Browse Prior Art Database

Table Driven Dual Tone Multi-frequency Receiver

IP.com Disclosure Number: IPCOM000107978D
Original Publication Date: 1992-Apr-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 6 page(s) / 210K

Publishing Venue

IBM

Related People

Nelson, RS: AUTHOR

Abstract

A Dual-Tone Multi-Frequency (DTMF) receiver with the following attributes is disclosed: - The input is blocks of digitized audio consistent with standard telecommunications sampling rates (8000 per second) and encodings (8 bits per sample, mu-law or A-law). - Computational complexity is within the capability of a general- purpose microprocessor being shared with other communication channels and/or other functions. - Energy at DTMF frequencies is evaluated using a non-recursive (inline) Discrete Fourier Transform (DFT). - A table provides simultaneous raw sample expansion and fast signed multiplication. - Different frequency resolution (different window functions) is used for different DTMF frequencies without increasing pathlength.

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Table Driven Dual Tone Multi-frequency Receiver

       A Dual-Tone Multi-Frequency (DTMF) receiver with the
following attributes is disclosed:
-  The input is blocks of digitized audio consistent with standard
telecommunications sampling rates (8000 per second) and encodings (8
bits per sample, mu-law or A-law).
-  Computational complexity is within the capability of a general-
purpose microprocessor being shared with other communication channels
and/or other functions.
     -  Energy at DTMF frequencies is evaluated using a non-recursive
(inline) Discrete Fourier Transform (DFT).
     -  A table provides simultaneous raw sample expansion and fast
signed multiplication.
     -  Different frequency resolution (different window functions)
is used for different DTMF frequencies without increasing pathlength.

      Given blocks of digitized audio, a microprocessor calculates
the strengths of the eight frequencies utilized in DTMF signalling.

      DFT techniques are utilized.  The blocks of sampled audio are
processed in non-overlapping frames of "N" samples each (N=104
assumed).  The amplitude of the cosine component "C" and sine
component "S" for each frequency of interest (697, 770, 852, 941,
1209, 1336, 1477, and 1633 Hz) are calculated in each frame (see Fig.
1).

      A table provides simultaneous raw sample expansion and fast
sign ed multiplication.  The table consists of an array of two's
complement numbers.  The array has two dimensions:
-  RAW SAMPLE:  This has 256 possible values.  Each raw sample
represents a signed expanded value that depends on whether mu-law or
A-law encoding is used by the equipment generating the digitized
tones.  Fig. 2 shows table values for mu-law samples.  A similar
table can be generated for A-law samples.
-  MODIFIER:  This has 127 possible values.  Each modifier represents
a signed multiplication factor between -1 and +1.

      Each array element contains the product of the corresponding
expanded sample and multiplier, rounded to 16 bits (14 bit integer +
2 bit fraction for mu-law, 13 bit integer + 3 bit fraction for
A-law).

      Fig. 3 shows the code structure.  The following conventions
apply:
1.  The code is called once for each buffer of received data.
2.  Receive buffer length is a multiple of 104, the DFT frame size.
3.  Input:
     -  TABLE@ is the start of the appropriate table (mu-law or
A-law).
     -  RECEIVE BUFFER@ is the start of the buffer containing the raw
samples.
     -  BUFFER REMAINING is the length of the buffer containing the
raw samples.
4.  There are 104 Sample Lookup and Accumulate Procedures (SLAPs)
coded inline, one for each time slot in the DFT frame.  Each SLAP
handles 1 sample (1 byte).
5.  The right side of Fig. 3 shows detail for the SLAP that handles
n=2.  The cosine and sine components of 8 frequencies are evaluated
in parallel, with a dedicated "C" and "S" accumulator for each
frequency. ...