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Vorbis Decoder

This example shows how to implement a Vorbis decoder, which is a freeware, open-source alternative to the MP3 standard. This audio decoding format supports the segmentation of encoded data into small packets for network transmission.

Vorbis Basics

The Vorbis encoding format [1] is an open-source lossy audio compression algorithm similar to MPEG-1 Audio Layer 3, more commonly known as MP3. Vorbis possesses many of the same features as MP3 while adding extra flexibility and functionality.

Encoding starts with the framing of the original signal. Vorbis allows the use of frames with different sizes. Therefore, Vorbis can adjust the frequency resolution across the signal if needed. Unlike MP3, there are no strictly enforced sampling frequencies and bit rates in the Vorbis format. The bit rate can vary throughout the entire signal for both Vorbis and MP3.

As with any lossy compression format, Vorbis performs transformation of a data frame. A psychoacoustic model is applied at the encoding stage. The model is not specified by the format and it is up to the developer of the encoder to ensure that the model provides significant data reduction while preserving most of the sound quality.

The Modified Discrete Cosine Transform (MDCT) [2] and its inverse counterpart are used in Vorbis to convert a signal into the transform domain, where energy concentration occurs. The Vorbis encoder then splits the spectrum image of a frame into a rough approximation called the 'floor,' and a remainder called the 'residue.'

The flexibility of the Vorbis format is illustrated by its use of different methods to represent and encode the floor and residue portions of the signal. The algorithm introduces 'modes' as a mechanism to specify these different methods and thereby code various frames differently.

Vorbis uses Huffman coding to compress the data contained in the floor and residue portions. In this step, Vorbis allows more efficient coding than MP3. Vorbis uses a dynamic probability model rather than the static probability model of MPEG-1 Audio Layer 3. Specifically, Vorbis builds custom codebooks for any particular audio signal, which can differ for 'floor' and 'residue' and from frame to frame.

After all Huffman encoding is complete, the frame data is bitpacked into a logical packet. In Vorbis, a series of such packets is always preceded by a header. The header contains all the information needed for correct decoding. This information includes a complete set of codebooks, descriptions of methods to represent the floor and residue, and the modes and mappings for multichannel support. The header can also include general information such as bit rates, sampling rate, song and artist names, etc.

Vorbis provides its own format, known as 'Ogg,' to encapsulate logical packets into transport streams. The Ogg format provides mechanisms such as framing, synchronization, positioning, and error correction, which are necessary for data transfer over networks.

Problem Overview and Design Details

The Vorbis decoder in this example implements the specifications of the Vorbis I format. It represents a subset of a "full powered" Vorbis. The example model decodes any raw binary .ogg file with an encapsulated compressed mono or stereo audio signal at a bit rate that might vary. The example model has the capability to decode and play back a wide variety of Vorbis audio files in real time provided that those files are correctly encapsulated into an Ogg transport filestream.

You can test this example with any Vorbis audio file by downloading an *.ogg file from such widely used resources as Wikipedia [3], where Vorbis is used as a primary format to store audio samples. To load the file into the model, replace the filename in the annotated code at the top level of the model with the name of the file to be tested. When this step is complete, click the annotated code to load the new audio file. The model is configured to notify you if the output sampling rate has been changed due to a change in the input data. In this case, the simulation needs to be restarted for a better listening experience.

In order to implement a Vorbis decoder in Simulink®, some technical issues need to be addressed. One such consideration is the fact that logical data packets do not have any specified size. This example deals with this variable size issue by capturing a whole page of the Ogg bitstream by detecting the 'OggS' synchronization pattern. For practical purposes a page is assumed to be no larger than 5500 bytes. After obtaining a segmentation table at the beginning of the page, the model extracts logical packets from the remainder of the page. Asynchronous control over such a decoding sequence is implemented using the Stateflow chart 'Decode All Pages of Data' depicted below.

Initially, the chart tries to capture the 'OggS' synchronization pattern persistently. After the pattern is detected the chart follows the decoding steps described above. Decoding the page is done in one call of the Simulink function 'decodePage'. This completes the decoding of the current page, and the model immediately goes back to detecting the 'OggS' sequence. The state 'ResetPageCounter' is added in parallel with the Stateflow algorithm described above to support the looping of the compressed input file for an unlimited number of iterations.

Data pages contain different types of information: header, codebooks, and audio signal data. The 'Read Setup Info,' 'Read the Header,' and 'Decode Audio' subsystems inside the 'decodePage' Simulink function are responsible for handling each of these different kinds of information.

Due to the iterative nature of the Huffman decoding process, a text-based implementation in MATLAB® code provides a more natural implementation than does a block diagram-based implementation. Therefore, the decoding process is implemented using MATLAB Function blocks. All nontrivial bit-unpacking routines in the example are implemented with MATLAB code.

Frames in Vorbis can have two different sizes within the signal. This means that the IMDCT should be implemented to account for precise inverse transform of the floor and residue portions of the signal. For this part, the model employs signals with variable size to ensure that only the valid part of a frame is processed for every IMDCT call. The variably sized signals are denoted in the subsystem below by the dark, dashed signal lines. In this example, the IMDCT is calculated using an FFT.

When the decoding process is concluded with an overlap-add operation, the output-ready data has a variable size in case of different frame lengths. Such bursts of data should be properly written into a sink in a timely manner. For this purpose, a sample count is maintained.

The Output block in the top level of the model feeds the output of the Decode Audio block to the audio playback device on your system. We deliver a valid portion of the decoded signal as a variable sized signal (Ogg Vorbis is a variable bit rate codec) into the 'To Audio Device' block, which has been updated to accept such inputs.

References

[1] Complete specification of the Vorbis decoder standard http://www.vorbis.com

[2] http://en.wikipedia.org/wiki/Modified_discrete_cosine_transform

[3] http://en.wikipedia.org/wiki/File:06_-_Vivaldi_Summer_mvt_3_Presto_-_John_Harrison_violin.ogg

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