Image Processing Reference
In-Depth Information
In the second waveform, the decoder curve fits the output to the samples it has been
given. In this case the period of the samples and the frequency of the audio being sampled
yield a waveform reconstruction that is nothing like the original waveform. It fits the sam-
ples but it is not the desired waveform.
This is the case that is argued for sampling at a far higher rate than Nyquist sug-
gested, and it is why audio sample rates go as high as 192 KHz. It is necessary to accu-
rately capture the highest audible waveform without this aliasing artifact. In fact any
aliasing that does happen at that sample rate is far above the audible range anyway, so it
will not matter.
CD Sample Rate (44.1 KHz) Versus DVD Sample Rate (48 KHz)
Quantization problems may rear their ugly head when you are converting between CD
sample rates (44.1 KHz) and DVD sample rates (48 KHz).
This is another example of when you should know something about the target out-
put platform before you commence your processing. If the video is being encoded for a
DVD product, you should ensure that your sample rate is 48 KHz (or a multiple thereof)
when you ingest the audio. It may also be necessary to check settings in all the tools you
use to ensure that the correct sample rate is preserved.
It is unlikely that the sample rate would get changed but it is possible if you flip a
switch the wrong way inadvertently.
Digital Filtering Versus Analog Filtering
You may want to filter the audio as part of the preparation for encoding. Here you have
a choice of doing some processing either before it is converted into a digital form or
The analog processing is going to be permanent and not easily altered after going
through the digitizing process, so you may want to defer any processing until after you
have the audio safely in a file on the hard disk and then use non-destructive DSP -based
Analog processing is often said to have a certain warmth and personality while the
digital processing is more harsh and cold. This may have been true of older processing
solutions, but these days the analog world is very well simulated using digital-filtering
algorithms that are very sophisticated indeed.
On the other hand, the analog hardware you have in your studio may yield better-
quality audio than the available plug-ins on your computer. Your mileage may vary of
course. The main advantage of applying the corrective filtering in the digital domain is
that the process can be automated and undone. Some tools support key-framed control
points. The processing is non-destructive.
In the end, it is likely to be a trade off between quality and convenience.
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