Contents
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Introduction
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Audio Signal Description
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Important Characteristics of an Audio Cable
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Balanced Audio Signal Cable
i. Interference Reduction
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Popular Connectors
i. TRS Connector
ii. XLR Connector
iii. Hybrid Jack
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Audio Multicore Cable
i. Composite Multicore Cable
a. Analog Multicore
b. Digital Multicore
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High-end Audio Cables
Amongst the variety of cables being used in the industry, audio cables form an integral part in electronics, sound engineering and telecommunication industry. This article discusses some of the important features and types of commercially available audio cables.
An audio signal is an electrical representation of sound waves in the form of an electrical voltage. An audio signal contains signals in the frequency range of 20 Hz to 20 KHz, referred to as the audible frequency range. Audio signals containing music and other audio signal like speech, are transmitted by a variety of means such as Radio or Satellite broadcast services; or using cables in a recording and studio applications. In the first instance, the audio band of signal is modulated onto a high frequency RF carrier and send over a transmission medium, whereas in the second case the output of an audio source (low frequency) like a microphone is transmitted over a cable. Audio signals are also transmitted over a fiber optic link using fiber optic cable such as TOSLINK. With the advent of digital audio processing techniques, audio signal is transmitted in digital audio formats specific to the transmission medium used, such as for coaxial medium: S/PDIF, XLR (AES/EBU), Ethernet (VOIP) and for optical medium: ADAT, TDIF.
The most important consideration for an Audio cable is that of preserving the signal integrity of the transmitted audio signal without introducing any distortion. Audio signal is transmitted over a cable, in either analogue form (such as mono or stereo channels of an audio amplifier) or in one of the digital audio format. We will be discussing some of these audio formats later in this article.
The quality of an audio cable is determined by the elements used in its construction, namely, the cable itself, the connectors and the terminations.
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Capacitance and shielding
Capacitance and shielding of the cable, play an important part in determining the quality of audio transmission. The capacitance is instrumental in preserving the audio signal integrity and determines the amount of audio signal absorbed or lost inside the dielectric medium of the cable. Lower the capacitance, lower is the signal loss; and typically it is below 40 pF/foot for good quality audio cables. As audio signal is sensitive to pick-ups from power line sources such as a power cords, fluorescent lights, grinder/mixies etc, manufacturer like Belden, Canare, Gepco, use enhanced shielding mechanism (foil or copper braid shield) to arrest external interference.
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Connector
The quality of audio connector plays an important role in determining the quality of audio transmission, and is determined by its construction methodologies. Good audio connectors are precision-machined cylindrical cases for larger contact area, shielded metal barrels and spring boots to protect the cable from twisting or kinking. The connectors use nickel-plated contacts.
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Termination
Termination involves joining the audio cable with a connector, either by soldering or crimping, and is an important factor determining the quality of audio transmission and signal integrity. Good soldering practices and guidelines are to be followed to ensure that the solder joints are not brittle that produce noise or short the signal. Silver based solder is preferred over tin or lead solder to avoid crack due to stress or temperature variations. Proper crimping of the cable clamp around the cable jacket ensures better pulling strength and stability.
The above three points are true for both analogue and digital audio signals, however, some more precautions need to be taken in the case of digital audio signal. If the audio transmitting equipment and the audio receiving equipment use a 75-ohm signal (as in S/PDIF or digital RCA audio transmission), the receiving equipment expects to receive an "accurate" 75-ohm signal. However, as the digital audio signal leaves the transmitting device at 75-ohms it will be altered by the components of the audio cable, such as the two connector joints at the cable ends and the cable itself. Care should be taken for the type of RCA connector used in audio signal transmission. Using a crimp style RCA connector (solder style is adequate for analog audio but not digital audio) will reduce the back reflection, preserve the 75-ohm impedance and maintain the signal integrity.
Balanced audio signal is used for sound recording, production and mixing in the audio industry. It uses impedance-balanced lines to counter noise, crosstalk and interference and supports long cable runs between sound recording equipments. Balanced connections use three-conductor connectors, such as the XLR or TRS connector. XLR connectors are used with microphones, while TRS jack plugs are used for mixer inputs and outputs.
Balanced audio cable contains two identical wires, which are twisted together and then wrapped with a third conductor (foil or braid) that acts as a shield.
Fig. Balanced audio cable construction
Each of the two wires is connected to identical source and load impedances and uses differential signalling technique to cancel out noise (and interference due to EMI) on both these lines. The outer shield in a balanced cable addresses the ground loop issues by separating the shield or chassis from the signal ground. For example the XLR connector uses Differential signalling technique by connecting normal polarity signal to pin 2 and inverted polarity signal to pin 3.
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TRS connector
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XLR connector
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Hybrid jacks
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TRS is an abbreviation for Tip-Ring-Sleeve. The tip of the TRS plug is positive (hot); the ring is negative (cold); and the sleeve is grounded. A TRS connector can be viewed as balanced transmission line in which two of its conductors carry the balanced signal with respect to a common terminal connected to the ground. TRS connectors are available in various sizes such as ¼", 1/8" etc. |
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XLR is an acronym for Extra Long Run. Besides, TRS, it is the most widely used audio connector. It is a 3-pin connector cable system which supports balanced audio signal. It can support long cable runs because of the way the balanced signal is transmitted. Two identical signals with their phases reversed are transmitted on its 2 pins with respect to the common ground. At the receiving end signal strength is doubled whereas the induced noise gets cancelled. |
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Hybrid jack is designed in such a way, that it mates with both TRS as well as XLR connector. The figure shows a hybrid jack connector that is compatible with the TRS male as well as XLR male connectors. |
Other examples of balanced audio cable are: the telephone line and data line such as digital audio format AES3 (AES/EBU) using XLR connector. Eight-channel analog balanced audio connectors like ADAT use DB25 connectors.
Lastly, unbalanced signal can be converted to balanced signal by using a balun.
A multicore cable, often referred to as a snake cable, is a compact cable consisting of multiple shielded pair microphone cable (bunch of 4 to 56 individual cables) housed inside a rugged, heavy-duty common outer jacket. The multicore cable is terminated at a patch panel consisting of connector options from: female XLR connectors, 0.25-inch jacks, and plugs.
As mentioned multicore cable consists of many microphone cables bunched together inside a common jacket, he inner microphone cables are each a pair of insulated, twisted-pair, multi-strand wires, surrounded by shielding (either foil or tightly-braided wire mesh). The cable comes in Plenum variety or more flexible variety. Popular manufacturers of this cable are Belden, Canare, Mogami and Gepco.
Composite multicore cable combines different cable types of signals into one cable, such as coaxial cores for video, twisted pair for data or low voltage cores for mains power. They are used in applications such as video camera and live sound recording (Yamaha PM1D) to the stage console.
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The multicore cable is used for stage to mixing console connections using XLR connectors. The figure shows a typical multicore XLR cable with 12 input connectors and 4 outputs. |
Important components of a analog multicore audio cabling system are briefed below:
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Balanced lines
Microphones use three pin connector (TRS connector) for balanced audio transmission to reduce pick up and electrical noise.
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Multi core termination
Multicore cables are terminated by XLR or TRS connectors.
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Breakout cable
An audio multicore can be used as a breakout cable inside a studio, by having a compound connector at one end and a component connector on the other.
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Multipin multicore
Multipin connectors are based on MIL C5015 standard. It uses multipin connections on the main and the sub core, which allows system expansion by channel count.
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Multicore |
MIL C5015 Multipin |
Sends/Returns |
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8 core |
25 pole |
8/0 |
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12 core |
37 pole |
8/4 |
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16 core |
54 pole |
12/4 |
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24 core |
85 pole |
20/4 |
The multipin multicore cable is used in TV production to cover sporting events at stadiums by connecting the TV vans, on field reporting and the press boxes.
Audio over Ethernet, and multichannel digital audio technologies such as AES10 (MADI) can be used in place of multicore cables.
Digital signal processing techniques such as A-to-D and D-to-A converters are used for audio transmission over a single wire. Audio signal is encoded into digital signal using ADC, transported over a single wire to the destination, and reconverted using a DAC. Most multicore systems use Ethernet wiring and one of the drawbacks is the latency due to A/D and D/A conversions.
High-end audio cables are designed for commercial, broadcast and hi-fidelity audio applications with importance given to electrical properties (L, C, R), shielding, construction geometries and materials used (Silver avoid oxidation and pure copper for conductivity) for conductors and shield construction. Digital audio signals such as S/PDIF and HDMI use error correction techniques to improve their transmission performance.
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S/PDIF CABLE
S/PDIF (short for Sony/Philips Digital Interconnect Format, or Sony Philips Digital Interface) is a specification for digital audio signal transmission between devices and components over either optical or electrical cable. S/PDIF is a IEC 60958 type II standard and is a modified version of AES3 standard. For example, S/PDIF interface is used to carry compressed digital audio from DVD output to a home theatre, and supports Dolby Digital or DTS surround sound.
The S/PDIF cable uses 75-ohm coaxial cable with RCA connector or fiber optic cable (TOSLINK) as shown in the diagram below. The RCA connector is usually colour-coded orange to differentiate from other RCA connector used to carry composite video.
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Not to Scale |
TOSLINK cable is useful in short cable run (limited to 20 feet) because of its high optical signal attenuation but exhibits tolerance to ground loops and RF interference.
SPDIF Specification are given below:
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Cabling |
75-ohm coaxial or fiber |
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Connector |
RCA or TOSLINK |
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Output level |
0.5 to 0.6 V peak to peak |
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Min Input level |
0.2 V |
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Max distance |
10 m |
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Modulation |
Bi phase mark code |
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Sub code information |
SCMS copy protection info |
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Max. Resolution |
20 bits (24 bits optional) |
Conclusion
The theme of the article is to present an overview of audio cable technology to the reader. We have covered some of the widely used audio cables in the industry. We have shown that the audio cable is characterized by the type of signal it carries and its prime objective is to maintain the audio signal integrity. Techniques such as balanced audio cable with differential signalling have been touched upon in this article. We have presented various connector and cable types used for analogue and digital audio transmission. Some of the audio cable presented here are used in high-end sound production and engineering application.
