Analog-to-digital conversion

Chapter Analog-to-digital conversion

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • number of digital states increases. An example of eight-level PAM/PCM is shown in Fig. 26-12D.Analog-to-digital conversionThe graph of Fig. 26-12D illustrates a method of analog-to-digital (A/D) conversion.A voice signal, or any continuously variable signal, can be digitized, or converted intoa string of pulses, whose amplitudes can achieve only a finite number of states.ResolutionThe number of states is always a power of 2, so that it can be represented as a bi-nary-number code. Fidelity gets better as the exponent increases. The number of statesis called the sampling resolution, or simply the resolution.You might think that the resolution would have to be very large for good reproductionto be possible. But in fact, a resolution of 23 = 8 (as shown in Fig. 26-12D) is good enoughfor voice transmission, and is the standard resolution for commercial digital voice circuits.A resolution of 24 = 16 is adequate for compact disks used in advanced hi-fi systems!Sampling rateThe efficiency with which a signal can be digitized depends on the frequency at whichsampling is done. In general, the sampling rate must have a frequency that is at leasttwice the highest data frequency.For an audio signal with components as high as 3 kHz, the minimum sampling ratefor effective digitization is 6 kHz, or one sample every 167 microseconds (µs). Ideally,the sampling rate should be somewhat higher; the commercial voice standard is 8 kHz,or one sample every 125 µs.For music and hi-fi digital transmission, the standard sampling rate is 44.1 kHz, orone sample every 22.7 µs. This is based on a maximum audio frequency of 20 kHz, theapproximate upper limit of the human hearing range.Image transmissionThe modulation techniques used for image transmission are similar to those employedfor sending voices. Nonmoving pictures can be sent within the same bandwidth as avoice. For high-resolution, fast-scan moving images, the necessary bandwidth is greater.A thorough discussion of image transmission is beyond the scope of this book. Thebasics of the three most common video communications modes are discussed here. Forfurther detail, a text on video communications is recommended.Facsimile“Still” images are transmitted by facsimile ( fax). If data is sent slowly enough, anyamount of detail can be transmitted within a voice band. This is how telephone fax works.A high-resolution commercial fax image has upwards of 1000 lines of data. The im-age is scanned from left to right and from top to bottom like reading a book. The com-plete image takes several minutes to send. Many of the black-and-white photographs inthe daily newspaper are sent via fax. Practically all weather satellite images are faxed.Image transmission487