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Claims  |
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We claim:
1. In a data reduction system of the type for preparing varying average
energy level analog input signals for storage or transmission, the
combination comprising
analog to digital converter means for converting the analog input signal to
equal length digital sample signals,
digital compression filter means responsive to digital sample signals from
said analog to digital converter means for generating a stream of equal
length compressed signals,
digital encoding means implementing a truncated variable word length code
for encoding the compressed signals from said digital compression filter
means,
entropy setting means for controlling the entropy of signals supplied to
said digital compression filter means to control the entropy of signals
from the compression filter,
mode control means for obtaining a measure of the average energy level of
the input signal prior to encoding by said digital encoding means and for
producing an output indicative of an energy level band within which said
signal falls, there being a plurality of different energy level bands
ranging from low to high energy levels through which said signal may
operate, and
means for controlling the entropy setting means in response to the output
from the mode control means for step control thereof with energy level
band changes, the entropy of signals supplied to the digital compression
filter means being reduced by said entropy setting means with changes from
a lower to a higher energy level band and being increased with changes
from a higher to a lower energy level band, the ratio .sigma./q being
changed by a factor of 2.sup.x with each change in energy level band,
wherein .sigma. is standard deviation of the compressed signal stream from
the digital compression filter means, q is quantization level of the
digital sample signals, and x is a non-zero integer.
2. In a data reduction system as defined in claim 1 wherein said mode
control means comprises
envelope detector means for obtaining a measure of the average energy level
of the input signal, and
threshold means responsive to the output from the envelope detector means
for producing a change in the output from the mode control means when the
output from the envelope detector means exceeds a threshold value of said
threshold means.
3. In a data reduction system as defined in claim 1 wherein said mode
control means is responsive to the input to the analog to digital
converter means for obtaining a measure of the average energy level of the
analog input signal to the digital converter means.
4. In a data reduction system as defined in claim 1 wherein said mode
control means is responsive to the output from the analog to digital
converter means for obtaining a measure of the average energy level of the
digital sample signals.
5. In a data reduction system as defined in claim 1 wherein said mode
control mean is responsive to the output from the digital compression
filter means for obtaining a measure of the average energy level of the
compressed signal stream.
6. In a data reduction system as defined in claim 1 wherein said entropy
setting means comprises means for setting at least one least significant
bit of the sample signal to a predetermined value with energy level
changes from a lower to a higher energy level, q, band to increase the
quantization level of the sample signal by a factor of 2.sup.N where N is
the number of least significant bits set to predetermined values.
7. In a data reduction system as defined in claim 6 wherein said mode
control means comprise
envelope detector means, and
threshold means responsive to the output from the envelope detector means
for producing a change in the output from the mode control means when the
output from the envelope detector means passes through a threshold value
of said threshold means.
8. In a data reduction system as defined in claim 1 wherein said digital
encoding means has a plurality of operating modes for implementing a
plurality of different truncated variable word length codes, and means
responsive to the output from said mode control means for selecting the
operating mode of the digital encoder means for operation with a code for
optimum reduction in the bit rate of the encoded compressed signal output
therefrom, changes in the output from the mode control means
simultaneously effecting changes in the operating mode of the digital
encoding means and entropy setting means.
9. In a data reduction system as defined in claim 8 including,
means responsive to the mode control signal output from said mode control
means for inserting an identifying code word in the encoded compressed
stream from the digital encoding means with changes in the output from the
mode control means for identifying the code being implemented by said
digital encoding means.
10. In a data reduction system as defined in claim 9 including,
means for periodically inserting said identifying code word in the encoded
compressed stream after every change in the identifying code word.
11. In a data reduction system as defined in claim 9 including,
code checker and stripper means for stripping the identifying code word
from the encoded compressed signal stream from the digital encoder means
and for producing a code identification signal corresponding to said
identifying code word,
digital decoder means responsive to the encoded compressed signal stream
from said code checker and stripper means and having a plurality of
different operating modes for decoding the different codes implemented by
said digital encoding means,
means responsive to the code identification signal from said code checker
and stripper means for selecting the operating mode of the digital decoder
means required for decoding the encoded compressed signal stream from said
code checker and stripper means, and
digital reconstruction filter means responsive to decoded signals from said
digital decoder means for reconstruction filtering thereof.
12. In a data reduction system as defined in claim 1 wherein said entropy
setting means for controlling the entropy of signals supplied to said
digital compression filter means comprises gain setting means for changing
the amplitude of the sample signal whenever the energy level of the input
signal changes from one to another energy level band, the standard
deviation, .sigma., of the compressed signal stream being changed by a
factor of 2.sup.x without a change in the quantization level of the sample
signal stream by operation of said gain setting means, and
means responsive to the mode control signal output from said mode control
means for inserting an identifying code word in the encoded compressed
signal stream from the digital encoding means with changes in the mode
control signal for identifying the setting of the gain control means.
13. In a data reduction system as defined in claim 12 wherein said digital
encoding means has a plurality of operating modes for implementing a
plurality of different truncated variable word length codes, and
means responsive to the output from the mode control means for selecting
the operating mode of the digital encoder means for operation with a code
which provides a reduction in the bit rate of the encoded compressed
signal output therefrom.
14. In a data reduction system as defined in claim 12 wherein said digital
encoding means implements a truncated Huffman code.
15. In a data reduction system as defined in claim 12 wherein said gain
setting means comprises a variable gain amplifier for reducing the
amplitude of the analog input signal to the analog to digital converter
means with input signal energy level changes from a lower to a higher
energy level band.
16. In a data reduction system as defined in claim 12 including,
code checker and stripper means for stripping the identifying code word
from the encoded compressed signal stream from the digital encoder means
and for producing a code identification signal corresponding to said
identifying code word,
digital decoder means responsive to the encoded compressed signal stream
from said code checker and stripper means for decoding said encoded
compressed signal stream,
digital reconstruction filter means responsive to decoded signals from said
digital decoder means for reconstruction filtering thereof,
controllable gain setting means for controlling the amplitude of the output
from the digital reconstruction filter means, and having a gain which is
controlled by the code identification signal from said code checker and
stripper means for increasing the amplitude of the digital reconstruction
filter output with input signal energy level changes from a lower to a
higher energy level band.
17. In a data reduction system as defined in claim 16 wherein said digital
encoding means implements a truncated Huffman code.
18. In a data reduction system as defined in claim 16 including,
digital to analog converter means for converting the output from the
digital reconstruction filter means to analog form, and
wherein said gain setting means for controlling the entropy of signals
supplied to said digital compression filter means, and said controllable
gain setting means for controlling the amplitude of the output from the
digital reconstruction filter means comprise variable gain amplifier means
for reducing the amplitude of the analog input signal to the analog to
digital converter means by a factor of 2.sup.-x and for increasing the
amplitude of the analog signal from the digital to analog converter means
by a factor of 1/2.sup.-x, respectively, with analog input signal energy
level changes from one energy level band to a higher energy level band.
19. In a data reduction system as defined in claim 1 including,
high frequency deemphasis filter means for reducing the amplitude of high
frequency components of digital sample signals supplied to said digital
compression filter means.
20. Digital decoding and decompression means for producing digital signals
f.sub.n (out) from a stream of encoded digital compressed signals,
different portions of said stream having been encoded using different
codes, said stream including an identifying code word at the start of each
portion of the stream encoded using a different code, said digital decoder
and decompression means comprising,
code checker and stripper means for stripping the identifying code word
from the stream of encoded compressed signals and for producing a code
identification signal corresponding to said identifying code word,
digital decoder means responsive to the encoded compressed signal stream
from said code checker and stripper means and having a plurality of
different operating modes for implementing different algorithms for
decoding the different codes,
means responsive to the code identification signal from said code checker
and stripper means for selecting the operating mode of the digital decoder
means required for decoding the encoded compressed signals from said code
checker and stripper means, and
means for reconstruction filtering of decoded signals from said digital
decoder means for producing digital signals f.sub.n (out).
21. In a data compression method for preparing analog input signals having
a substantially Gaussian distribution for storage or transmission, which
method includes converting the analog input signal to a digital sample
signal stream of equal word length sample signals, digital compression
filtering said sample signal stream for generating a stream of compressed
signals of equal word length, and digital encoding the compressed signal
stream to generate a stream of variable word length encoded compressed
signals, the improvement including,
reducing the ratio of .sigma./q by a factor of 2.sup.-x whenever the
average energy level of the analog input signal exceeds a predetermined
threshold level for entropy reduction of the sample signal stream, wherein
.sigma. is standard deviation of the compressed signal stream, q is
quantization level of the sample signal stream, and x is a non-zero
positive integer.
22. In a data compression method as defined in claim 21 wherein the step of
reducing the ratio of .sigma./q comprises restricting at least one least
significant bit of the sample signal to a selected value whenever the
average energy level of the analog input signal exceeds a predetermined
threshold level.
23. In a data compression method as defined in claim 22 which includes
restricting increased numbers of least significant bits of the sample
signals to selected values as the average energy level of the input
signals exceeds higher predetermined threshold levels.
24. In a data compression method as defined in claim 22 which includes
implementing a different code with changes in the input signal average
energy levels above and below the threshold level.
25. In a data compression method as defined in claim 24 which includes
inserting an identifying code word in the encoded compressed signal stream
with each change in the average energy level of input signal above and
below the threshold level to identify the code implemented for use in
subsequent decoding of the encoded compression signals.
26. In a data compression method as defined in claim 22 including,
obtaining a measure of the average energy level of the input signal by
envelope detection thereof before compression filtering.
27. In a data compression method as defined in claim 21 wherein the step of
reducing the ratio of .sigma./q comprises reducing the amplitude of the
analog input signal by a factor of 2.sup.-x.
28. In a data compression method as defined in claim 21 which includes
obtaining a measure of the average energy level of the analog input signal
before digital compression filtering said sample signal stream for control
of the reducing step.
29. In a data compression method as defined in claim 21 which includes
obtaining a measure of the average energy level of the analog input signal
following digital compression filtering before digital encoding of the
comprised signal stream for control of the reducing step.
30. In a data compression method as defined in claim 21 wherein the step of
digital encoding implements a truncated variable word length code.
31. In a data compression method as defined by claim 30 wherein the code
comprises a truncated Huffman code.
32. In a data compression method for preparing a digital sample signal
stream of equal word length sample signals including digital sample signal
streams having a substantially Gaussian distribution for storage or
transmission, which method includes digital compression filtering said
sample signal stream for generating a stream of compressed signals, and
digital encoding the compressed signal stream to generate a stream of
variable word length encoded compressed signals, the improvement
including,
restricting one or more least significant bits of the sample signals before
compression filtering thereof to selected values whenever the average
energy level of the signal stream before digital encoding thereof exceeds
a predetermined threshold level for entropy reduction thereof.
33. In a data compression method as defined in claim 32 which includes
restricting increased numbers of least significant bits of the sample
signals to selected as the average energy level of the signal stream
before digital encoding exceeds higher predetermined threshold levels.
34. In a data compression method as defined in claim 32 which includes
implementing a different truncated variable word length code with changes
in the compressed signal stream average energy levels above and below
threshold level.
35. In a data compression method as defined in claim 34 which includes
inserting an identifying code word in the encoded compressed signal stream
with each change in the average energy level of the compressed signal
stream above and below the threshold level to identify the code
implemented for use in subsequent decoding of the encoded compression
signals.
36. In a data compression method as defined in claim 34 wherein the code
comprises a truncated Huffman code.
37. In a data compression method as defined in claim 32 which includes
obtaining a measure of the average energy level of the signal stream
before digital compression filtering thereof for controlling the
restricting step.
38. In a data compression method as defined in claim 32 which includes
obtaining a measure of the average energy level of the signal stream after
digital compression filtering thereof for controlling the restricting
step.
39. In a data compression method as defined in claim 32 wherein the
restricting step comprises truncation of sample signals before compression
filtering thereof.
40. In a data compression method for preparing analog input signals
including those having a substantially Gaussian distribution for storage
or transmission, which method includes converting the analog input signal
to a digital sample signal stream, digital compression filtering said
sample signal stream for generating a stream of compressed signals, and
digital encoding the compressed signal stream using a truncated variable
word length code to generate a stream of variable word length encoded
compressed signals, the improvement comprising,
changing the amplitude of the analog input signal by a factor of 2.sup.x,
where x is a non-zero integer whenever the average energy of the analog
input signal crosses a predetermined threshold level for changing the
entropy thereof, and
inserting an identifying code word in the encoded compressed signal stream
with each change in the average energy of the analog input signal above
and below the threshold level to identify the magnitude of the amplitude
reduction.
41. In a data compression method as defined in claim 40 wherein the step of
changing the amplitude of the digital sample signal stream includes
controlling the amplitude of the analog input signal by means of a
variable gain amplifier.
42. In a data compression method as defined in claim 40 including inserting
said identifying code word in the encoded signal stream periodically after
each change in the average energy of the analog input signal above and
below the threshold level.
43. In a data reduction system of the type for preparing a digital sample
signal stream of equal word length sample signals for storage or
transmission,
digital compression filter means for compression filtering the digital
sample signal stream and generating a compressed signal stream of equal
word length compressed signals,
digital encoding means for encoding the compressed signal stream by use of
a variable word length code, and
means for controlling the quantization level of the digital sample signals
and entropy of the sample signal stream supplied to the digital
compression filter means by restricting one or more least significant bits
of the digital sample signals to selected values in response to a measure
of the average energy level of the digital sample signal stream above a
predetermined level.
44. In a data reduction system as defined in claim 43 wherein said digital
encoding means has a plurality of different operating modes for
implementing a plurality of different codes, and
means for implementing a different code with a change in the quantization
level of the digital sample signals. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
Systems which include means for converting analog signals to digital sample
signals, digital compression filter means for reducing the entropy of the
sample signals, and Huffman encoding means for encoding the output from
the digital compression filter in preparation for recording and/or
transmission to a remote location, together with a playback or receiver
means, Huffman decoding means, digital reconstruction means which is the
exact or nearly exact inverse of the digital compression filter means, and
means for converting the decoded and filtered digital signals back to
analog form are disclosed in an article by U. E. Ruttimann and H. V.
Pipberger entitled, "Compression of the ECG by Prediction or Interpolation
and Entropy Encoding", IEE Transactions on Biomedical Engineering, Vol.
BME-26, No. 11, pp. 613-623, November 1979. A similar system is shown in
an article by K. L. Ripley and J. R. Cox, Jr. entitled, "A Computer System
for Capturing Transient Electrocardiographic Data", Pro. Comput. Cardiol.
pp. 439-445, 1976. The present invention is directed to method and means
for further reducing the entropy of the digital sample signal prior to
recording and/or transmission thereof.
SUMMARY OF THE INVENTION
The present invention is particularly adapted for use in the compression of
audio analog signals, such as music signals. The analog music signals
typically are converted to 14 to 16 bit sample signals by analog to
digital conversion which provides for small quantization levels. For very
low level music signals such small quantization levels are necessary, but
with moderately high level signals, the ear is incapable of resolving the
waveform down to such levels i.e. from one part in approximately 16,000 to
one part in approximately 65,000. That is, the quantization level where
distortion is noticeable to the listener is higher when the music level is
higher. Experiments have shown that, depending upon the music level, the
word length may be reduced to as low as 8 bits with no, or extremely small
amount of noticeable distortion. It will be apparent that greater signal
compression is possible with shorter sample signals.
The present invention is directed to an arrangement for changing the
"effective" word length for different average music levels. With this
invention, the average music energy level is measured as by means of an
envelope detector, and the detector output is sensed by one or more
thresholds. In one embodiment of the invention, one or more least
significant bits (LSB) of the sample signal are set to a predetermined
value, i.e. to 1 or 0, when the threshold(s) is exceeded as for example by
truncation, round-off, a table look-up procedure, or the like, thereby
effectively reducing the entropy of sample signals when the music level
increases. These sample signals are supplied to a digital compression
filter to generate signals whose entropy is less than the entropy of the
input signals. When the LSBs are set to 1 or 0 the output entropy is
further reduced. The compressed signals are supplied to an encoder for
truncated Huffman encoding thereof. The digital output from the encoder is
recorded by digital recording means, and/or transmitted to a remote
receiving location. At a playback unit or receiving station the encoded
music signal is decoded by decoder means, and the decoded signal is
supplied to a digital reconstruction filter which is substantially an
inverse of the compression filter. Digital to analog converter means
converts the reconstruction filter output to analog form.
In a modified form of this invention, different Huffman codes may be
employed for encoding the compression filter output, the code employed
being dependent upon the threshold signal. The code employed is selected
for minimizing the average bit rate of the Huffman encoded signal. With
this arrangement, a code identification word is inserted in the signal
stream from the encoder to identify the code which is being employed at
the time. At playback, the code identification word is used to select the
proper operating mode of the Huffman decoding means.
In another modified form of this invention variable gain circuits are
included in the recording and playback systems for controlling input
signal levels to the digital compression filter means and from the digital
reconstruction filter means in accordance with the threshold signals.
Either analog or digital gain control circuitry may be employed. This form
of the invention also may be used with a plurality of different Huffman
codes for different encoding of the sample signals dependent upon the
threshold signals.
In yet another modified form of this invention, the setting of LSBs, or the
setting of variable gain circuits at the input to the digital compression
filter means, is controlled by means responsive to the average music
energy level at the output from the digital compression filter. With this
arrangement, different Huffman codes are employed for encoding the
compression filter output dependent upon of the number of LSBs which are
set to zero, or the gain setting. Code identification words are inserted
in the Huffman encoded signal stream for use at playback for correct
decoding of the signal stream.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood from the following description when
considered with the accompanying drawings. In the drawings, wherein like
reference characters refer to the same parts in the several views:
FIGS. 1A and 1B together show a block diagram of a data reduction system; a
digital recording section being shown in FIG. 1A and playback section
being shown in FIG. 1B;
FIG. 2 shows the frequency response of high frequency deemphasis and
emphasis filters included at the input and output, respectively, of the
data reduction system;
FIG. 3 shows a waveform and graphic representations of signals appearing at
various locations in the data compression system shown in FIGS. 1A and 1B;
FIG. 4 is a graphic representation of encoded difference signals showing
the format employed for encoding those difference signals which are
outside a predetermined signal range;
FIG. 5 is a block diagram showing details of the mode control unit shown in
FIG. 1A;
FIG. 6 is a table illustrating different truncated Huffman codes for use in
the data reduction system;
FIG. 7 is a graph for use in showing the relationship between the
probability that a digital sample signal value will occur within a certain
quantization level and size of the quantization level;
FIG. 8 is a table which illustrates the setting of least significant bits
of sample signals supplied to the digital compression filter;
FIG. 9 is a block diagram of a modified form of data reduction system which
is similar to that shown in FIGS. 1A and 1B but which employs a single
truncated Huffman code;
FIGS. 10A and 10B together show a block diagram of a modified form of data
reduction system which includes gain setting means instead of the least
significant bit setting means employed in the embodiment of FIGS. 1A and
1B;
FIG. 11 is a block diagram of yet another modified form of a digital data
recording system which embodies the present invention in which the mode
control unit is responsive to the output from the digital compression
filter means.
RECORDING SYSTEM
Reference first is made to FIG. 1A wherein a recording unit of a
combination recording-playback system embodying the present invention is
shown comprising a high frequency deemphasis filter 20 to which an analog
input signal f(t), such as a music signal, is supplied. The filter 20
deemphasizes the high frequency portion of the analog signal to reduce the
signal entropy. The frequency response of filter 20 together with the
frequency response of filter 20A included in the playback portion of the
system is shown in FIG. 2. There, it will be seen that the relative gain
of the filter 20 decreases beginning at approximately 0.4 KHz, to
deemphasize high frequency components of the analog signal. For
simplicity, the analog output from filter 20, as well as the analog input
thereto, is identified as f(t). At A of FIG. 3, an analog signal f(t) is
shown, comprising a music signal which may range in frequency from
approximately 15 to 20,000 Hz.
The filter 20 output is supplied to an analog to digital converter (A/D
converter) 22 for conversion of the analog signal into digital form, the
n.sup.th sample from the A/D converter being identified as f.sub.n. The
form of the A/D converter output, shown at C of FIG. 3, comprises samples
f.sub.n-1 through f.sub.n+i of equal length words. The A/D converter 22
operates in a conventional manner at a fixed sampling rate and fixed word
length output. As noted above, A/D conversion word length typically is 14
to 16 bits and, for purposes of illustration only, a 14 bit word length is
shown in FIG. 3. Also, for purposes of description only, a sampling rate
of, say, 44 Khz may be employed for conversion of the analog music signal.
The output from the high frequency deemphasis filter 20 also is supplied to
a mode control unit 24 over line 25 which, as described in detail
hereinbelow generates mode control signals at output 26 therefrom for step
control of several elements of the data compression circuit. Mode control
signals at output 26 are dependent upon the average energy level of the
music signal input to the unit.
The output from A/D converter 22 is supplied to an entropy control circuit
28 which, in the illustrated system comprises means for setting one or
more of the least significant bits of the sample signal supplied thereto
to a predetermined value of 1 or 0. A mode control signal from mode
control unit 24 is supplied to the entropy control circuit 28 for
controlling the setting of the least significant bits. For low level input
signals, sample signals pass unaltered through circuit 28. At a first
threshold level of the analog input envelope, the least significant bit
may be set to 0, for example; at a second threshold level, the two least
significant bits may be set to 0;l etc. The operation of setting least
significant bits to zero in response to mode control signals is described
in greater detail following a description of the mode control unit 24. For
simplicity, in FIG. 1A, the output from entropy control unit 28 is
identified as f.sub.n the same as the input thereto.
The output from the entropy setting unit 28 is supplied to a compression
filter 30 which, for present purposes, is shown to include an estimator 32
and subtracting means 34. The estimator 32 provides an estimate of
f.sub.n, here identified as f.sub.n, based upon actual samples occuring
both before and after the sample f.sub.n to be estimated. Estimators for
providing such estimated f.sub.n values are, of course, well known. A
difference signal .DELTA..sub.n is produced by the compression filter 30
comprising the difference between the actual signal input f.sub.n and the
estimated signal value f.sub.n by subtraction of the estimated value from
the actual value at subtracting means 34, as follows:
.DELTA..sub.n =f.sub.n -f.sub.n (1)
In the graphic signal representation of the compression filter output,
shown at D in FIG. 3, difference signals .DELTA..sub.n, .DELTA..sub.n+1,
.DELTA..sub.n+2, . . . .DELTA..sub.n+i are shown.
It here will be understood that the present invention is not limited to use
with the illustrated compression filter in which the output .DELTA..sub.n
comprises the difference between the actual signal input f.sub.n and an
estimated value f.sub.n. Other compression filtering may be used in which
the compression filter output .DELTA..sub.n is not a direct function of
the difference between the actual input f.sub.n and an estimated value
thereof, f.sub.n. The use of the term "difference signal values
.DELTA..sub.n ", therefore, is intended to identify the output from any
suitable digital compression filter which may be employed in the systems
of the present invention.
The compressed signal values .DELTA..sub.n are supplied to a digital
encoder 40 for coding the same using a truncated Huffman code. Truncated
Huffman encoding is, of course, well known. Briefly, the Huffman encoding
technique makes use of the fact that the compression filter 30 has signal
outputs, .DELTA..sub.n, having different probabilities of occurrence, and
uses this fact to achieve a reduction in the total number of bits in the
encoded signal over the input signal. A single code word is assigned to
infrequently occurring compressed signals, and supplied as a label for the
actual compressed signal value .DELTA..sub.n. In FIG. 1A, the encoder 40
output is designated h(.DELTA..sub.n) and, at E in FIG. 3, the values
h(.DELTA..sub.n), h(.DELTA..sub.n+1), etc. represent encoded values of
.DELTA..sub.n, .DELTA..sub.n+1, etc. The encoder 40 output comprises code
words for the most frequently occurring values of .DELTA..sub.n, together
a combined code word label and actual value of the compressed signal
.DELTA..sub.n for less frequently occurring values of .DELTA..sub.n. For
purposes of illustration only, if the compressed signal .DELTA..sub.n is
outside the range of -3 to +3 then the actual signal .DELTA..sub.n
together with a code word label is produced at the encoder output. In FIG.
4, wherein several encoded values are shown, it will be seen that the
encoded value for .DELTA..sub.n+2 comprises a label together with the
actual compressed signal .DELTA..sub.n+2, wherein .DELTA..sub.n+2
comprises an infrequently occurring compression signal value; that is some
value outside the range of .+-.3.
The digital compression filter 30 has a predetermined transfer function
which is unaltered in the operation of the system. For example only, the
compression filter may implement the following transform:
.DELTA..sub.n =f.sub.n+1 -2f.sub.n +f.sub.n-1 (2)
However, the probability that particular signals .DELTA..sub.n will be
produced by the compression filter is dependent upon the setting of
entropy setting unit 28. Therefore, in accordance with another aspect of
the present invention, the Huffman code implemented by Huffman encoder 40
may be selected for maximum reduction in the length of encoded compressed
signals produced thereby. In FIG. 1A, the digital encoder unit 40 is shown
to include a plurality of individual encoders 40-0, 40-1 . . . and 40-N.
Switches 42 and 44 at the inputs and outputs of the encoders select which
encoder is employed during system operation. The switches 42 and 44 are
under control of the output from mode control unit 24 for changing the
Huffman code employed simultaneously with changes in the number of least
significant bits which are set to a predetermined value by entropy setting
unit 28.
The encoded compressed signal h(.DELTA..sub.n), from the selected Huffman
encoder is supplied to a code identification generator and insertion
circuit 50 for insertion of a code word into the encoded signal stream to
identify the operating state of the system. The code identification
generator and insertion circuit 50 is controlled by the output from the
mode control unit 24 for the generation of a code word that is unique for
the energy level band of the analog input signal f(t). In the illustrated
arrangement, the code word identifies the Huffman code employed by digital
encoder 40. The code word may comprise a word which is not contained in
any of the Huffman codes, or it may be a code word that is the same for
each of the codes, but which is followed by a binary number that
identifies the state. The code word is sent after a switch in the output
from the mode control unit 24 and periodically thereafter in case some of
the subsequently recorded bit stream is destroyed by bit errors. For
example, placing an identifier in the encoded bit stream every 50 or 100
samples will identify the code every several milliseconds but cause
negligible increase in the average bit length. The encode bit stream, with
identifier word, is identified by h(.DELTA..sub.n) +ID in the drawings.
The encoded digital data stream, with identifiers, is recorded and/or
transmitted to a remote receiver. For recording, the data stream from code
identification generator and insertion circuit 50 is connected through a
buffer memory 52 to a recorder unit 54. Recorded encoded digital signals
such as those recorded at recorder unit 54 are reproduced using the system
shown in FIG. 1B, which system includes a playback unit 60.
Signals from the playback unit 60 are supplied through a buffer memory 62
to a code checker and stripper unit 64 which examines the bit stream for
the identifying code word, or words. This process may be accomplished
using one or more AND logic gate network means connected to a shift
register through which the bit stream is passed; one logic gate network
for each identifying code word. Alternatively, the code word may be
identified by use of a subroutine in a microprocessor. In any case, the
identifying code word is stripped from the bit stream, and the encoded
compressed signals h(.DELTA..sub.n) are supplied to a digital decoder 66
for decoding the same. A control signal is generated by the code checker
and stripper in response to the identifying code word, which signal is
connected over line 68 to the decoder 66 for control of the decoding
operation.
Since the compressed signal stream is encoded using different codes, it
will be apparent that the decoder 66 must be operable in different
decoding modes for decoding the coded signal stream supplied thereto. The
illustrated decoder is shown to include a plurality of individual decoders
66-0, 66-1 . . . and 66-N. Switches 70 and 72 at the inputs and outputs of
the decoders select which decoder is employed. The switches 70 and 72 are
under control of the control signal supplied thereto over line 68 from the
code checker and stripper 64. The appropriate decoder unit is switched
into the circuit for decoding the encoded difference signals supplied to
decoder 66, dependent upon which Huffman encoder 44-0 etc. is employed
during coding.
The compressed signal output .DELTA..sub.n from the digital decoder 66 is
supplied to a reconstruction, or decompression, filter 74 for conversion
of the compressed signals to equal length sample signals f.sub.n (out).
The reconstruction filter 74 is an exact, or substantially exact, inverse
of the compression filter 40 for exact, or nearly exact, reconstruction of
the input sample signals f.sub.n supplied to the digital compression
filter 30.
A digital to analog converter (D/A converter) 76 converts the signal
samples f.sub.n (out) from the digital reconstruction filter 74 to analog
form. An analog high frequency emphasis filter 20A, having a frequency
characteristic depicted in FIG. 2, emphasizes the high frequency
components of the analog signal from the D/A converter 76 whereby the
filter output closely matches the input which was supplied to the high
frequency deemphasis filter 20 included in the recording section shown in
FIG. 1A.
Reference now is made to FIG. 5 of the drawings wherein a mode control unit
24 of the type which may be included in the recording section of FIG. 1A
is shown to include an envelope detector 80 responsive to the analog music
signal f(t) supplied thereto over line 25 from the high frequency
deemphasis filter 20. It here will be noted that the input signal to the
envelope detector may be obtained from the analog music signal at the
input to the high frequency deemphasis filter 20 rather than from the
filter output, if desired.
The envelope detector measures the average music energy level, and the
detector output is connected to one or more threshold circuits for sensing
one or more thresholds of the detector output. In FIG. 5, threshold
circuits 82-1, 82-2 through 82-N are shown. The envelope detector output
identified by reference character 80A, is depicted at B of FIG. 3,
together with the input levels TR1, TR2 and TRN at which the threshold
circuits 82-1, 82-2 and 82-N, respectively, function to produce an output.
The threshold circuit outputs are supplied to a logic unit 84 which, in
turn, produces a first output when the analog input envelope equals or
exceeds TR1, a second output when the envelope equals or exceeds TR2, etc.
Obviously, output line 26 from the logic unit 84 may comprise a plurality
of conductors over which individual control signals dependent upon the
level of the analog input envelope are conducted to the various circuits
to be controlled. The logic may be implemented using a logic gate network,
computer subroutine, or the like.
Reference now is made to FIG. 6 which shows a table of truncated Huffman
codes which may be employed in the operation of encoder 40 shown in FIG.
1A. In FIG. 6, encoding operations at four different levels of the average
value of the analog input signal are illustrated wherein the input signal
envelope, IE, is less than threshold TR1 (see B of FIG. 3), i.e. IE<TR1;
IE>TR1<TR2; IE>TR2<TR3; and IE>TR3. The compressed signals, .DELTA..sub.n,
which occur most frequently are assigned a code word. In the illustrated
arrangement, these signals comprise values between +3 and -3. The most
frequently occuring compressed signals are assigned the shortest code
word. All other signals outside the range of .+-.3 are identified as
"else" in the table, and these are assigned a code word which, as
described above with reference to FIG. 4, comprise a label for the actual
compressed signal value .DELTA..sub.n which subsequently is recorded.
An examination of the table of FIG. 6 reveals, for example, that the
signals 2, -2, 3 and -3 have the least probability of occurance during
operation when IE>TR3, IE>TR2<TR3, IE>TR1<TR2, and IE<TR1, respectively.
Consequently, these signals are assigned the longest code word which, in
the table is 00000001. From the tab | | |
