Method for transmitting multiresolution audio signals in a radio frequency communication system as determined upon request by the code-rate selector

The present invention relates to a method for transmitting multiresolution audio signals via wireless devices in a radio frequency communication system wherein audio signals are decomposed into levels of resolution. The audio signal is decomposed into levels including a base signal at a base transmission rate and one or more signal details and input into a code rate selector, controlled by either party to the communication. The base signal represents the coarsest resolution or quality of the signal. Each signal detail, when added to the base signal, improves the resolution of the signal by increasing the detail and the transmission rate. An audio receiving unit transmits a request for audio transmission to the audio transmitting unit. In response to the initial request, the base signal is transmitted to the audio receiving unit. If the base signal is insufficient, the sound quality can be increased incrementally by sending further requests to transmit additional signal detail from the code rate selector. The additional signal detail is then recombined with the base signal at the audio transmitting unit to provide a higher level of sound quality to the signal. The higher quality audio signal is then transmitted to the audio receiving unit.

FIELD OF THE INVENTION 
The present invention relates to wireless communication systems and more 
particularly to a method for transmitting multiresolution audio signals 
via wireless devices in a radio frequency communication system. 
BACKGROUND OF THE INVENTION 
In current technology, a radio frequency communication system following a 
specific standard is governed by a maximum audio transmission rate. A 
typical maximum audio transmission rate is on the order of 64,000 bits per 
second (64 kbps) which corresponds to high fidelity audio. However, 
transmission of audio signals at full rate requires a significant amount 
of resources in terms of power and spectrum. Thus, full rate audio 
transmission is not cost-efficient for applications such as mobile 
cellular communications, but is more suited to wireline communication 
systems or high-power directive microwave links where these parameters are 
not as critical. 
In addition, for mobile cellular communication systems, the predominant use 
is for voice communication such as conversations between parties via 
radiotelephones. For simple voice communication, high fidelity audio is 
not critical for efficient communication. The human ear is capable of 
discerning audio signals at much lower resolution. In these instances, 
audio transmission rates on the order of 3,200 bits per second (3.2 kbps) 
may provide sufficient audio quality for the parties to the conversation. 
Communication at this level results in significant conservation of power 
and spectrum resources for the communication system. However, there may be 
instances where high fidelity audio is required, for example, where a 
musical recording must be transmitted. The problems lies in that the audio 
transmission rate is determined when the audio signal is coded and the 
coding device typically is set to code the signal at a single 
predetermined rate. Thus, there exists a trade-off between conservation of 
power and spectrum resources at a low transmission rate and providing an 
acceptable audio resolution to the system user at a high transmission 
rate. Therefore, a system is needed to eliminate this trade-off whereby 
the resolution or the quality of an audio signal transmitted via radio 
frequency communication systems can be varied by the users to meet 
specific situations. 
SUMMARY OF THE INVENTION 
The present invention relates to a method for transmitting multiresolution 
audio signals via wireless devices in a radio frequency communication 
system. 
This method uses wavelet techniques to decompose an audio signal. The 
decomposed original signal includes a base signal, representing the lowest 
resolution of the original signal, and one or more signal details which, 
when added to the base signal, provide increasing levels of resolution. 
The maximum number of levels of resolution into which the audio is 
decomposed may be determined by the resolution limits of the audio 
transmitting unit or by artificial limitation on the audio transmitting 
unit by the transmitting party. 
After establishing a communication channel, the base signal is transmitted 
to the audio receiving unit. The audio receiving unit includes means for 
incrementing the resolution of the signal by sending requests for 
additional signal detail to the audio transmitting unit. Additional signal 
details are then combined with the previous level of resolution, again 
using wavelet techniques, to create an audio signal of higher resolution. 
The higher resolution audio signal is then transmitted to the audio 
receiving unit. The audio receiving unit may send multiple requests for 
additional signal details. Each time, the additional signal detail is 
combined with the previous level of resolution to provide a new audio 
signal of higher resolution. 
One advantage to decomposing the audio signal into incremental levels of 
resolution is that only the level of resolution necessary to provide 
agreeable quality and intelligibility of the audio signal is transmitted. 
For example, audio signals such as an ordinary voice conversation may only 
require an intermediate or low level of audio signal resolution to be 
intelligible. Bandwidth and power are saved by transmitting a lower 
resolution. Some audio, such as musical works, may require high resolution 
to be effective for the purposes of the party receiving the communication. 
In such cases, the receiving party can request higher levels of 
resolution. 
The required level of resolution can be determined by either party to the 
communication or by the resolution limits of the communication devices. 
Representative applications of this concept include transmission of audio 
signals from hand-held cellular radio devices or from compact disc players 
connected via an interface to a hand-held cellular radio devices. 
Therefore, it can be seen that the main advantage of this method will be 
to lessen the power and spectrum requirements of radio frequency 
communication system while allowing the users of the devices to determine 
the optimal level of resolution necessary to suit their needs.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, FIG. 1 shows the multiresolution audio 
transmission system, indicated generally by the numeral 10. The basic 
elements of the multiresolution audio transmission system 10 are an audio 
transmitting unit generally numbered as 20, an audio receiving unit 
generally numbered as 70, and a communications media 110 providing a 
communications link between the audio transmitting unit 20 and the audio 
receiving unit 70. 
FIG. 2 shows the audio transmitting unit 20 in greater detail. The audio 
transmitting unit 20 basically comprises an audio processing unit 30, an 
audio quality control unit 50, and a display 65. The audio processing unit 
30 digitally codes the audio input, decomposes the signal, selects the 
level of resolution, and encodes transmission rate and error protection 
parameters before the signal is transmitted to the audio receiving unit 
70. The audio quality control unit 50 enables either the transmitting 
party or the receiving party to increase the level of the quality of the 
audio transmission. The level of audio quality or resolution used by the 
audio transmitting unit 20 for the audio transmission is indicated on the 
display 65. 
The audio processing unit 30 includes an audio input device 32, an 
analog-to-digital converter 34, a full-rate signal coder 36, a signal 
decomposition unit 38, a code rate selector module 40, an unequal error 
protection module 42, a signal combining unit 44, and a transmitter module 
46. An audio signal is input into the audio transmitting unit 20 via an 
audio input device 32. An audio input device 32 may include, for example, 
a microphone, a piezo-electric transducer, or an audio playback device 
such as a tape recorder or a compact disc player. The analog-to-digital 
converter 34 converts the voice signal to a digital form. The digital 
signal is then coded by the full-rate signal coder 36 to compress the 
signal. The output of the full-rate signal coder 36 is a standard 13 kbps 
audio signal. The output of the full-rate signal coder 36 is decomposed by 
the signal decomposition unit 38. The signal decomposition unit 38 
decomposes the full rate audio signal into a low rate (coarse) base audio 
signal and discrete incremental levels of signal detail. The signal detail 
increases at each level such that the addition of levels to the base 
signal increases the resolution or quality of the resulting transmitted 
audio signal by adding detail and increasing the transmission rate. The 
full rate audio signal may be decomposed, for example, by using wavelet 
techniques or other pyramidal decomposition schemes. Such methods are 
well-known to those skilled in the art and therefore are not described 
herein. For a more detailed explanation of wavelet techniques, see S. G. 
Mallat, A Theory for Multiresolution Signal Decomposition: The Wavelet 
Representation, IEEE Transactions on Pattern Analysis and Machine 
Intelligence, vol. 11, no. 7, pp. 674-693 (July, 1989), which is 
incorporated herein by reference. The code rate selector module 40 
determines which signals, S.sub.0, D.sub.1, D.sub.2. . . D.sub.0, to pass 
to the unequal error protection module 42 for channel coding. The signal 
combining unit 44 combines the base signal and/or signal details after 
channel coding. The transmitter module 46 is used to transmit the audio 
signal at a selected resolution to a remotely located audio receiving unit 
70. 
The audio quality control unit 50 controls the level of resolution 
transmitted by the audio transmitting unit 20. The audio quality control 
unit 50 includes a receiver module 52, a selector controller 54, a counter 
56, and an local control device 58. The receiver module 52 receives and 
demodulates signal detail requests from the audio receiving unit 70. The 
selector controller 54 processes the transmission request and instructs 
the code rate selector module 40 to pass the appropriate detail signals 
based upon the request. The counter 56 maintains a count of the total 
number of details transmitted by the image transmitting unit 20. Incoming 
requests for transmission of additional signal details, also referred to 
herein as transmission requests, are directed through the receiver module 
52. The code rate selector module 40 is then directed to increase the 
level of sound quality by adding the requested additional signal details 
to the existing signal. The resolution of the signal is increased since 
the addition of details includes a corresponding increase in the rate of 
signal transmission for each detail. In general, the higher the signal 
transmission rate, the higher the quality of the transmitted audio. At the 
same time that signal details are added, the counter module 56 is also 
updated to maintain count of the total levels of signal detail used for 
the audio transmission. 
The level of resolution may also be incremented without first receiving a 
transmission request from the audio receiving unit 70. This would be 
accomplished by a local control device 58 incorporated into the audio 
transmitting unit 20 and controlled by a man-machine interface such as a 
keypad connected to the audio transmitting unit 20. In such instances, the 
user of the audio transmitting unit 20 could increase the resolution of 
the audio signal by adding signal details in response to a verbal request 
from the user of the audio receiving unit 70. The code rate selector 40 
would then increase the resolution of the audio signal transmitted to the 
audio receiving unit 70 by adding signal details and increasing the 
transmission rate. In this situation, in order to maintain the resolution 
level indicated to both parties to the conversation, information coded 
with the transmitted audio signal would notify and/or update the counter 
96 in the audio receiving unit 70 to the resolution level of the signal. 
The audio receiving unit 70, shown in FIG. 3, comprises a reception unit 80 
and an audio quality adjusting unit 90. The reception unit 80 receives the 
audio signal and converts the signal to an audio output which is heard by 
the receiving party. The reception unit 80 includes a receiver module 82, 
an audio signal processor 84, and an audio output device 86. The received 
audio signal is demodulated by the receiver module 82. The demodulated 
signal is then decoded by the audio signal processor 84 and applied to the 
output device 86 to generate an audible signal. 
The audio quality adjusting unit 90 allows the receiving party to make an 
initial request to the transmitting party to begin the audio transmission. 
It also enables the receiving party to send requests for signal details if 
the received sound quality is insufficient. The audio quality adjusting 
unit 90 includes a audio quality control device 92, a timer 94, a counter 
module 96, a display 98, and a transmitter module 100. 
The audio quality control device 92 is used to increment the resolution of 
the audio signal. The audio quality control device 92 may, for example, 
comprise a pushbutton. Pushing the button 92 causes a transmission request 
to be sent via the transmitter module 100. The timer 94 is used to delay 
the transmission of any requests for signal details so that multiple 
presses of the button 92 can be accumulated and sent as a single request. 
The counter module 96 keeps a count of the number of transmission requests 
sent to the audio transmitting unit 20. The resolution level of the 
received audio signal is indicated on the display 98. 
The communications media 110 illustrated in FIG. 1 comprises the means by 
which the audio transmitting unit 20 communicates with the audio receiving 
unit 70. While the means of communication is not unique to the present 
invention, it may comprise such methods as wireline, radio frequency, 
infrared, or microwave. Subclasses to the means of communication may be 
any channels which are dedicated for specific roles in the communication 
between the audio transmitting unit 20 and the audio receiving unit 70. In 
the present invention, a radio frequency means of communication is 
assumed. 
To use the transmission system of the present invention, an audio signal S 
is first digitally coded and then decomposed using, for example, wavelet 
techniques into a base signal S.sub.0 and a series of signal details 
D.sub.1, D.sub.2, . . . D.sub.n. The signal details D.sub.1, D.sub.2, . . 
. D.sub.n can then be sequentially recombined with the base signal 
S.sub.0, coded at a base transmission rate, R.sub.0, to provide increasing 
levels of quality of the audio signal at increasing rates of signal 
transmission. For example, S.sub.0 combined with D.sub.1 would provide an 
audio signal of one resolution level above the base signal. This signal 
would have a corresponding audio signal transmission rate, R.sub.1. 
Similarly, S.sub.0 combined with D.sub.1 and D.sub.2 would provide a audio 
signal of two resolution levels above the base signal. This signal would 
have an audio signal transmission rate, R.sub.2, where R.sub.2 is greater 
than R.sub.1. Following this concept, S.sub.0 combined with D.sub.1, 
D.sub.2, . . . and D.sub.n would provide an audio signal with the highest 
available level of resolution corresponding to the original audio signal. 
Thus, accordingly, the highest level of resolution would be transmitted at 
the highest rate of transmission, R, corresponding to high fidelity audio. 
Note that due to the hierarchical nature of the multiresolution 
transmission system 10, the base signal is the most important data in a 
multiresolution audio transmission. The importance of the base signal lies 
in that it is the foundation upon which subsequent signal details are 
added. As such, it is necessary to preserve the integrity of this data 
upon transmission. For the same reason, the lower levels of signal detail 
that are added to the base signal to improve the resolution are more 
important than the higher level fine signal details. Thus, in addition to 
assigning progressively faster signal transmission rates as the requested 
resolution increases, the multiresolution audio transmission system 
assigns varying error protection to the components of the signal based on 
the relative importance of those components. The base signal is assigned 
the most error protection and each signal detail from D1 . . . D.sub.N is 
assigned a decreasing level of error protection. This process of assigning 
error protection to the components of the audio signal is termed unequal 
error protection and is encoded in each component of the audio signal 
prior to combining the components for transmission. 
After the original audio signal is decomposed, the audio transmitting unit 
20 determines the number of levels of the signal which will be transmitted 
to the audio receiving unit 70. When a transmission request is received 
from the audio receiving unit 70, the audio transmitting unit 20 responds 
by sending the requested level of the audio signal data. Usually, only the 
base signal is transmitted in response to the initial transmission 
request. This signal is most often an order of magnitude smaller than the 
original signal. The initial audio signal, however, may also comprise the 
base signal and one or more signal details. In such case, the base signal 
and the specified number of signal details are selected by the rate code 
selector module 40 prior to transmission. 
If more detail is required, the recipient can send additional transmission 
requests (in real time) for more details. For each additional transmission 
request, the audio transmitting unit 20 responds by directing the code 
rate selector module 40 to increase the level of signal detail which is 
sent to the audio receiving unit 70. Thus, after the original audio signal 
is decomposed at the signal decomposition unit 38, the full resolution of 
the signal (the base signal and all detail levels up to the maximum level 
of decomposition) are available for transmission to the audio receiving 
unit 70. Subsequently, the code rate selector module 40 acts as a "valve" 
which allows only the requested level of signal detail to pass through for 
transmission. When additional signal details are requested, the "valve" 
opens further to allow more signal details to pass through. Furthermore, 
as the level of detail increases, the rate of signal transmission also 
increases. The result is improved resolution of the audio signal. After 
the levels of detail are determined, unequal error protection is assigned 
to each level of the signal. However, prior to transmission, the audio 
signal details are recombined with the base signal and any previously 
added signal details such that a unified signal is transmitted. If a 
request for multiple signal details is received by the audio transmitting 
unit 20, the corresponding number of signal details are added to the 
existing level of resolution, allowing a higher quality signal to be 
transmitted. 
Note that the user of the audio transmitting unit 20 may be able to 
designate specific audio receiving units 70 which would be able to receive 
multiresolution audio signals. In practice, an example of this imposed 
limitation in a radiotelephone communication system would be where the 
user of the audio transmitting unit 20 programs certain authorized 
telephone numbers into the device. As a result, only callers from those 
specific telephone numbers with audio receiving units 70 would have full 
multiresolution audio signal reception capability. Callers from numbers 
other than those specifically authorized by the user of the audio 
transmitting unit 20 would receive either no audio signal or an audio 
signal at a preset level of resolution. Here, the blocking of all audio 
signal transmission would comprise a security feature to prevent reception 
of audio signals by unauthorized parties. The transmission of only the 
marginally usable base signal to unauthorized numbers may also have a 
similar security effect. On the other hand, transmission of any higher 
level of resolution, such as the base signal combined with a number of 
signal details, may also be set by the user of the audio transmitting unit 
20 in order to limit the transmission resolution. The user of the audio 
transmitting unit 20 may determine that an intermediate level of 
resolution is all that the receiving party needs and transmit only that 
level. Therefore, these features will assist both in the security and in 
the optimization of bandwidth and power parameters of audio transmission. 
Referring now to FIGS. 4 and 5, the operations of the audio transmitting 
unit 20 and audio receiving unit 70 are shown in greater detail. During 
the call initiation, each party to the communication establishes the 
initial level of resolution to be transmitted, such as S.sub.0 or S.sub.0 
combined with D.sub.1, etc. The step of establishing the minimum level of 
resolution is indicated by function block 120 (FIG. 4) for the audio 
transmitting unit 20 and function block 150 (FIG. 5) for the audio 
receiving unit 70. Typically, this level would be preset in both the audio 
transmitting unit 20 and the audio receiving unit 70 by the respective 
user. If the level specified by each party is different, the minimum of 
the two values would be the limiting initial parameter. 
In addition to setting a minimum level of resolution, the transmitting 
party may also be able to specify the maximum level of resolution 
available to the receiving party. The maximum level of resolution set by 
the transmitting party may be less than the resolution of the original 
audio signal when initially input into the system. For instance, the audio 
signal may have originally been decomposed into a base signal and seven 
additional signal details. The transmitting party could limit the maximum 
available resolution to four additional signal details. Accordingly, the 
receiving party would only be able to receive a maximum resolution of the 
base signal with four additional signal details, not the full resolution 
of the original signal. This feature is useful where the transmitting 
party bears the cost of the communication and seeks to limit the air time 
required for the audio transmission. 
After the minimum level of resolution has been established, audio 
transmission starts when a transmission request is sent from the audio 
receiving unit 70 to the audio transmitting unit 20 via a dedicated 
control channel DCC such as the fast associated control channel (FACCH) or 
the slow associated control channel (SACCH). Note that a dedicated control 
channel DCC may be utilized for the transmission request since no audio 
signal is sent. 
As previously described, the analog audio signal input through the audio 
input device 32 is converted to a digital signal by the analog-to-digital 
converter 34. The single digital signal is then coded by the full rate 
signal coder 36 at the full transmission block 126) to ensure that the 
number of signal details requested by the audio receiving unit 70 does not 
exceed the maximum level available for the audio signal. 
When the requested level of resolution has been selected by the code rate 
selector module 40, the base signal and any signal details comprising the 
request are each coded with error protection at the unequal error 
protection module 42 (function block 128). The base signal and the 
selected signal details are then combined by the signal combining unit 44 
(function block 130) into a single audio signal. Since the base signal and 
the signal details each include different transmission rates, the 
combination of these elements produces a cumulative transmission rate for 
the signal. The audio signal is then transmitted to the audio receiving 
unit 70 via the transmitter module 46 (function block 132). 
After transmission of the audio signal, the transmitting party may 
terminate the call (decision block 134). If the call is not terminated, 
the receiving party may send requests for additional signal detail 
(decision block 122). As before, if a request for additional signal detail 
is received, the resolution of the previous audio signal is increased 
accordingly prior to transmission. However, if no request for additional 
signal detail is received by the audio transmitting unit 20, the existing 
level of resolution is used for the remainder of the communication between 
the two parties (decision block 136 and function block 138). When the 
parties terminate the communication (decision block 134), the counter 60 
is reset (function block 140) before the call is ended. 
Referring now to FIG. 5, the operation of the audio receiving unit 70 is 
shown. The audio transmission process begins when the audio receiving unit 
70 sends a rate available for the particular audio transmission system in 
use. At this point, the signal is decomposed by the signal decomposition 
unit 38 into a base signal and a number of signal details. The number of 
signal details resulting from the decomposition of the original signal is 
called the maximum level of decomposition. The maximum level of 
decomposition is determined by the computational complexity of the audio 
transmitting unit 20 or it may be artificially limited by the transmitting 
party. Once decomposed, the components of the original audio signal are 
selectively transmitted to the audio receiving unit 70 as described 
herein. 
Referring now to FIG. 4, the operation of the audio transmitting unit 20 is 
shown. When a transmission request is received by the audio transmitting 
unit 20 (decision block 122), the audio transmitting unit 20 adjusts the 
resolution of the audio signal by selecting the appropriate levels of 
detail. As generally indicated by function block 124, when the first 
transmission request is received, the audio transmitting unit 20 directs 
the code rate selector 40 to select the base signal for transmission to 
the audio receiving unit 70. Thereafter, the audio transmitting unit 20 
directs the code rate selector 40 to select the next level of signal 
detail for addition to the base signal in response to each transmission 
request. If a request for multiple details is received, the audio 
transmitting unit 20 directs the code rate selector 40 to select that 
number of details for addition to the previous level of resolution over 
the base signal (function block 124). As indicated by function block 126, 
each time a transmission request for audio signal data is received, the 
counter module 60 increments the count (function transmission request to 
the audio transmitting unit 20 (decision block 152). The counter module 96 
is then incremented to take count of the number of audio signal details 
requested (function block 154). After the audio receiving unit 70 receives 
the audio transmission from the audio transmitting unit 20 at the 
requested resolution level (function block 156), the receiving party 
listens to the audio output from the audio output device 86 to determine 
if the sound quality is acceptable. 
After reception of an audio signal, the receiving party may opt to 
terminate the call (decision block 160). If the call is not terminated and 
if the receiving party is not satisfied with the resolution or the quality 
of the sound after reception of the initial audio signal S.sub.1, the 
audio resolution may be increased by pressing the button 92 the number of 
times corresponding to the number of additional signal detail levels 
desired. Activation of the button 92 (decision block 152) leads to three 
events: the timer 94 is activated, an increment corresponding to the 
extent of activation of the button 92 is registered by the counter module 
96 (function block 154), and the total resolution level is indicated on 
the display 98. After the specified time-out, the timer 94 commands the 
counter module 96 to send a request for additional signal details to the 
audio transmitting unit 20 corresponding to the increment registered by 
the counter module 96. For example, if the button 92 is pressed 2 times, a 
request for two additional signal details is sent via the dedicated 
control channel DCC. Similarly, if the receiving party had activated the 
button 92 three times, the audio transmitting unit 20 would be requested 
to add three additional signal details to the existing level of audio 
resolution transmitted to the audio receiving unit 70. 
The process of incrementally tuning the resolution of the audio signal 
continues as specified in the preceding paragraph until the receiving 
party is satisfied with the resolution of the audio output or until the 
maximum number of signal details has been reached. If no additional 
requests for signal details is sent by the receiving party (decision block 
152), the communication between the parties continues at the existing 
level of resolution (function block 162). On termination of the 
communication (decision block 160), the counter 96 is reset prior to the 
end of the call (function block 164). 
The described method for transmitting multiresolution audio signals 
illustrates the increased level of efficiency which may be realized by 
enabling the parties to a communication to determine the optimal level of 
resolution or sound quality. This method would result in significantly 
more efficient transmission of sound, especially if the highest level of 
resolution is not required for the user's needs. Therefore, when applied 
to radio frequency communications, this method would serve to increase the 
efficiency of audio communication by reducing bandwidth requirements and 
power consumption. 
The present invention may, of course, be carried out in other specific ways 
than those herein set forth without parting from the spirit and essential 
character of the invention. The present embodiments are, therefore, to be 
considered in all respects as illustrative and not restrictive, and all 
changes coming within the meaning and equivalency range of the appended 
Claims are intended to be embraced therein.