Re: 5.1 IN HDTV.
- From: "Joe" <joecan_ca@xxxxxxxx>
- Date: 11 Nov 2005 04:17:13 -0800
> We have always tried to appeal to the lowest common denominator in the
> television world and I would like to see an end to this in the HD market. I
> would assume that anyone that is watching an HD television is listening to a
> 5.1 sound system, and they are certainly not listening in mono because there
> are no mono HD sets in production.
>
> Brad Harper
>
>
below,is an interesting point of view concerning 5.1 in hdtv.yes,if
people are watching hd tv at home,they probably have a 5.1
setup........but,most people are still listening to stereo with their
regular tv's.maybe,hdtv should be broadcast in 4.0 surround presently
and then in the near future when most people at home are hd and
5.1,begin the 5.1 broadcasting.JOE.
By John Watkinson
It's important from the outset to stress that much of what is said
about surround sound is difficult to understand. In looking into that,
the reason that often emerges is that someone, somewhere, is trying to
violate the laws of physics. We are presently with surround sound where
the industry was with stereo in the 1960s. Back then, we had the same
situation inasmuch as much of what was said didn't make sense, and many
of the products were disappointing, but eventually the industry figured
out how to do it.
Quadraphonics went straight on its face, primarily because the
technology didn't exist to deliver four channels through analog media
without quality loss. Subsequently, Dolby would develop a surround
system that worked well with two-channel analog media to create
ambience at the rear.
But for true surround sound, with an arbitrary number of totally
independent channels, the only real solution is digital audio. Once
audio is digitized, any number of channels can be multiplexed into a
bitstream without crosstalk. That bitstream can be an interface in a
production environment, or it can be an MPEG transport stream for
program delivery. In a workstation, it's not much harder to manipulate
multiple channels than it is to handle stereo. The same is true of file
servers. All disk-based audio recorders lock the audio data to time
code, so it is not harder to have more channels. All that is needed is
more storage capacity and more processing power in proportion. That is
exactly what computers give us every year. Thus, the actual nuts and
bolts of producing and delivering multichannel sound is relatively
simple. What is not so simple is defining what the channels should do.
Cinema vs. television
When high-definition television began to emerge as a concept, it became
clear that if the picture was going to be better, then the sound should
also improve. It was a natural step for HDTV to adopt surround sound.
As HD requires more video bandwidth, a bit more audio bandwidth could
readily be found.
So far, the reasoning can't be faulted. However, what happened next
wasn't reasoning. Someone somewhere simply assumed that the way the
movie industry did surround sound was how broadcasters should do it. As
it turns out, it was a false assumption.
When the cinema started to use multichannel sound, it wasn't stereo as
we know it. Rather, it was just more channels feeding typically three
speakers behind the screen. A given sound would be routed to one of the
three channels in production. Cinemas are large places with a
significant number of seats way off-axis. The front left and right
speakers will be designed for power output rather than stereo imaging
capability. The center speaker pulls the image back to the screen.
Cinema screens are acoustically transparent, so it's easy to put a
speaker behind the center of the screen.
Unfortunately, most television displays aren't acoustically
transparent, and only a small number of people watch. Thus, the center
channel speaker just adds cost and complexity, as there is no need for
it in HDTV. If the front left and right speakers are any good, any
center channel signal can be reproduced just by feeding it equally to
left and right speakers. Consumer manufacturers seem to be aware of
this, because surround sound decoders can be set up to do just that.
You can explain this until you're blue in the face and no one believes
it, so I have taken to walking off with the center speaker in the
middle of surround demos to illustrate the point.
4.0 surround
Let us consider the lowest frequencies. This involves the sixth
channel. Because it has much lower bandwidth than the other channels,
the system is called 5.1. That sixth channel is called Low Frequency
Effects (LFE). This channel does not come out of any known surround
microphone technology, because the LFE signal in movies is entirely
synthesized in post-production.
LFE is designed to operate massive woofers in cinemas to scare the
audience during earthquakes and explosions. The sounds made do not
resemble those that can actually be heard during these events, or
indeed the waveform on the film, but that does not matter.
Unfortunately, the speakers needed to deliver cinema SPL at these low
frequencies are too big for the average home. Thus, for HDTV sound, the
LFE channel is quite unnecessary. Provided the remaining channels have
full bandwidth, the user can have whatever speakers he wishes, and the
LF can have directional information that LFE, being a single channel,
cannot.
Thus, selecting 5.1 for HDTV was a mistake because the center channel
and the LFE channel are unnecessary. What HDTV really needs is a
properly engineered 4.0 system. As mentioned above, four digital audio
channels is easy. It's all you need, and in many cases, it's more
realistic than 5.1, especially for drama and music. When it is
considered that the delivery mechanism will use compression, probably
MPEG AAC, it should be clear that the fewer the audio channels, the
better the quality will be for a given total audio bit rate. It is also
worth remembering that most of the masking models used in compressors
are based on mono. When sounds are in different places in a stereo or
surround image, the masking isn't as strong and a higher bit rate will
be needed.
There's no reason not to mix 4.0 surround and simply leave the center
and LFE channels muted. A center sound will be panned equally in the
front left and right signals. Low frequencies will come from the
appropriate speaker, unless the viewer has small speakers, in which
case his surround decoder will be set to filter off the LF from the
four channels, add it up and put it in the subwoofer. All surround
sound decoders can do that.
Monitoring surround
Whilst we are considering hard truths, it should be understood that
there is no way to create a virtual sound source outside a pair of
speakers. Thus, surround sound is a misnomer. It's stereo at the front,
with sounds positioned anywhere between front left and front right, and
stereo at the back, with sounds positioned anywhere between rear left
and rear right. With four or five channels, you can't pan sounds to the
side. You can try, but it doesn't give the illusion of a sound source
there. The only way to get sound from the side is to use reverberation.
That's why many rear speakers are designed to emit sound in more than
one direction.
So it follows that you can't mix surround sound in a traditional dead
control room because you won't hear anything from the side at all.
Instead, the control room needs to have some reverberation, preferably
representative of a domestic environment, or the viewers will have a
completely different experience.
Whilst surround monitoring in a fixed installation does not present any
serious problems, the real difficulty will come with outside broadcast
vehicles. These seem to be getting smaller, and it is difficult enough
to monitor stereo - let alone multiple channels.
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Surround sound needs a different approach to level metering. Whilst it
is possible simply to use a large number of conventional meters, this
isn't as intuitive. In stereo, the audio vectorscope display was useful
as it gave a graphic representation of where the dominant sound was
coming from. In surround sound, the equivalent to the audio vectorscope
is the so-called jellyfish display. This is a blob whose diameter
increases with level, and whose border distends in the direction of the
dominant channel.
For the foreseeable future, possibly even forever, there will be
viewers listening in stereo or, shock horror, mono. It is important to
realize that a stunning surround mix may be reduced to an
unintelligible cacophony when heard in stereo or mono. Thus, it is
important during the mixing process to check what the stereo and mono
versions sound like.
Another disparity between movies and TV is the amount of money
available for post-production. Movies can rebuild the soundtrack
entirely in post, using ADR and effects. Thus, sounds can be panned to
any desired channel. In TV, the sound has to come from microphones. One
approach that has a lot of merit is to record the four raw outputs of a
sound field microphone. Using the sound field control box, the best
directivity and direction of the virtual microphones can be determined
on replay.
In many situations, the dominant sound source is going to be at the
front. In the theatre or concert hall, the performers are typically in
front of the audience. In cases like this, stereo microphones can be
used to capture two channels, and the rear channels can be created
entirely artificially using suitable digital reverberators.
Surprisingly enough, it's virtually impossible in such an application
to tell the difference between artificial rear channels and real ones
from rear microphones. It wouldn't be surprising if a lot of TV sound
ended up done this way.
By John Watkinson
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