UniSon Line - A New Universal Analogue Audio Interface

Still no universal standard exists. Even in the professional world where XLR connectors are now increasingly the norm, signal levels vary; and when it comes to consumer equipment there are so many different levels in use, along with varying input and output impedances, that it is rare for sound levels from different boxes to be at all matched - even when they come from the same manufacturer.

UniSon results from of a great deal of though and innovation by Pete Skirrow of LinDev aimed at solving the problem once and for all. The outcome is an interface that is rigidly defined, yet versatile enough to be used on evrything from a portable MP3 player to a studio mixing consol; with the guarantee that all will be simply plug-in compatible in every respect.

Is it too late for such an idea? I don't think so, because UniSon is so devised as to actually work with all existing equipment using just the appropriate lead! A key feature is the novel differential input circuit that that can also accept unbalanced signals at half the balanced level and yet give the same output! By making power available to a laid-down standard on one pin of the connector UniSon enables all sorts of interface adaptors with gain to be used, as well as mics with built-in preamps to line level (+18dB FS) without the inconvenient and power-hungry 48V phantom power used to achieve the same end in most professional mics. This power capability functions as a power-bus with up to 1A (6-8V Lithium Ion) capability, such that if just one unit in a system is powered then all the others are too. A camcorder for example, might power a mini-mixer, Blumlein shuffler, PPM and numerous mic preamps, without any fear of the mic preamp batteries running down because they were left turned on! Unison is not intended to replace XLR's in the robust studio environment, but it can provide very high levels of sound quality, and could become the ideal standard on a new generation of miniature camcorders, replacing the microphone socket with a balanced line input that can power active microphones, miniature mixers, and radio receiver plug-ins. It is ideal for miniature production and post-production mixers and processors suited to the new filmaking-with-video movement, and could well replace phono's on domestic equipment, where a balanced audio connection is badly needed to eliminate the troublesome video buzz that can plague home cinema systems with long leads to the player. Another major area that could benefit from the new standard is that of computer and internet audio. By providing low cost balanced input and output on slim pcb mounted connectors it can solve the problems of noise that spoil most computer audio cards, many of which currently provide attrocious levels of performance at every level imaginable. UniSon can eliminate the need for a separate mic socket (using pre-amped mics) and also for compromised line/headphone amps, providing power instead for proper equalised headphones or in-line headphone amps, with proper stereo to binaural conversion, power amplification and hearing damage protection all engineered in. In the new world of MP3 Web audio, it offers a new standard for interfacing low cost miniature mixers, microphones and preamps to existing semi-pro digital portable recording equipment to produce live or miltitrack master recordings (with or without video) of very high quality, freeing music makers from costly and overpowering studios facilities.

Advantages of UniSon are:

1 It puts two channels on one connector - no more struggling to see which channel is Yellow or RED (or is it black!).

2 By providing power it makes the use of all sorts of active adaptors and preamps possible, including powered mics with high level output for use with long leads (a neater equivalent of the 'phantom power' mics used by professionals).

3 It puts everything on a connector small enough to fit on portable equipment and camcorders, giving high performance balanced operation without a pair of (XLR) connectors that are as big as the unit itself!

4 It uses male and female connectors such that inputs only connect to outputs (unlike phonos!)

5 By adopting centre grounding it retains compatibility with professional balanced lines, while operating from only 5V rails at low power, facilitating the use of balanced connection in portable and consumer devices.

6 It facilitates the interconnection of professional and consumer items, often with just a passive connecting lead.

7 It standardises signal levels once and for all, giving manufacturers a chance to simple state that inputs and outputs are 'UniSon Compatible'.

PHYSICAL INTERFACE and CONNECTIONS

UniSon is currently being implemented on 'D type' 9-way connectors, because they offer low cost, easy wirability, proven reliability, male and female versions both fixed and 'free', and sufficient pins to permit Y-C video to be added for some applications (like patching in post production). A smaller modern connector similar to the 'Firewire' or USB connectors might be even better in the future, but no suitable design offering 'in-line' female versions, or latching, exists at present. While there is an obvious disadvantage of confusion with RS232 'com' ports, this is no a great problem, and will be less so as USB takes over from RS232.

ELECTRICAL INTERFACE DEFINITION

OUTPUTS shall be balanced and centre grounded. The 'antisense' pin shall carry an inverted version of the 'sense' pin with an accuracy of +- 2%. Note that output level balance does not affect noise rejection on centre-grounded systems. INPUTS shall respond to differential signals, with a common mode rejection ratio of 30dB (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. Inputs must have the special property of giving the same output if the antisense input connection is removed (left open circuit), to facilitate single ended connection to existing consumer equipments, some of which already work at a level close to that required by UniSon (AL= -6dBu).

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 pole. 0dBu means a level of 0.775V rms.

HEADROOM 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 IMPEDANCE All inputs shall have an impedance to ground of 22k (+-10%) from the 'sense' pin. The impedance to balanced differential signals shall be 44k (+-10%). Note that for unbalanced connections to work as intended with no loss of signal level, only the proposed input circuit may be possible.
OUTPUT IMPEDANCE All outputs shall have an equivalent source impedance of 20ohms or less 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. It also complies with an often ignored IEC standard that requires outputs in a studio environment to have an impedance of 40 ohms.

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 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.

FOOTNOTE:

Though it may appear as heresy to some, especially in the USA where quality control was never formalised to the same extent as in the UK, the above, and other LinDev recommendations are in fact an amalgamation of the best current practice in the broadcast industry (as defined now in EBU standards), and based on over 20 years of cooperation with BBC and IBA quality control departments by Pete Skirrow and Lindos Electronics. Lindos Electronics designed special test equipment for the IBA quality control department which was used to commission all independant Radio and Television stations in the UK, and has been a major supplier of equipment for quality measurement to Australian and South African Broadcast organisations, the Royal Opera House, Rupert Neve, the UK Post Office, and many top studios and facilities worldwide.

Pete Skirrow is on the working group of the AES (Audio Engineering Society) for digital measurement, and was invited to write a chapter on Audio Measurement for the 'Audio Engineers Reference Book' (Focal Press - Butterworth Heinman).
© Pete Skirrow 2000

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