Understanding Video Quality Many Organizations Are Addressing the Requirements for Video Solutions

Home , Display resolution, Frame rate, Video quality

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Video Solutions: Understanding Video Quality Many organizations are addressing the requirements for video solutions. While it is important to construct video applications support systems quickly to address immediate needs, network operators need to select the right system the ﬁ rst time, to minimize the total life cycle cost of these systems.

Abstract medium. NTSC defi nes an aspect ratio of 4:3, which Operators need to clearly understand the require- provides a theoretical maximum resolution of 720 ments of their organization. This includes an (horizontal) by 486 (vertical) when specifi ed in terms understanding of the video image quality. While of non-square (10:11) pixels. In square pixels this there are many other issues that will signifi cantly would be 648x486, or more commonly 640x480. impact network design, this paper provides infor- mation on factors that address image quality. The The resolution issue is complicated by additional factors discussed include: common resolution standards for video encoding. Common Intermediate Format (CIF) is 352x288 @ • Resolution 30fps. It represents a compromise between the • Frames Per Second (FPS) NTSC frame rate (30fps) and PAL resolution. In • Video Codec practice, producing 352x288 from a NTSC camera • Packets Per Second (PPS) source is diffi cult. As such North American surveil- • Bit Rate lance vendors have created a ‘modifi ed’ CIF format of 352x240 (“NTSC CIF”) which is easily derived An understanding of the effect of these factors from an NTSC video source by removing every on bandwidth requirements will enable network other vertical scan line. Alternatives are Quarter operators to correctly design communications CIF (QCIF) 176x144 (or 176x120), 4CIF 704x576 (or infrastructure networks that will support video 704x480), and 16CIF 1408x1152 (or 1408x960). applications. The interaction between encoded resolution and Intricacies of Video Solution Image Quality display resolution of the decoded video can be Operators need to select video performance that is dramatic. By understanding the device which will appropriate to the needs of the application. Different be used to view the decoded video, the network performance levels will require different cameras operator can know the requirement for image to collect images. In general, higher performance collection and processing. The resolution of the levels will require a higher bandwidth infrastructure display device has an impact on overall perceived to transport data from the camera to the video video quality, since the encoded video will need command center. Network operators need to clearly to be scaled accordingly. understand the camera performance required in order to design the communications infrastructure Frames Per Second (FPS) Frames Per second appropriately. If there is not suffi cient capacity at any (FPS) is the number of “snapshots” of the video point in the communications infrastructure, video scene in one second. Recall that modern fi lms are images may be delayed or lost, defeating the 24 FPS, NTSC is 30 FPS, and PAL is 25 FPS. Video purpose of a video solution. surveillance cameras can be confi gured for a range of FPS. In many cases, 10 FPS is suffi cient. Resolution Resolution is the number of “pixels” (picture elements) contained within each frame of Video Codecs One might think that stipulating the video. For example, The National Television System encoder’s resolution and Frames Per Second (FPS) Committee (NTSC) as a standard specifi es a manda- would exactly result in a bandwidth throughput tory 525 analog scan lines of vertical resolution, of need. However, even when stipulating both the res- which 486 are typically visible. Horizontal resolution olution and FPS, the IP camera may be confi gured is variable dependent on the recording or display for a particular bandwidth within some bandwidth

2 WHITE PAPER - Video Solutions: Understanding Video Image Quality range. There are tradeoffs between confi gurations. Data Bit Rate Packet size and transfer rate In general, higher bandwidth allocation for a given fps (Packets Per Second – PPS) is signifi cant for video and resolution usually will lead to better video quality. applications. The overhead of encapsulating a video stream into a packet stream can be sub- An encoder device collects and produces a com- stantial depending on the data requirements and pressed video stream from the camera. The most network confi guration. In general, encoded video common codecs used in video surveillance are will be encapsulated in Real-time Transport Protocol MJPEG, MPEG4-SP, and H.264. (See Table 1) (RTP), RTP encapsulated in User Datagram Protocol (UDP), and UDP encapsulated in Internet Protocol version 4 (IPv4). Additionally IP is encapsulated into Table 1: Common Surveillance Video Codecs some Data Link Layer protocol, such as Ethernet. Codec Type Codec Characteristics The overhead sum of Ethernet/IP/UDP/RTP is MJPEG • Independantly decodable 18+20+8+12 = 58 octets, plus the video payload. frames • Greatest Bandwidth The bit rate consumption in kbps is typically stated required for a given video for a codec and associated parameters (like fps and quality resolution).

MPEG-4 • Frames are encoded in a Network Bandwidth It is intuitive that bandwidth Simple dependant manner will be predominantly ﬂ owing in an uplink direction Proﬁ le (SP) • Designed for motion video from the video encoder to the network. The net- • Most common in today’s work operator will need to allocate some bandwidth surveillance industry in the downlink direction from the network to the MPEG-4 • Builds upon MPEG-4 to camera. This downlink bandwidth is used to control Part 10, provide best quality vs. the camera for Pan, Tilt, Zoom (PTZ) and other H.264, AVC bandwidth required tradeoff functions. In many networks, the network operator can designate the up/down ratio of data to the

3 WHITE PAPER - Video Solutions: Understanding Video Image Quality Table 2: Typical Quality Settings and Network Bandwidth Consumption (MPEG4-SP) Frames per Seconds Resolution 5 FPS 10 FPS 15 FPS 25 FPS 30 FPS QCIF 25 kbps 50 kbps 75 kbps 125 kbps 150 kbps CIF 100 kbps 200 kbps 300 kbps 500 kbps 600 kbps 4CIF 400 kbps 800 kbps 1200 kbps 2000 kbps 2400 kbps

cameras in terms of a percentage. For many video surveillance applications, it is common to have 10% downstream (control signals to the camera) and 90% upstream (video images from the camera).

Putting It All Together While there are many factors affecting bandwidth consumption of a video stream, table 2 depicts typical quality settings and their use of network bandwidth for a variety of applications. In each case, an MPEG4-SP codec is used.

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