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UniSon - A Universal Audio Interface

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UniSon, is primarily a specification relating to analog interconnections, both professional and consumer, which offers a way out of various confusions that currently exist. By defining working levels, impedances, and other properties, in relation to XLR, Phono (RCA) and MiniJack connections, UniSon aims to ensure the proper interworking of equipment.

A key feature of the UniSon standard is the requirement that balanced outputs shall be centre-grounded, rather than floating, with an alignment level of 0dBu. This avoids the many subtle problems that arise with modern ‘electronically balanced and floating’ output circuits. It also provides a no-compromise way of interfacing balanced outputs to unbalanced inputs, by using one side of the signal and ground, rather than by grounding one side of the signal as is necessary with a floating output. Grounding one side is not good practice, as it forces the output circuit to handle twice the normal signal level on one side, a condition that is not usually included in the specification and which can cause clipping or distortion. By specifying an alignment level of –6dBu for all unbalanced inputs, the UniSon standard also facilitates direct connection between balanced professional equipment and unbalanced ‘consumer’ equipment, without any need for signal attenuation, and without compromising the balanced output performance.

Although the UniSon standard is appropriate to existing XLR, Phono, and Minijack interconnections, an additional UniSon-D specification gives pin allocations for a novel implementation of the standard using commonly available 9-way D-type connectors. UniSon-D connections are used on the Lindos MiniSonic range of test sets for compactness, permitting easy interfacing to professional or consumer equipment using appropriate conversion leads. They also include a PowerBus pin, supplying up to 500mA at a nominal 6V (6 to 8V for easy application in Lithium Ion powered battery equipment) which makes it possible to power external adaptors such as microphone preamps.

Electrical Interface Definition

shall be balanced and centre grounded. The 'cold' or (-) pin shall carry an inverted version of the 'hot' or (+) pin with an accuracy of +- 2%. Note that output level balance does not affect noise rejection on centre-grounded systems.

shall respond to differential signals, with a common mode rejection ratio of 30dB minimum (40dB or more if possible). This is easy to attain, using 1% resistors, but enough to ensure a massive improvement in noise rejection over single ended systems.

Signal Level Definition

Alignment Level
Signal levels shall be defined in terms of Alignment Level, which shall correspond to a sine-wave level of 0dBu (balanced) or -6dBu on each output pole. 0dBu means a level of 0.775V rms. Equipment that uses only unbalanced (single-ended) inputs and outputs, shall use an alignment level of –6dBu (rather than the commonly quoted –10dBu or –10dBV) if it is to labelled as ‘UniSon’ compliant.

Maximum level shall be defined as 18dB of Headroom, which for this purpose shall mean that all inputs and outputs must be capable of handling a sine wave signal of amplitude +18dB relative to Alignment Level without serious clipping (Distortion Residue less than -30dB Rel).
On digital equipment Alignment level is therefore 18dB below FS (full scale) digital level. On analogue equipment, less than 18dB of headroom may be available, as in the case of broadcast channels where the EBU standard of +8db (studio) (+9 absolute max) is normally adhered to. In this case the equipment specification should specify a figure for Headroom (as the level at which Distortion Residue is -30dB Rel). It should also specify whether internal soft limiting is applied to soften tbe effect of overload, so that users can avoid applying such limiting twice.

DC Offsets
Inputs may be AC or DC coupled. Outputs shall not have more than 10% of AL ie 39mV of DC offset on either pin. Where this is achieved by capacitive decoupling, a high value (100k ohm) resistor shall be provided tying each output pin to ground. Such rules are essential to avoid significant loss of headroom or loud clicks when plugging into outputs, and yet, surprisingly, something that is never guaranteed or specified currently!

Input Configuration
For full UniSon compliance, it is desirable that balanced inputs should be able to work from single-ended sources while providing the necessary +6dB of gain compensation. This may be achieved by the incorporation of gain controls with 0dB and +6dB markings, or by suitable switching. Alternatively, the UniSon Universal Input Circuit may be used, which automatically compensates, adding 6dB of gain when no (-) signal is connected.

Input Impedance
All inputs shall have an impedance to ground of 10kohm (+-10%) or greater. The preferred impedance to ground shall be 22k (+-10%) and the impedance to differential signals 44kohm.

Output Impedance
All outputs shall have an equivalent source impedance of 20ohms or less (10 ohm preferred) at each pin over the frequency range 20Hz to 20kHz. This ensures neglible signal level error when driving multiple inputs from one output, as well as minimal high-frequency capacitive loss on long lines.

Output Drive Capability
All outputs shall be capable of driving at least four UniSon inputs, equivalent to 5.5k from each pin to ground, without serious clipping. This also ensures that existing professional inputs (commonly 10k) can be easily driven. The circuits recommended will actually drive a 600ohm load to +18dB with quite low distortion, which is useful but need not be considered essential.

Working Levels
Working levels should be chosen such that the typical level for reasonably loud sounds is 0dB AL when monitored on a Peak Programme Meter to IEC286 (typeII) (BBC standard). Early CD's will be found to comply with this rule fairly well, though in recent years there has been a deplorable tendency for levels to creep up until peaks are clipped or have to be drastically compressed. This is wrong! CD's, unlike FM stations, do not have to compete for loudness, and there is no significant noise or distortion advantage in recording at such high levels on modern equipment using optimal dither signals. When this rule is adhered to it becomes possible for the user to play or record all sources without needing to adjust listening or recording levels for most of the time.
When levels are set in this way, programme level meters on home recording equipment working off-air will never go above +8dB AL (or they wouldn't if they were to the PPM dynamics standard). This is as it should be, since all broadcast sources are compressed to 8dB of Headroom. While they could be recorded at a level 10dB higher on digital equipment, this is not advantageous, as the dynamic range of the source is already limited, and little is gained. Sticking to the rules means that other recordings, which do contain occasional high peaks, will replay at the same loudness, but with the added clarity that large headroom confers.

Where equipment incorporates a ‘volume’ control, intended to change the listening level on headphones or speakers, this shall provide 12dB of extra gain (‘overvolume’) at maximum setting such that the maximum signal level (+18dB) drives the output circuit into clipping by 12dB. This is necessary if acceptable listening levels are to be achieved on recordings that only occasionally reach maximum level (as they should do). Where the same output is used for line-level signals and headphone output, it should be possible to set the output to provide a calibrated output level, either by an option setting or by a mark or dB readout associated with the volume control. The common practice of designing equipment to provide maximum possible output only at maximum signal levels has resulted in a flood of equipment, from iPod style devices to computer outputs, on which many recordings, especially those of classical music, are almost inaudible, with no possibility of turning up the level in quiet passages. (See ‘The Overvolume Problem’)

Condition of use:

UniSon is offered to the industry and to standards bodies free of charge for the benefit of all. The circuits and details may be used, and all material here reproduced on condition that the UniSon input and output logos are used on all documents and on inputs and outputs that comply with the specification. Also, reference must be made in literature to the website.

Unison Universal Input Circuit


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