Double Compression
Definition
The condition in which a video has been encoded at least twice through a lossy codec. The term is also used loosely for any multi-generation encoding. It is the most common form of re-encoding encountered in forensic casework and the condition most methods are designed to detect.
Related terms
- Bitrate Anomaly
- A departure from the expected relationship between scene complexity and compressed file size. Sudden drops, plateaux, or spikes in the per-second bitrate...
- DCT Coefficient Histogram
- A histogram of the discrete cosine transform coefficients across macroblocks or coding units in a frame or clip. After a single encode,...
- Group of Pictures (GOP)
- The periodic structure of I-frames, P-frames, and B-frames in a compressed video stream. Each encoding generation imposes its own GOP structure. When...
- Hash Verification
- The process of computing a cryptographic hash (SHA-256 or equivalent) of the exhibit file and comparing it against a previously recorded value...
- I-Frame (Intra-Coded Frame)
- A video frame that is encoded entirely from its own pixel data, without reference to any other frame. I-frames mark the start...
- Intra-Refresh Artefact
- The elevated count of intra-coded macroblocks or CTUs at frame positions that correspond to the I-frame interval of a previous encoding generation....
- Macroblock (MB) / Coding Tree Unit (CTU)
- The basic spatial coding unit: 16x16 pixels in H.264 (macroblock) and up to 64x64 pixels in H.265 (CTU). Each unit is classified...
- MEMS Microphone
- A Micro-Electro-Mechanical Systems microphone: the capsule type used in virtually all modern smartphones and tablets. MEMS microphones have a characteristically flat midrange...
- P-Frame (Predictive Frame)
- A frame encoded as a set of changes relative to a preceding reference frame. P-frames are smaller than I-frames because they only...
- Psychoacoustic Masking Model
- The perceptual model used by lossy codecs such as MP3 and AAC to decide which frequency components can be discarded or coarsely...
- Quantisation Noise
- The distortion introduced when a continuous audio signal is rounded to discrete digital values. Lossy codecs concentrate quantisation noise at predictable frequency...
- Quantisation Parameter (QP)
- A per-block or per-slice value that controls the coarseness of the DCT coefficient rounding step in H.264 and H.265. Higher QP means...
Explained in these topics
- Compression History Estimation in Video StreamsThe condition in which a video has been encoded at least twice through a lossy codec. The term is also used loosely for any multi-generation encoding. It is th...
- Microphone and Codec Identification in Audio ForensicsThe condition in which a lossy-encoded audio file has been decoded and re-encoded, leaving two overlapping sets of quantisation artefacts. The inner artefact l...
- Video Authentication and Integrity VerificationThe artefact introduced when a video that has already been compressed is re-compressed. Each compression cycle degrades quality and leaves statistical signatur...