Abstract:
The invention relates to a method and apparatus for achieving maximal coding gain for audio transmission. More particularly, at a chosen sample rate and frequency range value, an audio input signal is downsampled to the sample rate, encoded and transmitted at a given bit rate. At the receiving end, the downsampled signal is decoded and upsampled to the original or other suitable sample rate. The upsampled signal is then audibly output. Since resampling ratios using “small” numbers prove to be more computationally efficient, this method and apparatus supports resampling ratios which imply both standard and non-standard sampling ratios in the codec.

Description:
[0001]    This non-provisional application claims the benefit of U.S. Provisional Application 60/114,719, filed Dec. 30, 1998, the subject matter of which is incorporated herein by reference.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of Invention  
           [0003]    The invention relates to audio signal transmission, and more particularly to varying the sample-rate to improve coding gain for audio signals.  
           [0004]    2. Description of Related Art  
           [0005]    There are a number of decisions which must be made in setting up an audio compression system. Among the most important variables that affect audio quality during encoding are the sampling rate, bit rate, and the frequencies that will be encoded, such as 20 Hz-20 KHz or some lesser range, for example. For a given level of distortion and a given algorithm, more bits are required to transmit more signal frequencies. Therefore, there is a optimal match between bit rate and frequency range such that if the bit rate is specified, distortion will increase if more frequencies are encoded then is optimal for that bit rate.  
           [0006]    Most high-quality audio algorithms, such as MPEG AAC (MPEG Advanced Audio Coder), PAC (Perceptual Audio Coder), MPEG layer3, Dolby AC3 (Advanced Coder 3), and NTT&#39;s TwinVQ, encode a fixed number of samples into each frame which then represent a unit of time for a particular algorithm. Each audio frame carries side information. The number of bits needed to encode the side information per frame is roughly constant. This side information imposes a per-frame overhead.  
           [0007]    The frame frequency (i.e., the number of frames per second) used by an audio algorithm is proportional to the sampling rate because each frame encodes a constant number of samples.  
           [0008]    Decreasing the sampling rate decreases the number of frames-per-second, which in turn decreases the number of bits diverted for overhead, allowing more bits to be used for audio coding. Thus, lowering the sampling rate results in more bits being available for audio coding which results in a higher quality signal as long as sufficient frequency range is preserved.  
           [0009]    To a similar end, the statistical properties of music indicate that an optimal frame duration is about 40 ms. For AAC and PAC at sampling rates of 44100 sps (samples per second) (i.e., the CD sample rate) the frame duration is about 23 ms; at 22050 sps, the frame duration is 46 ms.  
           [0010]    The lower the sampling rate, the lower the frequency range that can be transmitted, as described by the Nyquist rule, which limits the maximum frequency range to half of the sampling rate. In practical implementations a “guard band” is needed which further lowers the achievable maximum frequency range. For example, for any algorithm (e.g. AAC), at a sampling rate of 22050 sps, the maximum frequency range is 8 to 10 KHz.  
           [0011]    Thus, for a given algorithm, and for a given bit rate b 0  that is not sufficient for encoding the entire human-audible frequency range in a transparent manner without audible distortion, and for a specified acceptable level of distortion, there is a maximum frequency range f 0  that one can encode, and that maximum will be associated with a sample rate f s0 .  
           [0012]    If there were no outside constraints, then one would use f s0  as the sampling rate. However, several outside constraints exist. For example, PCs and Macintoshes work mostly at 44100, 22050 and 11025 sps. Some PCs work at one or more of the rates 48000, 32000, 24000, 16000 and 8000 sps, but very few PCs will work at all of these sample rates. In fact, Macintosh audio hardware will not work at all at these latter sample rates, so a user is constrained to a small set of sample rates if he or she want to interact with PCs and an even smaller set of sample rates if one wants to interact transparently with Macs without involving potentially inferior resampling in the PC or Mac.  
         SUMMARY OF THE INVENTION  
         [0013]    The invention relates to a method and apparatus for achieving maximal coding gain for audio coding and reproduction. More particularly, at a chosen sample rate and frequency range value, an audio input signal is transduced, sampled, downsampled to the encoding sample rate, encoded and transmitted at a given bit rate. At the receiving end, the downsampled signal is decoded and upsampled to the original or other suitable sample rate. The upsampled signal is then audibly output.  
           [0014]    Resampling using “small-integer” ratios (e.g. 11:8) is computationally more efficient than using arbitrary resampling ratios. This method and apparatus support both arbitrary and small-integer ratio resampling. The use of small-integer resampling frequently implies the use of non-standard sampling rates in the transmitted channel, for example 32073 sps rather than 32000 sps.  
           [0015]    These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The invention will be described with reference to the accompanying drawings, in which like elements are referenced with like numbers, and in which:  
         [0017]    [0017]FIG. 1 is an exemplary diagram of an audio transmission system;  
         [0018]    [0018]FIG. 2 is a block diagram of a generic audio encoding/decoding system;  
         [0019]    [0019]FIG. 3 is a block diagram of a generic frame-based audio encoding/decoding which operates at a bit rate too low to support the full audio bandwidth implied by the sampling rate (thru Nyquist);  
         [0020]    [0020]FIG. 4 is a block diagram of a generic frame-based audio encoding/decoding system using a low-pass filter;  
         [0021]    [0021]FIG. 5 is a block diagram of a generic frame-based audio encoder/decoder that discards spectral coefficients;  
         [0022]    [0022]FIG. 6 is a generic frame-based audio encoding/decoding system that downsamples the audio input;  
         [0023]    [0023]FIG. 7 is a block diagram of a frame-based audio encoding/decoding system according to the invention;  
         [0024]    [0024]FIG. 8 is a block diagram of a frame-based audio encoding/decoding system of the invention utilizing a non-standard downsampling ratio;  
         [0025]    [0025]FIG. 9 is a flowchart of the encoding portion of the invention; and  
         [0026]    [0026]FIG. 10 is a flowchart of the decoding portion of the invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]    [0027]FIG. 1 is an exemplary block diagram of an audio transmission system  100  of the invention. An encoding terminal  110  that downsamples and encodes audio signals is connected to a multimedia communications network  140  through modem  120  and local exchange carrier  130 . A decoding terminal  170  that receives, decodes and upsamples the audio signals is also connected to the multimedia communications network  140  through modem  160  and local exchange carrier  150 . The encoding terminal  110  and decoding terminal  170  include memory units  180  and  190 , respectively, for intermediate storage of the compressed audio signal either prior to transmission or after reception of the audio signals, for example.  
         [0028]    The multimedia communications network  140  represents any combination of existing communications networks, such as a telephone network, Internet, intranet, etc.  
         [0029]    The modem devices  120 ,  160  may be ethernet interfaces, cable modems, ISDN modems, ADSL modems, or any other interface circuit intended to connect two networks or a network and a digital computing apparatus. The modem devices  120 ,  160  may contain a conventional RJ-11 outlet for connection to computer modem, facsimiles, printers or other equipment. The modem devices  120  and  160  may also be equipped with universal serial bus (USB), integrated system digital network (ISDN) or other standard data interfaces, as will be appreciated by the person skilled in the art. However, other similar devices may be used to permit sharing of large bandwidths over media already installed.  
         [0030]    Encoding terminal  110  and decoding terminal  170  may be any pair of devices that receive and send audio signals according to the invention through the multimedia communications network  140  via modems  120  and  160 . The encoding terminal  110  and decoding terminal  170  may represent such devices as a personal computer (PC), telephone, television, facsimile, or any other device capable of sending and receiving audio signals. It may be appreciated that the encoding terminal  110  and decoding terminal  170  may include software and/or hardware for performing the encoding and decoding functions, and further that the encoding and decoding terminals may be different types of devices.  
         [0031]    It may further be appreciated that while the encoding terminal  110  and the decoding terminal  170  include memory units  180  and  190 , respectively, for intermediate storage of the compressed audio signal, the compressed audio signal may be intermediately stored in one or more other intermediate storage devices located throughout the audio transmission system  100 , such as between the modem  120 , 160  and the local exchange carrier  130 , 150 , or in the multi-media communications network  140 .  
         [0032]    In providing a more detailed discussion of the encoding and decoding of audio signals, a discussion of conventional systems is set forth in FIGS.  2 - 6  to better to explain the features and advantages of the present invention.  
         [0033]    [0033]FIG. 2 shows a generic audio encoding/decoding system  200  operating at a bit rate which is sufficient to encode all of the frequencies in the input signal. An encoder  210  located within a computing unit, for example a PC, receives an audio input signal with frequency range fin (typically spanning the range of 20 Hz-20 KHz) and encodes the signal for transmission across a communications channel.  
         [0034]    The input signal may either be analog or digital. If the input signal is analog, the encoder  210  will include an analog-to-digital conversion apparatus. However, the input signal may already be digitized, such as stored signals retrieved from an audio compact disc, for example.  
         [0035]    A decoder  220 , located within another PC for example, receives and decodes the transmitted audio signal to produce an audio output f out  which is less than fin and less than f s /2. The encoder/decoder system  200  in this example has no other specified bandwidth limit and the distortion level is unspecified. If the bit rate b ch  and the sample rate f s  are high enough (for the encoding algorithm) then the reproduced audio will be indistinguishable from the original. If either is too low, then the audio will be perceived as degraded.  
         [0036]    [0036]FIG. 3 shows a generic frame-based audio encoding/decoding system  300  operating at a high sampling rate, such as 44100 sps. The audio encoder/decoder system of FIG. 3 is similar to that of FIG. 2, but the sampling rate of 44100 sps used for encoding is too high to permit transparent audio reproduction of the full human-audible frequency range (20 Hz-20 KHz) at the specified bit rate of 96 Kbps, so a degradation in audio signal quality is perceived. In this example, as well as in the examples in FIGS.  4 - 6 , the encoder is operating at 96 Kbps and 44100 sps, although the same principles apply at other sampling rates and other bit rates.  
         [0037]    One way to improve reproduced audio signal quality when the bit rate is too low to support the full frequency range of the input is to encode less than the full frequency range. By way of reference, for a production quality AAC codec, best reproduced signal quality at 96 Kbps and 44100 sps occurs for a signal bandwidth of about 13 KHz. FIGS.  4 - 6  show various ways to decrease the audio frequency range.  
         [0038]    [0038]FIG. 4 shows a generic frame-based audio encoding/decoding system  400  operating at a high sampling rate that uses a low pass filter  410  to limit the frequency range that is encoded. In many cases, a lower sampling rate would allow a wider frequency range or alternatively a higher quality audio signal (because of frame overhead and music statistics). Consequently, the system in FIG. 4 is sub-optimal.  
         [0039]    [0039]FIG. 5 shows a generic frame-based audio encoding/decoding system  500  that operates at a high sampling rate (44100 sps) that discards spectral coefficients in the input signal to limit the frequency range that is encoded and transmitted. This operation is similar but not identical to that of the low pass filter  410  discussed above.  
         [0040]    The audio input signal is input to the Modified Discrete Cosine Transform (MDCT)  510  (or other time-to-frequency domain transform) and the spectral coefficients are discarded by the spectral coefficient discard unit  520 . The signal is then input to a noise allocation unit  530  (which computes the masking thresholds for the audio frame and quantizes the spectral coefficients according to the thresholds) which emits the compressed signal. The compressed signal is then transmitted to the decoder  220  of another computing unit (for example, another PC, or a portable audio device similar to the Diamond Rio MP3 player) for decoding and output.  
         [0041]    [0041]FIG. 6 shows a generic frame-based audio encoding/decoding system  600  that downsamples the audio input signal to limit the frequency range that is encoded and transmitted. (Resamplers typically incorporate frequency-limiting filters.) The audio input signal is downsampled by the downsampler  610  at a 2:1 ratio and is then input into encoder  210  for encoding. The signal is then transmitted across a communication channel to the decoder  220  at the receiving PC that plays out the audio signal at the downsampled rate. This will generally be suboptimal because the decoder  220  must operate at a submultiple of 44100 sps. In this example, the suboptimal would be 2:1 to 22050, which is not the rate that provides optimal frequency response.  
         [0042]    [0042]FIG. 7 shows the encoding/decoding system  700  of the invention. The audio encoding/decoding system  700  includes an optimal triplet of sample rate f s0  (in this case 32 Ksps), bit rate 96 Kbps, and the maximum supportable frequency range f 0  which at 96 Kbps/32 Ksps is about 13 kHz. The optimal triplet could be determined in a number of ways, e.g. algorithmically or by searching a table. The analog signal (or a digitized version of the analog signal) is input to the encoding unit  710  of a PC, for example, where the signal is downsampled by downsampler  730  from 44100 to 32000 and encoded by the audio encoder  740 . The encoded audio signal is then transmitted across a communications channel, through a modem, for example, at a given bit rate of 96 Kbps to another PC for output.  
         [0043]    At the receiving PC, the received signal is input to a decoding unit  720 , where a bit stream decoder  750  decodes the downsampled signal. The decoded signal is then input to the upsampler  760  which upsamples the signal to the original or other suitable sample rate. An audio output is then produced with a frequency range f out  of about 13 kHz. Note that in the example of FIG. 7, 44100 sps and 32000 sps are standard AAC rates.  
         [0044]    As discussed above in reference to FIG. 1, the encoding unit  710  and the decoding unit  720  may include memory units for intermediate storage of the compressed audio signal either prior to transmission or after reception of the audio signals, for example.  
         [0045]    It may be the case that the codec (for example, AAC) is specified at a set of standard rates; and that f s0  does not match one of these standard rates. However many codecs (such as AAC) can be modified to run at an arbitrary sample rate, and although the resulting encoding unit  710  will generate AAC bit streams that will not reproduce audio accurately unless the decoding unit  720  incorporates this invention, the perceived quality of the reproduced audio signal will be better for the bit stream that uses the non-standard rate than for a bit stream that uses any standard rate.  
         [0046]    For example, as shown in FIG. 8, the downsampling process used in FIG. 7 may be more computationally efficient when the downsampling factor is the ratio of two small numbers. Consider the case where it is desired to downsample from the standard rate of 44100 sps to the standard rate of 32000 sps. Neither  441  nor  320  (the smallest integers which preserve the 44100:32000 ratio) qualify as a small integer in this context. If a ratio of 11:8 is used, which is equivalent to the ratio of 44000:32000, we can downsample to a comparable intermediate sample rate (32073 sps) in a computationally efficient way, without degrading significantly either frequency response or distortion levels from the optimal sample rate of 32000 sps.  
         [0047]    Accordingly, as shown in FIG. 8, the process is the same as that in FIG. 7 but 32073 sps is used as the intermediate sampling frequency. 32073 sps is sufficiently close to an AAC standard rate that audio signals can be encoded using the parameters for a standard AAC rate.  
         [0048]    When the intermediate sampling rate is close to a codec standard rate, the bit stream header, which generally carries information about the sampling rate at which the audio was encoded, can indicate the nearby standard rate. This is generally advantageous because it allows a conventional decoder (i.e. one which does not incorporate the current invention) to decode the bit stream and reproduce the audio, even though the audio reproduction strictly speaking is not accurate. In this case (32073 sps sampling rate rather than the 32000 sps indicated in the bit stream header), there will be a pitch shift in the audio reproduced by the conventional decoder. This may be acceptable for some applications but not for others.  
         [0049]    However, the invention is still useful when the resulting sampling rate is not close to a standard rate, as long as it is possible to modify the audio encoding unit  710  so that it supports the non-standard rate. For example, with a downsample ratio of 9:8 one obtains a sampling rate of 39200 sps, which with a production AAC codec would support a frequency range as high as 15-17 KHz at a bit rate of 112 Kbps at an acceptable level of distortion. Since the downsample factor is again the ratio of two small numbers, the resampling process would again be computationally efficient.  
         [0050]    It may be advantageous to indicate to the decoding unit  720  what resampling ratio has been used to encode the audio, since otherwise the codec system (FIGS. 7 &amp; 8) must operate at a fixed resampling ratio. As a particular embodiment of the method and apparatus of this invention, the resampling ratio is incorporated into the bit stream within a reserved bit field of the standard header. As an alternative embodiment, the resampling ratio can be incorporated as side channel information. In a specific example, AAC permits “data packets” to be incorporated in the bit stream. These data packets are ignored by a standard AAC codec. The resampling ratio can be specified in a data packet, possibly along with other information.  
         [0051]    While the invention above has been discussed from the point of view of supporting the maximum frequency range for a given bit rate and level of distortion, there are two alternative ways of looking at this problem. Rather than support maximum frequency at a given bit rate, a frequency range and a given distortion level at a minimum bit rate may be supported. Alternatively, a given frequency range at a given bit rate may be supported to achieve the lowest distortion levels. That is, there are three interrelated variables: bit rate, distortion level, and frequency support. One can fix any two variables and use the above embodiment to achieve the best possible results for the remaining variable.  
         [0052]    [0052]FIG. 9 is a flowchart of the encoding process according to the invention. Process begins at step  1000  and proceeds to step  1010  where the sample rate f s0  and maximum frequency range f 0  are determined as an optimal pair either algorithmically or by searching a table, for example. In step  1020 , an input signal is received by the encoding unit  710  and is downsampled by downsampler  730  to f s0 . The process proceeds to step  1030  where the signal is encoded by the audio encoder  740 . The process then proceeds to step  1040  where the signal (along with a header, data packet, etc. that includes the downsampling information), is transmitted at a given bit rate from a modem across a communication channel. The encoding process then goes to step  1050  and ends.  
         [0053]    [0053]FIG. 10 is a flowchart of the decoding process. Process begins at step  1100  and proceeds to step  1110  where the downsampled signal (along with a header, data packet, etc. that includes the downsampling information) is received by another PC&#39;s (for example) decoding unit  720 . The process proceeds to step  1120  where the downsampled signal is decoded by the bit stream decoder  750  and then upsampled at step  1130  by the upsampler  760  at a ratio corresponding to the downsampling ratio included with the received downsampled signal, for example. The upsampled signal is then output in step  1140 . The process then goes to step  1150  and ends.  
         [0054]    While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, preferred embodiments of the invention is set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.