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Audiovisual Signal Types and Interconnects

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Over the decades various technical standards have been adopted for analogue and then for digital video and audio. As the popularity of systems declined they were generally replaced by improved technologies, making many system configurations entirely obsolete. Obsolete interconnects can be difficult to use in conjunction with modern equipment.

This paper looks at the nature of audio and video signals and then surveys significant historical interconnects (such as RCA) and contemporary, digital equivalents (such as SDI and SPDIF).

Any method of transferring digital data, such as USB (Universal Serial Bus) can potentially be used to transfer digital audiovisual files, but such technologies are not primarily associated with audio video signal data and so are not discussed here.

Video signal types

No matter how different audiovisual systems might seem from the outside or how exotic the cabling, there are a surprisingly limited number of signal types likely to be passing through those cables. Video signals are either ‘component' or ‘composite' in nature. A brief description of the differences is given below, for more information see An Introduction to Digital Video.

Analogue component video signals

Video signals can be split into two or more component parts. In a typical component video system the signal will be divided into:

‘Y' or ‘luma' which is the brightness portion of the image and travels along one cable
‘Pb' which gives the difference between the luma information and the blue part of the image and has its own cable
‘Pr' is the final component and carries similar information describing the difference between luma and the red part of the image, again in a dedicated cable

A green signal is not required as it can be calculated form the other two colour components. This common ‘component' system (or colour space) is known as YPbPr.

S-Video (Separate Video) is another common form of component video. S-Video separates the signal into only two parts; luma and chroma (combined colour information).

Digital component video signals

Video specifications such as SDI (Serial Digital Interface) use a colour space very similar to YPbPr. This is known as YCbCr. Again, luma, blue/luma difference and red/luma difference form the coloured image, but in the YCbCr information is expresssed digitally.

Analogue composite video signals

In a composite video signal, a signal cable carries all the constituent parts of the video image. YUV is a common composite video signal type.

In YUV the ‘Y' stands for luminance. This monochromatic image contains more than just image information: it also provides a synchronising ‘pulse'
The ‘U' value is the difference between blue and luminance
‘V' is the red/luminance difference

In a composite system all of this information is carried in the same cable (the colour information is carried as a sub-carrier signal). YUV is versatile; it contains a fully formed monochrome image for display on black and white televisions.

Common interconnects

Composite video interconnects

RCA (Radio Corporation of America) connector
Commonly used in sets of three for composite video; video, left audio and right audio

BNC (Bayonet Neill Concelman) connector
BNC is an established analogue connector which has found new use in SDI (Serial Digital Interface) systems

TNC (Threaded Neill Concelman)
Less popular, threaded version of the BNC

C connector
Similar to BNC but larger, again less popular

F connector
Often used in satellite television systems and for terrestrial TV in the US

N connector
Typically used for in cable television systems

SCART (Syndicat des Constructeurs d'Appareils Radiorécepteurs et Téléviseurs)
SCART is a versatile connector which can be used for composite, and component signals and also carries sound

Sony's AV Multi
Similar to SCART, AV Multi can carry many different kinds of video including composite and component signals

UHF (Ultra High Frequency) connector
Common in 1970s video systems but now unpopular

TV aerial connector (a.k.a. Belling-Lee connector)
Relatively low-quality but ubiquitous, used with analogue and digital television sets

Component video interconnects

S-Video (separate video, a.k.a. 4-pin mini DIN)
Common domestic component format

Multiple RCA (Radio Corporation of America) connectors
Used in sets to carry composite information
Multiple BNC (Bayonet Neill Concelman) connectors
As with RCA, used in sets to carry composite information
SCART (Syndicat des Constructeurs d'Appareils Radiorécepteurs et Téléviseurs)
SCART is a versatile connector, which can be used for component and composite signals and also carries sound
Sony's AV Multi
Similar to SCART, AV Multi can carry many different kinds of video including component and composite signals
VGA (a.k.a. D-sub 15 also mini-VGA)
Ubiquitous computer monitor format, now largely replaced by DVI

DVI (Digital Visual Interface) and mini-DVI, Micro-DVI)
As with VGA not commonly used for anything but display purposes

HDMI (High-Definition Multimedia Interface)
Contemporary audiovisual consumer format, is used in many systems such as games consoles, digital television and Blu-ray players. Carries multiple audio channels in addition to image information

D-Terminal
Component format popular in Japan for use with digital satellite systems. Comes in backwards compatible types; D1, D2, D3, D4 and D5

Which one to use?

It is common for a single piece of equipment to have more than one type of interconnect. For instance, you might use VHS video player's aerial connector for plugging directly into a televison set, the SCART socket for connecting to a camcorder and RCA for sending aound to an amplifier. Different signal types will provide a different quality of signal, so how do we decide which to use? The general rule of thumb is:

Digital data transfer options (such as USB or Universal Serial Bus) if present, offer direct and quicker-than-real-time options for transferring video as digital information. However these methods have limited bandwidth and are typically used by consumer-level equipment
Digital component is often the best method of transferring professional-quality video and has a much higher bandwidth
Analogue component is the next best choice, although debate still rages over the nuances of digital vs. analogue, it is generally accepted that technical quality is higher in digital component systems
Analogue composite is the final choice, although in the older systems required by many digitisation projects, it is the only choice

Audio signal types

Similar to the world of video, the number of audio signal types is relatively small in comparison to the vast array of connectors and cable types available and in common use. The following section briefly explains these signals and then highlights the connectors used when transmitting and interfacing audio signals.

Analogue electrical voltage signal

Sound is represented in an electrical form as changes in voltage which are relative to changes in amplitude. This signal can be carried in a number of ways, such as the magnetised particles in tape, but through a cable it is carried as a balanced or an unbalanced signal.

Unbalanced and balanced signals

When transmitting electrical audio signals across a cable most pro-sumer and professional equipment uses a balanced signal to provides protection against any external noise picked up along the length of the cable. On the other hand most consumer or older equipment uses an unbalanced signal, and this provides less protection against noise interference.

Unbalanced Signal

An unbalanced signal consists of an electrical signal sent along one wire alongside an earth wire to eliminate ground loops between the components at each end of the connection, that can introduce noise.

Balanced signal

A balanced signal consists of a hot (positive) wire, a cold (negative) wire and the same source signal is fed twice, however the second signal is an inverted version of the original (the polarity is inverted from +ve to -ve). At the receiving end the second signal is inverted back to be identical to the source and any noise which has been added to the signal along the cable is cancelled out when the two signals are summed together. Diagram 2 explains this method of noise reduction.

Diagram 2

Two identical signals are transmitted with the second (lower) one inverted (note how it follows the exact opposite path of the upper signal). The signals are sent down a twisted pair of wires.

It is important that the pair of wires is twisted so any noise the cable picks up is identical on each wire. The noise picked up on each wire will then be in phase with (peak at the same time as) each other and can be seen in the spikes halfway along the waveform, shown below.

The second signal is then inverted back from -ve to +ve. Note that it is the same shape as signal one above, except for the spike (noise)

- Signal 2 inverted

And then summed with the first signal which results in the signal below. The noise picked up is then cancelled out because the receiving device will only respond to differences in the signal.

- Resultant signal

This noise reduction method of 'balancing' when transferring an analogue audio signal via cable is not a perfect solution which is why an earth or shield wire is used as well to prevent ground loops between connecting components.

Conversely, instead of a balanced mono signal, the twisted pair of wires in an analogue audio cable can be used to send two separate left and right audio signals to create a full stereo channel. This is a common feature in some consumer level audio equipment.

Analogue laser signal

A particularly rare but interesting method of transmitting an analogue audio signal was used in DiscoVision and LaserVision technology in the 1980's. This was a precursor to later Laserdisc technology that utilised laser technology to read an analogue audio signal.

Radio frequency (RF) signal

Like the laser signal mentioned above, radio frequency transmission of audio is a method of delivering audio wirelessly. Here a source signal is modulated by a carrier signal.

Analogue signal level

Analogue audio is often transmitted at different amplitude levels depending on the source and therfore needs to be attenuated when it is received.

The most common differentiation is between:

Line/Instrument level - nominal level of approximately 1 volt (where 1 volt = +4dbu (0.775v reference))
Mic level - level generated directly from microphone approximately 60db lower than line level
Speaker level - which is approximately three times the output value of line level.

Digital audio signal

In a digital audio signal, the information is transmitted as streams of binary code, a series of 1's and 0's. For more information on binary form and digital audio see the document An Introduction to Digital Audio.

Digital signals are becoming more prevelant as the need to interface audio with PC's and other digital signal processors is increasing. Digital signals can be connected, stored, read and written by a wide variety of devices and methods.

There are a number of interfaces and protocols for using digital audio signals, the most common of which are:

AES3 (AES/EBU)
The standard digital audio protocol developed by the Audio Engineering Society which is a balanced signal

AES2 (AES/EBU)
Unbalanced version of AES3

S/PDIF (Sony/Philips Digital Interconnect)
The consumer variant of AES3 is S/PDIF which can be transmitted via standard coaxial cable cable or over optical fibre

MADI (Multi-channel Audio Digital Interface)
A protocol found in high end professional systems which can support 28, 56 or 64 channels of audio at once

TDIF (Tascam Digital Interconnect)
Bi-directional protocol allowing 8-channels to be sent and/or received at any one time

Optical signal

Although digital by nature, optical audio has been mentioned in its own right due its prominent use within professional audio systems. Commonly sent via the ADAT Lightpipe protocol or S/PDIF, a digital signal is sent (uni-directionally) across fibre optic cable as a series of pulses of light.

Common Interconnects:

RCA (Radio Corporation of America) connector
Unbalanced analogue or AES2 digital connection. Commonly used in pairs for left and right audio

RCA Left and Right

XLR
3 pin connector providing balanced or unbalanced analogue or digital AES3 connection. Commonly used for microhone connection (analogue) or in professional recording and signal processing systems (digital)

XLR - Female XLR - Male

1/4" TRS Jack (tip, ring, sleeve)
Balanced analogue connection. Commonly used to connect instruments or signals at line level to an audio system. Often referred to as a 'stereo jack' as a left and right signal can be sent down the twisted pair instead of a balanced mono signal

TRS Jack

1/4" TS Jack (tip, sleeve)
An unbalanced analogue connection, otherwise known as a 'mono jack' as only a mono signal can be sent via this connection.

3.5mm TRS Jack (tip, ring, sleeve)Balanced (or 'stereo') analogue connection. Commonly used to connect headphones to an audio system and can be found on most portable audio players. Often referred to as a 'stereo mini-jack' connector

DIN connector (Deutsches Institut für Normung)
A variety of 13mm 2, 3, 4 and 5 pin connectors with varying configurations mostly found on older hi-fi and playback equipment The most common is 2-pin din (picutred below) used to connect speakers to amplifiers

2-pin DIN

Bantam Jack
2-pole or 3-pole (TRS or TS) providing balanced (or stereo) or unbalanced (or mono) connectivity. Commonly found in studio patchbays

Bantam Jack

D-Sub (or D-Subminature)
The 25-pin D-Sub is used throughout high end audio systems for connecting large amounts of analogue and digital signals

D-Sub

Banana Plug
A rare connection found in test systems and some synthesisers, banana plugs were also used with older hi-fi and playback equipment
EDAC
Analogue multi-connector. Commonly found in studio patchbays for connecting a large numbers of sources

Edac

TOSLINK connector
Connector for optical fibre (developed by Toshiba) primarily used for ADAT lightpipe and S/PDIF

Toslink

Which one to use?

With so many interconnects and signal types available and in use it can be quite confusing knowing which one should be used for the right application. The answer to this question lies largely with the equipment you will be using due the wide variation and inconsistency between different manufacturers and products. However, there are a few key points that you should consider when deciding which type of signal or interconnect to use.

Digital transfer signals offer higher quality than analogue signals for real-time audio transfer
Balanced signals provide more protection from noise intereference than unbalanced signals for recording an analogue audio signal
XLR and 1/4" TRS are the standard connections for most consumer and pro-sumer recording equipment
Always check that you have reasonable connectivity, i.e. inputs and outputs are compatible before buying new products (although some cheap adaptors are available for this purpose)

Conclusion

Obsolete technical standards now have renewed importance as specially funded digitisation projects acquire obsolete equipment in order to playback and digitally capture archival media. No matter how well planned or funded a digital asset management system, it is impossible to populate it with digitised analogue materials without some understanding of obsolete equipment. JISC Digital Media provide a range of advice documents covering all aspects of the digitisation process; from selection of materials to establishing a preservation policy. We also host a free helpdesk service for those collections seeking tailored advice.

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