MP2 To Wav

[stanleycup]

I demultiplexed an mpeg1 file that had the audio encoded at 224 kb/sec.

When I converted the .mp2 file to a .wav file, I noticed that the
.wav file has a bitrate of 128 kb/sec.

Is there a reason why the bitrate changed? I didn't see any options
to change the bitrate. I just wanted to convert the .mp2 file to a .wav file.

I used Winamp and VirtualDub to convert and got the same result.

[LigH]

A wave file with 16 bit stereo shall have a bitrate of 1536 kbps for 48 kHz or 1411.2 kbps for 44.1 kHz - are you sure you didn't save it as MP3 compressed wave? To ensure this, you shall try to select "PCM" as compressor if you are using a convertor whicch is able to write ACM compressed waves (which output plugin did you select in WinAmp, how did you set it up?). You can also try to use MADplay to decode your MP2 to WAV, it can be found e.g. at http://mitiok.cjb.net (I just hope it can handle MP2 too).

[stanleycup]

Converted to PCM format 44.1 khz, 16 bit stereo, 172 kb/sec

In Winamp, I used the Nullsoft Diskwriter pluin v2.0 (out_disk.dll)
output plugin.

Is this correct?

[LigH]

Originally posted by stanleycup
Converted to PCM format 44.1 khz, 16 bit stereo, 172 kb/sec

In Winamp, I used the Nullsoft Diskwriter plugin v2.0 (out_disk.dll)
output plugin.

Is this correct?

Almost - you selected a useful plugin, but you just didn't report the settings correctly: Upper or lower case of several letters is very important!

ACM codecs report the bitrate in unusual "KB/s" = "K-Bytes per second" = 1024 Bytes per second, but bitrates for streams are instead usually reported in "kbps" = "kilo-bits per second" = 1000 bits per second. Divide the 1411200 bps (for 16bit stereo at 44.1 kHz) by 8 (bits per Byte) and 1024 (K), then you get around 172.3 KB/s.

Of course, the bitrate changes when converting a compressed audio format back to an uncompressed one. The bitrate for uncompressed formats is easily calculated: 44100 Hz (samples per second) * 16 bits * 2 channels (stereo) = 1411200 bps. The bitrate for compressed formats instead may depend on the desired quality.

I need to ask back: How did you recognise that the wav file shall have only 128 kbps? If you are running Windows 9x, you can quickly check the bitrate and compression format by clicking on the wav file in the Windows Explorer with the right key and select the "Details" tab - I'm not sure if this works in the same easy way under Windows 2000 or XP, but it shall. Furthermore, many wave editors can detect the format when trying to open one.