PapaJohn Productions
 
Newsletter #113 - July 22, 2006
The Audio Sample Rate of DV-AVI Files - 32kHz or 48kHz?
 

 
There was an interesting exchange of posts about audio issues on the Movie Maker newsgroup about a week ago. They were saying that audio issues result from Movie Maker 'up-sampling' audio to 48kHz. The posters agreed that MM2 does such a poor job of it that you should up-sample with other software before using the source file in a project.
 
I added the essence of the posts to my Problem Solving > Audio Issues page, and it seemed like a good subject for a newsletter. Lots of users including myself have regular or intermittent audio issues. They drive me more and more to saving as high quality WMV files instead of DV-AVI.
                                                                                                      
First Pass by MM2From CamcorderMaybe the posters are right about the up-sampling being the cause... let's take a look. The properties of a typical type I DV-AVI source file from my camcorder, as shown by MM2, has an audio sample rate of 32 kHz... when captured with MM2 or WinDV.
 
Rendering it to a new DV-AVI file in MM2 shows the sample rate of saved movies is 48 kHz. According to the posters, it's this part of the process that introduces significant audio artifacts because Movie Maker doesn't do it very well.
 
VirtualDub Property CheckLet's use this newsletter to study it enough to see if the posters are right... and what we can do about it.
 

 
FLASH 
 
At the 11th hour toward the newsletter issue deadline, I got a clue that the audio sample rate being reported by MM2 in XP and Movie Maker in Vista might be in error, and there's really no up-sampling going on when the camcorder records the audio at 48 kHz.
 
Opening the files with other software such as VirtualDub (the screen shot at the right) shows them as being 48 kHz.
 
It appears to be a reporting issue by Movie Maker, not a resampling, and Microsoft is checking into it...
 

 
As the bottom line isn't conclusive yet, I'm issuing the newsletter with hopes that it raises your awareness of the audio sampling rate.
 
 
... before getting into it further, a few notes...
 

 
Notes...
 
My first for-sale video on Google Video... submitted May 3... still has the status of "Video is verified; stay tuned - it will be live shortly"... it's been over 2 months in the queue to be online.
 
I've stopped checking it's status every day and now taking bets as to whether or not it ever gets to the next step of 'being live'.
 

 
The MaximumPC issue with the Photo Story 3 tutorial is on magazine stands in the US. The 7 page article starts on page 68 in the Special Summer 2006 edition....
 

 
You can automatically open a second video at the end of the first one... this item is too short for a full newsletter issue, but worth a mini-mini tutorial.
 
My latest Video Doodle is a 2-part one. The first part is a 10 second countdown Photo Story that ends by opening another story... 
Doodle
 
Script Command
The first one opens the second by an embedded 'script command'.
 
Use the Windows Media File Editor included with the Windows Encoder software.
 
Open the story file (or WMV movie), go to the frame where you want to embed the command, press the Script Commands button, then the Add button, and enter the full URL. In this case it's
 
http://www.papajohn.org/video/Doodle-Part2.wmv
 
for the 2nd story.
 
You can have it open a website page, a movie, a story, an image... any URL address you usually enter in a browser.
 
With the high quality of photo stories and the average quality of online videos, it might be a neat way to make a 1-2 combo without lowering the quality of the story by re-rendering it as a clip in the movie.
 
It's not for everyone, as a security setting in the Windows Media Player may need tweaking from its default. Details are included at the end of the first story... if the 2nd story doesn't automatically start.
 

 

Vista Corner... I was using it to cross-check the audio sampling rate of captured files for this newsletter. Its reports are consistent with MM2 in XP, but now looking to be consistently wrong. 
 
 
.... back to the main topic...
 

 
The Audio Sample Rate
 
Real world audio waves are analog, not digital. The sample rate is like putting dots on the audio waves to approximate them... like pixels approximates visual images. Put 32,000 dots on the waves each second and you have a sample rate of 32 kHz. 48,000 dots gets you 48 kHz. Can you hear the difference?... probably not. Is it an academic discussion?... you hope... but it might depend on the quality of the process that's used to change from one rate to another.
 
I'll start with my digital camcorder, a Sony TRV80. The manual says the audio recording for 16 bit stereo (my normal setting) is at a 48 kHz sample rate.
 
Captured Files using various apps get you either 32 or 48 kHz, as reported by the properties in Movie Maker 
Rather than having to rip the audio from the type I files and do the up-sampling with another app, maybe using MM1 or WinDV to capture to type II files gets you there easier and with better results. (I said that before finding that the reporting might be in error and the files are actually starting at 48 kHz) 
 
Ripped or Re-rendered files from the captured files - using default settings

 
Test Project
 
I captured a one minute segment of a Christmas concert... the same segment 4 times, using MM1, MM2, and WinDV for each of types I and II. Here's the storyboard view of this simple project.
Test Project
 
I rendered it to new DV-AVI files a number of times to check how the 'upsampling' from the 32 kHz source files makes it versus those that don't need upsampling. After each render I made the project a bit more complex to see if and when the audio starts to run into problems.
Each of the rendered DV-AVI files sounded as good as the original. I couldn't hear any differences between the 32 kHz or the 48 kHz source file segments. I wasn't successful at getting the audio to breakdown, not even a hiccup or short blip. 
 
That kind of supports the issue being one of reporting the sample rate, not one of actual differences...
 

 
Conclusions and Closing
 
I asked the other digital media MVPs and Microsoft for comments about this subject, digested their collective thoughts, and mulled things over for a day as I edited the issue for the final time after finding out about the reporting issue. 
 
My conclusion was heading this way before the 11th hour finding...  
  • My Sony camcorder records 16 bit audio at a 48 kHz sample rate
  • Type II DV-AVI capture processes maintain the 48 kHz rate... but type I captures convert it to 32 kHz
  • Movie Maker 2 and Vista save DV-AVI files at the 48 kHz rate
Maintaining the sample rate at 48 kHz from camcorder tape to captured file to rendered movie seems preferable to going from 48 to 32 and then back to 48 kHz... because resampling processes can reduce quality or introduce audio artifacts.
 
I'll let you know next week or future issues what else is found.... now we're heading off to the Ann Arbor Art Fair to look at pictures and take some video... have a great weekend.
 

Have a great week...
 
PapaJohn