Double Quantisation Effect
Definition
The statistical artefact that appears in DCT coefficient histograms when a video or image is quantised twice with different step sizes. The histogram of the doubly compressed data shows periodic dips at multiples of the first quantisation step, because values near those multiples were rounded away in the first pass and are unlikely to reappear.
- Domain
- Digital video and image forensics
- Detects
- Recompression with a different quantisation step
- Signature
- Periodic dips in DCT coefficient histogram
- Cause of dips
- Values near first-pass rounding points rarely reappear
Common questions
What does detecting a double quantisation effect actually prove about a video file?+
It shows the file was compressed at least twice with different settings, which is consistent with editing, transcoding, or re-saving after the original capture. It does not by itself identify what was changed or when, so it is normally paired with other traces before an examiner concludes the footage was manipulated.
Can a video be re-encoded without leaving a detectable double quantisation signature?+
Yes, in some cases. If the second quantisation step matches the first, or if the recompression uses a very different codec pipeline, the periodic histogram artefact can be weak or absent, which is why this test is used alongside other compression-history indicators rather than as a standalone determination.
Related terms
- Discrete Cosine Transform (DCT)
- A mathematical transform that expresses a block of pixel values as a sum of cosine functions at different frequencies. JPEG applies the...
- Intra-Frame vs Inter-Frame Coding
- Intra-frame (I-frame) coding compresses each frame independently using DCT quantisation, like a JPEG image. Inter-frame (P-frame or B-frame) coding stores only the...
- Macroblock
- The fundamental coding unit in most video codecs: a 16x16 pixel block in luma (brightness) and the corresponding chroma samples. Macroblocks are...
- Quantisation
- The step in lossy compression where DCT coefficients are divided by a quantisation parameter and rounded to integers. Higher quantisation parameters produce...
- 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...