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.
- Definition
- Video encoded at least twice through a lossy codec
- Frequency in casework
- Most common re-encoding pattern encountered
- Detection target
- What most compression-history methods are built for
- Related term
- Multi-generation encoding (broader, looser usage)
Common questions
Why is double compression significant in video forensics?+
A video re-encoded a second time, for example after being edited and re-saved or transcoded for a different platform, leaves detectable statistical traces from the first compression pass overlaid by the second. Finding those traces helps establish whether footage has passed through an editing step rather than being a single, unaltered original.
Does double compression by itself prove a video was tampered with?+
No. Ordinary actions like re-uploading to a social media platform, converting file formats, or forwarding through a messaging app can all introduce a second compression pass without any deceptive editing. Investigators treat double compression as a signal that further examination is warranted, not as proof of manipulation on its own.
How is double compression typically detected?+
Analysts examine periodicities in the DCT coefficient histograms or macroblock statistics that arise when a second quantisation grid is applied on top of the first. These periodic artefacts differ depending on whether the two compression passes used the same or different quality settings and frame alignment.
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 Streams
- 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...