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Common Intermediate Format
Common Intermediate Format
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CIF (Common Intermediate Format or Common Interchange Format), also known as FCIF (Full Common Intermediate Format), is a standardized format for the picture resolution, frame rate, color space, and color subsampling of digital video sequences used in video teleconferencing systems. It was first defined in the H.261 standard in 1988.

Comparison of CIF formats and D-1

As the word "common" in its name implies, CIF was designed as a common compromise format to be relatively easy to convert for use either with PAL or NTSC standard displays and cameras. CIF defines a video sequence with a resolution of 352 × 288, which has a simple relationship to the PAL picture size, but with a frame rate of 30000/1001 (roughly 29.97) frames per second like NTSC, with color encoded using a YCbCr representation with 4:2:0 color sampling. It was designed as a compromise between PAL and NTSC schemes, since it uses a picture size that corresponds most easily to PAL, but uses the frame rate of NTSC. The compromise was established as a way to reach international agreement so that video conferencing systems in different countries could communicate with each other without needing two separate modes for displaying the received video.

Technical details

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The simple way to convert NTSC video to CIF is to capture every other field (e.g., the top fields) of interlaced video, downsample it by 2:1 horizontally to convert 704 samples per line to 352 samples per line, and upsample it vertically by a ratio of 6:5 vertically to convert 240 lines to 288 lines. The simple way to convert PAL video to CIF is to similarly capture every other field, downsample it horizontally by 2:1, and introduce some jitter in the frame rate by skipping or repeating frames as necessary. Since H.261 systems typically operated at low bit rates, they also typically operated at low frame rates by skipping many of the camera source frames, so introducing some jitter in the frame rate tended not to be noticeable. More sophisticated conversion schemes (e.g., using deinterlacing to improve the vertical resolution from an NTSC camera) could also be used in higher quality systems.

In contrast to the CIF compromise that originated with the H.261 standard, there are two variants of the SIF (Source Input Format) that was first defined in the MPEG-1 standard. SIF is otherwise very similar to CIF. SIF on 525-line ("NTSC") based systems is 352 × 240 with a frame rate of 30000/1001 frames per second, and on 625-line ("PAL") based systems, it has the same picture size as CIF (352 × 288) but with a frame rate of 25 frames per second.

Some references to CIF are intended to refer only to its resolution (352 × 288), without intending to refer to its frame rate.

The YCbCr color representation had been previously defined in the first standard digital video source format, CCIR 601, in 1982. However, CCIR 601 uses 4:2:2 color sampling, which subsamples the Cb and Cr components only horizontally. H.261 additionally used vertical color subsampling, resulting in what is known as 4:2:0.

QCIF means "Quarter CIF". To have one quarter of the area, as "quarter" implies, the height and width of the frame are halved.

Terms also used are SQCIF (Sub Quarter CIF, sometimes Sub-QCIF), SCIF (sometimes Sub-CIF ), 4CIF (4 × CIF), 9CIF (9 × CIF) and 16CIF (16 × CIF). The resolutions for all of these formats are summarized in the table below.

Format Video Resolution Storage aspect ratio (SAR)
SQCIF 128 × 96 4:3
QCIF 176 × 144 11:9
SCIF 256 x 192 4:3
SIF(525) 352 x 160 11:5 (≈13:6)
CIF/SIF(625) 352 × 288 11:9
4SIF(525) 704 x 320 11:5 (≈13:6)
4CIF/4SIF(625) 704 × 576 11:9
9CIF 1056 × 864 11:9
16CIF 1408 × 1152 11:9

xCIF pixels are not square, instead having a ″native″ aspect ratio (pixel aspect ratio (PAR)) of 12:11 (PAR = DAR : SAR = ⁠4/3⁠ : ⁠11/9⁠ = ⁠12/11⁠), as with the standard for 625-line systems (see CCIR 601). On square-pixel displays (e.g., computer screens and many modern televisions) xCIF rasters should be rescaled so that the picture covers a 4:3 area, in order to avoid a "stretched" look: CIF content expanded horizontally by 12:11 results in a 4:3 raster of 384 × 288 square pixels (384 = 352 * 12/11). (This can happen on larger graphics displays of any aspect ratio in a window of 384 × 288 square pixels or enlarged to full screen on any larger 4:3 graphic display.)[citation needed]

The CIF and QCIF picture dimensions were specifically chosen to be multiples of 16 because of the way that discrete cosine transform based video compression/decompression was handled in H.261, using 16 × 16 macroblocks and 8 × 8 transform blocks. So a CIF-size image (352 × 288) contains 22 × 18 macroblocks and a QCIF image (176 × 144) contains 11 × 9 macroblocks. The 16 × 16 macroblock concept was later also used in other compression standards such as MPEG-1, MPEG-2, MPEG-4 Part 2, H.263, and H.264/MPEG-4 AVC.

See also

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References

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Common Intermediate Format

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Common Intermediate Format () is a standardized format defined in the recommendation for video codecs in audiovisual services at bit rates of p × 64 kbit/s (where p ranges from 1 to 30), featuring a resolution of 352 × 288 pixels and components at half that size using subsampling, with typical frame rates of 29.97 fps for compatibility or 25 fps for PAL. This format was developed to provide a common resolution and structure for compressing and transmitting moving video sequences, enabling between different standards like PAL and without requiring full conversion. Introduced as part of the standard—the first internationally agreed compression specification— originated in November 1988 to support early videoconferencing over Integrated Services Digital Network (ISDN) lines, with a revised version approved in March 1993 that remains in force. The format's design emphasizes block-based hybrid coding, combining inter-picture prediction and (DCT) to achieve efficient data rates up to 1.92 Mbit/s, while maintaining compatibility with quarter-sized variants like QCIF (176 × 144 pixels) for lower-bandwidth applications. Beyond its foundational role in video telephony, CIF has been widely adopted in subsequent standards such as and in practical implementations like surveillance systems, where it serves as a baseline resolution for recording and streaming analog-derived footage, ensuring consistent quality in resource-constrained environments. Its enduring influence persists in legacy equipment and hybrid digital-analog transitions, though higher-resolution formats have largely supplanted it in modern applications.

History and Development

Origins in Video Coding Standards

The Common Intermediate Format (CIF) emerged as a pivotal component in early video compression efforts during the late 1980s, specifically developed within the framework of the ITU-T H.261 standard. Published in November 1988, H.261 was designed to enable low-bitrate video telephony and conferencing over Integrated Services Digital Network (ISDN) lines, targeting bitrates from 384 kbit/s to 1920 kbit/s in multiples of 384 kbit/s. This standard addressed the need for efficient compression of moving video signals, using a hybrid approach combining inter-picture prediction and discrete cosine transform (DCT) coding. The primary purpose of CIF was to establish a unified intermediate picture format that could reconcile incompatibilities between the predominant analog television standards worldwide, namely the 525-line system used in and and the 625-line PAL/SECAM systems prevalent in and elsewhere. By defining CIF as a common resolution, the format facilitated for international and conferencing, allowing source video from diverse regional standards to be encoded and decoded without region-specific adaptations. This bridged the gap in frame dimensions and scanning lines, promoting global standardization in early communication. CIF's development was led by the Study Group XV, which focused on creating practical solutions for services at these constrained bitrates to support emerging ISDN . The choice of CIF's initial resolution—352 pixels horizontally by 288 lines vertically for luminance—was deliberately derived from downsampling the active picture areas of full PAL or frames. This ensured alignment with H.261's macroblock-based compression structure, where each consists of 16x16 blocks, resulting in an number of macroblocks per frame (22 horizontally and 18 vertically) to optimize processing efficiency and minimize artifacts in the DCT-based encoding.

Standardization and Evolution

The Common Intermediate Format (CIF) was formally adopted within the Recommendation H.261, approved on November 25, 1988, as a standardized video picture format for services at multiples of 384 kbit/s. This initial specification established CIF's core parameters for source coding in hybrid inter-picture and transform-based compression, targeting early over integrated services digital networks (ISDN). In March 1993, H.261 was revised to support bit rates of p × 64 kbit/s, where p ranges from 1 to 30 (64 to 1920 kbit/s), enabling lower-bitrate applications over ISDN. CIF received further international recognition through its definition in , which served as the foundational standard for low-to-medium bitrate video coding. By the mid-1990s, aliases such as Full CIF (FCIF) emerged in related ITU documentation to distinguish it from lower-resolution variants like Quarter CIF (QCIF), reflecting its role as the baseline format in evolving telecommunication protocols. The format's evolution continued with references in Recommendation , approved on March 20, 1996, which extended support for enhanced low-bitrate communication while incorporating alongside optional higher-resolution modes like 4CIF. However, no major revisions were made to 's core specifications after its 1988 introduction; subsequent advancements focused on improvements rather than altering the format's resolution or structure. Key milestones in CIF's timeline include its 1988 initial release via , followed by commercial implementations in video phones around 1990 as ISDN infrastructure enabled practical deployment. By the 2000s, updates to CIF remained limited, as attention shifted to higher-resolution standards like those in H.264, though CIF persisted in legacy low-bitrate applications.

Technical Specifications

Resolution and Pixel Structure

The Common Intermediate Format (CIF) specifies a horizontal resolution of 352 and a vertical resolution of 288 for the component, providing compatibility with both PAL and systems through differences in rather than resolution. This resolution structure supports efficient video compression by ensuring the image dimensions are multiples of 16 in both directions—specifically, 22 macroblocks horizontally (352 ÷ 16 = 22) and 18 vertically (288 ÷ 16 = 18)—allowing seamless alignment with 16×16 macroblocks used in the compression algorithm. CIF pixels are non-square, with a pixel aspect ratio (PAR) of 12:11, which arises from the format's storage aspect ratio of approximately 1.222:1 combined with the intended 4:3 display aspect ratio. This non-square geometry requires rescaling or anamorphic adjustment when rendering CIF content on square-pixel displays to preserve the correct 4:3 proportions without distortion. The CIF resolution is derived by subsampling the full CCIR 601 digital video format (720×576 active pixels for PAL) by a factor of 2 in both horizontal and vertical directions, with the horizontal dimension precisely set to 352 pixels to optimize for macroblock-based processing rather than exact halving of 720 (which would yield 360). For NTSC sources (720×480 active pixels), vertical scaling is applied to reach 288 lines. Pixel values in CIF are represented in the YCbCr color space at 8 bits per component, where luminance (Y) is sampled at full resolution and chrominance components (Cb, Cr) are subsampled to half resolution horizontally and vertically.

Frame Rate and Timing

The Common Intermediate Format (CIF) employs a standard frame rate of 300001001≈29.97\frac{30000}{1001} \approx 29.97 frames per second (fps) for progressive scan video, aligned with NTSC timing requirements to ensure compatibility in transmission systems. This rate includes a clock tolerance of ±50 parts per million (ppm) to maintain synchronization. In PAL-based systems, CIF adopts a 25 fps rate to match the 625-line television standard. CIF's temporal structure is progressive, derived from interlaced broadcast formats through subsampling and scaling: for PAL, every other field from a 50 fields per second source contributes to 25 fps at 288 lines; for NTSC, scaling from the 60 fields per second (480-line) source achieves the 29.97 fps at 288 lines. This facilitates efficient conversion from analog sources. During conversion from native source formats to CIF, such as in pipelines, potential timing may arise due to resampling or buffering processes, which can disrupt audio-video alignment and impair lip synchronization in conferencing scenarios. H.261 implementations mitigate this through strict , though real-world conversions may require additional buffering to bound within acceptable limits (typically under 50 ms for perceptible effects). CIF is optimized for bitrates starting at a minimum of 64 kbit/s (corresponding to p=1 in the p×64 kbit/s structure), where the frame rate supports adjustability via temporal subsampling—skipping 0 to 3 intermediate pictures—but remains nominally fixed at 29.97 fps in the core profile to meet real-time constraints; higher multiples (up to 2 Mbit/s) allow fuller rates without skipping. The interplay with CIF's 352×288 resolution further constrains the effective data rate, balancing motion rendering against .

Color Space and Subsampling

The Common Intermediate Format (CIF) employs the color space to separate (Y) from (Cb and Cr) components, enabling efficient compression by prioritizing the human visual system's greater sensitivity to brightness over color details. This is derived from the CCIR Recommendation 601 sampling grid, which standardizes encoding for both and 625-line television systems. CIF utilizes a scheme, in which the components are sampled at half the horizontal resolution and half the vertical resolution relative to the . Specifically, for every 2x2 block of Y samples, there is one Cb and one Cr sample shared across the block, resulting in a subsampling ratio that discards finer color details while preserving perceptual quality. This approach aligns with the structure in encoding, where each 16x16 corresponds to 8x8 blocks. In CIF, the full luminance plane measures 352 pixels horizontally by 288 lines vertically, while the effective resolution for each Cb and Cr plane is reduced to 176x144 pixels due to the subsampling. This yields planes that are one-quarter the size of the plane individually, optimizing data representation for transmission. The subsampling in CIF achieves a 50% reduction in color data compared to a full sampling scheme, where would match resolution, making it particularly suitable for the low-bitrate constraints of videotelephony at rates up to 384 kbit/s. By halving the total information, this method significantly lowers bandwidth requirements without substantial visible degradation, as confirmed in analyses of early standards.

Format Variants

Lower-Resolution Variants

The lower-resolution variants of the (CIF) were developed to accommodate constrained bandwidth environments, such as early mobile networks and low-bitrate communication channels, by reducing the count while preserving compatibility with the underlying video coding frameworks. Quarter CIF (QCIF) provides a resolution of 176 × 144 for , resulting in exactly one-quarter the active picture area of standard CIF (352 × 288 ). This scaling is achieved through integer division by 2 in both horizontal and vertical dimensions from CIF, ensuring alignment with the 16 × 16 structure used in block-based coding; QCIF thus comprises 11 × 9 . QCIF was supported as an optional format in Recommendation (1990) for videoconferencing over ISDN lines and became a core format in (1996), enabling efficient compression for as low as 64 kbit/s. It found widespread use in mobile and early web-based video streaming due to its balance of visual quality and transmission efficiency over limited channels. Sub-Quarter CIF (SQCIF), also known as sub-QCIF, further reduces the resolution to 128 × 96 pixels for , offering a minimal footprint suitable for extremely low-bitrate scenarios like basic wireless video calls. This format maintains alignment with 8 × 6 blocks of 16 × 16 pixels each and was introduced as a standardized option in (1996) to support decoding at bit rates up to 64 kbit/s in resource-constrained devices. SQCIF's compact size minimized data requirements in early mobile and PSTN-based applications, though it was not part of the original specification.

Higher-Resolution Variants

Higher-resolution variants of the Common Intermediate Format (CIF) extend the base 352×288 resolution to support improved in compatible systems while maintaining alignment with video coding standards. These formats were introduced as optional picture sizes in ITU-T Recommendation , published in 1996, which built upon the core CIF and QCIF formats defined in the earlier standard from 1990. Unlike , which supported only CIF and QCIF, incorporated upscale options to accommodate higher- applications without altering the fundamental block-based coding structure. The primary higher-resolution variant is 4CIF, which doubles the horizontal and vertical dimensions of CIF to achieve a luminance resolution of 704×576 pixels. This format corresponds to the active video area of the PAL broadcast standard, enabling compatibility with professional television workflows while preserving the color subsampling scheme. In , 4CIF supports enhanced detail for applications requiring greater spatial fidelity, such as improved or archival recording, with the picture divided into macroblocks of 16×16 pixels to align with the codec's grid—resulting in 44 macroblocks horizontally and 36 vertically. A further extension is 16CIF, which quadruples the CIF dimensions to 1408×1152 pixels in , providing four times the area for high-end conferencing and professional encoding scenarios. This format maintains the 16-pixel alignment, yielding 88 horizontal and 72 vertical macroblocks, and was designed for systems where bandwidth permits higher quality without custom resolutions. Both 4CIF and 16CIF emerged as part of H.263's flexible source format support, allowing encoders and decoders to negotiate these sizes for optimized performance in diverse video codecs beyond initial low-bit-rate constraints.

Applications and Usage

Videotelephony and Conferencing

The Common Intermediate Format (CIF) played a pivotal role in the development of early digital and videoconferencing systems, particularly through its integration with the H.261 video compression standard. Approved in 1990, H.261 was designed specifically for real-time services over Integrated Services Digital Network (ISDN) lines, supporting bit rates of p × 64 kbit/s (where p ranges from 1 to 30), enabling practical deployment of video phones and conferencing endpoints in the late and 1990s. CIF, as the baseline resolution in H.261, facilitated efficient compression and transmission for these applications, allowing systems to operate within the bandwidth constraints of ISDN channels, typically requiring 6 × 64 kbit/s (384 kbit/s) for CIF at 30 frames per second. A key advantage of CIF in these systems was its role in unifying video formats across global standards, bridging the differences between (525-line, 29.97 fps) and PAL (625-line, 25 fps) television systems without necessitating complex conversions during international calls. By defining a common intermediate image format of 352 × 288 pixels with progressive scanning, and CIF ensured interoperability in H.320-based videoconferencing setups over ISDN, which became the for multipoint conferences in the . This compatibility reduced latency and equipment costs, making CIF-enabled systems viable for business and professional use worldwide. CIF's legacy endures in modern VoIP environments as a fallback option in certain codecs and protocols, where legacy support ensures backward compatibility for older endpoints during negotiations in SIP or sessions. However, its low resolution—often derided as "" video due to the small, blocky appearance on standard displays—limited visual quality and user satisfaction, prompting a phase-out by the early in favor of higher-resolution formats like VGA ( × ) and beyond, supported by advancements in and H.264 over networks.

Surveillance and Digital Recording

The Common Intermediate Format () gained prominence in () systems during the 1990s and 2000s as analog-to-digital conversion became widespread, serving as a foundational resolution for early IP cameras and recorders (DVRs). With a resolution of 352 × 288 pixels for PAL systems or 352 × 240 for , CIF enabled efficient of analog feeds in applications, allowing multiple cameras to be multiplexed onto limited storage and network resources in DVR setups. This format's structure aligned well with early compression standards, fitting neatly into 16 × 16 pixel macroblocks for block-based encoding. In digital recording contexts, was commonly employed in early camcorders and media players for archival purposes, where its modest resolution facilitated storage on emerging without excessive data demands. Typical bitrates for CIF video in these systems ranged from 1 to 2 Mbps when compressed using standards like , balancing quality and efficiency for non-real-time capture and playback in and devices. This made it suitable for recording extended footage on hard drives or tapes, with compression ratios often exceeding 80:1 to minimize file sizes. As of 2025, CIF persists in low-cost deployments, particularly where bandwidth is constrained, such as in remote or edge networks with limited . Its simplicity supports integration into budget IP and camera systems, often alongside higher resolutions for scalable monitoring. Quarter CIF (QCIF) variants, at 176 × 144 pixels, are frequently used in cameras for ultra-low-bandwidth transmission in mobile or battery-powered setups. A key advantage of CIF in contemporary systems is its seamless compatibility with modern codecs like H.264 and H.265 in hybrid DVR/NVR architectures, enabling legacy analog cameras to coexist with digital streams for cost-effective upgrades without full overhauls.

Comparisons and Relations

Relation to Source Input Format (SIF)

The Source Input Format (SIF), introduced as part of the video compression standard in the early , specifies a resolution of 352×240 pixels for (525-line) systems or 352×288 pixels for PAL (625-line) systems, with frame rates of 29.97 fps for NTSC and 25 fps for PAL, and was designed primarily for efficient storage and playback in consumer media like Video CDs and early digital video discs. A key similarity between SIF and the Common Intermediate Format (CIF) lies in their shared resolution structure, where both formats utilize 352 horizontal pixels and, in the PAL configuration, 288 vertical pixels, facilitating compatibility in processing pipelines derived from the CCIR 601 sampling standard. Despite these overlaps, notable differences distinguish the formats: SIF employs , maintains fixed frame rates aligned with broadcast standards (25 fps for PAL versus CIF's more flexible 29.97 fps adaptation for NTSC-like transmission), and targets compressed storage for consumer applications, whereas CIF prioritizes low-bitrate real-time transmission in systems. Owing to their parallel development from CCIR 601 principles and near-identical resolutions in many contexts, and are frequently interchanged or confused in early workflows, with SIF representing an optimized, non-interlaced variant tailored for MPEG-1's storage-focused encoding rather than CIF's transmission-oriented design.

Relation to Modern Video Standards

The Common Intermediate Format (CIF), originally specified in the H.261 standard for videoconferencing, was a core resolution (352×288 pixels) in H.323-based systems during the 1990s, often paired with Quarter CIF (QCIF) and VGA equivalents for low-bandwidth applications. By the early 2000s, the adoption of (Advanced Video Coding) in 2003 shifted focus to higher-definition formats like , effectively superseding CIF in mainstream videoconferencing and rendering it obsolete for bandwidth-intensive scenarios. , an interim standard from 1996, extended support for CIF while introducing multiples like 4CIF, but even this bridged only temporarily to the resolution-agnostic flexibility of . CIF's foundational elements continue to influence modern codecs. The 16×16 structure from , used for and in CIF encoding, evolved into variable block sizes in H.264 and was further refined into larger coding tree units (up to 64×64) in H.265 (, or HEVC) to handle ultra-high definitions like 4K while preserving efficiency principles. Similarly, CIF's —reducing color information to one-quarter of luma samples—remains a bandwidth-optimized default in H.264 and H.265 for streaming, enabling compatibility across diverse devices without full chroma fidelity. As of 2025, CIF sees limited deployment in new systems, primarily persisting in legacy (CCTV) installations and resource-constrained embedded (IoT) devices where low processing power favors its simplicity over higher resolutions. Post-2010 extensions in standards like MPEG-DASH include support for CIF media presentation descriptions (MPDs), allowing legacy compatibility in adaptive streaming for cable and IP-based services, though such usage is niche amid dominant and 4K adoption. For interoperability, CIF content can be upscaled to standard-definition formats such as (NTSC) or (PAL), but this introduces artifacts and inefficiency due to the original low count, making it suboptimal compared to native encoding in or 4K, which leverages modern codecs' superior compression without resolution losses.

References

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