Unit 6: DSP applications

DSP APPLICATIONS
Mrs Minakshi Pradeep Atre,
PVG’s COET, Pune
Courtesy: DSP by Li Tan

INDEX
1) Digital Crossover Audio Systems: Two-band
Digital Crossover
2) Interference cancellation in ECG
3) Speech Noise Reduction
4) Speech coding and compression
5) Compact Disc (CD) recording system

DIGITAL CROSSOVER AUDIO SYSTEMS:
TWO-BAND DIGITAL CROSSOVER
1st APPLICATION

1. NEED
 Many applications require entire range of
frequencies
 Not possible for single speaker to handle
 Beyond capability of single speaker driver
 So we engineers often combine several drivers
such as speaker cones and horns
 They cover different range of frequencies resulting
in full audio range

2 WHAT ARE HORN SPEAKERS?
 A horn loudspeaker is
a loudspeaker or loudspeaker element which
uses an acoustic horn to increase the overall
efficiency of the driving element(s).
 The horn serves to improve the coupling efficiency
between the speaker driver and the air.

WOOFERS SQUEAKERS AND TWEETERS
5.1 sound system
7.1 sound system

BLOCK SCHEMATIC OF TWO BAND DIGITAL
CROSSOVER

3 DESIGNING TWO-BAND DIGITAL CROSSOVER
SYSTEM
 Two speaker drivers
 Woofers: low frequencies ( 20 Hz to 5kHz)
 Sub-woofer: 20 to 200 Hz
 Tweeters: high frequencies (2kHz to 20 kHz) can go
high till 10kHz
 incoming digital audio signal is split into two bands
by using a low pass filter and a high pass filter in
parallel
 Amplification of separated signals
 Sending them to corresponding speaker drivers
 So the objective is

OBJECTIVE IS TO DESIGN LPF AND HPF
 objective is to design the low pass filter and the
high pass filter so that their combined frequency
response is flat, while keeping transition as
sharp as possible to prevent audio signal
distortion in the transition frequency range

 Although traditional crossover systems are
designed using active circuits (analog systems)
or passive circuits, the digital crossover system
provides a cost-effective solution with
 programmable ability, flexibility, and high
quality.

CROSSOVER SYSTEM SPECIFICATIONS

CHOOSING FILTERS
 In the design of this crossover system, one
possibility is to use an FIR filter, since it provides a
linear phase for the audio system
 However, an infinite impulse response (IIR) filter
can be an alternative.
 Based on the transition band of 800 Hz and the
pass band ripple and stop band attenuation
requirements, the Hamming window is chosen for
both low pass and high pass filters.
 We can determine the number of filter taps as 183,
each with a cutoff frequency of 1,000 Hz.

 frequency responses for the designed lowpass filter
and highpass filter are given in Figure 7.26(a),
 and for the lowpass filter, highpass filter, and
combined responses in Figure 7.26(b)
 The crossover frequency is 1000 Hz for both the
filters

LPF AND HPF : MAGNITUDE RESPONSES
COURTESY: DSP BY LI TAN

60 HZ HUM ELIMINATOR AND HEART
RATE DETECTION USING
ELECTROCARDIOGRAPHY (ECG)
2nd APPLICATION

ECG
 Electrocardiogram
 ECG is a small electrical signal captured from an ECG sensor
 ECG signal is produced by the activity of the human heart,
thus
can be used for
 heart rate detection,
 fetal monitoring,
 and diagnostic purposes
 Unwanted 60 Hz interference in recorded data
 interference comes from
 the power line and
 includes magnetic induction,
 displacement currents in leads or in the body of the patient,
 effects from equipment interconnections, and other imperfections

ALTHOUGH USING PROPER GROUNDING OR
TWISTED PAIRS MINIMIZES SUCH 60-HZ EFFECTS,
ANOTHER EFFECTIVE CHOICE CAN BE
USE OF A DIGITAL NOTCH FILTER, WHICH
ELIMINATES THE 60-HZ INTERFERENCE WHILE
KEEPING ALL THE OTHER USEFUL INFORMATION

HUM NOISE
 Corrupted signal is useless without signal
processing
 It is sufficient to eliminate the 60-Hz hum frequency
with its second and third harmonics in most
practical applications.
 We can complete this by cascading with notch
filters having notch frequencies of 60 Hz, 120 Hz,
and 180 Hz, respectively

BLOCK SCHEMATIC OF HUM ELIMINATOR

ECG WAVE
 Single pulse of the ECG is depicted in Figure
 It’s characterized by five peaks and valleys, labeled P, Q, R, S,
and T.
 Highest positive wave is the R wave.
 Shortly before and after the R wave are negative waves called Q
wave and S wave.
 P wave comes before the Q wave, while the T wave comes after
the S wave.
 Q, R, and S waves together are called the QRS complex.
 Properties of the QRS complex, with its rate of occurrence and
times, highs, and widths, provide information to cardiologists
concerning various pathological conditions of the heart.
 The reciprocal of the time period between R wave peaks (in
milliseconds) multiplied by 60,000 gives the instantaneous heart
rate in beats per minute.
 On a modern ECG monitor, the acquired ECG signal is displayed
for diagnostic purposes.

BLOCK SCHEMATIC OF SIGNAL ENHANCEMENT

 a major source of frequent interference is the
electric-power system
 Such interference appears on the recorded ECG
data due to
 electric-field coupling between the power lines and the
electrocardiograph or patient, which is the cause of the
electrical field surrounding mains power lines
 Another cause is magnetic induction in the power line,
whereby current in the power line generates a magnetic
field around the line.
 Sometimes, the harmonics of 60-Hz hum exist due to
nonlinear sensor and signal amplifier effects.
 If such interference is severe, the recorded ECG data
become useless

ECG ENHANCEMENT FOR HEART RATE
DETECTION
 To significantly reduce 60-Hz interference, we apply
signal enhancement to the ECG recording system
 The 60-Hz hum eliminator removes the 60-Hz
interference and has the capability to reduce its second
harmonic of 120 Hz and its third harmonic of 180 Hz.
 The next objective is to detect the heart rate using the
enhanced ECG signal.
 We need to remove DC drift and to filter muscle noise,
which may occur at approximately 40 Hz or more.
 If we consider the lowest heart rate as 30 beats per
minute, the corresponding frequency is 30/60 = 0.5 Hz.
 Choosing the lower cutoff frequency of 0.25 Hz should
be reasonable

 Thus, a bandpass filter with a passband from 0.25 to 40
Hz (range 0.67– 40 Hz, discussed in Webster [1998]),
either FIR or IIR type, can be designed to reduce such
effects.
 The resultant ECG signal is valid only for the detection
of heart rate.
 Notice that the ECG signal after bandpass filtering with
a passband from 0.25 to 40 Hz is no longer valid for
general ECG applications, since the original ECG signal
occupies the frequency range from 0.01 to 250 Hz
(diagnostic-quality ECG), as discussed in Carr and
Brown (2001) and Webster (1998).
 The enhanced ECG signal from the 60-Hz hum
eliminator can serve for general ECG signal analysis

SUMMARIZING THE DESIGN SPECIFICATIONS FOR
THE HEART RATE DETECTION APPLICATION
 System outputs: Enhanced ECG signal with 60-Hz
elimination
 Processed ECG signal for heart rate detection
 60 Hz eliminator:
 Harmonics to be removed: 60 Hz (fundamental)
120 Hz (second harmonic) 180 Hz (third harmonic)
 3 dB bandwidth for each filter: 4 Hz
 Sampling rate: 600 Hz ( 5 times 120 for child)
 Notch filter type: Second-order IIR
 Design method: Pole-zero placement Band-pass
filter:

 Passband frequency range: 0.25–40 Hz
 Passband ripple: 0.5 dB
 Filter type: Chebyshev fourth order
 Design method: Bilinear transformation
 DSP sampling rate: 600 Hz

CD RECORDING SYSTEM
courtesy: DSP by Li Tan

CD PLAYBACK SYSTEM
courtesy: DSP by Li Tan

CD RECORDING
 A compact-disc (CD) recording system is described
in Figure
 The analog audio signal is sensed from each
microphone and then fed to the anti-aliasing
lowpass filter.
 Each filtered audio signal is sampled at the industry
standard rate of 44.1 kilo-samples per second,
quantized, and coded to 16 bits for each digital
sample in each channel.
 The two channels are further multiplexed and
encoded, and extra bits are added to provide
information such as playing time and track number
for the listener.

 The encoded data bits are modulated for storage,
and more synchronized bits are added for
subsequent recovery of sampling frequency.
 The modulated signal is then applied to control a
laser beam that illuminates the photosensitive layer
of a rotating glass disc.
 When the laser turns on and off, the digital
information is etched onto the photosensitive layer
as a pattern of pits and lands in a spiral track.
 This master disc forms the basis for mass
production of the commercial CD from the
thermoplastic material.

 During playback, as illustrated in Figure, a laser
optically scans the tracks on a CD to produce a
digital signal.
 The digital signal is then demodulated.
 The demodulated signal is further oversampled by
a factor of 4 to acquire a sampling rate of 176.4 kHz
for each channel and is then passed to the 14-bit
DAC unit.
 For the time being, we can consider the
oversampling process as interpolation, that is,
adding three samples between every two original
samples in this case

 After DAC, the analog signal is sent to the anti-
image analog filter, which is a lowpass filter to
smooth the voltage steps from the DAC unit.
 The output from each anti-image filter is fed to its
amplifier and loudspeaker.
 The purpose of the oversampling is to relieve the
higher-filter-order requirement for the anti-image
lowpass filter, making the circuit design much
easier and economical

 Software audio players that play music from CDs,
such as Windows Media Player and RealPlayer,
installed on computer systems, are examples of
DSP applications.
 The audio player has many advanced features,
such as a graphical equalizer, which allows users to
change audio with sound effects such as boosting
low-frequency content or emphasizing high-
frequency content to make music sound more
entertaining

Unit 6: DSP applications

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