asp.net-core - 为什么将wav转换为mp3会得到不同的长度

Question

我使用 NAudio 和 NAudio.Lame 将 wav 文件转换为 mp3 文件，代码如下:

using var reader = new WaveFileReader(file);
using var ms = new MemoryStream();
var writer = new LameMP3FileWriter(ms, reader.WaveFormat, 32);
reader.CopyTo(writer);
writer.Flush();
File.WriteAllBytes(file.Replace("wav", "mp3"), ms.ToArray());

wav文件长度为1500ms，而mp3文件长度为1557ms。
我通过使用 ffmpeg.exe 来做同样的事情，命令如下:

ffmpeg -i c:\2\xxx.wav c:\2\xxx.mp3

输出信息:

ffmpeg version 5.0.1-essentials_build-www.gyan.dev Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 11.2.0 (Rev7, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-zlib --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-sdl2 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libgme --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libtheora --enable-libvo-amrwbenc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-librubberband
  libavutil      57. 17.100 / 57. 17.100
  libavcodec     59. 18.100 / 59. 18.100
  libavformat    59. 16.100 / 59. 16.100
  libavdevice    59.  4.100 / 59.  4.100
  libavfilter     8. 24.100 /  8. 24.100
  libswscale      6.  4.100 /  6.  4.100
  libswresample   4.  3.100 /  4.  3.100
  libpostproc    56.  3.100 / 56.  3.100
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, wav, from 'c:\2\xxx.wav':
  Duration: 00:00:01.50, bitrate: 768 kb/s
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, mono, s16, 768 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
Output #0, mp3, to 'c:\2\xxx.mp3':
  Metadata:
    TSSE            : Lavf59.16.100
  Stream #0:0: Audio: mp3, 48000 Hz, mono, s16p
    Metadata:
      encoder         : Lavc59.18.100 libmp3lame
size=      12kB time=00:00:01.51 bitrate=  66.2kbits/s speed= 137x
video:0kB audio:12kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.928711%

我得到一个长度为 1536 毫秒的 mp3 文件。
那么，为什么长度改变了？
谢谢阅读。

Answer 1

我认为 Mokubai's comment会对你有用。
在这种情况下，我也尝试过做一些测试。无法重现问题。
如需更多帮助，可以在Mark Heath下留言的最新回答。他是 NAudio 的专家。

我的测试代码

using System;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Threading.Tasks;
using Azure.Storage.Blobs;
using IdentityModel.Client;
using NAudio.Lame;
using NAudio.Wave;
using Newtonsoft.Json.Linq;

namespace ClientCredentialsConsoleApp
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            var tempAudioUrl = "https://****/file_example_WAV_10MG.wav";
            var audioFile = await AudioConverter.ConvertToMp3Async_YourCode(new Uri(string.Format(tempAudioUrl)));
            File.WriteAllBytes("wav" + DateTime.Now.ToString("yyyyMMddHHmmss") + ".mp3", audioFile);

        }
       
    }
    public static class AudioConverter
    {
        public static async Task<byte[]> ConvertToMp3Async(Uri uri)
        {
            using (var client = new WebClient())
            {
                var file = await client.DownloadDataTaskAsync(uri);
                var target = new WaveFormat(8000, 16, 1);
                using (var outPutStream = new MemoryStream())
                using (var waveStream = new WaveFileReader(new MemoryStream(file)))
                using (var conversionStream = new WaveFormatConversionStream(target, waveStream))
                using (var writer = new LameMP3FileWriter(outPutStream, conversionStream.WaveFormat, 32, null))
                {
                    conversionStream.CopyTo(writer);

                    return outPutStream.ToArray();
                }

            }
        }
        public static async Task<byte[]> ConvertToMp3Async_YourCode(Uri uri)
        {
            using (var client = new WebClient())
            {
                
                var file = await client.DownloadDataTaskAsync(uri);
                using var reader = new WaveFileReader(new MemoryStream(file));
                Console.WriteLine("wav time is :"+reader.TotalTime);
                var ms = new MemoryStream();
                //var writer = new LameMP3FileWriter(ms, reader.WaveFormat, 32)
                using (var writer = new LameMP3FileWriter(ms, reader.WaveFormat, 32))
                {
                    reader.CopyTo(writer);

                    return ms.ToArray();
                }
                //reader.CopyTo(writer);
                //writer.Flush();
                //File.WriteAllBytes("wav"+DateTime.Now.ToString("yyyyMMddHH:mm:ss")+ ".mp3", ms.ToArray());

            }
        }
    }
}