Computers or CD players convert musical information from disc or hard disc to an audio signal with a certain voltage. This result is routed through a mixer into a power amplifier, which drives the speakers. Generally, we use stereo systems, which divide the signal into a left and a right channel. The mixer is an instance, where multiple signals are mixed together (f.e. two CD players, one computer, two microphones) all these inputs are mixed together as channels and the mixer allows for adjusting gain and frequency spectrum (low vs. high tones) for each channel. Please do not confuse this mixer with the actual DJ-Mixer. We will call the mixer typically provided at big venues a house mixer. The house mixer typically is just the instance, where signals are received into a professional house sound system and it is the place where DJs can hook up their equipment. As this mixer is typically located away from the DJ-Booth or DJ table, it is not convenient to use that for DJ mixing. So typically an individual DJ mixer will be connected to the house mixer. The DJ has to connect his Equipment. This means, he has to provide the right cables and has to know the parameters of an electrical cable connection between different devices.
Be connected !
A music signal is electrically just an alternating current, provided at a certain voltage and a certain impedance (the power, a device can draw from the cable). For preamplified audio inputs of consumer equipment, this is called a line input. A certain voltage is defined as a reference and the actual signal strength is given as a logarithmic ratio comparing the actual signal with the reference (for consumer audio products, this reference is 1 V(RMS) and the ratio describing this is called dBV) [1,2]. If a signal exceeds this scale, it will be very likely clipped and distorted, so the signal should be generally kept below that threshold. The signal should also be kept at a high level to avoid the noise generated by the electronics at really low signal levels. The distance from the actual system to the 0 dBV threshold is called the headroom and this is one of the most important things in professional sound engineering.
The message here is that usually a music signal is handed over as a line signal which should be kept below a certain threshold (usually 0dBV) to avoid distortion.
Normally DJs bring their own primary equipment (CD Players, DJ mixers and Computers) and hook that up to the mixer of the venues sound system (equipment from some 1000 $$ to more than 100.000 $$). As professional mixers have sometimes more than 20 lines (Picture 1) and are usually located very unconveniently, many DJs have their own DJ-mixers with just a few channels and a crossfader that allows for smooth transition between both CD decks. For consumer equipment, the output of the CD player is usually two RCA (or sometimes called cinch) connectors . A cable with two RCA plugs (Picture 2) connects that to the line input of the mixer. Usually this is also a pair of RCA sockets (Picture 3). Typically the right channel has a red band (memorize: Red=Right) and the left channel has a white band.
Computers usually have a TRS socket for 3.5 mm TRS plugs (Picture 4) . In this case a special cable or a combination with an adapter (Picture) need to be used. TRS plugs (TRS = tip, ring, sleeve) come in different sizes. For consumer communication equipment, diameters of 2.5 or 3.5 mm are used. For consumer audio 3.5 and for professional or semiprofessional equipment 6.35 mm (1/4 inch) TRS connectors are used. For professional equipment each channel (left/right) is transferred into one plug. TRS-Adapters are usually quite safe, so adapters from TRS to RCA are acceptable, but not always perfect. Inputs with TRS are typically seen at house mixers, but also at semiprofessional DJ equipment (Picture 3).
Just a short note on cables. Always buy premium quality cables. A cable for 30$ is strong and reliable, the cable for 5$ will give you trouble when you really don’t need it. I also do not believe in adapters. Some adapters are reliable, some are not. It is always better to have a custom made cable than to deal with adapters. So I have different cables RCA to RCA and RCA to 3.5 mm TRS. In a small box, I carry cables with lengths of 2, 5 and 10 meters, and adapters for emergency situation (Picture 5 and 6). The RCA to RCA cable I normally use is gold plated and comes with a lifelong warranty.
The DJ mixer is now plugged into a line input of the big house mixer and thus connected with the power amplifier and the speakers.
ATTENTION: Whenever you plug in a cable into a mixer, be sure that that respective audio channel is turned off (muted or faded with the channel fader or the master fader) or you are at risk of damaging the PA or the speakers. The reason of this is, that during the insertion of the plug, sudden loud buzz can appear because the hot part of the plug connects before the ground is closed. Generally, equipment should be powered on from periphery (player, mixer) to speaker (PA) and powered off from speaker (and PA) to periphery.
Sometimes you will hear a humming sound when you connect consumer DJ equipment. This is due to the formation of a so called ground loop, which catches the 50 Hz signal from the power line. In that case, a so called ground loop protector is the usual solution which just has to be inserted into the line cable. (Picture 7)
Balanced Cables for Professional Equipment
You may imagine, that with increased length of cables, the probability of ground loops is rising. Even if typical cables have one hot wire which is shielded by a so called ground plaiting, this (so called unbalanced) cable can not completely avoid interferance by electrical signals. In the professional and semiprofessional world, audio signals are transported with differential cables. Here the signal is split into one hot (+) signal and another signal with a 180° phase shift (-, or cold). Both signals have equal strength and impedance. These signals are carried on two wires, shielded by the ground braid. If Interference occurs, it affects both signals identically and can be easily blended out in the receiving circuitry. For professional and semiprofessional equipment, balanced cables are used to connect devices and especially microphones to mixer and PA. Typically they use XLR type (Picture 8 and 9) connectors , some devices (like mixers) also can use quarter inch TRS connectors [6,7].
Usually unbalanced signals can be fed into professional equipment (with adapters, feeding the signal into the (+) input and connecting the (-) input to ground level), because these inputs have an adaptive electronic circuit. Older equipment may require a special active adapter to convert unbalanced signals into balanced signals.
A problem arises here, because the reference level of signals is higher in professional equipment (it is measured in dBU). So connecting an unbalanced low level line signal to a balanced input of a professional device expecting a higher level can be a problem. It might be necessary to amplify the line signal and electronically balance it using a special converter (Picture 10) .
Problems like this are far above the normal expertise of a DJ. If you are unsure, please ask the venue technician, he typically helps to connect your equipment to the house mixer, by providing the proper adapters. I personally do not rely on adapters, so if I operate frequently at a certain venue, I provide my own cables.
Anyway, a certain knowledge is necessary to deal with signal connections between DJ equipment. Another usual piece of equipment is a cable tester. I know this can be done with a cheap multimeter, but with a tester, everybody can do it.
The Future – pure Digital Connections
Digital connections are perfect, because no interference or quality loss can occur. There are some typical connections between consumer devices, working in a pure digital way.
One of them is the frequently used SPdif connection, which can be an optic glassfibre cable (Toslink) or an electrical connection (BNC or RCA) (Pictures 11 and 12).
For professional audio equipment the standard AES/EBU with XLR plugs/sockets is frequently used. Again, it is difficult to connect both worlds. As the digital signal is not completely the same, it may work easily, it may also not work without using a device to translate both signals (Picture 13).
CD players generally and DJ-Decks rarely support digital outputs. The problem here is that pure digital mixers are difficult to find. To my knowledge, only the Pioneer DDM 900-Nexus (situation in April 2015) supports full digital inputs and outputs . Anyway, in the future, the whole signal pathway will be purely digital and the D/A conversion then can be at the end of the chain.
– Richard (DJ Ricardo)
P.S.: Next time we will look at digital audio files and the workflow of grabbing a Tango CD, tagging it and storing it into a music database.