Digital Encoding Circuitry

This invention relates to digitally encoding and decoding electrical signals, e.g., to efficiently store music on magnetic tape.

In a continuously variable slope delta (CVSD) encoding system, the encoded signal represents the slopes of individual line segments which together approximate the input signal, and is generated by determining, for each segment, the difference between the input signal and a reference signal whose value is determined by the previous segments of the input signal. In the most rudimentary delta encoding system, the digital output of the encoder represents, at sequential points in time, whether the input signal is greater or less than the reference, and the reference is increased or decreased, at a constant rate, accordingly.

To improve the accuracy of the encoding and reduce the quantization noise introduced by the encoding process, a CVSD system increases and decreases the reference signal at a variable rate corresponding as closely as possible to the magnitude of the instantaneous slope of the input signal.

The Harris, Inc. HR-3210 and the Motorola, Inc. XC3417/18 modulators/demodulators have syllabic filters examining the digital output to detect continuous series of 1's or 0's, indicating a too great or too small rate of change of the reference, and modifying that rate accordingly. In De Boer et al. U.S. Pat. No. 3,490,045, the digital output is integrated to obtain a voltage representing the relative occurrence of 1's and 0's in the output and, if the resulting voltage exceeds a threshold, the rate of change of the reference is changed accordingly. Linder U.S. Pat. No. 3,109,987 and Gaunt U.S. Pat. No. 3,582,784 show encoders wherein the previous rate of change of the reference is detected and utilized to determine the future rate of change.

My invention features a reference signal generating circuit having a filter for extracting from the pattern of digital output bits information relating to the time derivative of the input signal, to provide a control signal for an integrator which, in turn, generates the reference. As the control signal varies with the derivative of the input signal, the rate of change of the reference is greater for more rapidly varying input signals and less for less rapidly varying input signals, so that the reference closely follows the input signal and the quantization noise is minimized.

In preferred embodiments, the control signal is fed back to the input of the filter and gated by the digital output, so that the rate of change of the control signal, and thus of the reference, is dependent upon previous values of the control signal, thus providing excellent response against frequency and amplitude variations in the input signal.

We turn now to the circuitry and operation of a preferred embodiment of the invention, after first briefly describing the drawings.

FIG. 1 is a schematic diagram of a CVSD encoder embodying the invention.

FIG. 2 is a block diagram of a CVSD decoder embodying the invention.

Referring to FIG. 1, D type flip-flop 10 is connected to sample output 13 of comparator 12 at a rate controlled by 250 KHz clock 14, to provide mutually inverted outputs 16 and 18. Output 16 is connected to encoded signal output 17 of the encoder and to differential input 20 of mode select switch 22, and outputs 16 and 18 are connected to the control inputs 19 and 21 of integrator switch 24. Output 26 of switch 24 is connected to (differential).sup.2 input 28 of switch 22, and output 23 of switch 22 is connected to the series combination of filter 30, envelope detector 32, switch 24, and differential integrator 34. Output 35 of integrator 34 and output 37 of audio signal source 36 are connected to summing point 38 to provide the input to comparator 12.

Filter 30 is a pi section lowpass filter having a bandwidth of 25 KHz, approximately 1.7 times the 15 KHz bandwidth of the input signal, with resistor 31 assisting to shape the frequency response of the filter. Envelope detector 32 is a half wave rectifier having a hold time constant in the range of 50 to 70 ms, and preferably 60 ms, and a full scale (10 v) slew time of 1667 .mu.s. Integrator switch 24 is a solid state C-MOS analog switch containing four independently controlled switches and shown here in block diagram form to illustrate the operation of the encoder. Integrator 34 is self-stabilized with a selectable positive or negative integration slew rate of 0.25 v/.mu.s at a maximum control signal of 10 volts and a slew rate of 2.5 mv/.mu.s at a control signal of 0.1 volts.

The following table contains the circuit components used in the circuit of FIG. 1, and are optimum for the (differential).sup.2 mode of operation.

R1-10 K.OMEGA., 5%

R2-10 K.OMEGA., 5%

R3-510.OMEGA., 5%

R4-6.8 K.OMEGA., 5%

R5-510.OMEGA., 5%

R6-40.2 K.OMEGA., 1%

R7-40.2 K.OMEGA., 1%

R8-20 K.OMEGA., 1%

R9-200 K.OMEGA., 1%

R10-10 K.OMEGA., 1%

R11-10 K.OMEGA., 1%

R12-10 K, 5%

Resistor 31 (encoder) 1 M.OMEGA., 5%

Resistor 31 (decoder) 2 M.OMEGA. carbon resistance potentiometer

C1-1000 pf, 5%

C2-1000 pf, 5%

C3-10 .mu.f, 10%

C4-3.3 .mu.f, 10%

C5-330 pf, 5%

C6-680 pf, 5%

C7-15 .mu.f, 10%

A1-Operational Amplifier (1/2) Motorola, Inc. MC 1458 or equivalent

A2-Operational Amplifier (1/2) Motorola, Inc. MC 1458 or equivalent

Comparator 12-National Semiconductor LM311 or equivalent

D1-Diode 1N4148

D2-Diode 1N4148

Switch 24-C-MOS Quad Analog Switch RCA CD4016BE or equivalent

Flip-Flop 10-C-MOS Flip-Flop (1/2) RCA CD4013BE or equivalent

Referring to FIG. 2, the decoder, shown in block diagram form, contains the same circuit elements connected in the same manner as the decoder of FIG. 1, except that comparator 12 and summing point 38 are omitted and resistor 31 is variable. The encoded signal is connected to the input of the decoder flip-flop while the integrator provides the output.

Referring again to FIG. 1, output 13 of comparator 12 and output 16 of flip-flop 10 are digital signals which are high when output 37 of audio signal source 36 (the input signal) is greater than reference signal output 35 of integrator 34 and low when the input signal is smaller than the reference. The value of the reference is determined by the previous values of the input signal so that output 16 represents the difference between the present and previous values of the input signal. The change with time, i.e., duty cycle, of output 16 represents the derivative (with respect to time) of the input signal.

When the encoder is operating in the differential mode, with switch 22 in the A position, output 16 is ac coupled into the input of filter 30, and the filter 30 extracts the information relating to the derivative of the input signal by separating the fundamental frequency of the duty cycle of output 16 from the other frequencies present in output 16, e.g., clock 14 and the higher harmonics of output 16. Resistor 31 assists in shaping the response of filter 30 and detector 32 by loading the input of detector 32 with a known, fixed resistance. Resistor 31 in the decoder performs the same function but is a variable resistor with a mid-range value equal to resistor 31 in the encoder, to allow the response of the decoder to be matched to that of the encoder. In the embodiment shown, the encoder and decoder may be matched to within .+-.0.5 db.

Envelope detector 32 half-wave detects the output of filter 30 to generate output 33 whose value is proportional to the duty cycle of output 16 and thereby to the derivative of the input signal. Standby reference input 40 to detector 32 is provided to provide a known offset for the detector when the input signal, and hence the output of filter 30, are zero, and may typically be as low as -40 db referenced to the maximum output of filter 30.

Integrator switch 24, operating under the control of inputs 19 and 21 from outputs 16 and 18, applies output 33 of detector 32 to inputs 42 and 44 of integrator 34 when, respectively, the input signal is greater than or less than the reference. Reference output 35 of integrator 34 increases at a rate proportional to the value of output 33 of detector 32, and thus to the derivative of the input signal, when detector output 33 is applied to input 42 and decreases accordingly when detector output 33 is applied to input 44. Inputs 42 and 44 are connected by switch 24, to, respectively, output 46 of integrator 34 and ground when they are not switched to detector output 33, thereby making integrator 34 self-stabilizing by chopping inputs 42 and 44 with outputs 16 and 18.

Reference output 35 of integrator 34 is the integrated derivative of the input signal and thereby closely follows the input signal by being a reconstruction of the input signal delayed in time by the period of clock 14.

The dynamic range of the encoder and decoder, when operating in the differential mode, is on the order of 60 to 70 db when the frequency of clock 14 is at least 10 times, and preferably between 15 and 25 times, the highest input signal frequency; and is frequency weighted by the ratio between the bandwidths of output 16 and output 37 of signal source 36.

The dynamic range of the encoder and decoder are further improved when operating in the preferred (differential).sup.2 mode. Dynamic ranges of better than 85 db may be achieved when the frequency of clock 14 is at least 15 times the highest input signal frequency, and to better than 90 db when the ratio is as high as 25 to 1. In this mode, switch 22 is placed in the B position, so that the input of filter 30 is output 26 of switch 24. The operation of switch 24 is controlled by outputs 16 and 18 of flip-flop 10, so that the input to filter 30 is the control signal modulated by the encoded signal, being a series of pulses which are time coincident with outputs 16 and 18, but, effectively, amplitude modulated by output 33 of detector 32, i.e., proportional to the derivative of the input signal. Future values of control signal output 33 are thereby determined, in part, by the present value of output 33 so that output 33 changes rapidly when the derivative of the input signal is large, i.e., the input signal is changing rapidly, and changes less rapidly when the derivative is small. This provides a further improvement in the response of the reference to changes in the amplitude or frequency of the input signal.

Referring to FIG. 2, an encoded signal from source 48 is applied to the input of decoder flip-flop 10 and is sampled at the same clock rate as in the encoder, so that outputs 16 and 18 of the decoder flip-flop are the same as those in the encoder. Audio filter 30, detector 32, switch 24, and integrator 34 generate reference signal output 35 in the same manner as, and identical to, that generated by the encoder. The integrator output is the decoder output and (as is the encoder reference signal) is an accurate reconstruction of the encoder input signal, tracking the encoder input to within .+-.0.5 db.

Other embodiments are within the following claims. E.g., while the embodiment of the invention described above relates to a CVSD system, the invention may be used in other encoding and decoding systems wherein the encoded signal represents the difference between a present value of the input signal and previous values of the input signal. Examples of such systems are voltage to frequency modulators, pulse width modulators, delta sigma modulators, and some differential pulse code modulators.