by W.A. Steer PhD
In the context of consumer-electronics, 'digital' has become something of a buzzword in recent years. Although marketing has strived to make 'digital' synonymous with 'good quality' in the public consciousness, it is evident that, particularly with the new digital TV and radio (in the U.K. at least), this is not always the case. This document attempts to explain what 'digital' is, the benefits and drawbacks, and why the electronics and entertainment industries are so keen to push it.
Very simply, 'analogue' is used to describe systems which operate using the principle of signals whose characteristic varies in proportion to some other function which they represent. An example is a gramophone record where the depth of the groove varies in exact proportion with the sound pressure level recorded. By contrast, 'digital' systems work on the principle of numerical representations and calculation.
Application to recording and transmission systems
What is 'analogue'?
From the earliest days of gramophone record, telephones, and radio, engineers worked to replicate the form of audio sound pressure waves in some other medium. In a gramophone record, the depth of the groove is varied in exact sympathy with the sound; in telephones an electrical voltage (or current) is generated in proportion to the sound pressure; in amplitude-modulated (AM) radio transmissions the strength of the radio wave is varied in direct accordance with the sound. All these systems are classed as 'analogue' (or American, 'analog'), because the stored or transmitted signal bears a direct analogy to the original information.
Despite their very attractive simplicity, the most obvious problem with analogue systems is that the reproduction is similarly an analogue (like-copy) of the stored or transmitted signal ...and so any slight distortion or interference of the stored or transmitted signal will cause likewise distortions and interference in the recovered sound. For example, dust on a gramophone record results in pops and crackles; on an AM radio receiver, interference (from sources such as strip-lights or electric motors) often causes buzzes and whistles in the received sound.
What is 'digital'?
The word 'digital' is a derivation of 'digit' and refers to systems which process numbers. In the case of digital recording or transmission a numerical description of the original signal is made, and it is this description, rather than a direct analogue, which is stored. In practice, this numerical description is created by sampling (measuring) the original signal to a finite resolution, at a series of discrete time intervals. The result can be thought of as a log-book of numerical measurements. As an example, in the case of compact-disc audio, that log-book has 44100 entries for every second of the recording, and each entry represents the sound pressure to 5 decimal digits (in fact, a resolution of 1 part in 65536), for the left- and right-hand stereo channels.
These digital signals (think: numerical description) are inherently more resilient to corruption during storage or transmission than their analogue counterparts. Visualise a laboratory notebook from school; even if the page (the recording) becomes blotched and stained, for as long as your numbers remain legible, the data you took remain exactly the same, and can be re-written without loss or damage of the information contained.
Furthermore, 'checksums' (a sophisticated sort of built-in tally) can be appended to digital signals so that even if some of the digits do get irrecoverably corrupted or confused (up to some finite limit of course) the receiver or playback device can still deduce the original signal with no impairment at all.
Compact-disc audio is an excellent example of the consumer benefits of digital recording technology. When compared to analogue records or cassette tape, the CD has demostrably greater dynamic range (which means practically no hiss during quiet parts of the music), very good and flat frequency response (so the sound is bright, crisp, and lifelike; not 'coloured' or 'muddy'), and the format is notably resilient to dust, dirt and scratches, and to environmental conditions. Furthermore both the media and equipment is very cheap compared to any (professional) analogue equipment of comparable sound quality.
Analogue systems are usually very simple in concept; generally simple and cheap in practice, to achieve at least reasonably-good quality results. Always susceptible to interference and distortion, and any such degradation is irrecoverable. Very high quality components and careful design permit high quality, but at an increasingly high price. Always some measurable degradation through every stage of processing.
Digital is more complicated in concept, but provides what I call 'assured quality' (the 'quality' is defined and fixed at the recording/encoding stage, and you're pretty-much guaranteed to get that back with any playback or receiving device). Resilient to distortion and degradation, perfect (loss-less) copies can be made. Processing (such as studio mixing or effects) can be performed numerically with negligible loss of fidelity. Possible to increase quality for relatively marginal increases in cost. Almost any sort of specialised analogue processing can be done at least as well in the digital domain, and in fact the scope for digital processing is almost unbounded.
It is quite possible to have digital sub-sections in a basically analogue piece of equipment. A common example is analogue (VHF/FM or MW/AM) radios with "digital tuning". In this case, digital counting circuits are used to generate a programmable frequency reference (from a quartz crystal) to provide stable and accurate tuning and convenient presets. The radio still only receives conventional analogue broadcasts, and in fact the majority of the radio circuitry is very similar to that in any analogue-tuning radio!
Another example would be a VHS video recorder claiming "digital tracking". Again it's basically a classic analogue video recorder, but uses a little bit of digital analysis to keep the head-drum accurately aligned with the recording on the tape. This module replaces the user-set "tracking" control found on older machines. The basic recording and playback process is still essentially the classic analogue design.
Just to confuse you, it is possible to make a radio receiver which uses almost entirely digital techniques to receive analogue signals... Although it would probably offer little real performance advantage over a conventional receiver, it may well be more versatile or (in the future) work out cheaper to produce - at the present time, this is still research-lab stuff and not something you'll find in the high-street shops!
Bit-rate Reduction ("Digital Compression")
Complete digital descriptions of high-quality analogue audio and television signals require a lot of data, which itself costs money to store or transmit. Digital data can be easily processed by computer-like hardware, and 'compression' algorithms exist which reduce the data required by filtering out redundancy and predictability in the data. Furthermore, (ideally) perceptually insignificant approximations can often be made which result in very substantial savings. MP3 for audio, and JPEG for still photographic images are well-known compression standards. For both formats, even a 'knowledgeable and critical observer' would be unlikely to detect the approximations (compression artifacts) when the data is compressed to one fifth of its native size. At compression ratios of about 10, the artifacts are detectable if you know what to look for, but the quality is broadly 'acceptable'. Further compression results in increasingly objectionable degradation of the programme material. Motion-video can often be compressed more because very little usually changes from one frame to the next.
Digital compression example
|'Acceptably-compressed' JPEG image, 1/10th native size
||Over-compressed JPEG image, 1/50th native size
JPEG compression breaks an image into 8×8 blocks and then reduces the level of detail within the blocks by using a what is known as a 'discrete cosine transformation' and then quantising the coefficients. Typical artifacts include 'blocking' and loss-of-detail, self-evident in the sky and distant trees; and 'ringing' around edges, particularly noticeable around the railings in the right of the picture.
Similarly, audio compressed to 100kbps or less (e.g. many MP3s) can sound 'muddy', 'thin', and generally indistinct when critically compared to a CD original.
It is far easier to implement secure encryption, leading to access-control (region-restriction, pay-per-view, copy-prevention) in digital media-distribution networks compared to the analogue equivalent.
Apart from the cynical view of merely being able to sell new equipment, more channels and secure encryption, facilitating pay-per-view (revenue) must be a big driving force within the industry! In principle, the low (technical) cost of channels opens up the market to smaller minority-interest programming, although a sizeable audience is still needed to justify the cost of making the programmes in the first place.
Digital television and radio broadcasting
In real-world digital broadcasting (c.2004) usually several programmes are transmitted in one radio signal, known as a 'multiplex'. For technical reasons that are way too complicated to explain here, by lumping many programmes together in one big signal, it is possible to create a transmission channel which is much more robust to interference and multipath echos, shadows, and fading. Although the total amount of data in a multiplex is fixed, the degree of compression of the programmes can be traded against the total number of programmes in the multiplex. Furthermore, the number of programmes or the distribution of quality can be changed dynamically, hourly, minutely, or even by the second.
Unfortunately economic pressures (coupled with the ease with which parameters can be adjusted) can oblige a broadcaster to use much greater compression ratios than engineering common-sense would dictate, with the result that the audio/visual quality suffers.
Every viewer or listener in the region who receives the digital signal should see exactly the same quaility image on their screen. Sadly, owing to excessive compression, the actual technical quality of the presentation is often substantially poorer than they might receive with a conventional analogue radio or TV and a good aerial!
In the UK, typically 4 or 5 digital TV programmes are broadcast on digital-terrestrial ('Freeview') in the space previously allocated to one analogue channel (digital satellite TV is also compressed to a comparable degree). That means that fundamentally the digital programmes have at best only one fifth of the 'information' of an analogue broadcast. In reality (thanks to reduced transmitter power, and substantial error-correcting codes etc) it can be shown that a digital TV programme has closer to one thirtieth of the information content of an analogue broadcast received with a decent aerial! Despite the fact that the compression is fairly smart, at that level of compression, I certainly notice that something's missing.
Digital TV offers more channels, and widescreen presentation. But in the U.K., unless your analogue reception is particularly bad, you get a worse quality picture with digital TV!
Digital compression artifacts seem to be more visible on LCD (and plasma) screens than CRTs, as well as becoming generally more objectionable as screen size get bigger or the viewing-distance becomes shorter.
Notes on 'quality' for analogue and digital systems
With analogue transmissions, the real technical quality of the transmission can be characterised by just two objective parameters: 'signal bandwidth' and 'signal to noise ratio'. The bandwidth describes the highest frequency in the signal (highest pitch for audio, or the picture sharpness for TV), and the signal-to-noise-ratio defines the level of background hiss for audio, or random-graininess for TV. As well as providing an objective standard, given these two parameters the subjective (or perceived) signal quality can easily be visualised. Non-compressed digital signals can be rated by "sampling rate" and "bit depth", which are essentially comparable to the analogue measures.
|Effect of reducing analogue 'bandwidth'
Very approximately 1/50th the information-content of original picture
|Effect of poor analogue signal-to-noise
Unfortunately there is no objective way of measuring the 'quality' of digitally-compressed programme material. It is straightforward enough to measure the data rate (or bit-rate) of the compressed data [typically around 1 to 5 mega bits per second (Mbps) for standard-definition digital TV broadcasts], but the corresponding subjective quality depends very much on the type of programme, and may vary by the second.
This lack of objective quality measurement (perhaps coupled with a lack of political will) makes it very difficult for the national standards bodies, or the technical arm of broadcasting organisations to enforce any given grade, in contrast to their traditional role -at least in the UK- for analogue broadcasting.
Created: July 2004
Last modified: 7 July 2004
©2004 William Andrew Steer