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Questions and Answers
What is the typical threshold for humans to perceive audio/video synchronization errors?
What is the typical threshold for humans to perceive audio/video synchronization errors?
- 100 ms
- 50 ms
- 80 ms (correct)
- 120 ms
What is the McGurk effect?
What is the McGurk effect?
Hearing Lips and Seeing Voices
What process involves embedding content and time references into a single data flow?
What process involves embedding content and time references into a single data flow?
- Synchronization
- Multiplexing (correct)
- Demultiplexing
- Codec
- Modulation
Transport Streams are mainly aimed at error-free applications.
Transport Streams are mainly aimed at error-free applications.
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In MPEG Systems, two clock values present in headers are PTS (Presentation Time Stamp) and ____. (Decoding Time Stamp)
In MPEG Systems, two clock values present in headers are PTS (Presentation Time Stamp) and ____. (Decoding Time Stamp)
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Match the media container formats to their descriptions:
Match the media container formats to their descriptions:
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Study Notes
Synchronization in Internet Video Streaming
- Synchronization is crucial in media streaming to avoid a bad quality of experience, which can occur when audio and video are not in sync.
- Humans typically perceive audio-video synchronization errors if the difference in time ("skew") is higher than about 80 ms.
Audio and Video Synchronization
- To achieve synchronization, different media streams (audio and video) need to be synced using time references for specific points in media, such as video frames and audio frames.
- All content and time references need to be embedded into a single data flow, which requires data multiplexing.
Audio and Video Multiplexing
- MPEG standards provide a mechanism to achieve multiplexing with proper synchronization.
- Multiplexing is typically achieved through Time Division Multiplexing (TDM), where audio, video, and data are inserted as chunks in a single stream.
- These chunks are named "packets" in the MPEG standard.
Stream Types
- There are two types of streams: Elementary Stream (ES) and Program Stream (PS) / Transport Stream (TS).
- Elementary Stream (ES) is a sequence of encoded bits (video) or NALUs (since AVC) and sequence of encoded frames (audio).
- Program Stream (PS) is aimed at error-free applications, such as files on reliable storage, while Transport Stream (TS) is adapted from PS and aimed at error-prone applications, such as digital TV.
Packetized Elementary Stream
- Packetized Elementary Stream (PES) is a packetized version of ES, where each packet contains a header, timestamp, and data bits.
- PES packets can be further divided into groups called "packs".
PES Stream IDs
- PES streams can support up to 32 audio streams, 16 video streams, and other streams for information and control, such as private, proprietary, or program guide information.
Transport Streams
- Transport Streams (TS) have fixed-length (188 bytes) packets with headers derived from PES streams.
- These packets contain a transport header and stuffing bits that will be discarded.
- TS headers contain a master clock, typically 27 MHz, which is used for synchronization.
Synchronization
- TS headers contain a master clock (PCR) and two clock values: PTS (Presentation Time Stamp) and DTS (Decoding Time Stamp).
- PTS is the time at which decompressed data should be played back, while DTS is the time at which compressed data should be decoded.
Container Formats
- Container formats specify a binary syntax suitable to contain media data encoded with various codecs.
- Examples of container formats include MP4, AVI, MKV, MOV, VOB, 3GP, and ASF.
- These formats typically include additional metadata, such as seek support and indexing.
Mixed-Media Formats
- Mixed-media formats can contain audio, video, and metadata, such as MP4, AVI, and MKV.
- MP4 is a popular container format derived from MPEG-4 Part 12 (ISO Base Media File Format) and is widely used for online video streaming.
Matroska Multimedia Container
- Matroska is an open standard, open format, royalty-free container format (.mkv).
- It is based on Extensible Binary Meta Language (EBML) and is suitable for streaming, with support for out-of-the-box playback on Windows 10 and WebM.
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