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Three Easy Steps to Create
Your Own Notch Filter



for the Agilent U8903A audio analyzer
using the Agilent VEE software
Application Note




Introduction
This application note assists you in
creating your own customized notch
filter for your audio analysis by using
a VEE program. The VEE program will
help you to create an FIR digital filter
to be used in your U8903A. You can
have the option of installing the notch
filter permanently into your U8903A
nonvolatile memory or of recalling
the filter when it is needed from a PC
through IO connectivity.
Back to Basics
What is an audio filter? Digital or analog filter?
Any medium that an audio signal An analog filter is any filter that operates on continuous-time signals. In other
passes through, whatever its form, respects, it is just like a digital filter. Analog filters were used successfully for
can be regarded as a filter. Filters are decades before digital filters come along. Although analog filters are excellent
usually used to discriminate between in some aspects, especially in cost, they do have some serious drawbacks. One
a frequency or a band of frequencies of the drawbacks of analog filters is the non-linear phase characteristics. This
from a given signal, which is normally does not affect most of the common applications, but it is heavily involved in
a mixture of both desired and applications like telecommunications and voice processing. Another drawback
undesired signals. The undesired in analog filters is the lack of sharpness at the cut-off frequency. It is possible
portion of the signal commonly comes to increase roll-off rate by cascading filter stages, but this will increase the cost
from noise sources such as power and complexity of the system. On the other hand, it is possible to achieve all
line hum, which are not required for a these characteristics fairly by using a digital filter.
particular application.
A digital filter is just a filter that operates on digital signals, such as sound
However, we do not usually think represented inside a computer. It is a computation, which takes one sequence
of something as a filter unless it of numbers (the input signal) and produces a new sequence of numbers (the
can modify sound in some way. filtered output signal). The filters mentioned are not for digital signals only as
For example, a speaker wire is not they can be operated on signals that are not digital as well. It is important to
considered a filter, but the speaker realize that a digital filter can do anything that a real-world filter can do. That
is. The tone control circuit in an is, all the filters alluded to above can be simulated to an arbitrary degree of
ordinary car radio is a filter, as are precision digitally.
the bass, midrange, and treble boosts
in a stereo preamplifier. Graphic Similar to any other digital signal processing system, the implementation of the
equalizers, echo devices, phase digital filter also requires an ADC, DAC, and a processor. A simple system with
shifters, and speaker crossover these devices are shown below.
networks are further examples of
Digital signal
useful filters in audio. There are also analog input ADC DAC analog output
processor
examples of undesirable filtering,
such as the uneven reinforcement of
certain frequencies in a room with Here the filtering action is accomplished through the software running in the
"bad acoustics." digital signal processor. The algorithms used for this purpose are known as
digital filter algorithms. This algorithm takes an input from the ADC, calculates
the output, and sends it to the DAC. There are mainly two types of filter
algorithms: