Patent Application
20050283370
[Not Applicable]
[Not Applicable]
Audio standards, such as MPEG-1, Layer 3 (also known as, and now referred to as MP3) employ lossy and lossless compression to reduce the memory and bandwidth requirements for storing and transmitting audio data.
During lossy compression, some of the original data is lost. Lossy compression includes digitization, windowing, time to frequency domain transformation, and quantization. A stochastic model of the human ear determines imperceptible portions of the original data. Accordingly, lossy compression realizes significant compression without perceptible degradation of the original signal. After lossy compression, the audio signal is represented by a series of symbols.
Lossless compression uses a variety of variable length codes for coding the symbols. The variable length codes for the symbols are designed to assign shorter codes to the most frequently occurring symbols and longer codes to the least frequently occurring symbols. The coding schemes include a number of tables that map the different symbols to different codes.
The encoded audio signal can then be transmitted and stored at a receiving terminal with an audio decoder. During play of the audio signal, the audio decoder decodes the variable length codes, inverse quantizes, transforms to the time domain, and dewindows the encoded audio signal, thereby reconstructing the original audio signal. Preferably, the foregoing occurs in real time, because most applications would require playing the audio signal at a specified speed.
The audio decoder is usually an integrated circuit. The audio decoder uses tables that map the different symbols to different codes to decode the variable length codes. The tables occupy approximately 50 KB of memory. In an integrated circuit, the amount of on-chip memory is limited and expensive. Although off-chip memory is less limited and less expensive, accessing off-chip memory is typically slower. Accessing the tables from off-chip memory may be too slow for audio decoding in real time.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.
Presented herein are system(s), method(s), and apparatus for reducing on-chip memory requirements for audio decoding.
In one embodiment, there is presented a method for decoding encoded audio signals. The method comprises fetching a first one or more tables from an off-chip memory; loading the first one or more tables into an on-chip memory; applying a first function to the encoded audio signals using the first one or more tables; fetching a second one or more tables from an off-chip memory after applying the first function; loading the second one or more tables into an on-chip memory; and applying a second function to the encoded audio signals, using the second one or more tables.
In another embodiment, there is presented an integrated circuit for decoding encoded audio signals. The integrated circuit comprises a direct memory access module, a memory, and an audio decoder. The direct memory access module fetches a first one or more tables from an off-chip memory. The memory stores the first one or more tables. The audio decoder applies a first function to the encoded audio signals using the first one or more tables. The direct memory access module fetches a second one or more tables from an off-chip memory after the audio decoder applies the first function. The memory stores the second one or more tables. The audio decoder applies a second function to the encoded audio signals, using the second one or more tables.
In another embodiment, there is presented an integrated circuit for decoding encoded audio signals. The integrated circuit comprises a memory, a direct memory access module, and an audio decoder. The direct memory access module is connected to the memory, and operable to fetch a first one or more tables from another memory and write the first one or more tables to the memory. The audio decoder is operably connected to access the first tables from the memory, and equipped to apply a first function to the encoded audio signals using the first one or more tables. The direct memory access module is operable to fetch a second one or more tables from the another memory after the audio decoder applies the first function and write the second one or more tables to the memory. The audio decoder is equipped to apply a second function to the encoded audio signals, using the second one or more tables.
These and other advantages, aspects and novel features of the invention, as well as details of illustrative aspects thereof, will be more fully understood from the following description and drawings.
Accordingly, the first 1024 samples of a window 830C Wx are the same as the last 1024 samples of the previous window 830C Wx-1. A window function w(t) is applied to each window 830C (W0 . . . Wn), resulting in sets (wW0 . . . wWn) of 2048 windowed samples 840C, e.g., (wWx(0) . . . wWx(2047)). The modified discrete cosine transformation (MDCT) is applied to each set (wW0 . . . wWn) of windowed samples 840C (wWx(0) . . . wWx(2047)), resulting sets (MDCT0 . . . MDCTn) of 1024 frequency coefficients 850C, e.g., (MDCTx(0) . . . MDCTx(1023)).
The sets of frequency coefficients 850C (MDCT0 . . . MDCTn) are then quantized. The frequency coefficients 850C are then coded with Huffman symbols 870. Header information 855, side information 860, and scale factors 865 are also added. The header information 855, the side information 860, and the scale factors 865 are encoded with variable length codes.
The Huffman coding and the variable length codes for the symbols are designed to assign shorter codes to the most frequently occurring symbols and longer codes to the least frequently occurring symbols. The coding schemes include a number of tables that map the different symbols to different codes.
In MPEG-1, layer 1 or 2, what is known as the audio elementary stream AES, comprises the header information 855, sample information 857, and scale factors 865. In MPEG-1, Layer 3, the AES comprises the side information 860, the scale factors 865, and the Huffman data 870. The AES can be multiplexed with other AESs. The multiplexed signal, known as the Audio Transport Stream (Audio TS) can then be stored and/or transported for playback on a playback device. The playback device can either be local or remotely located.
Where the playback device is remotely located, the multiplexed signal is transported over a communication medium, such as the Internet. During playback, the Audio TS is de-multiplexed, resulting in the constituent AES signals. The constituent AES signals are then decoded, resulting in the audio signal.
Referring now to
Referring now to
The on-chip memory 310 can comprise Static Random Access Memory (SRAM). The on-chip memory 310 is generally expensive, and consumes a significant portion of the physical area of the integrated circuit. The off-chip memory 320 can comprise Dynamic Random Access Memory (DRAM) and is generally cheaper than the on-chip memory 310. However, the off-chip memory 320 is also slower than the on-chip memory 310.
The off-chip memory 310 stores each of the tables required by the portions of the audio decoder 205. When specification portions of the audio decoder 205 decode the AES, a direct memory access module 315 fetches the appropriate tables from the off-chip memory 310 and loads the tables to the on-chip memory 320.
The tables that are stored in the off-chip memory 310 are listed below for Layers 1, 2 and 3.
As can be seen, storing each of the foregoing tables in the on-chip memory 310 would disadvantageously increase the requirements for the on-chip memory 310. However, accessing the tables from the off-chip memory by each component of the audio decoder 205 would be inefficient and slow.
The processing speed requirements are less memory requirements by storing the tables in the off-chip memory 320 [Mirut—this clause does not make sense—do you mean to say that the processing and memory requirements are reduced by storing?], and loading the tables used by each portion (e.g., header and bit allocation information processing module 210, a side information decoder 215, a scalar 220, a Huffman decoder 225, an inverse quantizer 230, joint stereo module 235, an alias reducer 240, an IMDCT module 245, synthesis sub-band filter 250) when the portion is decoding the encoded AES.
Referring now to
At 415, the audio decoder 205 parses the bit allocation table. Additionally, during 415, the audio decoder 205 makes a direct memory access (DMA) to fetch and load the algorithm specific tables for decoding the remaining part of the header information into the on-chip memory 310. During 420, the audio decoder decodes the scale factors with the tables stored in the on-chip memory 310.
At 430, the audio decoder 205 decodes the Huffman coding. Additionally, during 430, the audio decoder 205 makes a direct memory access (DMA) to fetch and load the Huffman tables for decoding the Huffman code into the on-chip memory 310. During 435, the audio decoder dequantizes the scale factors with the tables stored in the on-chip memory 310.
At 440, the audio decoder 205 reduces the aliasing. Additionally, during 440, the audio decoder 205 makes a direct memory access (DMA) to fetch and load the tables for alias reduction and data from a previous block for overlap add into the on-chip memory 310, and writes output data for the overlap add to the off-chip memory 320.
At 445, the audio decoder 205 synthesizes and filters sub-bands. Additionally, during 445, the audio decoder 205 makes a direct memory access (DMA) to fetch and load the tables for alias reduction and delay buffer data from earlier, into the on-chip memory 310, and writes output delay buffer data to the off-chip memory 320.
Referring now to
At 515, the audio decoder 205 parses the side information. Additionally, during 515, the audio decoder 205 makes a direct memory access (DMA) to fetch and load the algorithm specific tables for decoding the remaining part of the header information into the on-chip memory 310. During 520, the audio decoder parses the scale factors with the tables stored in the on-chip memory 310.
At 525, the audio decoder 205 decodes the Huffman coding. Additionally, during 525, the audio decoder 205 makes a direct memory access (DMA) to fetch and load the Huffman tables for decoding the Huffman code into the on-chip memory 310. During 530, 535, and 540, the audio decoder dequantizes, reorders the spectrum, and processes joint stereo information using the tables stored in the on-chip memory 310.
At 545, the audio decoder 205 reduces the aliasing. Additionally, during 545, the audio decoder 205 makes a direct memory access (DMA) to fetch and load the tables for alias reduction and data from a previous block for overlap add into the on-chip memory 310, and writes output data for the overlap add to the off-chip memory 320.
At 550, the audio decoder 205 synthesizes and filters sub-bands. Additionally, during 550, the audio decoder 205 makes a direct memory access (DMA) to fetch and load the tables for alias reduction and delay buffer data from earlier, into the on-chip memory 310, and writes output delay buffer data to the off-chip memory 320.
The circuit as described herein may be implemented as a board level product, as a single chip, application specific integrated circuit (ASIC), or with varying levels of the system integrated on a single chip with other portions of the system as separate components. The degree of integration of the monitoring system may primarily be determined by speed of incoming MPEG packets, and cost considerations. Because of the sophisticated nature of modern processors, it is possible to utilize a commercially available processor, which may be implemented external to an ASIC implementation of the present system. Alternatively, if the processor is available as an ASIC core or logic block, then the commercially available processor can be implemented as part of an ASIC device wherein the memory storing instructions is implemented as firmware.
While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
