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Audio

Real-time audio applications are not as complicated as video, but they do have their own set of unique characteristics. The bandwidth for concert hall stereo sound is 20 KHz. For full life-like stereo reproduction of sound, many reproduction machines sample the original analog signal at 8 or 16 times the peak frequency. If we have a machine that oversamples at x8, the bandwidth required to transport the digitized signal to and fro is 20 KHz x 8 samples / Hz x 8 bits / sample = 1.28 Mbps. For a machine that oversamples the audio signal x16, the bandwidth required for transport is 20 KHz x 16 samples / Hz x 8 bits / sample = 2.560 Mbps. Both of the preceding examples utilized an 8-bit / sample sampling machine. If the digitized signal were created on a 16-bit / sample machine, the results would be double.

There is a trade-off between how well we can hear and how much bandwidth is necessary to carry the digitized audio signal. The more bandwidth we use, to a point, the better the reproduced signal matches the original signal. However, reproducing the signal to a greater accuracy than we are capable of distinguishing with the limitations of our hearing abilities is not sound economics and wastes bandwidth. So, some folks argue that oversampling x4 and limiting the upper bandpass to 10 KHz (bandwidth = 10 KHz x 4 samples / Hz x 8 bits / byte = 320 Kbps) is sufficient for virtually all audio applications.

However, audiophiles want x8 or even x16 oversampling and 16 bits / sample. Typically, 1.28 Mbps is sufficient for most audiophiles and 320 Kbps is sufficient for the rest of the world.

For telephone quality audio connections, the bandwidth required is 4 KHz x 2 samples / Hz x 8 bits / sample = 64 Kbps or the basic T1 rate. Of course, anyone listening to the sound reproduced with this set of parameters would not mistake the sound as originating from Carnegie Hall.

ATM can transport the bit rates required for audio without any effort. However, if the sound is to arrive at the destination in a live transmission and, after conversion to an analog signal, be a faithful reproduction of the original sound, the transmission latency (delay) must be acceptable. A latency greater than 100 microseconds begins to become noticeable at the higher frequencies. To keep latency from noticeably degrading the audio signal, a transmission rate of 4.2 Mbps (3xDS-1 UNI) or greater is required.


Figure 7-1.  Simplified composite video test signal

Full Motion Color Video

Real-time video applications have unique characteristics that require special consideration for encoding and transporting the data. The key words are "real time." For an audio and/or video signal to arrive at its destination (your monitor/TV set) with an acceptable level of distortion (less than that required for the human eyes and ears to perceive), the time relationship between the data bits must be preserved within the acceptable limits.

Figure 7-1 shows a composite baseband video test signal. The signal varies from –40 IRE to +120 IRE over a 62-microsecond period. The timing relationship between the various components of the composite signal, which vary over a 62-microsecond period, must be maintained within strict limitations, or else the signal received, and viewed, will contain a varying degree of distortion. The amount of distortion is dependent upon the shift in the relationships, but it usually doesn’t take much to muck up a color signal sufficiently to say the picture is not viewable.

A closer examination of Figure 7-1 in the 0- to 15-microsecond time period reveals the waveform in Figure 7-2. The timing values given have typical 0.1 microsecond tolerances. With 0.1 microsecond tolerance, you can readily ascertain there are critical timing relationships involved. The particular video waveform segment shown in Figure 7-2 is the video blanking pulse which includes the chrominance subcarrier. Picture effects of improper timing relationships include: no picture, picture too light, picture too dark, apparent color saturation, picture breakup, incorrect color saturation, color smearing or bleeding, poor reproduction of sharp luminance transitions, fuzzy vertical edges, brightness variations between the left and right side of the screen, horizontal streaking and smearing, top-to-bottom brightness inaccuracies, picture flicker, changes in hue, incorrect reproduction of picture colors, loss of detail, transient brightness effects, grainy or snowy picture, color sparkles, picture blurriness, and an audible buzz in the audio channel. For most of the video problems just listed, it does not take much change in the composite signal relationships to produce the undesired picture effects. What causes changes in signal relationships in ATM transmission? Primarily, signal delay as cells transit the network. There is the finite time a cell takes to travel from point A to Point B. Also, there is a queueing time, called latency, associated with every network node the cell must transit. Again, all cells must wait their turn before they are processed by the equipment at every node.


Figure 7-2.  Simplified video pulse width requirements


Figure 7-3.  Composite color video bandwidth diagram


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