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Line Inputs

After amplifying and buffering our tape and disc sources, the next task is routing, mixing crossfading and balancing the music sources. This Zenlike task is as much the heart of a DJ's individuality as the speed and stress of his repartee. There can be no dogmatic rules here. Casual rock DJ's á la Radio Caroline turn off one turntable, read out the biography of the band, take a bite out of a sandwich and then proceed with the show. Heavy metal DJ's crossfade relentlessly and with ever increasing gusto, whilst Soul and Disco-Funk operators double-beat, blend and segue.

Figure 1 depicts the basic layout of a typical UK broadcast console; although it appears similar to a mixing desk in that the level of each source can be set individually, this is a deceptive view, for instead of mixing all the signals, all we wish to do is to butt two of them together, with perhaps a little mixing at the moment of changeover. At the output of each input amplifier is a three-position switch: Post Office style lever switches are recommended here for their positive action.

Figure 1. Basic UK broadcast console layout.
(Click image for higher resolution version)

Assume a record is playing on turntable one. S1 will be on and the signal will be routed to the audience via the sound system. If we're not using any taped material, then S3 and S4 will be in the mute position. Meanwhile, S2 will be in the cue position, and we'll be able to cue up the record on turntable two via the monitor 'phones.

Records can vary greatly in level — if the disc on turntable one is a twelve inch single and the record on two is an album, there may be 6-8dB difference in level. Instead of equalising the levels at the moment of changeover, we can set disc two at the appropriate level by switching the VU meter (or PPM) between the cue and live positions on S5 to compare the average levels of both discs. Channel two fader can then be adjusted to give a meter reading similar to the record in progress. Because of the need to equalise levels, especially in broadcast work, the normal position of the faders should be about three-quarters up to allow adjustment in both directions together with sufficient track below the knob to allow long, smooth fade-up and down strokes. The three-quarters position naturally causes a notional loss of level which is compensated for by adjusting the volume control at the output of the console. Disc two is now cued up and when disc one finishes, S2 is quickly flicked over to on whilst the channel one fader is swiftly pulled down. S1 is then moved to cue, and the cueing up process is repeated.

This is just one way of using this very flexible arrangement. Alternatively, disc two could be brought in on the channel fader whilst one is faded out — or any other combination of switch and fader controlled change-overs, including the use of monitor 'phones to detect the end of a soporific intro, swiftly jumping in on the lever switch when the music livens up. In general, switched changeovers give tight, broadcast style programming and are ideal when the music is for presentation rather than participation.

Faded changeovers provide a range of smooth effects, characteristically á la Disco from the subtlety of the Soul session, where you feel the music change to the heavy Rock show, where you simply hear it change. Though versatile, the broadcast style disco console demands a lot of skill from the DJ.

Figure 2. Standard crossfader.
(Click image for higher resolution version)

Figure 2 shows an alternative arrangement, using a single fader, which commutes between disc (or tape) signal sources in a single stroke — hence a crossfader. S1 and S2 allow either the tape one or disc one input to follow music emanating from either of the number one inputs, and vice versa.

Although inherently simple to use, skill is still necessary to achieve useful results. As the fader traverses the centre position, the level falls abysmally, and it's normally necessary to ride the volume control, raising it at this point to give a smoother transition. Alternatively, this dead spot can be used to inject brief repartee, whilst for a hard driving Rock show, the fader moves so swiftly that the dip in level is inaudible.

Rapid transitions can also be achieved using switches, but however rapidly it's shunted across, the crossfader is always smoother and has the advantage that it can be operated by a single finger or an unemployed elbow whilst the hands are frantically occupied with other controls. The crossfader then is ideal for the Rock DJ, though it can be used by almost anyone (Simon Bates used one of mine quite happily!) and the results are purely a function of the DJ's skill, in the same way that a gifted photographer can capture evocative scenes with a very ordinary camera.

Although the broadcast arrangement (Figure 1) is best suited for Soul DJ's who frequently want to double beat, some may covet the crossfader for its simplicity; though the 'hole in the middle' effect shown in Figure 2(a) appears to render it unsuitable. Figure 3, however, shows how the crossfader can be tweaked to remove the dip in level around the fader's central position using linear pots tweaking resistors Rx and Ry.

Figure 3. Crossfader with modified taper.
(Click image for higher resolution version)

Because the ideal crossfader response is a tenuous idea in the mind of a DJ, we cannot easily plot a graph of a desirable fader characteristic, so the values of Rx and Ry have to be determined by trial and error. As a guide, their values usually lie within 20 to 200 per cent of the fader value. Whilst this trick overcomes the limitation of the crossfader shown in Figure 2, it is less satisfactory from the viewpoint of equalising levels.

Like the broadcast arrangement, the crossfader shown in Figure 2 can be used to equalise the levels; reducing the level of disc one slightly will make two audible, but rarely to a significant degree. However, the crossfader shown in Figure 3 must be kept hard over otherwise events on the other turntable (such as cueing up) will be plainly audible to the audience. So it's necessary to anticipate major changes in level, using the metering or past experience, and deftly adjust the volume control immediately after crossfading.

Finally, on club consoles, it can be useful to have a foolproof method of crossfading. Figure 4 shows the elements of an automatic crossfader operated by flicking switch S1. Simultaneously, the appropriate turntable is started (via a relay and S1c). Once started, the turntables are locked on by latching the relays. Once the new disc is underway, the other turntable can be stopped by touching SA or SB which unlatch their respective relays. Note that SA (for example) only takes effect when S1 is set at 'fade to B', therefore if hit accidentally, they cannot abort a record in progress. RC networks known colloquially as 'snubbers' are wired across these switches and the relay contacts to attenuate switching clicks.

Figure 4. Automatic crossfader.
(Click image for higher resolution version)

This arrangement allows the boss or another member of the club staff to stand in for a skilled DJ who's just phoned in to say that his car has been engulfed by a monster snowdrift on the M62. The crossfade speed can be preset for acolytes to produce a tolerable performance, albeit monotonous. The automatic crossfader is also a gift for the skilled DJ who wishes to concentrate on the audience and his own showmanship rather than subtle juxtapositions of musical texture.


The best music — whether Debussy, Mingus or Motorhead is an arcane and powerful language that transcends the ethos of rational thought, so it's not unnatural to feel anger welling when a (nameless!) radio DJ natters senselessly over a note that is trying hard to form a chord — a record you want very much to hear.

In a sense, certain radio DJ's have set a bad example, for whilst voiceovers can be used creatively (viz: rapping), they are certainly unwelcome if their content is out of sympathy with the music or the mood of the occasion. Mainstream Rock records are very definitely discrete entities, but soul and disco material is very amenable to sympathetic voices. It's even possible to make administrative announcements over laid back soul without disturbing the atmosphere at all — given a suitably accented voice.

The onset of acoustic feedback frequently limits the DJ's microphone gain setting to a level below or just above the ambient level of the music, yet for the minimum intelligibility, your vocals must be 10-15dB above the mean SPL of the music. The obvious solution is to attenuate the music whilst talking, by turning down the volume control. However, this requires considerable dexterity and sobriety if for any reason both hands are already occupied.

The classic alternative is the microphone activated autofader or 'ducking' circuit. Lamentably, this gizmo owes its popularity more to a common desire to emulate radio DJ's than to any real expedience. Perhaps the best known aberration of the autofader is its predisposition to pump the music even whilst the circuit's sensitivity to the DJ's vocals is barely sufficient. The autofader works well in the broadcast studio because any monitoring is at low levels or on headphones. Once the music level approaches or exceeds the DJ's vocal level as heard by the microphone, the music attempts self-attenuation, once attenuated, it stops attenuating itself, the level rises and once again, it begins to act on itself — hence the pumping effect. Whilst close miking can help, there is no easy solution to this problem. Screaming into the microphone will also help the autofader to differentiate between your own voice and ambient Hazel O'Connor, but a raised voice is subtly different in character to a quiet one, and isn't necessarily sympathetic to your own performance or the music!

Another problem is the idiosyncrasies of DJ's vocal chords! An exciting voice full of character is really a voice full of distortion, and the result is a highly assymetrical waveform (Figure 5(a)); note the very large negative peaks. This assymetry is not a problem in itself, but the vast majority of autofaders derive their control voltage from only one-half of the microphone signal; usually the positive half. This being so, the sensitivity control will have to be set at a much higher level than strictly necessary if the DJ has a voice with negative assymetry as in Figure 5(a). This aggravates pumping. If the DJ's voice produces large positive peaks, then the sensitivity control can be set at a low level, but if another DJ with negative assymetry uses the autofader, he will find it a little insensitive. The symmetry is also dependent on microphone phasing.

Figure 5. a) Sample speech waveform. b) Full-wave rectification control circuit for autofader VCA.
(Click image for higher resolution version)

Figure 5(b) shows a full-wave rectifier circuit which alleviates the assymetry syndrome; the DC control voltage is derived from both the positive and negative peaks in the vocal waveform. Although the autofader can be used successfully if the music is quiet or your microphone is abnormally insulated from the ambient SPL, when Rock music is reproduced at its live levels of 105-125dB, one begins to appreciate that the autofader is passé.

An elegant alternative is the switched 'ducking' circuit (Figure 6). When S1 is in its normal position, the gate of TR1 is earthed and presents a high resistance across its drain-source terminals. Provided this impedance is much greater than R1, and the value of R1 is much less than the input impedance of the next stage, no attenuation will occur. When S1 is moved to fade, C2 charges via RV3 and a rising negative potential appears at the gate of TR1. The FET responds by lowering its source-drain resistance (ideally to a terminal value about one tenth of R1) and the music signal is progressively attenuated over a period of say half a second.

Figure 6. Switched 'ducking' circuit.
(Click image for higher resolution version)

When the announcement finishes, S1 is returned to its normal position, C2 discharges via RV2, the negative voltage on the gate of TR1 returns slowly towards 0V and the music fades up. C1, C3 and R3 prevent switching glitches and other unwholesome noises.

Although the fade up-down characteristic is audibly non-linear, it is normally acceptable, and the circuit can be improved by using a ramp voltage carefully matched to the Vgs/RDS-on parameter of the FET in place of the simple CR network. The beauty of this circuit is that like the microphone activated autofader, it needs no hands to operate it — only a footswitch and a co-operative, well trained foot!

Alternatively, if panel mounted, the switch can easily be stabbed at by a spare elbow if your hands are fully occupied. Yet more adventurous, the switch can be placed on the microphone's body: This is ideal for possessive DJ's who like to clasp their microphones. With a little practice and ingenuity, the switched ducking circuit can possess all the attractions of the autofader without pumping and capricious sensitivity.

In Part 3 we will look at the art of fading and mixing signals.

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jun 1981

Feature by Ben Duncan

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