AUDIOCLINIC (Q and A) (Dec. 1975)

by Joseph Giovanelli

50- and 75-µS Pre- and De-emphasis

Q. Please explain 50-microsecond and 75-microsecond pre-emphasis at the FM transmitter and 50-microsecond and 75-microsecond de-emphasis at the FM receiver. I have noticed that some receivers and tuners have a switch for selecting either one. Does the 50-microsecond de-emphasis circuit produce a brighter sound?

-Larry Cook, Albany, Ga.

A. In FM broadcasting, as with tape and disc, we boost treble during production or broadcast and reduce it during reproduction to compensate for the boost. This results in less noise, but the amount of initial boost must equal the amount of later frequency cut or the frequency response will be altered. This is accomplished with a combination of a resistor and a capacitor. Any network of this kind will have a specific frequency boost or cut, starting at a given point and continuing at the given rate to the desired frequency. Beyond the designed-in frequencies, however, the network may not remain effective.

Simply, when a capacitor and resistor are connected in series and a voltage is impressed across it, it takes time for the capacitor to charge. If the capacitor is already charged and then connected in parallel with the resister, it will take time for the capacitor to discharge into the resistor. These charge and discharge periods are said to be the "time constant" of the combination. Since several combinations of resistor/capacitor elements will produce this same time constant, the amount of boost or cut can be stated by using the time constant of the components which make up the circuit, rather than the amount of boost or cut involved and at what frequency.

Most FM stations use a 75-microgecond boost requiring 75/1,000,000 of a second to charge or discharge a network. (Actually, the formula is based on a charge of 68 percent of the maximum possible charge that the capacitor can hold, and a discharge to 32 percent of the full original charge.) Using a 50-microsecond cut while listening to a station broadcasting with a 75-microsecond boost, the treble will be a bit brighter than otherwise. FM stations which broadcast n the Dolby system use a 25-microsecond frequency boost at their transmitters for increased listener compatibility between Dolby and non-Dolby equipped receivers.

Loudspeaker Flux

Q. Please explain what the word "flux" means, as applied to loudspeakers. Does more flux make a speaker sound better?

-Richard McHale, Upper Darby, PA.

A. Flux describes the amount of magnetism, in this case, in the voice coil gap of a loudspeaker. It is measured in oersteds (the old term was gauss). Generally, the more gauss present, the better. The amount of flux will affect the efficiency of a speaker and the amount of control which amplifier damping can exert on the motion of the speaker cone.

Direct-coupled Amplifiers

Q. What is a "direct-coupled" amplifier and how is it different from other amplifiers?

-Ronald L. Ambrogi Brooklyn, N.Y.

A. A direct-coupled amplifier is one in which the signal is transferred from one stage to the next without coupling capacitors. Because capacitors introduce phase shift, their elimination removes this problem. Their elimination also saves parts and therefore lowers cost a bit.

The disadvantage is that the voltage of the early stages affects the operating point of the later stages. A failure in any of the early stages may bring about failure of the later stages. This can prove costly. Fortunately small-signal transistors and ICs used in these early stages are not highly prone to failure.

Direct-coupling is widespread in transistor amp circuitry. It was used only very occasionally in tube amp circuitry.

Equalizers

Q. Please explain the use of an equalizer in a four-channel component system. Are two equalizers necessary? Where is the equalizer connected within the system? Some manufacturers claim their equalizer enables the user to increase the volume of certain instrumental sections and enhance the performance of a given soloist. ff this claim is true, how does an equalizer accomplish this?

-Roy Clark, Chicago, Ill.

A. Amplifiers have bass and treble tone control. They enhance the sound quality of the system by enabling the listener to add or remove bass or treble from specific program sources. Such controls are a form of equalizer. Today's "full" equalizers are more sophisticated. Rather than dividing the audio spectrum into just two segments, these devices take narrower segments of the response and boost or cut them. A segment usually is one octave wide, but can be narrower depending on the design of the equalizer and the desired uses.

Considering we can hear over a frequency range of perhaps 10 octaves, we might have 10 separate tone controls for each channel. If we divide the spectrum into even smaller segments, we would have even more controls and, in so doing, can correct peaks or dips in room or speaker response. But this flexibility brings with it the complications involved in trying to use all these controls effectively.

If an equalizer can correct for problems in a stereo or mono system, it can also be used successfully for four channel sound systems. It would probably be easiest to install the equalizer after the matrix decoder on a matrix four-channel system. For discrete channels, it would be placed between the discrete source (decoder, tape machine, etc.) and the individual channel inputs.

Most equalizers sold are stereo units. Such a unit can be used to control the equalizations for two channels, requiring two such units for a four-channel unit. Some equalizers have a set of controls which affect both channels at one time. But if each channel has its own set of controls to adjust for room acoustics, more flexibility is available. Some equalizers are sold as single-channel units. You would then need four of them, one for each channel.

Equalizers can occasionally be used to increase the volume of certain instrumental and vocal timbres, though this is not always successful. if an instrument has most of its acoustical energy concentrated in a certain portion of the frequency spectrum, the equalizer can be adjusted to bring up only the portion containing that instrument. Conversely, an instrument can be subdued by turning down its portion of the spectrum. But if more than one instrument or vocalist shares a portion of the spectrum, everything will be affected when that portion is adjusted.

Equalizers can totally destroy the sound of a system when used improperly. The highest quality sound system can be made to sound like a table radio if the equalizer is not adjusted properly.

(Audio magazine, Dec. 1975, JOSEPH GIOVANELLI)

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