FFMpeg/video4linux2，v4l2：v4l2帧为46448字节，但预期为153600字节（网络摄像头捕获速度不够快？）

我试图在linux/ubuntu中捕获网络摄像头的输出。我有一个chicony网络摄像头（联想笔记本电脑）。我在VMWARE虚拟机中运行。捕获过程不会超过10秒。你能帮忙吗

我使用的命令是：

$ ffmpeg -y  -f video4linux2 -r 20 -s 160x120 -i /dev/video0 -acodec libfaac -ab 128k  /tmp/web.avi

我得到的最重要的信息是：

[video4linux2，v4l2@0x9e43fa0]v4l2帧为46448字节，但预期为153600字节

来自ffmpeg的完整消息：

ffmpeg version N-55159-gf118b41 Copyright (c) 2000-2013 the FFmpeg developers
  built on Aug 18 2013 09:09:13 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-libass --prefix=/opt/ffmpeg --enable-debug --enable-libfreetype
  libavutil      52. 40.100 / 52. 40.100
  libavcodec     55. 19.100 / 55. 19.100
  libavformat    55. 12.102 / 55. 12.102
  libavdevice    55.  3.100 / 55.  3.100
  libavfilter     3. 82.100 /  3. 82.100
  libswscale      2.  4.100 /  2.  4.100
  libswresample   0. 17.103 /  0. 17.103
[video4linux2,v4l2 @ 0x9e43fa0] The V4L2 driver changed the video from 160x120 to 320x240
[video4linux2,v4l2 @ 0x9e43fa0] The driver changed the time per frame from 1/20 to 1/15
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 6424.338678, bitrate: 18432 kb/s
    Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 320x240, 18432 kb/s, 15 fps, 15 tbr, 1000k tbn, 1000k tbc
Codec AVOption ab (set bitrate (in bits/s)) specified for output file #0 (/tmp/web.avi) has not been used for any stream. The most likely reason is either wrong type (e.g. a video option with no video streams) or that it is a private option of some encoder which was not actually used for any stream.
Output #0, avi, to '/tmp/web.avi':
  Metadata:
    ISFT            : Lavf55.12.102
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 320x240, q=2-31, 200 kb/s, 20 tbn, 20 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo -> mpeg4)
Press [q] to stop, [?] for help
[video4linux2,v4l2 @ 0x9e43fa0] The v4l2 frame is 46448 bytes, but 153600 bytes are expected
/dev/video0: Invalid data found when processing input
frame=   29 fps= 14 q=3.5 Lsize=      87kB time=00:00:01.45 bitrate= 490.0kbits/s    
video:80kB audio:0kB subtitle:0 global headers:0kB muxing overhead 7.760075%
[video4linux2,v4l2 @ 0x9e43fa0] Some buffers are still owned by the caller on close.