Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

2017-07-07 02:10:07

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4.4 VIDEO

A video signal occupies a bandwidth of 5MHz. Using the Nyquist sampling theorem, we need to sample the video signal at 10 samples/msec. If we use 8-bit PCM, video signal requires a bandwidth of 80Mbps. This is a very high data rate, and this coding technique is not suitable for digital transmission of video. A number of video coding techniques have been proposed to reduce the data rate.

For video coding, the video is considered a series of frames. At least 16 frames per second are required to get the perception of moving video. Each frame is compressed using the image compression techniques and transmitted. Using this technique, video can be compressed to 64kbps, though the quality will not be very good.

Video encoding is an extension of image encoding. As shown in Figure 4.6, a series of images or frames, typically 16 to 30 frames, is transmitted per second. Due to the persistence of the eye, these discrete images appear as though it is a moving video. Accordingly, the data rate for transmission of video will be the number of frames multiplied by the data rate for one frame. The data rate is reduced to about 64kbps in desktop video conferencing systems where the resolution of the image and the number of frames are reduced considerably. The resulting video is generally acceptable for conducting business meetings over the Internet or corporate intranets, but not for transmission of, say, dance programs, because the video will have many jerks.

Moving Picture Experts Group (MPEG) released a number of standards for video coding. The following standards are used presently:

MPEG-2: This standard is for digital video broadcasting. The data rates are 3 and 7.5Mbps. The picture quality will be much better than analog TV. This standard is used in broadcasting through direct broadcast satellites.

A variety of video compression standards have been developed. Notable among them is MPEG-2, which is used for video broadcasting. MPEG-4 is used in video conferencing applications and HDTV for high-definition television broadcasting.

MPEG-4: This standard is used extensively for coding, creation, and distribution of audio-visual content for many applications because it supports a wide range of data rates. The MPEG-4 standard addresses the following aspects:

Representing audio-visual content, called media objects.

Describing the composition of these objects to create compound media objects.

Multiplexing and synchronizing the data.

Figure 4.6: Video coding through frames and pixels.

The primitive objects can be still images, audio, text, graphics, video, or synthesized speech. Video coding between 5kbps and 10Mbps, speech coding from 1.2kbps to 24kbps, audio (music) coding at 128kbps, etc. are possible. MP3 (MPEG Layer–3) is the standard for distribution of music at 128kbps data rate, which is a part of the MPEG-4 standards.

For video conferencing, 384kbps and 2.048Mbps data rates are very commonly used to obtain better quality as compared to 64kbps. Video conferencing equipment that supports these data rates is commercially available.

MPEG-4 is used in mobile communication systems for supporting video conferencing while on the move. It also is used in video conferencing over the Internet.

In spite of the many developments in digital communication, video broadcasting continues to be analog in most countries. Many standards have been developed for digital video applications. When optical fiber is used extensively as the transmission medium, perhaps then digital video will gain popularity. The important European digital formats for video are given here:

Multimedia CIF format: Width in pixels 360; height in pixels 288; frames/ second 6.25 to 25; bit rate without compression 7.8 to 31 Mbps; with compression 1 to 3 Mbps.

Video conferencing (QCIF format): Width in pixels 180; height in pixels 144; frames per second 6.25 to 25; bit rate without compression 1.9 to 7.8 Mbps; with compression 0.064 to 1 Mbps.

Digital TV, ITU-R BT.601 format: Width 720; height 526; frames per second 25; bit rate without compression 166 Mbps; with compression 5 to 10 Mbps.

HDTV, ITU-R BT.109 format: Width 1920; height 1250; frames per second 25; bit rate without compression 960 Mbps; with compression 20 to 40 Mbps.

Note

Commercialization of digital video broadcasting has not happened very fast. It is expected that ultilization of HDTV will take off in the first decade of the twenty-first century.

Summary

This chapter presented the details of coding text, voice, image, and video into digital format. For text, ASCII is the most commonly used representation. Seven bits are used to represent characters. Unicode, which uses 16 bits is now being used for text representation. Characters of any world language can be represented using Unicode.

For audio, Pulse Code Modulation (PCM) is the most widely used coding technique. In PCM, voice is coded at 64kbps data rate by sampling the voice signal at 8000 samples per second and representing each sample by 8 bits. Using Adaptive Differential Pulse Code Modulation (ADPCM), the coding rate can be reduced to 32kbps without any reduction in quality. Another technique used for voice coding is Linear Prediction Coding (LPC), with which the data rate can be reduced to as low as 1.2kbps. However, as the bit rate goes down, quality goes down. Variants of LPC are used in many applications such as mobile communications, Internet telephony, etc.

For image compression, the Joint Photograph Experts Group (JPEG) standard is used, through which compression ratios up to 30:1 can be achieved. For video coding, the most widely used standard was developed by Moving Picture Experts Group (MPEG). MPEG-2 is used for broadcasting. MPEG-4 defines standards for video encoding from 5kbps to 10Mbps. MPEG-4 is used in mobile communications as well as in multimedia communication over the Internet.

References

J. Campbell. C Programmer's reference guide to Serial communication. Prentice-Hall, Inc., 1997.

G. Karlsson. "Asynchronous Transfer of Video". IEEE Communications Magazine, Vol. 34, No. 8, August 1996.

G. K. Wallace. "The JPEG Still Picture Compression Standard". Communications of the ACM, Vol. 34, No. 1, April 1991, pp. 30-44.

D. LeGall. "MPEG: A Video Compression Standard for Multimedia Applications". Communications of the ACM, Vol. 34, No. 1, April 1994.

http://www.cdacindia.com Web site of Center for Development of Advanced Computing. You can obtain the details of the ISCII standard from this site.

Questions

What are the different standards for coding of text messages?

What is waveform coding? Explain the PCM and ADPCM coding techniques.

What is a vocoder? Describe the speech production model.

Explain the LPC coding technique.

Explain the JPEG compression technique.

What are the salient features of the MPEG-4 standard?

Exercises

1.	On your multimedia PC, record your voice and observe the speech waveform. Store the speech data in a file and check the file size. Vary the sampling rate and quantization levels (bits per sample), store the speech data, and observe the file sizes.
2.	Install a desktop video camera on your PC and, using a software package such as Microsoft's NetMeeting, participate in a video conference over the LAN. Observe the video quality.
3.	Calculate the bit rate required for video transmission if the video is transmitted at the rate of 30 frames per second, with each frame divided into 640 × 480 pixels, and coding is done at 3 bits per pixel.
4.	Describe the Indian Standard Code for Information Interchange.
5.	Download freely available MP3 software and find out the compression achieved in MP3 software by converting WAV files into MP3 files.
6.	Calculate the memory required to store 100 hours of voice conversation if the coding is done using (a) PCM at 64kbps (b) ADPCM at 32kbps and (c) LPC at 2.4kbps.
7.	If the music signal is band-limited to 15 kHz, what is the minimum sampling rate required? If 12 bits are used to represent each sample, what is the data rate?
8.	An image is of size 640 × 480 pixels. Each pixel is coded using 4 bits. How much is the storage requirement to store the image?

Answers

1.	To record your voice on your multimedia PC, you can use the sound recorder available on the Windows operating system. You can also use a more sophisticated software utility such as GoldWave (http://www.goldwave.com). You will have the options to select the sampling rate (8kHz, 16kHz, etc.) and the quantization levels (8 bits, 16 bits, etc.). GoldWave provides utilities to filter background noise, vary the pitch, and so on.
2.	When you use a software package such as Microsoft's NetMeeting over a LAN, the video will be transmitted at very low bit rates, so the video appears jerky.
3.	If the video is transmitted at the rate of 30 frames per second, with each frame divided into 640 × 480 pixels, and coding is done at 3 bits per pixel, the data rate is 30 × 640 × 480 × 3 bits per second = 3 × 64 × 48 × 3kbps =27,648kbps = 27.648Mbps
4.	The Indian Standard Code for Information Interchange (ISCII) is a standard developed by Department of Electronics (Ministry of Information Technology), Government of India. ISCII code is used to represent Indian languages in computers. Center for Development of Advanced Computing (CDAC) supplies the hardware and software for Indian language processing based on ISCII. You can obtain the details from the Web site http://www.cdacindia.com.
5.	MP3 software can be obtained from the following sites: http://www.dailymp3.com http://www.mp3machine.com http://www.mp3.com
6.	To store 100 hours of voice, the memory requirement is given below if the coding is done using (a) PCM at 64kbps, (b) ADPCM at 32kbps, and (c) LPC at 2.4kbps. To store 100 hours of voice using PCM at 64kbps data rate, Total duration of voice conversation = 100 hours = 100 × 3600 seconds Memory requirement = 100 × 3600 × 64 kbps = 100 × 3600 × 8 Kbytes = 360 × 8 Mbytes = 2880Mbytes 1440Mbytes 100 × 3600 × 2.4kbps = 100 × 3600 × 0.3Kbytes = 36 × 3 Mbytes = 108Mbytes
7.	If the music signal is band-limited to 15kHz, the minimum sampling rate required is twice the bandwidth. Hence,Minimum sampling rate = 2 × 15kHz = 30kHzIf 12 bits are used to represent each sample, the data rate = 30,000 × 12 bits/ second = 360,000 bits per second = 360kbps.
8.	The image is of the size 640 × 480 pixels. Each pixel is coded using 4 bits. To store the image,Memory requirement = 640 × 480 × 4 bits = 153.6Kbytes.

Projects

Develop a program to generate Morse code. The output of the Morse code (sounds for dot and dash) should be heard through the sound card. The duration of a dash is three times that of the dot.

Study the Durbin–Levinson algorithm for calculation of linear prediction coefficients. Implement the algorithm in software.

Develop software for compression of images using the JPEG standard.