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| Portable computer having latching hooks |
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| Data set network diagnostic system |
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| Recording medium of flat surface with resin filled ring grove |
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| Analog subscriber carrier system repeater with automatic gain and slope correction |
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| Plasma panel display selectively updatable on pel line basis |
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| Display device |
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| Multiport memory device |
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Convolution filter arrangement for digital multifrequency receiver
| Details |
Inventors: Metz, Reinhard;
Assignee: Bell Telephone Laboratories, Incorporated (Murray Hill, NJ)
Primary Examiner: Brigance; Gerald L.
Assistant Examiner:
Attorney, Agent or Firm: Padden; F. W.
A digital multifrequency (MF) receiver for use in telephone switching systems is disclosed. Input MF signals in a PCM (pulse code modulation) format are detected by simultaneously computing the convolution of the digital input signal with discrete portions of the impulse response of a narrow bandpass filter. The convolution functions are performed by multiplication or log addition circuitry with adder and accumulator apparatus. A 10-millisecond PCM sample comprising 80 discrete samples of an input signal is simultaneously applied to four convolution circuits, each of which applies a different portion of the impulse response of a bandpass filter, also comprising 80 samples, to the input signal as it is received. The complete impulse response signals are applied to the convolution circuits but out-of-phase with impulse response signals as applied to the remainder of the convolution circuits. When four such circuits are used, a quadrature is established and the amplitude of the desired signal is computed at 2.5 millisecond intervals by computing the square root of the sum of the squares of adjacent convolution circuits. |
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DETAILED DESCRIPTION In the illustrative digital switching system of FIG.
1, a digital switching network 10 operates under control of a program controlled facility 11 and provides for the switching of digitally encoded signals among the communication channels 12 and 13 via terminal interface circuits 14 and 15 and other circuits such as the service circuits 16.
Signaling from a distant telephone office is accomplished over one of communication channels 12 or 13 and is received in the switching system of FIG.
1 via an interface circuit 14 or 15 and the switching network 10 in an MF digital receiver 17.
The receiver comprises the exemplary convolution circuitry of this invention.
Receiver 17 receives input MF signals illustratively in PCM format and the different MF signals are detected as later explained by the convolution of the digital input signals with variably delayed coefficient signals representing discrete portions of the impulse response of a narrow bandpass filter and supplied by a controller and memory arrangement 18 under control of facility 11.
Receiver 17 detects and interprets the received MF signal and supplies them to the facility 11 to complete the call processing operations.
Multifrequency signaling consists of the transmission and reception of two out of a possible six tones, which are 700, 900, 1100, 1300, 1500 and 1700 Hertz.
Expressed in another way, an MF tone is of the form 500+200i Hertz where i=1,2,3,4,5 and 6.
For practical applications there are a number of constraints and specifications on the level and duration of the tone pairs.
Each pair of tones of the possible 15 combinations indicates a particular number or symbol.
The MF tone detection process thus consists of determining whether there are spectral components at the MF frequencies, and performing subsequent timing and level tests.
Convolution Filtering Spectrally, the six MF tones can be thought of as impulses on a frequency axis, as shown in FIG.
2.
In practice, a narrow bandpass filter is used at each of the frequencies, such that each filter gives an output if and only if a spectral component is present in its bandpass
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I/O Systems 
