)
5. ?頻幀的數(shù)據(jù)量計(jì)算
6. ?頻播放時(shí)間計(jì)算
（1）10241000/44100=23.21995464852608ms ->約等于 23.2ms，精度損失了0.011995464852608ms，如果累計(jì)10萬(wàn)幀，誤差>1199毫秒，如果有視頻?起的就會(huì)有?視頻同步的問(wèn)題。 如果按著23.2去計(jì)算pts（0 23.2 46.4 ）就會(huì)有累積誤差。
（2）1024*1000/48000=21.33333333333333ms
4. FFmpeg重采樣API
5. ?頻重采樣?程范例
1. 簡(jiǎn)單范例（resample）
/** Copyright (c) 2012 Stefano Sabatini** Permission is hereby granted, free of charge, to any person obtaining a copy* of this software and associated documentation files (the "Software"), to deal* in the Software without restriction, including without limitation the rights* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell* copies of the Software, and to permit persons to whom the Software is* furnished to do so, subject to the following conditions:** The above copyright notice and this permission notice shall be included in* all copies or substantial portions of the Software.** THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN* THE SOFTWARE.*//*** @example resampling_audio.c* libswresample API use example.*/#include <libavutil/opt.h> #include <libavutil/channel_layout.h> #include <libavutil/samplefmt.h> #include <libswresample/swresample.h>static int get_format_from_sample_fmt(const char **fmt,enum AVSampleFormat sample_fmt) {int i;struct sample_fmt_entry {enum AVSampleFormat sample_fmt;const char *fmt_be, *fmt_le;} sample_fmt_entries[] = {{AV_SAMPLE_FMT_U8, "u8", "u8"},{AV_SAMPLE_FMT_S16, "s16be", "s16le"},{AV_SAMPLE_FMT_S32, "s32be", "s32le"},{AV_SAMPLE_FMT_FLT, "f32be", "f32le"},{AV_SAMPLE_FMT_DBL, "f64be", "f64le"},};*fmt = NULL;for (i = 0; i < FF_ARRAY_ELEMS(sample_fmt_entries); i++) {struct sample_fmt_entry *entry = &sample_fmt_entries[i];if (sample_fmt == entry->sample_fmt) {*fmt = AV_NE(entry->fmt_be, entry->fmt_le);return 0;}}fprintf(stderr,"Sample format %s not supported as output format\n",av_get_sample_fmt_name(sample_fmt));return AVERROR(EINVAL); }/*** Fill dst buffer with nb_samples, generated starting from t. 交錯(cuò)模式的*/ static void fill_samples(double *dst, int nb_samples, int nb_channels, int sample_rate, double *t) {int i, j;double tincr = 1.0 / sample_rate, *dstp = dst;const double c = 2 * M_PI * 440.0;/* generate sin tone with 440Hz frequency and duplicated channels */for (i = 0; i < nb_samples; i++) {*dstp = sin(c * *t);for (j = 1; j < nb_channels; j++)dstp[j] = dstp[0];dstp += nb_channels;*t += tincr;} }int main(int argc, char **argv) {// 輸入?yún)?shù)int64_t src_ch_layout = AV_CH_LAYOUT_STEREO;int src_rate = 48000;enum AVSampleFormat src_sample_fmt = AV_SAMPLE_FMT_DBL;int src_nb_channels = 0;uint8_t **src_data = NULL; // 二級(jí)指針int src_linesize;int src_nb_samples = 1024;// 輸出參數(shù)int64_t dst_ch_layout = AV_CH_LAYOUT_STEREO;int dst_rate = 44100;enum AVSampleFormat dst_sample_fmt = AV_SAMPLE_FMT_S16;int dst_nb_channels = 0;uint8_t **dst_data = NULL; //二級(jí)指針int dst_linesize;int dst_nb_samples;int max_dst_nb_samples;// 輸出文件const char *dst_filename = NULL; // 保存輸出的pcm到本地，然后播放驗(yàn)證FILE *dst_file;int dst_bufsize;const char *fmt;// 重采樣實(shí)例struct SwrContext *swr_ctx;double t;int ret;/*if (argc != 2) {fprintf(stderr, "Usage: %s output_file\n""API example program to show how to resample an audio stream with libswresample.\n""This program generates a series of audio frames, resamples them to a specified ""output format and rate and saves them to an output file named output_file.\n",argv[0]);exit(1);}*/dst_filename = "/Users/lijinwang/Downloads/course/study/believe.pcm";dst_file = fopen(dst_filename, "wb");if (!dst_file) {fprintf(stderr, "Could not open destination file %s\n", dst_filename);exit(1);}// 創(chuàng)建重采樣器/* create resampler context */swr_ctx = swr_alloc();if (!swr_ctx) {fprintf(stderr, "Could not allocate resampler context\n");ret = AVERROR(ENOMEM);goto end;}// 設(shè)置重采樣參數(shù)/* set options */// 輸入?yún)?shù)av_opt_set_int(swr_ctx, "in_channel_layout", src_ch_layout, 0);av_opt_set_int(swr_ctx, "in_sample_rate", src_rate, 0);av_opt_set_sample_fmt(swr_ctx, "in_sample_fmt", src_sample_fmt, 0);// 輸出參數(shù)av_opt_set_int(swr_ctx, "out_channel_layout", dst_ch_layout, 0);av_opt_set_int(swr_ctx, "out_sample_rate", dst_rate, 0);av_opt_set_sample_fmt(swr_ctx, "out_sample_fmt", dst_sample_fmt, 0);// 初始化重采樣/* initialize the resampling context */if ((ret = swr_init(swr_ctx)) < 0) {fprintf(stderr, "Failed to initialize the resampling context\n");goto end;}/* allocate source and destination samples buffers */// 計(jì)算出輸入源的通道數(shù)量src_nb_channels = av_get_channel_layout_nb_channels(src_ch_layout);// 給輸入源分配內(nèi)存空間ret = av_samples_alloc_array_and_samples(&src_data, &src_linesize, src_nb_channels,src_nb_samples, src_sample_fmt, 0);if (ret < 0) {fprintf(stderr, "Could not allocate source samples\n");goto end;}/* compute the number of converted samples: buffering is avoided* ensuring that the output buffer will contain at least all the* converted input samples */// 計(jì)算輸出采樣數(shù)量max_dst_nb_samples = dst_nb_samples =av_rescale_rnd(src_nb_samples, dst_rate, src_rate, AV_ROUND_UP);/* buffer is going to be directly written to a rawaudio file, no alignment */dst_nb_channels = av_get_channel_layout_nb_channels(dst_ch_layout);// 分配輸出緩存內(nèi)存ret = av_samples_alloc_array_and_samples(&dst_data, &dst_linesize, dst_nb_channels,dst_nb_samples, dst_sample_fmt, 0);if (ret < 0) {fprintf(stderr, "Could not allocate destination samples\n");goto end;}t = 0;do {/* generate synthetic audio */// 生成輸入源fill_samples((double *) src_data[0], src_nb_samples, src_nb_channels, src_rate, &t);/* compute destination number of samples */int64_t delay = swr_get_delay(swr_ctx, src_rate);dst_nb_samples = av_rescale_rnd(delay + src_nb_samples, dst_rate, src_rate, AV_ROUND_UP);if (dst_nb_samples > max_dst_nb_samples) {av_freep(&dst_data[0]);ret = av_samples_alloc(dst_data, &dst_linesize, dst_nb_channels,dst_nb_samples, dst_sample_fmt, 1);if (ret < 0)break;max_dst_nb_samples = dst_nb_samples;}// int fifo_size = swr_get_out_samples(swr_ctx,src_nb_samples);// printf("fifo_size:%d\n", fifo_size);// if(fifo_size < 1024)// continue;/* convert to destination format */// ret = swr_convert(swr_ctx, dst_data, dst_nb_samples, (const uint8_t **)src_data, src_nb_samples);ret = swr_convert(swr_ctx, dst_data, dst_nb_samples, (const uint8_t **) src_data, src_nb_samples);if (ret < 0) {fprintf(stderr, "Error while converting\n");goto end;}dst_bufsize = av_samples_get_buffer_size(&dst_linesize, dst_nb_channels,ret, dst_sample_fmt, 1);if (dst_bufsize < 0) {fprintf(stderr, "Could not get sample buffer size\n");goto end;}printf("t:%f in:%d out:%d\n", t, src_nb_samples, ret);fwrite(dst_data[0], 1, dst_bufsize, dst_file);} while (t < 10);ret = swr_convert(swr_ctx, dst_data, dst_nb_samples, NULL, 0);if (ret < 0) {fprintf(stderr, "Error while converting\n");goto end;}dst_bufsize = av_samples_get_buffer_size(&dst_linesize, dst_nb_channels,ret, dst_sample_fmt, 1);if (dst_bufsize < 0) {fprintf(stderr, "Could not get sample buffer size\n");goto end;}printf("flush in:%d out:%d\n", 0, ret);fwrite(dst_data[0], 1, dst_bufsize, dst_file);if ((ret = get_format_from_sample_fmt(&fmt, dst_sample_fmt)) < 0)goto end;fprintf(stderr, "Resampling succeeded. Play the output file with the command:\n""ffplay -f %s -channel_layout %"PRId64" -channels %d -ar %d %s\n",fmt, dst_ch_layout, dst_nb_channels, dst_rate, dst_filename);end:fclose(dst_file);if (src_data)av_freep(&src_data[0]);av_freep(&src_data);if (dst_data)av_freep(&dst_data[0]);av_freep(&dst_data);swr_free(&swr_ctx);return ret < 0; }2. 復(fù)雜范例
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