Studio Maunoir
in Geneva –
a certified THX
pm3 post
production
theatre.
Getting the
Force –
THX pm3
With surround sound rapidly becoming an expected facility in post-production
environments, how do you ensure that everyone works to the same standards?
TERRY NELSON explains the concept of the increasingly adopted THX pm3
format.
ertain procedures and labels often manage to
obtain a semi-mythical status without people
really being informed about the substance –
possibly why myths are often so popular! I remember
my first film production seminar at the Disney Studios
back in 1986 and the buzz in the audience when the
THX logo flashed up on the screen for the opening
trailer to the film extracts. A lot of us were aware of
THX at this point, but no one really had a clear idea of
what it was all about. However, one thing was certain –
from that point onwards I was a fan!
So what is THX? Again, the myths abound and one
often hears the comment that it is really all marketing
hype and of no real use to anyone, while others think
that it is a coding process, or that it is vaguely associated
with standards. To clear the air, let’s do a bit of basic
history – don’t worry, this will not take long!
C
A Long, Long Time Ago...
It really all stems from the Star Wars films and George
Lucas’ obsession with quality – quite a worthy attitude
when all said and done. Dolby Stereo hit the big screen
in the mid-70s and though it was not the first, Mr.
Lucas’ ‘nice little film’ did much to popularise surround
sound for general cinema release (as opposed to first
release 70mm theatres which already had an established
surround format).
After several visits to local cinemas to do a ‘quality
control’ on the projection, George Lucas was horrified
to find that the soundtrack often had very little to do
with what he heard back at the Ranch. He discussed
the situation with Tomlinson Holman, his Technical
Director, and decided to make the expertise that had
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gone into the Stag dubbing Theatre at Skywalker Ranch
available to cinemas. This became the ‘THX Program’
(first known as the THX Sound System) and coincided
with the release of Return of the Jedi in 1983.
This expertise essentially amounted to the proper
implementation of various SMPTE recommendations
already in existence, combined with some R&D work
from Tom Holman and his team. The THX Program
established an electro-acoustical standard in terms of
acoustics, monitor performance and projection, and
specified approved components for the audio system
which could be selected from a list of equipment tested
and found suitable in the Lucasfilm workshops. The
guiding principle was that film audiences should enjoy
the same experience as near as possible to that of the
production team on the dubbing stage – no more, no
less. The only piece of equipment actually sold by THX
was its custom-designed electronic crossover for the
screen systems.
The success of the professional programme led to
the development of ‘Home THX’, where the same
electro-acoustic principles were applied to the
domestic environment and the certification of hi-fi
equipment, together with comprehensive installation
guidelines.
Son of THX
Fast forward to almost the present day. The evolution of
the studio business generally saw the rise of smaller
facilities and rooms for AV and mastering work where
high-end Home THX components were often being
used. In response to customer requests, THX decided to
develop a programme aimed specifically at these
smaller facilities, covering AV production, DVD
mastering, multichannel music production, and so on –
and this resulted in THX pm3.
The first pm3 studios were DVD mastering suites at
Hollywood’s 20th Century Fox facility, and since then
the list of accredited studios has been increasing
steadily. So, what does it require to be a pm3 studio
and, just as importantly, what’s in it for you?
I think the major advantage is to know what you
actually have rather than what you think you’ve got.
One of my tenets has always been that if a system is
classed as a monitor system, it is there to let you know
what you have – like it or not. If you want something
that ‘sounds good’, that’s fine provided it is for your
own listening pleasure and not for quality control. Our
friends that look after the picture side of things need to
know that what they are seeing is correct and take a lot
of time making sure that their monitors and projection
systems reflect this – the situation should be no
different for audio.
One of the wonderful myths we have about
loudspeakers is that we can all compensate for a
particular unit’s deficiencies or idiosyncrasies. It is a bit
like saying that one could drive a car with three wheels
so why bother with four? Well, I am sure we can
accommodate the characteristics of different
loudspeakers and a lot of people do... but wouldn’t it be
better to actually know? In any case, the increasing
requirements for multichannel work – not to mention
the increase of mastering studios and suites – make it
imperative that programme material should translate
well from room to room as well as to the outside world,
and this is where THX pm3 comes in.
The Standard
Lets start with the basics and look at the isolation and
acoustics of the room. In order to hear everything
properly you will need good isolation from the outside
world and the studios around you. “You don’t say?” I
hear you cry. Well, I used to think it was common sense
as well until I was in a large AV facility next to a very
noisy bank of video machines in a transfer bay, and
someone was stressing the importance of audio quality!
If I am kind, the ambient noise was at least 70dBA SPL!
OK, so we are down to the basic essentials of studio
construction and this will mean standard practice such
as floating floors, double-wall systems where possible,
and freedom from structure-borne vibrations and noisy
air-conditioning systems. You may be lucky enough to
be in a building that is already well isolated and the
rooms in question are in pretty good shape.
Once the isolation is considered satisfactory we can
turn to the acoustics of the room and also to its
intended purpose. Whereas the original THX Program is
designed for large spaces such as dubbing stages and
cinemas, pm3 is for smaller rooms and these have a
different set of problems. Also, due to the diversity in
the types of production, the pm3 portfolio is a lot wider
in its requirements.
For example, the studio in question could be audio
only (for radio or music productions) and thus with no
picture. In this case, the monitor layout would use the
standard 60° angle between the left and right main
speakers, possibly with identical surround speakers as
per the EBU/ITU recommendation for surround sound
production.
Another situation could be multichannel sound
production for video or DVD mastering. Here we are
talking about sound-for-picture where the audio and
video are of equal importance with a monitor layout
that is suited for picture. This will typically mean a 45°
angle between the left and right speakers and possibly
more diffuse surrounds (such as dipoles or tripoles) in
order to provide a more ambient sound field instead of
discrete sources. (So much for standards – Ed)
So why use 45° instead of 60° for the L-R monitor
angle? In a sound-for-picture situation, if the speakers
are too wide, the sound field will tend to ‘go outside
the picture’ which is undesirable (possibly in film
production, but perhaps not for television – Ed) The
size of the room will govern how the picture is shown
but I would strongly recommend wherever possible
using projection with the speakers behind a microperforated screen (such as a Stewart) and ideally
mounted in a baffle wall. After all, we all know that the
better the picture, the better the sound (certainly to the
producer!). However, a multichannel audio-for-picture
studio by no means excludes audio-only productions.
The Design Phase
In terms of design, there are various ways to go about
things. You could design the room yourself (assuming
that you have the requisite expertise), have it designed
by a studio designer, or turn the whole thing over to
THX. In the case of the last option, to get the ball rolling
you will need to fill in site forms giving information
concerning the studio project, together with such plans
as have already been established. The THX Design
The monitoring
amplifiers and
THX processing
rack
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25
Office will then make any necessary corrections or
suggestions and provide you with a design package that
lays down the criteria to be met, as well as details like
speaker placement and angles in both the horizontal
and vertical planes.
So what sort of criteria are we looking at? The RT60
(or reverberation time) curve limits will be laid down
according to the volume of the room, and the room
characteristics must be free of any flutter echoes or
noises, such as rattles due to poor construction. Solidity
of construction is not the end of the story though, as it
is important to have a low ambient noise in the control
room. Again, this should appear obvious to most people
but reality can often be different. Noise sources can be
quite varied and these range from equipment with loud
fans – such as VTRs, computers and even consoles – to
projectors and air conditioning. We can also mention at
this point that the audio system, particularly the
monitoring, should be suitably quiet. There is no
absolute limit for the ambient noise floor but better
than NC25 should be aimed for with NC20 being a
good basis.
The size of the room and the distance of the mix
position from the front monitors will short-list which
speakers and amplifiers may be selected for a particular
project from the THX pm3 approved equipment list.
The reference sound pressure level at the mix position
is 85dBC (per channel) for a signal level of -20dBFS (the
same spec as for dubbing stages) though smaller rooms
will probably prefer to work at a level of 75dBC for
most of the time (the ‘Home Theatre’ level), possibly
coming up 10dB higher for final checks.
For optimum performance, the front speakers
should be flush-mounted into a monitor wall and it is a
good idea to do the same with the surrounds if at all
possible. However, this is not a hard and fast rule and
will depend on the speakers used.
One part of the monitor system that can be
problematic concerns the LFE – or sub-bass – channel.
It is also the part that is probably the most prone to
folklore and myth. Small rooms tend to pose more
acoustic problems than large ones due to the more
pronounced effect of room modes, and this is especially
true with low frequencies, thus making the correct
placement of the subwoofer(s) critical. The proposed
placement by your studio designer will be verified by
THX and changed if deemed necessary. If you are
feeling a bit sceptical at this point, the case can easily be
demonstrated by placing a subwoofer on a trolley and
wheeling it around the room while running signal –
revealing some interesting ‘now-you-hear-it-now-youdon’t’ effects.
Subwoofer placement is almost a book in itself, but
the most recent pm3 project I was involved in (Studio
Maunoir in Geneva, Switzerland) used two subwoofers
placed laterally as recommended by David Griesinger of
Lexicon. His researches have shown that most people
are looking for what he calls ‘envelopment’ by the bass
frequencies, where the sound appears to roll around
them as in a good concert hall. The actual placement of
the speakers in this case was on the floor in the same
vertical plane as the first surround speakers (the 110°
position as per ITU).
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Bass Management
This brings us on nicely to the question of Bass
Management and this function is integrated in the THX
monitor controller, again the only piece of hardware
that THX actually supply. Many domestic systems now
employ Bass Management in order to cater for the
increasing use of satellite and subwoofer speaker
systems. Taken at its most basic level, this means that
the lower frequencies – generally below the region of
100 to 120Hz – are filtered out from the L, C, R, and
surround channels, and summed together into the
subwoofer channel, together with any information in
the discrete .1 or LFE channel. The reason for doing this
should be obvious: the satellite speakers have a poor
low frequency response due to their reduced size but
the signal still needs to be reproduced somehow.
Though this system may seem a bit of an anathema,
it boosts the acceptance factor for smaller (and not so
small) living rooms, or for people who do not like seeing
large speakers in their home. The principle also assumes
that due to the reduced directivity of low frequencies,
this segregation is acceptable. In order to be able to
monitor what your programme will sound like in
domestic situations, the monitoring chain has to
incorporate similar Bass Management stategies.
However, I would stress that this is in the monitor chain
and in no way affects the individual multichannel signals
being recorded. The THX controller also provides the
various solo and mute functions for the various
channels, plus overall level control and calibration.
A question often asked is “why do I have to use the
THX controller – there are other models on the market
that do a good job?” and this is a perfectly valid point.
However, as is often the case, different people have
different ideas as to how to do things and Lucasfilm
found that after discussion with various manufacturers,
the best way to maintain a common standard between
pm3 rooms would be to have their own dedicated
model.
The final step in the pm3 procedure will be the visit
of a THX field technician to inspect the overall
installation and make the necessary measurements to
verify that all of the criteria for the particular studio have
been met according to the design package. Assuming
that all is well, the studio is now certified THX pm3 and
this status can be used in studio promotional literature
and in programme credits. The studio also receives
various promotional materials from THX including wall
plaques and certificates. To ensure consistency the
studio is also re-checked and certified annually against
the prescribed standard.
What’s in it for You?
Well, there are really two basic viewpoints to take: a) the
performance of your studio is as it is and will rely on
reputation to bring in the productions; or b) the studio is
recognised as achieving a reference performance level
equivalent to other studios of the same calibre, and this
will facilitate the exchange of programme material
between rooms and dubbing stages. The overall cost of
having a certified room, taken as a percentage of a
complete professional studio installation, is minimal, so
at the end of the day, the choice is yours.