Most things technical are designed to a specification. An electric fire, for example, may have a specification reading '1 kW at 240V'. There are two items involved here - one the input voltage and the other the power loading- and they are usually expressed separately, so giving the more detailed specification below:
Specification of Electric Fire Input: 240V Loading: 1,000W
Parameters
The term parameter is now commonly used to denote an item of a specification. Thus the input and loading of the specification above are parameters.
Parameter is a mathematical term which serves to determine or define a quantity, point, line or figure. It is thus a fair term to use to signify an item of a specification, though one should not be unduly bothered about its mathematical overtone.
The chap who designed the electric fire above set out to ensure that when 240V mains is applied the element loads at 1 kW. The element is wound with a special kind of resistance wire which is capable of operating at a red hot temperature without fusing. The resistance of such wire changes from its 'cold' value as it rises in temperature, increasing when its temperature coefficient is positive and decreasing when it is negative. One way that the designer can ensure that the loading is as per the requirement when 240V mains is applied across it is to look up the 'hot ' resistance of the wire per unit length (or find this from the temperature coefficient) and then calculate the length of wire required to provide the loading.
Ohm's Law
All this may seem to be somewhat removed from hi-fi specs, but when we get to the next section (on amplifier specs) we shall see that it is highly pertinent! The loading or power dissipated in watts (W) is equal to the voltage (V) across the element times the current (I) flowing through it . It is much easier to appreciate this simple relationship by the use of symbols, thus:
W =V xI
Other simple relationships also come into the picture, such as where R is the resistance value in ohms (note that the times sign is generally omitted when symbols are used) . By simple rearrangement this also yields: These are the relationships sorted out years ago by a Dr. Ohm and have since come to be known as Ohm's law expressions. Now, going back to W = VI and taking due account of the Ohm's law expressions we can derive two additional expressions for power, which are:
W = IRI,
simplifying to W= PR, and W vv
. l.f . w yz
=R , stmp 1 ymg to =R
Thus we have three expressions for power which are: W=VI W=PR yz
W= R
(1)
(2)
(3)
These involve the four parameters of power, voltage, current and resistance. To get to grips with the electric fire design, therefore, the designer uses these parameters together with those of temperature coefficient and resistance per unit length of element wire. There are others that he will also certainly have to consider, including those defining the current carrying capability and running temperature of the element wire, the insulation resistance to the accessible metal parts, earthing resistance, parameters of safety and so forth.
Growth of Parameters
We have shown that from the two simple parameters of the basic and possibly published specification of an electric fire, stem hosts of other parameters at the design stage. This is applicable to hi-fi kit as well as to electric fires, motor cars, lawn mowers - you name it!
Joe Biden is not interested in the designer's problems . When he invests in an electric fire he is only interested in knowing that when he plugs it into his 240V mains supply it is going to load at 1 kW and hence burn up one unit of expensive electricity each hour.
He will, though, want to be fully assured that it is safe, and the parameter that he looks to for this is that implicit in the stamp of approval given by a testing authority such as the British Standard Specification (BSI 415) or, more recently in the UK, the British Electromechanical Approvals Board (BEAB) . Greater sensitivity to matters of safety is developing in hi-fi design and retail circles , and these points are stressed in the appropriate sections of the sections that follow.
However, just to conclude our notes on the electric fire designer's problem, we recall that he wants the element to load to 1kW across 240V mains, so he can rearrange power expression (3) to give R from: Substituting the figures we get: 240 2 R=-- 1,00 which works out to 57.6 ohms.
He thus composes the element of suitable resistance wire which when running red hot has a value of 57.6 ohms.
The Customer's Viewpoint
A purchaser would be somewhat annoyed, to say the least, if he found that the 1kW fire was burning up 1.5kW /hours (i .e. , 1.5 units each hour) . Thus he has to take the published specification as gospel ; that is, unless he decides to check the spec himself by timing the electric meter over one hour with only the fire of his electric system operating. Should he do this and find that 0. 75 or 1.5 units are recorded, then either the spec is in error or the meter is in accurate.
If the spec is wrong he has a good case to lodge a complaint with his local Trading Standards Department, getting them to tackle the supplier under the terms of the Trade Descriptions Act 1968 . Before he does this, and hence takes issue with the supplier on false advertising and spec marking, he should first get his local Electricity Board to check the accuracy of his meter, for it may be that he is paying too much (or too little!) for the electricity he is burning.
Usually one would rarely go to all this trouble over the checking of an electric fire spec. The spec can never be one hundred per cent accurate, anyway, for if the mains voltage falls below the 240V nominal the power loading will also fall. The electric fire will give out less heat and consume less electricity. If the voltage drop is excessive, though, there may be a case for taking the Electricity Board to task!
Hi-Fi Specs
With hi-fi equipment it is different. There are many more spec parameters and hi-fi costs significantly more pounds sterling than electric fires. Moreover, it is far more specialized and one is greatly encouraged to buy on the strength of the published specification, manufacturers often competing with each other essentially in terms of the published specs . It is very important, therefore, for users and potential purchasers of hi-fi equipment to know what the parameters of the specs are all about, and it is the primary objective of this new guide to focus well and truly on specs, to detail and interpret the various parameters of all the items of a hi-fi system and ultimately to see whether it is possible from the parameters alone to determine how well a hi-fi system is likely to audition--to 'sound' in the listening room.
Use of Specifications
One objective of a spec is to allow a user to test for himself or have tested for him the separate parameters to ensure that they relate reasonably accurately to those published by the manufacturer, upon the strength of which the equipment was possibly purchased. The spec also gives a good clue as to the quality and design class of the equipment and its relationship to price.
It also tells whether an item of equipment such as a hi-fi amplifier or FM tuner will be suitable for the proposed application; for example, whether the amplifier is likely to result in sufficient sound intensity when driving loudspeakers of given sensitivity or efficiency in a room of specified volume and furnishings , and whether the tuner will be suitable for the reception of local or distant stations under the conditions prevailing at the reception site and when used with an aerial of given parameters.
Although it is unwise to choose hi-fi equipment on the specifications alone (it is highly desirable to judge by ear as well, and this applies particularly to loudspeakers and FM tuners) , one cannot really make an intelligent choice without resource to the specs.
Hi-Fi Amplifier
Before making a choice of any hi-fi item, a number of questions need to be asked and answered. For example, when deciding on the purchase of an amplifier the questions go something like this: How large is the room? Is it heavily furnished with soft chairs and thick carpets or is it sparsely furnished with wooden floor and hard walls? Are large or relatively small loudspeakers to be used? Is hard rock, pop or classical music mostly to be indulged in? All these things can be translated to spec parameters and it then becomes possible to choose an amplifier from a technical standpoint, if not from one of musicality, purely from the spec parameters.
Hi-Fi Tuner or Receiver
Will the tuner be required for the reception of stations in the a.m. bands as well as the mono and stereo ones in the FM band (remembering that only FM is capable of hi-fi quality and stereo) . If the answer is 'yes', then what a.m. bands will be required (medium, long and medium or long, medium and short)? Most FM/a.m. tuners and receivers have medium wave only. Long wave will now also be important owing to imminent a.m. wavelength changes by the BBC (November, 1978). Will the near service area regional and 'local ' FM stations only be used? If not, will the tuner be used for the reception of more distant stations, such as the nearest stereo station located beyond the accepted service area and hence in the fringe area or will it be used for more esoteric DX-ing (i .e. , long distance and freak reception trials)? What are the local reception conditions like - i.e. , is the reception site on the main road, partly or heavily screened or in open country? Is it possible to use an outside aerial or is it proposed to use an indoor or roof-space aerial? Is the tuner or receiver to be used for the best possible hi-fi quality or will station seeking have greater priority - or will both be important? The answers to questions like these will determine the tuner parameters to look out for.
Loudspeakers
Having decided on the amplifier parameters, the plan then is to study the parameters of the loudspeakers to ensure compatibility. For example, are they capable of handling the input required for the output sound pressure in the listening room? Are they sufficiently sensitive (or efficient) to provide the required listening room peak sound pressure from the output power or voltage of the amplifier? Is the listening room sufficiently large to do full justice to the extended bass output of the larger and hence more costly type of floor-standing loudspeakers (if not, then there is little future in spending quite a lot of extra cash in an endeavor to reproduce down to the lower octave - it will not be audible, see Section 6)? Are the cosmetics of the loudspeakers acceptable to the distaff side( !)? The parameters alone will determine all these factors , but as loudspeakers are more 'personal ' than the electronics hardware side of hi-fi the parameters of the spec alone certainly cannot tell how a pair of loudspeakers will audition (to you) in the listening room. A home trial is the answer- in your own room on your own equipment.
Ancillary Equipment
The spec parameters of ancillary equipment, such as the record and cassette (or tape) playing decks, headphones , FM aerial, etc., tell quite a lot about the technical performance but less about the aesthetics of the resulting sound and this - like loudspeakers - applies in particular to pickup ,cartridges and headphone sets.
Pickups especially have a characteristic 'sound' of their own. They are not absolutely neutral in this respect since they add their own bits of flavoring or coloration. In these areas choice is influenced more by quality compatibility than by technical compatibility. Pretty well any contemporary pickup cartridge will work with any contemporary amplifier or hi-fi receiver.
If you are spending (1 pound is approx $1.30 USD) £ 60 on an amplifier then you· would choose a pickup which suits the quality of the amplifier at that low price - not one costing itself (1 pound is approx $1.30 USD) £ 60 to (1 pound is approx $1.30 USD) £ 80! Conversely, if you are running an amplifier or receiver of (1 pound is approx $1.30 USD) £ 300 or (1 pound is approx $1.30 USD) £ 400 quality it is hardly likely that you would partner it with a cartridge costing a fiver or so.
Pickups and loudspeakers tend to work hand-in-hand so far as the resulting sound quality is concerned. For example, if a loudspeaker suffers an idiosyncrasy at the treble end, say, and the pickup is similarly harassed, the net result will be an unnecessary emphasis of the treble end affliction. By tutored choice of pickup with respect to the loudspeaker, reproduction of much more palatable quality can be achieved. The scheme is to seek a pickup which has a tendency to cancel rather than enhance the loudspeaker's problem.
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1.1(a) 1.1(b)
1.1(c) 1.1 (d)
1.1(e) 1.1(f)
1.11gl 1.11hl 1.11il
Fig. 1.1: Examples of some interesting hi-fi equipment parameters. (a)
intermodulation products generated by a hi-fi receiver, scale 2kHz/ 10dB/div.
(b) 1kHz squarewave response of hi-fi amplifier. (c) Ringing produced by
a hi-fi amplifier across a loudspeaker load . (d) phase shift parameters
of a hi-fi amplifier. (e) Frequency response top and stereo separation
below of FM tuner, scale 10dB/div. vertically and logarithmic sweep from
20Hz-43kHz horizontally. (f) Frequency response top and stereo separation
below of the pick up input of a hi-fi amplifier, scale 5 and 10dB/div.
vertically and sweep Hz x 10. (g) Frequency response top and stereo separation
below of pickup playing a test tone of constant amplitude to 40Hz and constant
velocity to 20kHz, scale 5dB/div. vertically and sweep Hz x 10. ( h) Run-up
time of turntable unit, scale 0.55/div. horizontally. ( i ) Rumble spectrogram
of high quality direct-drive turntable unit ref. 10cm/S recorded velocity,
scale 10dB/div. vertically and 10Hz/div. horizontally with 1 Hz analyzing filter.
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Fig. 1.2: Stereo cassette deck by Japanese Akai, Model CS-7070, which
includes Dolby noise reduction, memory and bias selection for different
tapes.
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Headphone Sets
Headphone sets should also be partly selected by audition rather than by the technical parameters alone. You will, of course, want to be assured that the headphone set will suit your amplifier or receiver. The parameters of the spec will tell you this, but they cannot express how the headphone set will sound to you.
Technical compatibility is high in this area also; pretty well any contemporary headphone set will work adequately by plugging it into the jack socket of almost any contemporary amplifier or receiver. Other aspects are user's comfort and the degree of attenuation afforded by the design to extraneous sounds.
Some designs let through more outside sounds than others . They are designed deliberately to do this; some people find that total intimacy and hence almost complete sound cut-off from the outside world disconcerting when listening from a headphone set.
Tape Machines
Tape machines (both reel-to-reel and cassette), although often used in a hi-fi system, have a greater autonomy than the other items and are often chosen for their own particular facilities and merits from the spec parameters alone.
The selection is rather less influenced than the other items by quality compatibility.
Technically, a high degree of compatibility obtains between the latest decks (as distinct from complete recorders with their own power amplifiers and loudspeakers) and modern amplifiers and receivers, always provided, of course, that the latter are equipped with suitable inputs and outputs for tape replay and recording.
Record Decks
Record decks, too, are fairly compatible and there is generally less of a problem with regard to cartridge/arm compatibility than there is when the arm and cartridge are purchased separately. This is because some of the technical parameters of the cartridge have to be carefully chosen to suit certain parameters of the arm, and vice versa.
FM Aerial
The parameters of the FM aerial become critical when the requirements are for fringe area reception and DX-ing, and when there are certain problems associated with the receiving site. For example, in hilly country or where lofty buildings and structures lie in relative proximity to the receiving aerial, the parameter of high directionality may well be desirable to minimize the aerial's response to the signals so reflected.
This allows the aerial to be carefully orientated for maximum discrimination against the reflected signals relative to the wanted, direct signal . The high response of the aerial to the reflected signals, which arrive a small fraction of a second after the direct signal , gives rise to a condition known as multipath distortion, which can severely mar the quality of the reproduction and impair the stereo separation.
Where interference from passing cars on a main road can also cause trouble, there is also a good case for a directional aerial . The aerial must always be regarded as an important item of the hi-fi system, for it is economic folly spending a lot of hard-earned pounds on a state-of-art FM tuner or receiver and then to feed it with poor or indifferent signals. This is tantamount to running a high performance motor car on low quality petrol ! The parameters of the spec can thus provide a good deal of information on the performance of the various items of the hi-fi setup; but they cannot always tell how well - or not so well - the system will sound and hence audition in the listening room. The parameters are nevertheless extremely important, but are best used in conjunction with overall listening tests.
Parameter Accuracy
The parameters do, though, solely determine the technical excellence of each item, and from them you can prove whether the equipment is behaving as it should (whether it is fully up to scratch) . From them you can also find out whether the equipment does really live up to the published specifications . Nowadays most designers and manufacturers are highly sensitive to ensuring that the parameters are, in fact, accurately stated to avoid confrontation with the Trading Standards people. The parameters are monitored by the hi-fi magazines, the various equipment reviews in this respect doing a great service to the hi-fi buying public.
If you have bought a piece of equipment on the strength of the specification and later discover from your own tests, by those commissioned by an outside authority or by those conducted by a reputable magazine reviewer, that the spec is far from being met, then you certainly have a just cause of complaint to your local Trading Standards Department.
Before you make such a complaint , however, be absolutely sure of your facts and if possible have the parameters in dispute rechecked by a qualified engineer who will issue a signed test sheet. It should be remembered that there are various ways of testing and expressing the various technical parameters, so you must always compare like with like.
Following Sections
The following sections in this guide endeavor to bring these points into focus. Each section is concerned with the specifications and the parameters of specific items of equipment, and each one starts with a typical specification which is analyzed parameter by parameter and interpreted in a way that is not specifically technical . The emphasis is not on how to test parameters (these more technical aspects are fully dealt with in my Audio Technician's Bench Manual, by the publishers of this guide, and some more recent ones in a forthcoming guide to be published by Newnes-Butterworth (by Gordon King) entitled Audio Equipment Tests) nor particularly how to choose hi-fi equipment, as this subject is fully embraced in my Choose and Use books on Tuners and Amplifiers, Pickups and Loudspeakers, etc., also by the publishers of this guide.
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