USO0RE40281E
(19) United States (12) Reissued Patent
(10) Patent Number: US (45) Date of Reissued Patent:
Tzannes et a]. (54)
SIGNAL PROCESSING UTILIZING A TREE STRUCTURED ARRAY
RE40,281 E Apr. 29, 2008
OTHER PUBLICATIONS
Vaidyanathan, “Quadrature Mirror Filter Banks, MiBand
(75) Inventors: Michael A. Tzannes, Lexington, MA (US); Peter N. Heller, Somerville, MA (US); John P. Stautner, The Woodlands, TX (US); William R. Morrell, Seattle, WA (US); Sriram
J ayasimha, Hyderabad (IN)
Extensions and PerfectiReconstruction Techniques,” IEEE ASSP Magazine, Jul. 1987, pp. 4*20.*
(Continued) Primary ExamineriDavid D. Knepper (74) Attorney, Agent, or FirmiSheridan Ross P.C.; Jason H. Vick
(73) Assignee: Aware, Inc., Bedford, MA (US)
(57)
ABSTRACT
(21) Appl. No.: 10/994,925
[A communication system for sending a sequence of sym
(22) Filed:
bols on a communication link. The system includes a
Nov. 23, 2004 (Under 37 CFR 1.47)
transmitter for placing information indicative of the sequence of symbols on the communication link and a receiver for receiving the information placed on the com munication link by the transmitter. The transmitter includes a clock for de?ning successive frames, each of the frames
Related U.S. Patent Documents
Reissue of:
(64) Patent No.:
6,252,909
Issued:
Jun. 26, 2001
Appl. No.:
08/804,909
Filed:
Feb. 25, 1997
including M time intervals, Where M is an integer greater than 1. A modulator modulates each of M carrier signals With a signal related to the value of one of the symbols
thereby generating a modulated carrier signal corresponding
U.S. Applications: (62)
to each of the carrier signals. The modulated carriers are
Division of application No. 10/603,833, ?led on Jun. 26, 2003, which is a continuation-in-pait of application No. 08/307,331, ?led on Sep. 16, 1994, now Pat. No. 5,606,642, Which is a division of application No. 07/948,147, ?led on
(52)
(58)
Int“ Cl“ G10L 19/02 H04K 1/10 H043 1/38
communication link. The carrier signals include ?rst and second carriers, the ?rst carrier having a different bandwidth than the second carrier. In one embodiment, the modulator includes a tree-structured array of ?lter banks having M leaf
Sep. 21, 1992, now Pat. No. 5,408,580.
(51)
combined into a sum signal which is transmitted on the
(2006-01) (2006-01) (2006-01) (2006-01)
nodes, each of the values related to the symbols forming an input to a corresponding one of the leaf nodes. Each of the nodes includes one of the ?lter banks. Similarly, the receiver
H03D 10" U.S. Cl. ..................... .. 704/205- 375/260' 375/219’ ’ 329/357,
can be constructed of a tree-structured array of sub-band ?lter banks for convening M time-domain samples received on the communication link to M symbol values.] Signal
Field of Classi?cation Search None See application ?le for complete search history
processing is performed by splitting a signal into subbands using a plurality of?lter banks connected to form a tree structured array. The ?lter banks are connected so that the
(56)
References Cited
signal is split into subbands ofdijferent size. The subbands
U_S_ PATENT DOCUMENTS
can be designed to approximate the bands of the human
auditory system for audio signal processing applications. 3,947,827 A *
3/1976 Daunemont et al' """" " 365/45
(Continued)
banks connected to form a tree-structured array is also
FOREIGN PATENT DOCUMENTS EP
0400222
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Reconstruction ofsignals using aplurality ofsynthesis?lter
Performed
5/1990
121 Claims, 16 Drawing Sheets TIME-DOMAIN INPUT SIGNAL
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US. PATENT DOCUMENTS 8/1980
5,479,447 A 5,517,435 A
4,216,354 A
*
4,622,680 A
* 11/1986 ZlIISGI ...... ..
4,766,562 A
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........................ .. 708/313
4,815,023 A * 3/1989 Arbeiter * 4’829’378 A 5/1989 LeGall 4,852,179 A
* 12/1995 Chow et al. .............. .. 375/260 * 5/1996 Sugiyama ................. ..708/322
1/1990 V6161huis 61 31.
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704/201 704/200.1
4,918,524 A : 4/1990 Ansan 6161. ........ .. 375/240.11 4’949’383 A 8/1990 KOh. et 31' """""""" " 704/229 4,972,484 A * 11/1990 Thelle 6161. .. 704/200.1 * 5,048,054 A 9/1991 Eyuboglu 6161. 375/222 5,144,569 A * 9/1992 Kunold ........ .. 708/300 5,170,413 A * 12/1992 H6ss 6161. 375/260 5,243,629 A
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9/1993
5,285,474 A
*
2/1994 Chow et a1. .............. .. 375/231
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K- Brandenburg & J-D- Johnson, “Second Generanon Per
ceptual Audio Coding: The Hybrid Coder”, 1990. K. Brandenburg & Gerhard Stoll, “The ISO/MPEGiAudio d
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Brandenburgs Karlheinz and Johnston, JD” AT&T Bell Laboratories, Second Generation Perceptual Audio Coding: .
The Hybrld Coder, Presented at the 88
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Convention, Mar.. .
13719, 1990’ Montreuxa AES’ PP- 1712, An Aule Engl'
neenng $001er Prepnm, PrePr1n12937A
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US RE40,281 E 1
2
SIGNAL PROCESSING UTILIZING A TREE STRUCTURED ARRAY
utilize a uniform bandwidth. Hence, the number of sub bands is at least as great as the total bandwidth available for
Matter enclosed in heavy brackets [ ] appears in the original patent but forms no part of this reissue speci?
transmission divided by the bandwidth of the smallest sub-band. The size of the smallest sub-band is determined by need to characterize each channel by a single attenuation and
phase shift. Thus, the sub-band having the most rapidly
cation; matter printed in italics indicates the additions made by reissue.
varying distortion sets the number of sub-bands and the computational workload in the case in which white noise is the primary contributor to the signal-to-noise ratio. In systems in which the major source of interference is narrow band interference, the minimum sub-band is set with
[This application] This application is a Division of US. Reissue application No. 10/603,833?ledJun. 26, 2003, now abandoned, which is a Reissue of US. application No. 08/804,909, ?led Feb. 25, 1997, now US. Pat. No. 6,252, 909, issued Jun. 26, 200]. US. application No. 08/804,909, ?led Feb. 25, 1997, is a Continuation-in-Part of US. patent
reference to the narrowest sub-band that must be removed from the communication channel to avoid the interference. Consider a communication channel consisting of a twisted
application Ser. No. 08/307,331, ?led Sep. 16, 1994, Pat. No. 5,606,642, which is a division of US. Patent Applica
pair of wires which is operated at a total communication band which overlaps with the AM broadcast band in fre
tion Ser. No. 07/948,147, ?led Sep. 21, 1992, Pat. No.
5,408,580.
quency. Because of the imperfect shielding of the wires, interference from strong radio stations will be picked up by the twisted pair. Hence, the sub-bands that correspond to
FIELD OF THE INVENTION
The present invention relates to data transmission systems, and more particularly, to an improved multi-carrier transmission system. The present invention further relates to
20
audio compression and decompression systems.
these radio signals are not usable. In this case, prior art systems break the communication band into a series of uniform sub-bands in which certain sub-bands are not used. Ideally, the sub-bands are suf?ciently narrow that only the
portion of the spectrum that is blocked by a radio signal is BACKGROUND OF THE INVENTION
25
lost when a sub-band is marked as being unusable.
In prior art multi-carrier systems, a communication path
Broadly, it is the object of the present invention to provide
having a ?xed bandwidth is divided into a number of
an improved multi-carrier transmission system. It is a further object of the present invention to provide a multi-carrier transmission system having a lower computa
sub-bands having different frequencies. The width of the sub-bands is chosen to be the same for all sub-bands and small enough to allow the distortion in each sub-band to be
tional workload than imposed by systems having bands of equal band-width.
modeled by a single attenuation and phase shift for the band. If the noise level in each band is known, the volume of data
These and other objects of the present invention will
sent in each band may be maximized for any given bit error
become apparent to those skilled in the art from the follow
rate by choosing a symbol set for each channel having the maximum number of symbols consistent with the available signal-to-noise ratio of the channel. By using each sub-band
nying drawings.
at its maximum capacity, the amount of data that can be transmitted in the communication path for a given error rate is maximized. For example, consider a system in which one of the sub-channels has a signal-to-noise ratio which allows at least 16 digital levels to be distinguished from one another with an acceptable error rate. In this case, a symbol set having 16
ing detailed description of the invention and the accompa
While digital audio recordings provide many advantages over analog systems, the data storage requirements for high-?delity recordings are substantial. A high ?delity 40
recording typically requires more than one million bits per
second ofplayback time. The total storage needed for even a short recording is too high for many computer applica tions. In addition, the digital bit rates inherent in non
compressed high ?delity audio recordings makes the trans
possible signal values is chosen. If the incoming data stream
mission of such audio tracks over limited bandwidth
is binary, each consecutive group of 4 bits is used to compute the corresponding symbol value which is then sent on the communication channel in the sub-band in question.
transmission systems di?icult. Hence, systems for compress ing audio sound tracks to reduce the storage and bandwidth requirements are in great demand.
In digitally implemented multi-channel systems, the actual synthesis of the signal representing the sum of the
One class ofprior an audio compression systems divide 50
various modulated carriers is carried out via a mathematical transformation that generates a sequence of numbers that
interval represented by each segment, the sound track is analyzed to determine the signal components in each ofa plurality offrequency bands. The measured components are
represents the amplitude of the signal as function of time. For example, a sum signal may be generated by applying an inverse Fourier transformation to a data vector generated from the symbols to be transmitted in the next time interval.
then replaced by approximations requiring fewer bits to 55
approximation to the original sound track is generated by reversing the analysis process with the approximations in
corresponding inverse transformation. The computational workload inherent in synthesizing and
There are two factors that determine the number of
sub-bands in prior art systems. First, the prior art systems
represent, but which preserve features of the sound track that are important to a human listener. At the receiver, an
Similarly, the symbols are recovered at the receiver using the
analyzing the multi-carrier signal is related to the number of sub-bands. For example, if Fourier transforms are utilized, the workload is of order NlogN where N is the number of sub-bands. Similar relationships exist for other transforms. Hence, it is advantageous to minimize the number of sub bands.
the sound track into a series of segments. Over the time
place ofthe original signal components. 60
The analysis and synthesis operations are normally car ried out with the aid ofperfect, or near perfect, reconstruc
65
tion?lter banks. The systems in question include an analysis ?lter bank which generates a set of decimated subband outputs from a segment of the sound track. Each decimated subband output represents the signal in a predetermined frequency range. The inverse operation is carried out by a synthesis ?lter bank which accepts a set of decimated
US RE40,281 E 3
4
subband outputs and generates therefrom a segment of audio sound track In practice, the synthesis and analysis ?lter banks are implemented on digital computers which may be general purpose computers or special computers designed to more e?iciently carry out the operations. Ifthe analysis and synthesis operations are carried out with
could be merged to provide a decomposition into critical bands. This approach imposes the same temporal con straints on allfrequency bands. That is, the time window over which the low frequency data is generated for each band is the same as the time window over which each
high-frequency band is generated. To provide accuracy in
su icient precision, the segment ofaudio sound track gen
the low frequency ranges, the time window must be very
erated by the synthesis ?lter bank will match the original
long. This leads to temporal artifacts that become audible at higher frequencies. Hence, systems in which the audio segment is decomposed into uniform sub-bands with adequate low-frequency resolution cannot take full advan
segment of audio sound track that was inputted to the
analysis ?lter bank. The dijferences between the recon structed audio sound track and the original sound track can
be made arbitrarily small. In this case, the specific ?lter bank characteristics such as the length of the segment analyzed, the number of?lters in the ?lter bank, and the location and shape of?lter response characteristics would be oflittle interest, since any set of?lter banks satis?/ing the perfect, or near-perfect, reconstruction condition would exactly regenerate the audio segment.
tage of the critical band properties of the auditory system. Prior art systems that recognize this limitation have
attempted to solve the problem by utilizing analysis and synthesis?lter banks based on QMF?lter banks that analyze a segment of an audio sound track to generate frequency components in two frequency bands. To obtain a decompo
sition of the segment into frequency components represent ing the amplitudes ofthe signal in critical bands, these two
Unfortunately, the replacement of the frequency compo nents generated by the analysis ?lter band with a quantized
20
approximation thereto results in artifacts that do depend on the detail characteristics of the ?lter banks. There is no single segment length for which the artifacts in the recon structed audio track can be minimized. Hence, the length of the segments analyzed in prior art systems is chosen to be a
structured con?guration. That is, each of the outputs of the ?rst level ?lter becomes the input to another ?lter bank at 25
compromise. When the frequency components are replaced
That is, a low frequency component represents the signal 30
?lter characteristics of the corresponding ?lter in the ?lter
amplitude in an audio segment that is much longer than a high-frequency component. Hence, the need to choose a
single compromise audio segment length is eliminated.
bank. The noise signal will be present over the entire
segment of the reconstructed sound track. Hence, the length of the segments is re?ected in the types of artifacts intro duced by the approximations. If the segment is short, the
least one ofwhose two outputs isfed to yet another level, and so on. The leafnodes ofthis tree provide an approximation to a critical band analysis ofthe input audio track It can be
shown that this type of?lter bank used dijferent length audio segments to generate the dijferent frequency components.
by approximations, an error is introduced in each compo nent. An error in a given frequency component produces an
acoustical ejfect which is equivalent to the introduction of a noise signal with frequency characteristics that depend on
frequency based QMF ?lters are arranged in a tree
While tree structured?lter banks having many layers may be used to decompose the frequency spectrum into critical
bands, such ?lter banks introduce signi?cant aliasing arti
35
facts that limit their utility. In a multilevel ?lter bank, the aliasing artifacts are expected to increase exponentially with the number of levels. Hence, ?lter banes with large numbers of levels are to be avoided. Unfortunately, ?lter banks based
40
hand, ifthe segment is too long, temporal resolution ofthe human auditory system will detect artifacts.
signals require large numbers of levels.
Prior art systems also utilize ?lter banks in which the frequency bands are uniform in size. Systems with a few (16-32) sub-bands in a 0-22 kHz frequency range are
suited to applications in which the playback ofthe material
artifacts are less noticeable. Hence, short segments are
preferred. However, the segment is too short, there is insu icient spectral resolution to acquire information needed to properly determine the minimum number ofbits needed to represent each frequency component. On the other
on QMF?lters which divide the signal into two bandlimited
Prior art audio compression systems are also poorly is to be carried out on a digital computer. The use ofaudio 45
generally called “subband coders ” while those with a large
for computer applications is increasingly in demand. Audio is being integrated into multimedia applications such as computer based entertainment, training, and demonstration
number of sub-bands (.gtoreq.64) are called “transform coders”. It is known from psychophysical studies of the
systems. Over the course of the next few years, many new
human auditory system that there are critical bandwidths which vary with frequency. The information in a critical
personal computers will be out?tted with audio playback and recording capability. In addition, existing computers will be upgraded for audio with the addition ofplug-in
50
band may be approximated by a component representing the time averaged signal amplitude in the critical band.
peripherals.
In addition, the ear’s sensitivity to a noise source in the presence of a localized frequency component such as a sine
limited to the use of costly outboard equipment such as an
tone depends on the relative levels ofthe signals and on the relation of the noise spectral components to the tone. The
Computer based audio and video systems have been 55
analog laser disc player for playback of audio and video. This has limited the usefulness and applicability of such
errors introduced by approximating the frequency compo
systems. With such systems it is necessary to provide a user
nents may be viewed as “noise”. The noise becomes sig
with a highly specialized playback con?guration, and there is no possibility of distributing the media electronically. However, personal computer based systems using com pressed audio and video data promise to provide inexpensive playback solutions and allow distribution ofprogram mate
ni?cantly less audible ifits spectral energy is within one critical bandwidth of the tone. Hence, it is advantageous to use frequency decompositions which approximate the criti cal band structure of the auditory system.
60
rial on digital disks or over a computer network.
Systems which utilize uniform frequency bands are poorly suitedfor systems designed to take advantage of this type of approximation. In principle, each audio segment can be
analyzed to generate a large number of uniform frequency bands, and then, several bands at the higher frequencies
Until recently, the use ofhigh quality audio on computer 65
platforms has been limited due to the enormous data rate
required tier storage and playback Quality has been com promised in order to store the audio data conveniently on
