Common Intermediate Format
CIF (Common Intermediate Format or Common Interchange Format), also known as FCIF (Full Common Intermediate Format), is a format used to standardize the horizontal and vertical resolution and frame rate and color subsampling of YCbCr video sequences. It has been commonly used in video teleconferencing systems, and was first defined in the H.261 standard.
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 colour encoded using YCbCr 4:2:0. 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.
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.
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 subQCIF), 4CIF (4× CIF) and 16CIF (16× CIF). The resolutions for all of these formats are summarized in the table below.
| Format | Video Resolution |
|---|---|
| SQCIF | 128 × 96 |
| QCIF | 176 × 144 |
| SCIF | 256 x 192 |
| SIF(525) | 352 x 240 |
| CIF/SIF(625) | 352 × 288 |
| 4SCIF | 512 × 384 |
| 4SIF(525) | 704 x 480 |
| 4CIF/4SIF(625) | 704 × 576 |
| 16CIF | 1408 × 1152 |
| DCIF | 528 × 384 |
xCIF pixels are not square, instead having a native aspect ratio of ~1.222:1. On older television systems, a pixel aspect ratio of 1.2:1 was the standard for 525-line systems (see CCIR 601). On square-pixel displays (computer screens, many modern televisions) xCIF rasters should be rescaled horizontally by ~109% to 4:3 in order to avoid a "stretched" look: CIF content expanded horizontally by ~109% results in a 4:3 raster of 384 × 288 square pixels.
The CIF "image sizes" were specifically chosen to be multiples of macroblocks (i.e. 16 × 16 pixels) because of the way that discrete cosine transform based video compression/decompression is handled. So, by example, a CIF-size image (352 × 288) corresponds to 22 × 18 macroblocks.
DCIF means Double CIF, proposed as a compromise resolution between CIF and 4CIF that is more balanced (in terms of horizontal vs vertical resolution) and suited to common CCTV equipment (with 480+ scanlines but a maximum of about 560 TVL) than 2CIF (704x288). The pixel and therefore data rate is exactly double that of CIF [1], but the 1:1.375 image aspect ratio is a lot closer to standard 4:3, with essentially square pixels.