I’ll bet you dollars to donuts that the audio is uncompressed.
Open the video in VirtualDub and go to File >> File Information…
The File Information dialog will tell you the sampling rate, audio compression, and also the total size of the audio portion of the AVI.
MP3 is a very common audio compression for DivX. The tricky part is getting an MP3 compressor that works with VirtualDub. If you don’t see an option in VirtualDub for MP3 encoding in VirtualDub then you’ll need to install a version of LAME that includes LameACM. VirtualDub uses the ACM (Audio Compression Manager) to encode the audio so you need an MP3 codec that works with the ACM to be able to encode MP3 in VirtualDub.
Not all distributions of LAME include the LameACM. http://www.doom9.org/ has a version of LAME with LameACM. Go to download >> audio tools. Here is a direct link to the download. <http://www.doom9.org/Soft21/Audio/lame-3.93.1.zip>
Extract the zip file. Right-click on the LameACM.inf file and select Install. That will install the LameACM codec.
Now we’re ready to compress audio in VirtualDub.
In VirtualDub go to the Audio menu and select Full processing mode. Full processing mode lets us play with the audio compression features.
Compressing to MP3 can be a little tricky because the MP3 encoder only works with certain sampling rates and bit precision settings. If your audio isn’t at the right sampling rate and bit precision then you won’t see any selections in the “Select audio compression” dialog when you select Lame MP3. A sampling rate of 44100Hz and a precision of 16-bit will give you the most options for MP3 encoding.
First we need to make sure the audio is at 44100Hz and 16-bit.
Audio >> Conversion…
This will display the Audio Conversion dialog
If the audio is not already at 44100Hz then change the sampling rate to 44100Hz and select the High quality checkbox option.
If the Precision is not at 16-bit then change it to 16-bit.
The Channels can be either mono or stereo
Click OK when done
Now we can compress
Audio >> Compression…
This will display the “Select audio compression” dialog
Select Lame MP3
You should now see some options in the right panel of the dialog for different bitrates that you can encode at. If you don’t see anything in the right panel then your sampling rate and precision rate are not compatible with Lame MP3 and you’ll need to change them. That’s why we changed to 44100Hz and 16-bit in the previous step.
Pick a bitrate for encoding
Click OK when done
Some notes on picking settings:
Lame MP3 can encode at sampling rates other than 44100Hz, but it needs a precision of 16-bit. It can’t handle 8-bit audio. In the “Select audio compression” dialog you can select the “Show all formats” checkbox to see all the sampling rates that Lame can handle.
Lame can do ABR (Adaptive BitRate) and CBR (Constant BitRate). ABR encodes with a variable bitrate with higher bitrates for more complex parts of the audio and lower bitrates for less complex parts of the audio. Even though ABR will give you better compression it is better to use CBR for video. Using ABR can cause problems with audio and video sync with the audio getting out of sync with the video. Going with a constant bitrate makes it easier on the video player to keep the audio and video in sync. There are also some video players that don’t play ABR audio very well.
When using DivX with MP3 audio it works better to use two pass encoding with DivX. The two pass encoding results in better audio video sync so that the audio doesn’t drift out of sync with the video.
If you’re going to be doing a lot of video stuff the Pro version of DivX will give you more options and better compression. DivX just recently reduced the price of the Pro version to $19.99 USD. It’s a good deal if you use it enough. Otherwise the free basic version does well enough.
