Telephonic communications – Centralized switching system – Switching controlled in response to called station...
Reexamination Certificate
1999-09-28
2003-05-06
Tieu, Benny Q. (Department: 2642)
Telephonic communications
Centralized switching system
Switching controlled in response to called station...
C379S386000
Reexamination Certificate
active
06560331
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The field of the invention relates to communications over telephone lines using dual-tone multiple frequency (DTMF) signaling. Specifically, the invention relates to the detection of DTMF signaling by a modem using a DTMF decoder that applies a non-uniform sample index by including compensation for the phase error introduced by the ITU standards, thereby reducing signal leakage.
2. Prior State of the Art
Dual-tone multiple frequency (DTMF) signaling, increasingly being deployed worldwide with push-button telephone sets, offers a high dialing speed compared with the dial-pulse signaling used in conventional rotary telephone sets. DTMF signaling is also used in applications requiring interactive control such as in voice mail, phone messaging, e-mail, telephone dialing, voice mail, and telephonic banking systems. In addition to signaling, tone detection is also used in line probing techniques to estimate the quality of phone lines. The DTMF signal standard, initially developed by Bellcore, was redefined in 1989 by the International Telecommunication Union (ITU). Since the Bellcore DTMF standard is a subset of the ITU standard, an ITU-compliant DTMF device must also be Bellcore compliant. To be commercially viable, all modems presently offered for sale must include DTMF signaling generation and detection functionality.
A DTMF signal consists of a sum of two tones with frequencies taken from two mutually exclusive groups of preassigned frequencies. Although alternative frequencies may be detected using the same methods employed by DTMF detectors, modern applications are optimized for detecting the frequencies of two tones from the internationally accepted ITU standard frequencies. The mutually exclusive groups of preassigned ITU frequencies consist of four low frequency tones and four high frequency tones. The four low frequency tones are 697 Hz, 770 Hz, 852 Hz, and 941 Hz. The four high frequency tones are 1,209 Hz, 1,336 Hz, 1,477 Hz, and 1,633 Hz. Each pair of tones, consisting of a low frequency tone and a high frequency tone, correspond to a unique number or symbol, one of sixteen push-button digits (0-9, A-D, #, *). The four alphanumeric keys (A-D) are not yet available on standard telephone handsets and are reserved for future use. Since the DTMF signaling frequencies are all located in the frequency band used for speech transmission, DTMF signaling systems are considered in-band.
The digital generation of DTMF signals is accomplished by adding two finite duration digital sinusoidal sequences. Table 1 demonstrates how the four low frequency tones and four high frequency tones are combined in a DTMF signal to create sixteen touch-tone digits consisting of numbers, symbols, and letters.
TABLE 1
Low f
High f (Hz)
(Hz)
1209
1336
1477
1633
697
1
2
3
A (ASCII 65)
770
4
5
6
B (ASCII 66)
852
7
8
9
C (ASCII 67)
941
*
0
#
D (ASCII 68)
To be commercially viable, DTMF decoders are subject to the constraints created by the ITU recommendations concerning frequency resolution, time duration, and signal power. Under the ITU recommendations a detected frequency must be within 3.5% of the expected frequency or be rejected as a DTMF tone. The guidelines also require that a qualified detected frequency within 1.5% of the target frequency register as a DTMF tone. According to the ITU recommendations, a DTMF signal of less than 23 ms should be rejected, while a signal duration of 40 ms or more should be accepted. Signals between 23 ms and 40 ms can either be accepted or rejected. Signal strength is measured by a Signal-to-Noise ratio and signal power. A detected signal must have at least a 15 dB Signal-to-Noise ratio before it can be considered. The detected signal must also have a signal power of at least −26 dBm. The ITU recommendations are shown in table 2 below.
TABLE 2
ITU Recommendations
Signal Frequencies
low group
697, 770, 852, 941 Hz
high group
1209, 1336, 1477, 1633 HZ
Frequency
operation
≦1.5% of signal frequencies
Tolerance
non-operation
≧3.5% of signal frequencies
Signal Duration
operation
40 ms
minimum
non-operation
23 ms
maximum
Signal Exception
pause
40 ms
maximum
interruption
10 ms
minimum
Twist
forward
8 dB
reverse
4 dB
Signal Strength
S/N
15 dB
minimum
power
−26 dBm
minimum
These ITU recommendations place stringent constraints on DTMF detection performance in both the time and frequency domains and are not always satisfied by conventional DTMF decoders relying on a standard DFT.
Decoding a DTMF signal involves identifying two tones in the sampled signal and distinguishing the two tones from signal noise, human voice signals, and other signal interference. Although a number of chips with analog circuitry are available for the generation and detection of DTMF signals in a single channel, these function can also be implemented digitally on DSP chips. The digital implementations surpass analog approaches both in cost and performance. The digital DSP based tone detection can be performed by computing the discrete Fourier transform (DFT) of the DTMF signal and then measuring the energy present at the eight fundamental DTMF tones and the eight associated second harmonic frequencies. The second harmonic energy measurement is made to distinguish DTMF signals from human voices. In general, the spectrumn of the human voice contains energy components at the second harmonics, while the DTMF contains negligible energy at the second harmonics. Thus, if energy is present at both the DTMF fundamental tone and the second harmonic then the signal is probably not a DTMF signal.
A traditional DTMF decoder computes the DFT samples closest in frequency to the eight fundamental frequencies in the ITU standard for DTMF. Most of the approaches in the prior art are based on the DFT of equation 1.
X
(
k
)
=
∑
n
=
0
N
-
1
x
(
n
)
ⅇ
-
j
2
π
kn
/
N
(
1
)
Given a sequence of N samples, the DFT uniformly samples the discrete-time Fourier transform of the sequence at N evenly spaced frequencies,
ω
=
2
π
k
N
where k, the frequency bin index, is equal to 0, . . . , N-1. Making the width (resolution) of each frequency bin equal to 2&pgr;/N. Each frequency bin is centered at an integer multiple of 2&pgr;/N, these uniform blocks do not correspond exactly to the eight fundamental DTMF frequencies, nor do they correspond to the eight associated second harmonic frequencies. This means no single value of N can meet all of the ITU frequency resolution recommendations. AT&T states that N=205 is the best value of N at a 8000 Hz sampling rate to detect the eight fundamental DTMF tones. A common method used to detect DTMF signaling assumes that the DTMF decoder does not need all samples X(k), since only eight frequencies are initially relevant. Since only a small subset of samples is required a fast Fourier Transform (FFT) algorithm is not efficient to use for DTMF detection. It is well known to those skilled in the art that Goertzel's algorithm is a more efficient and effective algorithm when only a small subset (8 fundamental tones and 8 harmonic tones) of samples X(k) are required. Numerous DTMF decoders in the prior art are based on using a DFT employing Goertzel's algorithm. In one previous embodiment the DTMF decoder used two banks of eight filters, one bank using Goertzel's algorithm for the fundamental tones and the other bank for the harmonics. This enables the device to avoid computing all N DFT coefficients for each fundamental DTMF frequency. The Goertzel filter is typically implemented as a second order infinite impulse response (IIR) band pass filter. The Goertzel filter requires 2N real multiplication/addition operations by the DSP. Other implementations employ the use of a non-uniform DFT (NDFT) to detect energy at fundamental DTMF frequencies. By setting k to yield an exact DTMF frequency of interest, i.e. k=N f
i
/f
s
where f
s
is the sampling rate. This approach in effect creates sliding windows for the
Cooklev Todor
Gray Mark
3Com Corporation
Tieu Benny Q.
Workman & Nydegger & Seeley
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