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626 Chapter 12 APPLICATIONS IN COMMUNICATIONS
bits assigned to the model parameters per frame is 72. If the model parameters
are changed every 20 µsec. the resulting bit rate is 3, 600 bps.
Since the reflection coefficients are usually transmitted to the receiver, the
synthesis filter at the receiver is implemented as an all-pole lattice filter,
described in Chapter 6.
12.5.1 PROJECT 12.5: LPC
The objective of this project is to analyze a speech signal through an
LPC coder and then to synthesize it through the corresponding PLC
decoder. Use several .wav sound files (sampled at 8000 sam/sec rate),
which are available in MATLAB for this purpose. Divide speech signals
into short-time segments (with lengths between 120 and 150 samples) and
process each segment to determine the proper excitation function (voiced
or unvoiced), the pitch period for voiced speech, the coefficients {a p (k)}
(p ≤ 10), and the gain G. The decoder that performs the synthesis is an
all-pole lattice filter whose parameters are the reflection coefficients that
can be determined from {a p (k)}. The output of this project is a synthetic
speech signal that can be compared with the original speech signal.
The distortion effects due to LPC analysis/synthesis may be assessed
qualitatively.
12.6 DUAL-TONE MULTIFREQUENCY (DTMF) SIGNALS
DTMF is the generic name for push-button telephone signaling that is
equivalent to the Touch Tone system in use within the Bell System. DTMF
also finds widespread use in electronic mail systems and telephone banking
systems in which the user can select options from a menu by sending
DTMF signals from a telephone.
In a DTMF signaling system a combination of a high-frequency tone
and a low-frequency tone represent a specific digit or the characters *
and #. The eight frequencies are arranged as shown in Figure 12.14, to
accommodate a total of 16 characters, 12 of which are assigned as shown,
while the other four are reserved for future use.
DTMF signals are easily generated in software and detected by means
of digital filters, also implemented in software, that are tuned to the 8 frequency
tones. Usually, DTMF signals are interfaced to the analog world
via a codec (coder/decoder) chip or by linear A/D and D/A converters.
Codec chips contain all the necessary A/D and D/A, sampling, and filtering
circuitry for a bidirectional analog/digital interface.
The DTMF tones may be generated either mathematically or from
alook-up table. In a hardware implementation (e.g., in a digital signal
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