In Paper No.2 the many different microphone rigs, used by the writer for capturing 5.0 Surround were discussed. However in a sense, it all really ‘starts’ with the need for a ‘standard’ playback system. This has got to be the ITU 5.0 arrangement of five loudspeakers, equally distributed in circle around the main, ‘sweet spot, listening position.

The front L and R are intended to be at 30 degrees either side of the Centre loudspeaker, but the writers pair are slightly wider, due to their original use just for Stereo! The rear Ls and Rs loudspeakers are wider apart at around 110 degrees. These, and the Centre one, are all equidistant to the listener’s ‘sweet spot’.

An achievable objective with 5.0 surround, is that the performers are still heard from the front, between the front L & R loudspeakers, but the ambience is ‘lifted out’ from between the front pair of loudspeakers, and distributed around the listener. The wider spaced rear pair of loudspeakers do assist the ‘diffusion’ of the recorded ambience.

With the many different surround mic rigs used this is achieved with a sort of 3D effect from the performers, plus an enhanced low frequency reality, assumed to be due to the various venue’s ‘standing wave’ effect at LF, being captured due to the 360 degree reproduction of the acoustics.

SURROUND MONITORING NEEDS

Monitoring room constraints can affect the performance of loudspeakers greatly, as we know with our stereo monitoring! For the ITU layout, you have five loudspeakers to keep away from walls and room corners along with the effect of nearby reflections, absorptions and resonances, to consider.

All that apart, the ideal would obviously be five identical loudspeakers in the array! Practicalities and cost can assuage that desire! The front pair must be considered the most important and three ‘smaller’ others, for the Centre and Rears, must the best aim in most cases. However having a reasonable LF performance from the rear pair, has proved to be a valuable asset

The writer now has a ‘transportable’ 5.0 system using active Klein & Hummel 300 active monitors for the front L and R, and their smaller 110 powered monitors for the Centre and Ls and Rs surrounds.

It is vital that the output levels of all five be closely matched. Whether active, powered or run from separate amplifiers, there needs to be individual level controls on all five feeds going to the loudspeakers. From a standard source the controls should be set to individually produce the same levels from each loudspeaker, when heard in the ‘sweet spot’.

Turning your head around to judge the levels from the rear units is very important. It can be useful to judge the level balance and the phase by feeding the same test signal to two loudspeakers at the same time and judge that you get a centre image, when listening exactly between each of them.

If you have access to something like Neutrik Test Instruments (NTI) Minirator for Pink Noise, their Minilyser for level metering and their Mini SPL Test Mic, the judgement can be taken further. With pink noise you will also be able to hear the likely timbre differences that each speaker delivers, and what effect their position in the room is causing!

FACILITIES NEEDED IN THE MONITORING CHAIN

The writer’s use of many different mic rigs has necessitated the following approach to the Monitoring and Mastering feeds to the loudspeakers. Most mic combinations used, need their recorded signals ‘processed’ to the basic five L, R, Ls, Rs & C loudspeaker feeds. Simple plug-in ‘virtual earth’ circuit modules with 5532 ICs are used (Pic. One), the rest of the circuitry being ‘hard wired’. A ‘virtual earth’ circuit inherently provides the phase inversion involved with M&S processing.

These specific DIY units, with the L, R, Ls, Rs & C signals, then plug in to a separate set of facilities. You have around 6dB level trim knob controls on each of the feeds, and a set of five faders for the setting of listening levels, with the front and rear pair of faders, providing ‘stereo’ width adjustments. This width facility is easily done with the virtual earth modules mentioned earlier, by initially creating Sum and Difference signals, and then back again, with M&S decoding, you have the variable width.

For whatever way one’s own 5.0 signals are recorded, and even more so for most ‘commercially’ issued recordings, it is absolutely essential to have at least five bar-graph level meters in the chain, prior to the feeds getting to the matched loudspeaker outputs.

The writer goes further and finds the following equally essential. This is the DK Technologies ‘Jelly Fish’ display, and its level bargraphs. The essential aspect to note, about this visual representation of the Surround audio distribution, is that an equal line-up tone on all five channels produces an accurate full circle in the display (Pic. Two).

Whilst monitoring with ones ears in the ‘sweet spot’ position, the writer has found that with all the various mic arrangements used, when it sounds right to the ears, the Jelly Fish display always has similar variations in its varying circular display and its level bargraphs. In some commercial releases there are certainly some very wide variations away from an assumed ‘standard’.

It is very handy to arrange a ‘remote’ control to allow individual muting of the front and rear pairs and also the centre feed. This really impresses listeners when they compare the Surround to what they hear, when they collapse all of it to Stereo!

Spot mics in the Surround mix? It is likely that some recordings, will need spot mics used for getting a good musical balance. Be it just one mic on a soloist or a few more, it is best to get a separate mix for the stereo, and use it later for the later Surround via an extra pair of channels on an eight channel recorder.

In the surround mix, this can be appropriately added to the front L and R feeds. Best done after the monitoring feeds to the ‘Jelly Fish’ display and its level bargraphs, otherwise it will emphasize the front area of the display and affect how you judge the contribution of that visual aid. A useful facility in the spot mix feed to the front L & R, can be to have pan pots, width trim controls, and a phase invert switch?

IN SUMMARY

It seems to the writer that there are always going to be even greater differences in all Surround playback systems, than that is so evident in all our Stereo playback systems, as discussed in Paper No.1 ‘The Black Art of Audio Recording & Reproduction’. However, that hasn’t stopped some practical ‘research’ in to the missing aspect of 5.0 Surround, and that is the Height aspect. So do look out for Paper No.4.