Uncompressed audio quality is usually dependent on
the samplerate and bit depth that it was sampled with. Lets me first
explain what these are:
When you record sound into the PC it is broken up
into a series of slices. These can be likened to the framerate of
a video file, each moment in time is frozen and the information
about it is recorded. In the same way, each audio slice represents
a number that defines how high or low the audio is at that moment
in time. These slices are called samples and the number of samples
per second is measured in Hertz (Hz).
A 'Bit' is a standard storage size used by your computer
just like kilobytes and megabytes, it is actually the smallest unit
of storage. Each bit can define itself as on or off (1 or 0). Eight
bits make a byte, 1024 bytes make a kilobyte (1K) and 1024K make
a megabyte (1 MB).
Going back to our video analogy, if the Samplerate
is how many "frames" the audio has, then the bit depth is how large
the frames are, the resolution or the amount of memory allocated
to each frame. The large the bit depth the more possible amplitudes
each audio 'slice' can have. Eight bit audio uses 256 amplitudes,
16-bit allows 65536 and 24-bit can have 16 Million!
Flask Mpeg and Samplerates
Firstly, it means that the higher the samplerate and
the higher the bit depth the higher the quality audio you will get.
It also means it will take much more storage space too! 44100Hz
16-Bit represents CD quality audio and 48000Hz is actually even
higher than CD quality. This means to get the best quality audio
you should also aim for the highest samplerate and bit depth your
sound card can handle. But more seriously the analogy to framerate
and samplerate is almost perfect because just as a higher framerate
will produce a smaller video length a higher samplerate will produce
a smaller audio length!!
I know it sounds strange because if you record an
hours audio at 44100Hz or at 48000Hz its should still last an hour!
And in fact it usually does because it was recorded that way. But
when you start converting samplerates that is a completely different
matter. Unless the converter takes into account this framerate change
there will by audio synch problems.
This is a very common reason Flask Mpeg will give
audio synch problems. If you convert a DVD 48000Hz audio down into
441000Hz will very often result in audio synch problems. If you
find the audio has been recorded at the wrong samplerate with Flask
Mpeg it may fix it if you convert it back to the original DVD samplerate
i.e. 48000Hz. Theoretically, the same film of 60 mins recorded both
separately at 44100Hz and 48000Hz will play for exactly 60 minuets.
But when it comes to ‘downsampling’ i.e. converting 48000Hz down
to 441000Hz, then problems arise because many programs do not seem
to keep the file length the same! Solution? Grab a DVD audio at
48000Hz 16 Bit only! You may then use VirtualDub to convert the
audio down to 44100Hz after because VirtualDub is
able to keep the audio length perfect!
Fixing that Dull Sound
Because many sound cards cannot handle 48000 Hz very well you may
find that any method you use to extract the sound will give a fairly
dull sound output. This can be prevented by converting the audio
to 44100 Hz in VirtualDub. This can be done at the same time that
you multiplex the audio and video files together or after when the
movie is finished. I'm assuming you are fixing a finished movie
here. So open VirtualDub as usual by going to File
> Open video file...
We don't want to effect the video itself so we use Video
> Direct stream copy. Whenever we select an option, VirtualDub
adds a black dot by it to tell us it is selected.
Then select Audio > Full processing mode.
Choose Audio > Conversion...
Choose 44100Hz and make sure the 'High
quality' is selected, and press OK.
Fixing Crackly Sound!
Actually, cracks pop's and hiss are usually a result of badly decoded
sound and cannot be fixed other than getting the sound all over
again! Ac3Dec is usually the culprit, and since Flask Mpeg uses
the same decoding engine as Ac3Dec then it is possible that it will
also get some crackly sound.
To be fair I think perhaps Ac3Dec has improved since it was first
made, so you may not need to worry. But nevertheless, if you do
get bad audio from it I suggest you try another method. The best
way I have found is to get the audio with GraphEdit, the quality
is much better in my option. Check out my 'Audio
with Graphedit' section for details on this. Then follow
the 'multiplexing' guide to merge it to the video file.
Fixing Very Quiet Sound!
As you would expect, DVD audio has a much higher Dynamic Range
than normal TV or Video. This means that it can go from mind-numbingly
loud to so quiet that you cannot hear anything! This is fine for
our DVD systems but usually a PC will not be able to give enough
range. So what you end up with is a soundtrack that is mostly quiet
with some very loud bits in it! Software DVD players such as WinDVD
have a built in 'Dynamic Normalizer' which usually makes sure the
audio is loud enough to hear at all times.
Since you probably used Flask Mpeg or Ac3Dec to create your audio
it may be the case that your audio is just too quiet to hear. Again,
the best way (in my opinion) to fix this is to get the audio again
using GraphEdit and the WinDVD audio filters. Of course, if the
audio is badly quiet this still may not be enough! The alternative
(slower way) is to normalize the sound. Normalizing is a way to
make the sound as loud as it will go without clipping. Clipping
is when the audio goes off the scale and looses parts of its information.
Clipping would cause pops, clicks and other annoying glitches that
we obviously don't want. So normalizing is a much better method
The old method I suggested was to use Cool Edit or SoundForge or
a similar audio editing software to normalize because they have
this option built in. But there is a more effective way and it doesn't
require you buy such expensive software like Cool Edit either =).
There is a really good free tool called DeDynamic. It does standard
normalization, but it is also able to perform dynamic normalization!
What this means is instead of just normalizing the whole file, it
normalizes and also adjusts the quiet to loudness ratios. So a loud
sound will remain just as loud but a normally quiet sound will be
made louder, cool huh! Whatever method you use, the only problem
with fixing the audio by normalizing is that it takes anything upto
about 4GB of hard drive space to do for a whole movie!
Step 1. Extracting the Audio
I'm assuming you used Flask Mpeg to create your movie so before
we go any further we need to extract the audio from your finished
movie. Open the movie in VirtualDub:
Choose File > Save Wav...
Don't worry about size yet, it shouldn't be larger
than about 100MB; in fact, its usually only about 60MB. VirtualDub
doesn't change the audio to uncompressed PCM audio, it literally
takes out whatever is in there. So if you used Mp3 that is exactly
what it will extract...although it will still call it a something.wav
Step 2. Converting it to a PCM Wave
Now we must convert it to uncompressed PCM. For this
we can use Winamp because everyone knows how to use Winamp and its
free of course ;^). Open your Wav file and press the stop button
to stop it playing. Right-click and choose:
Options > Preferences...
Choose the Audio I/O tab. Now instead of the normal
OUT_WAVE.DLL option, choose OUT_DISK.DLL. Press 'Apply' and Winamp
will ask where you wish to save the file.
Choose a location and wait until the audio is converted
into a huge one and a half Gigabyte wave file :(.
Step 3. Normalizing
The bad news is DeDynamic is a DOS operated tool.
The good news is I'm gonna show ya how to use it :). Make a folder
on your main hard drive (usually C:) and call it something simple
like dynamic. Cut an paste the gigabyte Wave file you just created
with Winamp into it (if you didn't already extract it to there).
Also put the DeDynamic.exe file in
there as well, so you will have something like this in your dynamic
Great! Now go to Start Menu
> Run and type Command.com
to bring the DOS Prompt up i.e.:
Type: CD\ and press Enter.
Type: CD dynamic and press Enter. Now,
cut and paste the following line into the DOS prompt:
(I have encircled the DOS paste button in red in
the picture below).
Obviously the text will not be those colours, I'm just using colours
so you can see what the line you pasted means. The -a
6 (highlighted in green text) is the amount of dynamic
normalization. You can probably quite happily use anything from
a setting of 1 to 50 without any problems. If, for example, you
wanted to dynamically normalize by 50 you'd put -a
50 where the -a 6
was in my above line. If you just want to normalize without the
extra dynamics just put -a 0.
Using an amount of 50 is severe overkill and it wasn't really designed
to do that much. But I've tried it at 50 and above and it sounds
quite good, very loud and there is no audio corruption either. But
this is a very powerful program, so I would suggest you try a setting
of between 4 and 6 and see how that goes. If you still want it louder
then you can keep going up until you are happy with it :).
Anyhoo, when you've decided on the setting, paste it, press the
'Enter' key and it will ask what output you'd like it converted
into with the following box:
I would choose the same as the input. So if you had 48000Hz audio
originally (all DVDs use this format), then I would choose it again.
You can choose another format such as MP3 if you want but this may
cause synch troubles and/or difficulty on merging the audio back
to the video.
In the end we should end up with another huge gigabyte file, but
this time it will be normalized to a really loud kick ass level!
That's it! Now go to my 'multiplexing'
guide to see how we use VirtualDub to recombine the new audio to
the old video file =o).
Duplication of links or content is strictly
prohibited. (C) NICKY PAGE 2000