APPARATUS AND METHOD FOR VOICE BAND REDUCTION

Abstract: An apparatus for frequency band reduction of a voice signal having an upper and lower band limit including a voice signal input. A clipping circuit (2) clips the positive and negative peaks of the voice signal connected to the voice signal input. A bandpass filter (3) is connected to the clipping circuit for rejecting harmonics created by the clipping circuit. A low and high end roll-off filter device is provided for producing a given db/octave frequency roll off at the lower bandlimit. The geometric mean value of the upper and lower bandlimit falls within a given lower and a given upper frequency range.

Description: APPARATUS AND METHOD FOR VOICE BAND REDUCTION This application is based on Patent Application 680,035 filed March 28, 1991, which is a continuation of U.S. Patent Application 325,239 filed March 16, 1989. TECHNICAL FIELD The invention relates to an apparatus and method for frequency band reduction of a voice signal, and more particularly to frequency band reduction by means of clipping the peaks of a voice signal and rejecting the harmonics created by the clipping, by means of a bandpass filter having special characteristics. As a result, the average voice energy level is increased, leading to increased noise reduction and savings in frequency band use. BACKGROUND Voice band reduction is desirable because it increases the number of voice channels within a frequency band of a given width. Unlike the prior art, the invention is applied on a broad band basis and the harmonics created in the clipping process are eliminated by means of suitable filtering. It is an object of the invention to provide frequency band reduction of a voice signal having a given original bandwidth without significant loss of voice signal quality and speaker recognition. SUMMARY OF THE INVENTION The invention provides a voice signal clipping circuit that clips the positive and negative peaks of the voice signal, followed by a bandpass filter having upper and lower bandlimits such that the square root of the product of the bandlimits falls within a range of 650 and 900 Hz and both the low and high end of the band has a roll-off of preferably 6 db/octave. The geometric mean value of the bandlimit frequencies is within range of preferably, but not necessarily 650-900 Hz. The bandpass filter removes the harmonics created by the clipping process, which would otherwise lead to sever distortion of the voice signal. A low and high end roll-off at a rate of approximately 6 db/octave is necessary to maintain an acceptable voice signal quality. Without the roll-off, the clipping ' and bandpass filtering causes the signal to sound hollow and causes a loss of speaker recognition. The low and high end roll is necessary to obtain the desired frequency band reduction. The clipping of the peaks of the voice signal causes an evening-out of the variations in the signal energy of the voice signal. The human voice is modulated such that the root-mean-square value of the voice energy constantly fluctuates within a wide range during conversation, resulting in a low energy level. The peak energy level of most transmission apparatus is limited to a given upper value. The clipping and filtering of the voice signal therefore operates to increase the average voice signal level which results in improved noise reduction and speech comprehension. The use of a geometric mean value of the signal band in the range of preferably, but not necessarily, 650-900 Hz is lower than that of conventional non- reduced telephone voice channels, which usually have a bandwidth of 300- 3000 Hz, giving a geometric mean value of 949 Hz. In reducing the signal bandwidth by clipping and filtering, applicant has found that it is important to move the geometric mean value of the bandlimits downward by controlling both the upper and lowerband limit. A voice band of for example 300 to 2000 Hz has been found to be attainable without significant loss of voice quality and speaker recognition. Such a band limited signal has a bandwidth of 1700 Hz, which is significant band reduction compared with the conventional bandwith of 2700 Hz, resulting in 1.6 times more usable voice channels within a given frequency band. The voice passband can be reduced to as low as 1 kHz while still retaining useable syllabic, word and sentence articulation in addition to some degree of speaker recognition. The bandpass filter and the filter means for creating the 6 db/octave roll-off can be realized by means of active or passive filters or a combination thereof, or by means of a single filter or several cooperating filters. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of the invention, showing the major function blocks, including a preamplifier, a clipping circuit and bandpass filter. Fig. 2 is a schematic circuit diagram of a band-reduction apparatus showing the details of the circuit. Fig. 3 is a graph showing bandlimits for various degrees of voice quality. Fig. 4 is a graph showing bandlimits for a typical band-reduced voice signal. nFTATT .FD DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 1 a voice frequency signal is connected to an input VSIN of an amplifier stage 1, having an input terminal connected to a potentiometer 5, having an output connected to the noninverting input of an operational amplifier 8. A negative feedback network having resistors 6, 7 provides negative feedback from the output of amplifier 8 to its inverting input to provide stability. A clipping circuit 2 has series-connected diodes 10, 11 having a negative clipping potential -CL connected to the anode of diode 10 and a positive clipping potential +CL connected to the cathode of diode 11. Clipping potentials -CL and +CL are of equal magnitude for symmetrical clipping of the voice signal, and are selected such that the potentials clip a given part of the peaks of the voice signal. The clipping signals are typically 60 to 80% of the maximum peak amplitudes of the voice signal. Diodes 10, 11 are joined at a junction point connected via a resistor 9 to the output of amplifier 8. Resistor 9 provides a degree of isolation between amplifier 8 and circuit 2 for sharper clipping. The clipped voice signal of the junction point is connected to the input of bandpass filter 3 consisting of two series sections formed of capacitors 14, 15 and inductors, 13, 16; and a shunt section formed of a parallel circuit of inductor 17 and capacitor 18. A filter input series resistor 12 may optionally be provided to isolate the circuit 2 from filter 3 and to provide improved impedance matching for filter 3. The output of filter 3 is connected to a roll-off network 4 consisting of an input potentiometer 19, a series LC branch, consisting of capacitor 21 and inductor 22 in series with the sliding contact of the potentiometer 19, and a shunt capacitor 23 connected between the output of inductor 22 and ground. The band-reduced voice signal appears at output terminal VSOUT, connected to the junction of inductor 22 and capacitor 23. The values of the components of the network 4 are selected such that capacitor 21 and inductor 22 have a broad maximum around the center frequency of the voice signal measured as the geometric mean of the upper and lower bandlimit, with roll-off to both sides of that frequency. Potentiometer 19 determines the degree of roll-off and is typically set such that the roll-off is approximately 6 db/octave, due to the damping effect of potentiometer 19. Fig. 2 is a more detailed circuit diagram of a band-reducing circuit. It is built of solidstate components and active filters. The voice frequency signal enters the circuit at input terminal VS in and is connected via input capacitor Cl to the gate of the unijunction transistor Ql, the output of which is connected via coupling capacitor C2 to a signal level control potentiometer PI, the output of which is connected via coupling capacitor C5 to the base of transistor Q2, which provides a second gain stage. The collector of transistor Q2 is connected via coupling capacitor C7 to a double pole double throw switch DPDT having an upper position A and a lower position B. In position B the coupling capacitor C7 is connected via switch DPDT to one side of a clipping circuit consisting of two antiparallel diodes Dl, D2, having their other side connected to ground potential. The diodes are of the silicon type, which have greater forward voltage drop and therefore in this application require no clipping potentials. In position A switch DPDT connects the voice frequency signal from capacitor C7 directly via an output circuit consisting of potentiometer P2, capacitor C8 and inductor L to the output terminal VS out, bypassing diodes Dl, D2. In position B of switch DPDT diodes Dl, D2 are connected into the voice signal path from transistor Q2, which continues from the clipping circuit to the input of a three stage active bandpass filter consisting of stages Al, A2 and A3. The filter has a passband of 300 to 2000 Hz, giving a geometric mean frequency equal to the square root of 300 times 2000, which equals 775 Hz, with a passband of 1700 Hz. The components of the filter are selected such that a roll-off of 6 db/octave is attained in connection with the switch output network LI, C8, measured from the bandlimits 300 and 2000 Hz. Switch DPDT bypasses the clipping circuit and bandpass filter in situations where best results may be obtained without the band-reduction circuit. Fig. 3 is a graph in logarithmic scale where the ordinate represents the lower limits of a family of voice bands and the abscissa represents the corresponding upper bandlimits. The line labelled "OPTIMUM BAND LIMITS" shows the relationship of the corresponding upper and lower bandwidth for voice frequency bands of optimal quality with the indicated bandlimits. The quality is evaluated in regard to voice articulation parameters that include syllabic, word and sentence recognition as well as speaker recognition. The right-hand ordinate is graded in "LIMINAL UNITS" which is an approximate measure of the quality of articulation of a band-reduced voice signal. One liminal unit is a rough measure of the just discernable increment in voice articulation. The upper line labelled "QUALITY" indicates bandlimits for a voice signal of relatively high grade, with bandlimits giving the greatest degree of articulation for a given bandwidth. Fig. 4 shows graphs of an unclipped voice signal (upper curve) and a clipped and band-filtered signal according to the circuit shown in Fig. 2 (lower curve). It has a bandwidth of 300-2050 Hz and has been judged by a poll of listeners to provide a good degree of voice articulation combined with satisfactory speaker recognition.

Claims: CLAIMS -1- Apparatus for frequency band reduction of a voice signal having an upper and lower bandlimit, comprising a voice signal input, a clipping circuit for clipping the positive and negative peaks of the voice signal connected to said voice signal input; a bandpass filter connected to said clipping circuit for rejecting harmonics created by said clipping circuit; low end roll off filter means for producing a given db/octave frequency roll off at the lower bandlimit; and wherein the geometric mean value of said upper and lower bandlimit falls within a given lower and a given upper frequency range. -2- Apparatus according to claim 1 wherein said geometric means value of said upper and lower bandlimit falls between 650 and 900 Hz. -3- Apparatus according to claim 1 wherein said bandpass filter is a passive filter of the third order. -4- Apparatus according to claim 1 wherein said bandpass filter is an active filter of the third order. -5- Apparatus according to claim 1 wherein said roll off filter means include a passive LC-filter of the second order. -6- Apparatus according to claim 1 wherein said roll off filter means include an active filter of the second order. -7- Apparatus according to claim 4 wherein said clipping circuit includes first and second clipping diodes joined at a junction point connected to an input of said active filter, and connected with their respective anode and cathode to a negative and positive clipping potential for clipping the peaks of said voice signal. -8- Apparatus according to claim 1 including switch means for selectively bypassing said clipping circuit and bandpass filter. -9- Apparatus according to claim 1 including high end roll off filter means for providing a roll off of 6 db/per octave at the high end of the voice band. -10- A method for voice band frequency reduction, comprising the steps of: clipping symmetrically the peaks of a voice band signal, and band-limiting the clipped voice band signal in a bandpass filter having upper and lower bandlimits, such that the square root of the product of the bandlimits falls within a given range.

Inventors: NEIL J PHILIP, NEIL, J., PHILIP
Applicants: NEIL J PHILIP, NEIL, J., PHILIP
Application year: 1992
Main classification: G01L3
IPC codes: G10L11, H04B1