How Audio Bitstreaming Works: VOB to AV Receiver

Audio bitstreaming allows a media player to extract compressed multi-channel audio formats—such as Dolby Digital (AC-3) or DTS—directly from a DVD VOB file and transmit them unaltered to an AV receiver (AVR). Rather than decoding the compressed audio internally into uncompressed Pulse Code Modulation (PCM), the source device preserves the native binary data. It packages the raw stream into standard transmission bursts over S/PDIF, optical, or HDMI connections, delegating all decoding, digital-to-analog conversion, and speaker calibration to the receiver’s dedicated hardware.

1. Demultiplexing the VOB Container

A VOB (Video Object) file is formatted as an MPEG-2 Program Stream containing multiplexed packets of video, audio, and subtitle data. When you initiate playback with bitstreaming enabled (often labeled as "Pass-through" or "Raw" in software settings):

• The player’s demultiplexer reads the VOB's Packetized Elementary Stream (PES) headers.
• It separates the audio packets from the video frames.
• Instead of routing the audio payload through a software decoder (like liba52 or ffmpeg) to create a multi-channel PCM stream, the player leaves the raw AC-3 or DTS frames intact.

2. IEC 61937 Framing and Encapsulation

Physical digital audio connections such as S/PDIF (coaxial or TOSLINK) and standard HDMI were originally standardized to carry two-channel linear PCM. To transport multi-channel compressed data across these pipelines without corrupting the signal, the system uses the IEC 61937 standard.

• Data Bursts: The player wraps the raw AC-3 or DTS frames into IEC 61937 data bursts.
• Preambles: Each burst includes a 16-bit preamble (labeled Pa, Pb, Pc, and Pd) that identifies the data type (e.g., AC-3 type 1, DTS type 11), stream specifications, and length.
• Clock Matching: The packaged bursts are padded with zeros to match the sampling frequency (typically 48 kHz for DVD audio) and bit rate of a standard stereo PCM channel. To the physical transport layer, the payload mimics normal stereo data, preventing the transmission line from rejecting the signal.

3. Digital Transmission

The framed bitstream is transferred across the digital interface:

• S/PDIF (Optical/Coaxial): Has sufficient bandwidth to support legacy compressed surround formats like standard AC-3 (up to 640 kbps) and full-bitrate DTS (up to 1.5 Mbps).
• HDMI: Offers substantially higher bandwidth, easily transporting the framed AC-3 and DTS streams alongside high-definition video using HDMI Audio Sample Packets.

4. Detection and Decoding by the AV Receiver

Once the bitstream reaches the AV receiver, the signal bypasses the AVR's standard stereo PCM pipeline:

• Identification: The AVR's input receiver detects the IEC 61937 preambles. It recognizes the payload not as two-channel PCM audio, but as compressed surround data, muting standard output to prevent loud digital noise.
• DSP Routing: The stream is routed to the AVR’s internal Digital Signal Processor (DSP) equipped with licensed Dolby Digital or DTS decoders.
• Decompression and Processing: The DSP decompresses the data into discrete multi-channel audio channels (such as 5.1 surround). It applies user-defined room correction, speaker delays, crossovers, and equalization.
• Digital-to-Analog Conversion (DAC): The individual decoded digital channels are converted into analog signals and sent to the internal amplifiers to drive the speakers.

By handling the audio this way, the source device acts purely as a transport, eliminating the risk of dynamic range compression, incorrect channel mapping, or downsampling errors by the host operating system.