Introduction to Digital Signal Processing - Digital Signal Processing: Fundamentals and Applications - page 9

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 on 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 1.10 B, 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, as we shall
see in Chapter 12. 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 (Ambardar, 1999).
Software audio players installed on computer systems that play music from CDs, such as Windows
Media Player and RealPlayer, are examples of DSP applications. These audio players often have many
advanced features, such as graphical equalizers, which allow users to change audio through techniques
such as boosting low-frequency content or emphasizing high-frequency content (Ambardar, 1999;
Embree, 1995; Ifeachor and Jervis, 2002).
1.3.5 Vibration Signature Analysis for Defective Gear Teeth
Gearboxes are widely used in industry and vehicles. During their extended service lifetimes, the gear
teeth will inevitably be worn, chipped, or go missing. Hence, with DSP techniques, effective diag-
nostic methods can be developed to detect and monitor the defective gear teeth in order to enhance the
reliability of the entire machine before any unexpected catastrophic events occur. Figure 1.11 (a)
shows the gearbox; two straight bevel gears with a transmission ratio of 1.5:1 inside the gearbox are
shown in Figure 1.11 ( b). The number of teeth on the pinion is 18. The gearbox input shaft is connected
a sheave and driven by a “V” belt drive. The vibration data can be collected by a triaxial accelerometer
installed on the top of the gearbox, as shown in Figure 1.11 ( c). The data acquisition system uses a
sampling rate of 12.8 kHz. Figure 1.11 ( d) shows that a pinion has a missing tooth. During the test,
the motor speed is set to 1,000 RPM (revolutions per minute) so the meshing frequency is determined as
f m ¼ 1000 ð RPM Þ 18 = 60 ¼ 300 Hz and input shaft frequency is f i ¼ 1000 ð RPM Þ= 60 ¼ 16 : 17 Hz.
The baseline signal and spectrum (excellent condition) from the x -direction of the accelerometer
are displayed in Figure 1.12 , where we can see that the spectrum contains the meshing frequency
component of 300 Hz and a sideband frequency component of 283.33 (300 16.67) Hz.
Figure 1.13 shows the vibration signature for the damaged pinion in Figure 1.11 (d). For the
damaged pinion, the sidebands ( f m f i , f m 2 f i . ) become dominant. Hence, the vibration failure
signature is identified. More details can be found in Randall (2011).
1.3.6 Digital Photo Image Enhancement
Digital image enhancement is another example of signal processing in two dimensions. Figure 1.14 (a)
shows a picture of an outdoor scene taken by a digital camera on a cloudy day. Due to the weather