W H I T E PA P E R

Video Solutions:Understanding Video Quality

2 WHITE PAPER - Video Solutions: Understanding Video Image 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.

AbstractOperators need to clearly understand the require-ments of their organization. This includes an understanding of the video image quality. While there are many other issues that will signifi cantly impact network design, this paper provides infor-mation on factors that address image quality. The factors discussed include:

• Resolution• Frames Per Second (FPS)• Video Codec• Packets Per Second (PPS)• Bit Rate

An understanding of the effect of these factors on bandwidth requirements will enable network operators to correctly design communications infrastructure networks that will support video applications.

Intricacies of Video Solution Image QualityOperators need to select video performance that is appropriate to the needs of the application. Different performance levels will require different cameras to collect images. In general, higher performance levels will require a higher bandwidth infrastructureto transport data from the camera to the video command center. Network operators need to clearly understand the camera performance required in order to design the communications infrastructure appropriately. If there is not suffi cient capacity at any point in the communications infrastructure, video images may be delayed or lost, defeating the purpose of a video solution.

Resolution Resolution is the number of “pixels” Resolution Resolution is the number of “pixels” Resolution(picture elements) contained within each frame of video. For example, The National Television System Committee (NTSC) as a standard specifi es a manda-tory 525 analog scan lines of vertical resolution, of which 486 are typically visible. Horizontal resolution is variable dependent on the recording or display

medium. NTSC defi nes an aspect ratio of 4:3, which provides a theoretical maximum resolution of 720 (horizontal) by 486 (vertical) when specifi ed in terms of non-square (10:11) pixels. In square pixels this would be 648x486, or more commonly 640x480.

The resolution issue is complicated by additional common resolution standards for video encoding. Common Intermediate Format (CIF) is 352x288 @ 30fps. It represents a compromise between the NTSC frame rate (30fps) and PAL resolution. In practice, producing 352x288 from a NTSC camera source is diffi cult. As such North American surveil-lance vendors have created a ‘modifi ed’ CIF format of 352x240 (“NTSC CIF”) which is easily derived from an NTSC video source by removing every other vertical scan line. Alternatives are Quarter CIF (QCIF) 176x144 (or 176x120), 4CIF 704x576 (or 704x480), and 16CIF 1408x1152 (or 1408x960).

The interaction between encoded resolution and display resolution of the decoded video can be dramatic. By understanding the device which will be used to view the decoded video, the network operator can know the requirement for image collection and processing. The resolution of thedisplay device has an impact on overall perceived video quality, since the encoded video will need to be scaled accordingly.

Frames Per Second (FPS) Frames Per second Frames Per Second (FPS) Frames Per second Frames Per Second (FPS)(FPS) is the number of “snapshots” of the video scene in one second. Recall that modern fi lms are 24 FPS, NTSC is 30 FPS, and PAL is 25 FPS. Video surveillance cameras can be confi gured for a range of FPS. In many cases, 10 FPS is suffi cient.

Video Codecs One might think that stipulating the Video Codecs One might think that stipulating the Video Codecsencoder’s resolution and Frames Per Second (FPS) would exactly result in a bandwidth throughput need. However, even when stipulating both the res-olution and FPS, the IP camera may be confi gured for a particular bandwidth within some bandwidth

range. There are tradeoffs between confi gurations. In general, higher bandwidth allocation for a given fps and resolution usually will lead to better video quality.

An encoder device collects and produces a com-pressed video stream from the camera. The most common codecs used in video surveillance are MJPEG, MPEG4-SP, and H.264. (See Table 1)

3 WHITE PAPER - Video Solutions: Understanding Video Image Quality

Table 1: Common Surveillance Video Codecs

Codec Type Codec Characteristics

MJPEG • Independantly decodable frames • Greatest Bandwidth required for a given video quality

MPEG-4 Simple Profi le (SP)

• Frames are encoded in a dependant manner • Designed for motion video • Most common in today’s surveillance industry

MPEG-4 Part 10, H.264, AVC

• Builds upon MPEG-4 to provide best quality vs. bandwidth required tradeoff

Data Bit Rate Packet size and transfer rate (Packets Per Second – PPS) is signifi cant for video applications. The overhead of encapsulating a video stream into a packet stream can be sub-stantial depending on the data requirements and network confi guration. In general, encoded video will be encapsulated in Real-time Transport Protocol (RTP), RTP encapsulated in User Datagram Protocol (UDP), and UDP encapsulated in Internet Protocol version 4 (IPv4). Additionally IP is encapsulated into some Data Link Layer protocol, such as Ethernet. The overhead sum of Ethernet/IP/UDP/RTP is 18+20+8+12 = 58 octets, plus the video payload.

The bit rate consumption in kbps is typically stated for a codec and associated parameters (like fps and resolution). Network Bandwidth It is intuitive that bandwidth Network Bandwidth It is intuitive that bandwidth Network Bandwidthwill be predominantly fl owing in an uplink direction from the video encoder to the network. The net-work operator will need to allocate some bandwidth in the downlink direction from the network to the camera. This downlink bandwidth is used to control the camera for Pan, Tilt, Zoom (PTZ) and other functions. In many networks, the network operatorcan designate the up/down ratio of data to the

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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 Putting It All Together While there are many Putting It All Togetherfactors 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.