The world has changed; it has changed in profound ways because of digital technology. Yes, the proliferation of computers is the obvious outward sign, but there is much more to it than that. Consider the mobile phone and digital communication via the internet. The DVD and the venerable CD are digitally recorded media which provide us with infinitely more convenient and rewarding sight and sound experiences than did the non-digital, clumsy, and fragile VHS tapes and the noisy vinyl LP album.
The term “digital” is ubiquitous in today’s society – it is so casually used, and yet few people on the street would be able to accurately define what the term means, explain why digital is better, or describe what existed in its stead before the technological revolution of the last half century. A digital process is one which represents information using numbers. Representing information using numbers (digits) implies a certain granularity or limited resolution (like counting to ten by ones; there are no in-between values such as 1.2 or 8.5!). Also, a digital process is a sampled-data procedure where the information to be digitized and stored is typically captured in a time-sequence of instantaneous samples. The information being digitized is only quantified at discrete-time instants. No information is gathered between sequential samples. Due to the granularity of the number representation and the finite sampling rate, there is some information content that is lost in the process. A more accurate digitization would demand more expensive equipment in terms of increased data bits and higher sampling rates. Today’s electrical and communication engineers are well-versed in the cost/performance tradeoffs involved and how to make those tradeoffs for optimal “results.”
Consider the old vinyl LP record. The musical/tonal content of a Mozart symphony was relegated to undulations in a continuous groove cut in a vinyl plastic substrate. The higher the frequency content of the music, the faster the “squiggles” in the groove. The louder the music, the wider the “swerve.” That groove and its subtle characteristics were subject to wear and tear in the form of scratches and stylus erosion as well as age-degradation of the vinyl. A CD stores its audio information using number sequences coded in binary bits. A binary bit is either considered a “1” or a “0” depending on its “state.” Like vinyl, the CD substrate and the optical data bits recorded on it will not last forever either. The key difference between the two is that the numbers which comprise the digital content of the CD can be re-recorded exactly on a new substrate at any time. Imagine the difficulties inherent in “copying” the subtle undulations of a vinyl groove!
Computers and digital devices work almost exclusively with the binary (two-state) number system. Why? Because a transistor switch which is either “on” or “off” is cheap and easy to fabricate in the huge quantities required in computers. Imagine the additional complexity of an electronic “switch” which would require the ten different states required for the decimal number system!
Here is what happens to music which is recorded on a CD:
The electrical signal which emanates from the microphone(s) at the recording session is “sampled” many times each second by assigning a binary-coded number to each sample; the larger the audible volume and its microphone signal at that particular instant, the larger the number assigned to that sample. The assigned number represents the signal only at the precise instant of sampling. In between the last sample and the next sample to come, the electrical signal will change somewhat, but the samples are taken often enough so as to not lose much information content. The huge sequence of samples and their numbers which capture a Mozart symphony is then recorded on a CD device using a laser which either activates a tiny spot on the disk ….or does not. That spot is thus either a binary “1” or “0” accordingly.
I would like to leave you with a concrete example which illustrates the difference between the modern DIGITAL approach and the ANALOG processes of times past. Imagine a staircase with a straight plank of wood placed upon it to form a ramp which connects the top level to the bottom level. The ramp represents the ANALOG world in the sense that a fine-tipped pen could place a tiny dot anywhere along it in an essentially infinite number of vertical positions from bottom to top. The staircase represents the DIGITAL world in which a similar dot can only be placed only at a finite number of discrete vertical positions from bottom to top – eleven in this example. From a layperson’s point of view, it might seem that the ANALOG world is less restrictive by offering more choices – better “resolution” as engineers like to say. However, as we just discussed, the DIGITAL world can “sample” quantities at a predetermined periodic rate and store the samples in a computer using numbers to represent their “magnitude” (value) at the particular instant of the sampling – all to great ultimate advantage as we will see.
To more easily grasp the concept of sampling, try this using the illustration: Pick any point along the red line – along the “plank.” If our digital sampler were ready to sample the elevation of that point above the ground floor level, it would assign a number to the sample which corresponds to the height of the step just below our point of interest. The height of the next step up would represent the next available binary number in our sampling number system (there are no fractions of a step).
In summary, here is the real advantage of digital signal processing: All of the information captured is stored as numbers in a computer or related digital device like a CD. It can be easily read and manipulated as is done during the restoration of old, grainy movie prints. The stored information is amenable to digital signal-processing techniques for improving both audio and video. And, most interesting of all, once digitized, the source information can be endlessly copied and reproduced for archival purposes with no further degradation in quality! This means that Michelangelo, Van Gogh, Mozart, Chopin, Gone With the Wind, and Astaire and Rodgers can live forever without fear of being lost or degraded. For so many of our classic movies, digital technology arrived just in the nick of time. Anything and everything pertaining to our sight and hearing can be represented by nothing more than huge collections of ordered “1’s” and “0’s” in our digital devices. Wonderful, wonderful, and really quite profound when you stop to think about it!
More on the permanence of art, sound, and documents via digital data in a future post!