Audio and Video Bandwidth Requirements

Technical information about CAN-8 software.

Audio and Video Bandwidth Requirements

Unread postby Richard » Wed Jan 23, 2013 4:02 pm

Bandwidth requirements for video and audio in CAN-8

Video compressed using MPEG 1 or 2 may have its compression rate adjusted such that the quality of the video is sacrificed to reduce the bandwidth required to transfer it. What follows is an analysis of bandwidth required based on our own experience with compressed video.

    MPEG-1 was designed to use a standard frame size of 352x240 (SIF dimension) and in most cases delivers good quality video (and audio) at a rate of 64 KBytes/sec. MPEG-1 may use other frame sizes, the data rate will vary when the frame size and also the frame rate is changed. The maximum "nominal" data rate for MPEG-1 is 115 KBytes/sec, this may be used to calculate maximum bandwidth requirements.

    Calculating for the number of stations:

    30 Stations * 115 KBytes/sec(peak) = 3.4 MBytes/sec (peak)

    For average video:

    30 Stations * 64 KBytes/sec = 1.9 Mbytes/sec

    This data rate is well within the limits of a network running at 100Mbits/sec (10MBytes/sec).

    The MPEG-2 standard is in many ways similar to MPEG-1. The primary motivation behind MPEG-2 was to have a standard that is more practical to edit in such systems as non-linear video editing. MPEG-2 also increases the quality of the video slightly by allowing a greater number of bits of quantization for DC components along with more flexibility in the encoding scheme. This said, the data rates for MPEG-2 tend to be higher for similar frame sizes and rates than for MPEG-1. The benefit in the lab situation is a slight increase in the quality of the video. Some sample data rates for MPEG-2 are calculated below using a frame size of 352 X 480 (Half Horizontal CCIR 601 displays as 352 X 240) and frame rate of 24 fps. The difference in video quality at the stations may not be noticed by most viewers, especially if the source material is a VHS tape.

    Calculating for 30 stations:

    30 Stations * 200 KBytes/sec(peak) = 6 MBytes/sec (peak)

    Average rate using 352 x 240 frame size:

    30 Stations * 80 KBytes/sec = 2.4 Mbytes/sec

    The average data rate is within the limits for 100Bit/sec ethernet but the peak rate may exceed it. Typical limits for ethernet are at approximately 30-40% of the bit rate (ie: 3-4 MBytes/sec).

In comparison, the data rates for the audio used by the CAN-8 system are very small. Our system uses our own format of compression that is done in real-time at the station when audio is recorded and is also decompressed when played back. The audio data transferred by the CAN-8 system is always in compressed form on the network.

The average data rate for the audio in compressed form is 6 Kbytes/sec.

Calculating for 30 Stations:

30 Stations * 8 Kbytes/sec = 240 Kbytes/sec

Each station may play and record at the same time. If the stations are on a fully duplex network, the peak rate is 240 kbytes per sec for 30 stations.

If the network is not fully duplex, the peak rate would be double this number, 480 Kbytes/sec. However in most cases the students are not playing and recording at the same time.

The CAN-8 system also has a live video broadcast capability in the "SCAN DVC" addition to the product. This feature allows the instructor to tune in, or play from a VCR a source of video and audio and have it broadcast in real-time to all the stations in the classroom. The video is digitized in the instructors station using appropriate video capture hardware such as the ATI All-in-wonder Pro and then the software performs a quick compression and broadcasts the video out to all the stations on the network. The typical data rate is 720 Kbytes/sec and does not change with the number of students as the packets are proagated on the network using broadcasts and all stations receive the same data. This data rate is within the capabilities of a 100MBits/sec network.
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