 produces four times the QCIF resolution (a little over 100,000 pixels per screen). The H.320 standard guarantees compatibility; QCIF is mandatory, CIF is optional. Using either format, a vendor can claim standards-compliance. In May 1996, the ITU-T approved the H.263 Recommendation, which it described five picture formats. Along with QCIF (mandatory) and CIF (optional) H.263 describes Sub-QCIF (SQCIF), 4CIF, and 16CIF (all optional and all covered in Chapter 7). SQCIF is not applicable to group systems because it produces very low- resolution images (it could be very useful in video communication from handhelds or in low-bandwidth applications). What is relevant to group systems is 4CIF, which delivers NTSC television quality, and 16CIF, which resolves an image with roughly the same clarity as HDTV. For obvious reasons, H.263 is replacing H.261. Group-oriented systems may also offer proprietary formats. These will be described (should a buyer inquire) using vertical and horizontal pixel counts, relative to CIF (352 pixels by 288 lines) and QCIF (176 pixels by 144 lines). FRAME RATES The best compression techniques allow forwarding of more frames of information per second, therefore, better image, and sound quality and lower bandwidths. To achieve television's smooth motion handling, images must be refreshed at 30 frames per second (fps). At 30 fps, the eye cannot detect that the picture is being reassembled, SO the brain perceives continuous motion. A refresh rate as low as 12 fps Group Systems yields an acceptable picture. Below that, almost any motion appears jerky. When distinguishing between group systems, frame rate comparisons can be made, but without sophisticated test equipment, they will be subjective. NTSC cameras capture video at 30 fps. What the codec does with those frames determines whether the motion on the screen will resemble a smooth waltz or the bunny hop. The H.320 standard does not specify a frame rate, and some companies slow the frame rate down to improve resolution. Nevertheless, all code