Voiceband signal classifier

• US 6,708,146 B1
• Filed: 04/30/1999
• Issued: 03/16/2004
• Est. Priority Date: 01/03/1997
• Status: Expired due to Fees

• Current Assignee: Telecommunications Research Laboratories
• Original Assignee: Telecommunications Research Laboratories
• Inventors: Sarda, Deepak P., Sewall, Jeremy S., Cockburn, Bruce F.
• Primary Examiner(s): Knepper, David D.
• Application Number:
  US09/303,160
• Time in Patent Office: 1,782 Days
• Field of Search: 704/205-218, 704/231-234, 704/237, 704/238, 704/239, 704/255, 382/155, 455/226.1, 706/13
• US Class Current: 704/217
• CPC Class Codes: G10L 25/78 Detection of presence or absence of voice signals switching of direction of transmission by voice frequency in two-way loud-speaking telephone systems H04M9/10

Abstract

A method and apparatus for classifying signals into a multiplicity of signal classes which employs discriminant functions of low-complexity discriminant variables that are computed directly from the passband signal. The method can be applied to the problem of classifying voiceband data (VBD), facsimile (FAX), native binary data, and speech on a 64 Kbps digital channel. In a hybrid two stage classification system, the first stage employs linear discriminant functions to make classification decisions into a smaller number of possible preliminary signal classes. The decisions of the first stage are then refined by a second stage that uses nonlinear discriminant functions such as quadratic or pseudo-quadratic functions. The second stage of a hybrid classifier then assigns the signal into a larger number of possible classes than does the first stage of the classifier alone.

Claims

1. A signal classifier for classifying a passband signal into one of a plurality of signal classes, the passband signal being carried by a communications network and having at least one segment with N samples, the signal classifier comprising:

an autocorrelator having the passband signal as input and having more than one autocorrelation coefficient as output;

a discriminator operable on a vector of more than one of the autocorrelation coefficients to discriminate between signal classes and classify the passband signal as being a member of at least one of the signal classes; and

the discriminator implementing both a linear decision sub-system and a non-linear decision sub-system, in which the linear decision sub-system and the non-linear decision sub-system each operate on a vector containing autocorrelation coefficients.

2. The signal classifier of claim 1 further comprising means to compute a normalized central second-order moment of the segment, and in which the discriminator is operable on the normalized central second-order moment.

3. The signal classifier of claim 2 in which the means to compute the central second-order moment of the segment includes a rectifier for rectifying the passband signal before computation of the central second-order moment.

4. The signal classifier of claims 1 or 3 in which the discriminator uses a non-linear decision sub-system to classify some but not all of the signal classes, and a linear decision sub-system to classify signal classes not classified by the non-linear decision sub-system.

5. The signal classifier of claim 4 in which the discriminator implements a non-linear decision sub-system to classify all classes for which it is trained, and a linear decision sub-system is used to classify all other classes.

6. The signal classifier of claim 5 further comprising an idle channel detector for identifying when the signal power is below a threshold for a given segment.

7. The signal classifier of claim 1 further comprising means, connected between the autocorrelator and the discriminator, for normalizing the autocorrelation coefficients with respect to the power of the signal segment.

8. The signal classifier of claim 1 in which the passband signal is a voiceband signal.

9. Apparatus for classifying a passband signal, the passband signal being carried by a communications network, the apparatus comprising:

autocorrelation means for forming an autocorrelation value of the passband signal at two or more delay intervals; and

means for combining mathematically the autocorrelation values to classify the passband signal as being a member of at least one of a plurality of expected classes;

the means of mathematically combining the values comprising means for using linear combinations operable on a vector of the autocorrelation values to classify the passband signal into one of a plurality of preliminary classes, and means for using nonlinear functions operable on a vector of the autocorrelation values for refining the classification decision to form a final decision assigning the passband signal into one of the plurality of expected classes.

10. The apparatus as defined in claim 9 where the passband signal is processed first by means that map, using a memoryless transformation, the signal into a processed signal which is then input to the autocorrelation means.

11. The apparatus as defined in claim 10 where the memoryless transformation is a nonlinear function.

12. The apparatus as defined in claim 9 where the passband signal is a sequence of codes representing samples of an originally analog signal taken at a regular sampling interval, and where the delay intervals are multiples of the sampling interval.

13. The apparatus as defined in claim 12 where the passband signal is processed using a memoryless one-to-one mapping from the codes to a sequence of processed codes, which represent a processed signal, and where the processed codes are input to the autocorrelation means.

14. The apparatus as defined in claim 13 where the passband signal is classified using a fixed number of consecutively received processed codes representing a finite-length segment of the originally analog signal.

15. The apparatus as defined in claim 14 where the autocorrelation values are normalized with respect to a normalization factor formed from the fixed number of processed codes.

16. The apparatus as defined in claim 15 where the normalization factor is an estimate of the average power of the passband signal contained in the finite-length segment of the originally analog signal.

17. The apparatus as defined in claim 16 where the means of mathematically combining the values of the autocorrelation of the signal use linear combinations of the values.

18. The apparatus as defined in claim 16 where the means of mathematically combining the values of the autocorrelation of the passband signal use nonlinear combinations of the values.

19. The apparatus as defined in claim 16 where the means of mathematically combining the values of the autocorrelation of the signal use quadratic combinations of the values.

20. The apparatus as defined in claim 16 where the means of mathematically combining the values of the autocorrelation of the passband signal use pseudo-quadratic combinations of the values.