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In practice, different parts of a DSP system are often designed
to operate at different sampling
rates because of the advantages it offers. Since real-time
digital filters must complete all
algorithmic operations in one sampling interval, a smaller
sampling interval (i.e. a highersampling rate) can impose an added
computational burden during the digital processing stage. It
is for this reason that the sampling rate is often reduced (by
decimation) before performing DSP
operations and increased (by interpolation) before
reconstruction. This leads to the concept ofmultirate signal
processing where different subsystems operate at different sampling
rates best
suited for the given task.
Digital Signal Processing Applications
Area DSP algorithm
General-purpose Filtering and convolution, adaptive filtering,
detection and correlation,
spectral estimation and Fourier transform
Speech processing Coding and decoding, encryption and
decryption, speech recognitionand synthesis, speaker
identification, echo cancellation, cochlea-implant
signal processing
Audio processing hi-fi encoding and decoding, noise
cancellation, audio equalization,
ambient acoustics emulation, audio mixingand editing, sound
synthesis
Image processing Compression and decompression, rotation, image
transmission anddecompositioning, image recognition, image
enhancement, retina-
implant signal processing
Information systems Voice mail, facsimile (fax), modems,
cellular telephones,
modulators/demodulators, line equalizers, data encryption
anddecryption, digital communications and LANs,
spread-spectrumtechnology, wireless LANs, radio and television,
biomedical signal
processing
Control Servo control, disk control, printer control, engine
control, guidance and
navigation, vibration control, power system monitors, robots
Instrumentation Beamforming, waveform generation, transient
analysis, steady-state
analysis, scientific instrumentation, radar and sonar
There are many applications where the signal at a given sampling
rate needs to be converted into
another signal with a different sampling rate.
Example1: In digital audio, three different sampling rates are
presently employed: 32 KHz in broadcasting, 44.1 KHz in
digital CD, and 48 KHz in digital audio tape (DAT) and other
applications. Conversion of sampling rates of audio signals
among these three different rates is
necessary in many situations.
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Example 2: In Video applications, the sampling rates of NTSC
(National Television Systems
Committee) and PAL (Phase Alternate Line) composite video
signals are, respectively,
14.3181818 MHz and 17.734475 MHz, whereas the sampling rates of
the digital componentvideo signal are 13.5 MHz and 6.75 MHz for the
luminance and the color difference signals,
respectively.
Example 3: DTMF Receivers
A touch-tone phone or dual-tone multi-frequency (DTMF)
transmitter/receiver makes use of
digital oscillators to generate audible tones by pressing
buttons on a keypad. Pressing a button produces a two-tone
signal containing a high- and a low-frequency tone. Each button
is
associated with a unique pair of low-frequency and
high-frequency tones. For example, pressing
the button marked 5 would generate a combination of 770-Hz and
1336-Hz tones. There are fourlow frequencies and four high
frequencies. The low- and high-frequency groups have been
chosen to ensure that the paired combinations do not interfere
with speech. The highest
frequency (1633 Hz) is not currently in commercial use. The
tones can be generated by using a
parallel combination of two programmable digital
oscillators.
Programmable filters are used to adjust the frequency-selective
properties of the filters. Multirate
filters are used in the processing of many complex signals with
different rates of fluctuation,
whereas two dimensional digital filters are the filters used in
image processing. Adaptive filters
are used invariably when the transmission medium between the
transmitter and receiver changes.
For example, when a telephone conversation is switched from one
point to another and the cable
or the microwave link changes, or when the mobile phone moves as
the talker moves over a wide
territory, adaptive filters are absolutely necessary to
compensate for the distortion of the signal as
it passes through the transmission link.
Sampling Rate Alteration in Different Applications
S.No. Area Application Frequency
Range
Sampling Rate
Alteration
1. Video format
conversion
Film (24 frames/sec) to
Television (NTSC format-30 frames/sec)
24 frames/sec to
30 frames/sec
30/24
5/4 Rate Change
2. Speech compression Speech
on DAT (48 KHz) toSpeech forTelephone (8 KHz)
48 KHz to 8
KHz
8/48
1/6 Rate Change
3. Conversion betweenaudio formats (CD-
DAT)
CompactDisc (44.1 KHz) to Digital
Audio Tape (48 KHz)
44.1 KHz to 48KHz
48.0/44.1
48.0/44.1 RateChange
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