6.111 Final Project ProposalHarrison HallVirtual Surround Sound from a Dolby Digital Source! The goal of this project is to extract the data coded in a continuous stream of Dolby Digital media and simulate the 5 satellite speakers and subwoofer on a pair of conventional, stereo headphones. The basic breakdown is to decode the information in the stream into the 6-channels of PCM data that typically flow to a home theater audio system and then to use this data as input to a Head-Related Transfer Function(HRTF) for each ear of the user to simulate surround sound. ! Similarly, this is the most logical breakdown for the modules in the system. Therefore the main modules are the Dolby Decoder and the HRTF calculation unit. There are other modules used incorporated into the model, namely the Spatial Location Interface, but these are not the primary focus of the project and the degree of planning and expansion that goes into preparing them will depend greatly on the primary modules in the system.! The module diagram corresponding to the system can be seen below. It is a very primitive diagram at this point as each module lacks a full specification and the necessary control signals have yet to be determined. However, the flow of audio data through the system will follow a path almost identical to the main path described, in the diagram. Dolby Digital DecoderAdvanced Television Systems Committee, Inc. Document A/52B22two so that each block contains 256 frequency coefficients. The individual frequency coefficientsare represented in binary exponential notation as a binary exponent and a mantissa. The set ofexponents is encoded into a coarse representation of the signal spectrum which is referred to asthe spectral envelope. This spectral envelope is used by the core bit allocation routine, whichdetermines how many bits to use to encode each individual mantissa. The spectral envelope andthe coarsely quantized mantissas for six audio blocks (1536 audio samples per channel) areformatted into an AC-3 frame. The AC-3 bit stream is a sequence of AC-3 frames.The actual AC-3 encoder is more complex than indicated in Figure 1.2. The followingfunctions not shown above are also included:1. A frame header is attached which contains information (bit-rate, sample rate, number ofencoded channels, etc.) required to synchronize to and decode the encoded bit stream.2. Error detection codes are inserted in order to allow the decoder to verify that a received frameof data is error free.3. The analysis filterbank spectral resolution may be dynamically altered so as to better matchthe time/frequency characteristic of each audio block.4. The spectral envelope may be encoded with variable time/frequency resolution.5. A more complex bit allocation may be performed, and parameters of the core bit allocationroutine modified so as to produce a more optimum bit allocation.6. The channels may be coupled together at high frequencies in order to achieve higher codinggain for operation at lower bit-rates.7. In the two-channel mode, a rematrixing process may be selectively performed in order toprovide additional coding gain, and to allow improved results to be obtained in the event thatthe two-channel signal is decoded with a matrix surround decoder.1.3 DecodingThe decoding process is basically the inverse of the encoding process. The decoder, shown inFigure 1.3, must synchronize to the encoded bit stream, check for errors, and de-format thevarious types of data such as the encoded spectral envelope and the quantized mantissas. The bitFigure 1.3 The AC-3 decoder.Input from TOSLink CableLaptopHRTFRear LeftFront LeftCenterFront RightRear RightSummation LeftSummation RightSpatial Location InterfaceGUIDecodedPCM Time Samples, 6 streams each 16-24 bits wideBRAM Memory16 Bit WordAddress[7:0]AC97EnablePCM[7:0]PCM[7:0]Audio OutLocationDataUser InputsFigure 1: Primitive Module Diagram of the Major Components of the SystemDolby Digital Decoder! The signal we will be processing will be brought to the board via a TOSlink cable through through a TOSLink receiver package that takes the light pulses and converts them directly to digital signals at a theoretical maximum of 13.2 megabits per second, but for this application we only are interested in 320 kilobits per second to 640 kilobits per second, or , similarly, we have to clock it to at least 640kHz and then compare the synchronizer packet at the head of each frame.! The Dolby Digital(AC-3) signal is a variable length encoding depending upon the frequency of the samples in the original work and the bit quality in the original work. The encoding is very high gain meaning that a studio master copy of a 5.1 channel audio program is converted from a PCM representation requiring more than 5 Mbps (6 channels × 48 kHz × 18 bits = 5.184 Mbps) into a 384 kbps serial bit stream in AC-3. Despite this major reduction in size, a memory module will still be needed to store the data. Clearly the on chip BRAM is enough for streaming data, since we don’t want more more audio lag than necessary. We will store the data in 8-bit segments because each Sync Frame is a multiple of the word-length. Then we can read out a sync frame as necessary to buffer the audio effectively.Advanced Television Systems Committee, Inc. Document A/52B285. BIT STREAM SYNTAX5.1 Synchronization FrameAn AC-3 serial coded audio bit stream is made up of a sequence of synchronization frames (seeFigure 5.1). Each synchronization frame contains 6 coded audio blocks (AB), each of whichrepresent 256 new audio samples per channel. A synchronization information (SI) header at thebeginning of each frame contains information needed to acquire and maintain synchronization. Abit stream information (BSI) header follows SI, and contains parameters describing the codedaudio service. The coded audio blocks may be followed by an auxiliary data (Aux) field. At theend of each frame is an error check field that includes a CRC word for error detection. Anadditional CRC word is located in the SI header, the use of which, by a decoder, is optional.5.2 Semantics of Syntax SpecificationThe following tables describe the order of arrival of information within the bit stream. Theinformation contained in the tables is roughly based on C language syntax, but simplified for easeof reading. For bit
View Full Document
Unlocking...