In the last chapter, we made an overview of the SPL needs. Now we can have a look at the Home Cinema sound system configurations.
A professional approach to sound is often disturbing the typical
Hi-Fi salesman’s or consumer audio professionals.
It only takes to read a so-called « technical » brochure
about a loudspeaker to understand how deep is the problem: You will find supposedly
meaningful words like “rhythm”, “pace”, “precision”, “fast”, but nothing that
will really help the installer to design a system, like:
- Directivity response vs. Frequency either polar or Matlab
- Directional gain
- Maximum SPL @1m without cheating (like when forgetting the thermal compression)
- Frequency response at maximum level (generally different from what it looks like at 1W)
- Distortion curves at various levels (at least 10V)
- Waterfall response diagrams
- A CAD
drawing
Without this information, the system designer is left to either do
an approximate job, or to perform all these measurements himself. This is
particularly critical when matching the loudspeaker to the room acoustical
environment and the audience geometry.
Such information is compulsory and most commonly provided with professional (I mean here: non-residential) loudspeakers, as
sound engineers simply do not accept working without it.
I have to say, strangely enough, I never found a directivity curve provided with any loudspeaker aiming at the residential market.
I have to say, strangely enough, I never found a directivity curve provided with any loudspeaker aiming at the residential market.
Now, if you start believing that such a concept as directivity
control has never been considered in the design of residential loudspeakers, I
cannot really say that it is 100% true.
Maybe only 99%... (I'm kidding ;-) )
I am not suggesting here that most residential loudspeakers are ill
designed; there are good choices around. But in many cases, you will have to
pay for a nice veneer or a glossy lacquer when your speakers will end up hidden
behind a screen!
And the sound system designer is left alone without the proper
information…
Unbalances
A Home-Cinema sound system comprises much more components than a
Hi-Fi system, needless to say.
So it is easy to understand why the most demanding customer,
willing to get only the best available components for each function of his home
cinema, will end up postponing the purchase of his new Bugatti Veyron.
For the vast majority of even affluent customers, there is a limit
to the budget, and hence compromises.
To illustrate this, let me first describe a typical Hi-Fi system. It comprises:
- 1 CD player
- 1 vinyl turntable (optionally)
- 2 analog line-level cables
- 1 integrated amplifier
- 2 loudspeaker cables
- 2 loudspeakers
Diagram 1
Now if we describe the audio inventory of a basic Home Cinema sound
system, there must be at least:
- 1 Blu-ray player
- 1 HDMI cable
- 1 digital audio cable or a 2nd HDMI cable
- 1 AV receiver
- 7 loudspeaker cables, at least 100m total
- 1 line-level audio cable
- 3 front loudspeakers
- 4 surround loudspeakers
- 1 amplified Subwoofer
This, of course, is not including the video
part of the installation.
Diagram 2
And for a more sophisticated installation, we will find:
- 1 Blu-ray player
- 1 AV server
- 1 switcher
- 2 HDMI cables
- 1 power conditioner
- 1 preamplifier-AV processor
- 8 audio line-level cables
- 2 DSP crossovers
- 12 XLR balanced audio cables
- 12 channels of power amplifiers
- 2 Subwoofers (passive)
- 3 bi-amped front loudspeakers
- 4 passive surround loudspeakers
- 12 loudspeaker cables, total at least 150m
Diagram 3
It is now easy to understand how important the cables budget could
be, and that esoteric audiophile cables are to be discarded (in addition, their
stiffness and diameter can become a nightmare for the installer).
It is also easy to understand that mono-block valve
amplifiers weighing 50 kg each and rated 12 Watts are not appropriate.
So you will have to forget about spending days doing A/B
comparisons between single components or cables.
An HC system needs to be optimized in the perspective of its total
budget, even if large.
So, in this case, optimization means balance. Let me get to
the point:
It is known that what you hear in a system is its weakest link.
Balancing a system means changing the weakest link to a better one, until there
is no weakest link at all.
Now, where do you start?
Well, you must have an opinion about components quality.
This is where the image becomes plain white and the sound an
unbearable silence…
How to evaluate the respective quality of the various elements in a
complex system as pictured above in diagram 3?
I’m sure my answer will disappoint all fanatic readers of AV
magazines: You need professional experience.
If you read tests in the medias, you will never find anything that
justifies the 1 to 20 price ratio between components which are all rated as
“excellent”. So, how will you select your components?
A tiny minority only selects world-renowned brands, top class
ranges, and very, very expansive stuff. Their customers are the like who will
not even have to postpone the Bugatti Veyron purchase…
Others (they are more) spend their time on forums. Actually, they
don’t need to get involved in HC installation, they do not have time for this
as they can spend all days on the internet.
Some others rely on 3 letters products certifications which prove
that their suppliers have paid the flat fee plus the royalties (it’s funny, all
certifications have 3 letters labels! none is for free)
What I consider as the best approach is to rely on your own
experience with the equipment, as you don’t have the time to spend your life
testing and comparing components. As a professional, you have installation jobs
on your schedule.
However, you need to get a new component under test from time to
time, otherwise the risk is to become outdated.
When designing video systems, things are crystal clear: You choose a screen (and Excellent one, preferably) and a
projector after checking their compatibility (lumens vs. dimension) and then
you calibrate the projector, play an image and evaluate the result.
Simple!
When designing a sound system, it is not so simple
An HC sound system is involving quite a few components, and the
complexity of the system is an exponential function that has the number of
components as exponent. Then, the results are audible instead of visible. This makes a real difference, as the sound is
by definition vanishing immediately, whereas an image can be made steady.
There are some measurement methods which are rigorous and reliable,
fortunately. Again, they are better known in the professional audio industry
than in the residential one.
The mains supply
Considering the number and diversity of devices connected to the
mains, it is quite difficult if not impossible to have a thorough evaluation of
the power supplies of each element.
A well designed power supply will not incur any problem even with a
relatively unsteady or polluted mains supply.
However, many electronic devices, even in the “high-end” ranges,
suffer from dependence with respect to the mains supply quality.
This quality varies with the location of the installation (it is
generally better in cities than in the countryside) and with the time of the
day.
Therefore, it is wise to be careful about the electrical part on
the installation, not only for safety reasons, but also for sound quality.
A few things to do:
1)
Draw a direct line from the mains connecting
board. Only audio devices must be connected to this line. Any other electrical
appliance, especially light control devices and/or machines comprising an
electrical engine have to be discarded.
2) Check
the grounding quality (< 10 Ohms preferably)
3) The
cross sectional area of the conductors must be sufficient to handle at least
twice the total maximum power consumption of all connected devices.
4) Use
a power conditioner providing sufficient available power for at least all
devices that are operating at line level. When estimating the necessary power.
Provide a large headroom.
5)
Install a
surge protection device.
As far as 4) is concerned, typical « audiophile » sockets strips are not providing sufficient
available power and are sold at a price that can be justified by the amount of
silver, gold or complex engineering, but not by the insulation they provide.
Ideally, the power
amplifiers should also be connected to a power conditioner. However, their
power consumption requires a very powerful, bulky and expansive power conditioner
(see image below). For this reason, such devices are not common in Home Cinema
installations.
22 KVA mains conditioner
The sources
Many audio/video sources provide a decent
quality at very affordable prices. This does not mean that you should use a
Blu-ray player sold at £99.99 to feed a £100,000 installation !
There are typically 3 types of A/V sources :
Ø Blu-ray players
These devices are the simplest to use and to
install. They are also the most affordable ones. Still you need to checK
- HDMI standard (which generation ?)
- The nerve-wrecking duration of the starting process
- The mechanical sturdiness
- The image and sound quality, by comparing it to the competition
- At the moment, one brands seems to have a leading edge (check our stand at the past 2014 ISE)
-
Ø The HTPC
One advantage of HTPC devices is the
« all in one » aspect. You can record blu-rays, download
video, shortcut lousy legal announcements, get a fast access to recorded
material, in short, happiness!
But…
Either you are a true Geek, and configuration problems,
compatibility issues etc. are peanuts for you,
And
You know how to feel like Mr.Everybody and design
interfaces that even your client’s grandma will find funny to use
Or
You
do not have both of these skills, and you are heading to a real nightmare.
Ø
Audio/Video servers
Audio / Video servers are a bit like HTPC, but
they are pre-configurated and (nearly) ready to use. They are
seemingly the ideal solution, combining a user-friendly interface and a huge
storage capacity (count in Terabits).
In fact, most still suffer from two limitations:
-
Very high
prices
-
Or outdated performances (Full
HD was not available on some servers for quite a while, for instance, or no
lossless sound format)
-
Or sometimes
both problems
So, the remarquable comfort of use of these AV servers has a price
The choice is yours…
Decoder-preamplifiers
For once, this is easy: Since lossless audio formats are available (DTS
Master and Dolby True HD), you don’t really feel like listening to anything else
(in Home Cinema, I mean).
So you first need to check that decoders support both these
formats.
Other things you will need to check are:
- HDMI quality and stability. It sometimes “bugs”
- Menus clarity and ease of use
- Clarity of the various functions and configuration, especially the ambiguous “Bass Management” and “LFE Mix”. You definitely need to understand what the device is doing, otherwise you will not be able to achieve what you want.
- In the case of integrated “receivers”, the availability of pre-out connectors is essential. It will allow you to insert control devices, like a digital crossover or an equalizer.
- Availability of a manual “lip-sync” adjustment.
Now, if your preamp is provided with an automatic equalization
function using a measurement, do not worry. What you only need to do is to
bypass this automatic function.
Anyway, never use it !
We will see why in a further chapter about EQ.
We will see why in a further chapter about EQ.
Now that the first elementary requirements are met, what makes a
difference between AV pre/pro is the sound quality.
I do not mention here the video quality, as I strongly believe that
the video signal of the main source should be connected directly to the video
projector. You will need 2 HDMI outputs from your source, as one will be
sending the audio signal to the decoder.
Checking the sound quality is something, I believe, you do not need
to be advised on. You’ll make your own evaluation.
Amplifiers
The amplification of a Home Cinema system can
be complex, as there are quite a few amplifier channels involved.
The above diagrams #2 and #3 illustrate this. According to the setup on diagram #2, 8 channels are needed, and on diagram #3, 12 channels.
The above diagrams #2 and #3 illustrate this. According to the setup on diagram #2, 8 channels are needed, and on diagram #3, 12 channels.
People used to 2 channel audio may think it is too much, but this
is only because they are so used to traditional Hi-Fi that they are reluctant
to get into the logic of real AV sound systems.
So, rule number one is like when scuba diving: never panic!
Looking closely at diagram #3, you will see that the loudspeakers are bi-amped. This means that the HF amplifiers do not necessarily need to be as powerful as the LF ones.
Looking closely at diagram #3, you will see that the loudspeakers are bi-amped. This means that the HF amplifiers do not necessarily need to be as powerful as the LF ones.
It is your choice: If you are budget-oriented, you will use less
powerful amps for feeding the treble channels and the surround channels, whilst
using the most powerful channels for L,C,R bass channels and LFE.
If you are simplicity –oriented, then you will use large power amplifiers
throughout.
In the setup pictured at #2, we can choose between 3 types of
solutions. The simplest (and most common) one is to use an amplified subwoofer
and a 7.1 integrated receiver. However, this does not deliver the best…
In the vast manjority of the AV receiers, the 7 channels (usually
identical) are supplied by one single power supply, to minimize manufacturing
costs. If some of these receivers can
offer 7 x 200w, most are limited to 7 x 175 W and oddly enough cost twice the
price of entry level devices rated 7 x 155 W.
Ain’t that strange?
Looking more closely, you will find that this power rating is for a non-standard 6 Ohms load. Even more interesting…
Then, you will discover that the rated 155 W per channel is only possible when only 2 channels are in use, but that all channels driven simultaneously, you will only have 7 x 112.63 Watts!
Looking more closely, you will find that this power rating is for a non-standard 6 Ohms load. Even more interesting…
Then, you will discover that the rated 155 W per channel is only possible when only 2 channels are in use, but that all channels driven simultaneously, you will only have 7 x 112.63 Watts!
This of course is written in extremely small fonts.
This image is not displaying the virtual AV receiver I am describing above.
It only shown that there is a single power supply for all channels.
There is an easy explanation to this: The power supply is designed
for providing a cost-optimized juice for the 2 so-called “main” channels (this
is irritating: there is only one main channel in Home Theater, the centre one).
When more channels need power simultaneously, the power supply just gives up.
Pursuing our investigations, we will discover that this magic
do-it-all device affords a distortion level which is less than 0.5% and an s/n
ratio > 90 dBA.
Not bad for a 60 years old single-ended valve amplifier!
Well, it is not, actually.
I am not trying to discourage the thorough and smart attempts of
the marketing departments to dissimulate the fact that the receiver is an
entry-level, low cost and low performance product by presenting it as a
high-end one. The marketing manager job is difficult these days, and I really
appreciate their creditable efforts.
In the pro-audio industry the use is to nickname an amplifier
“black box with gain”. This really gives the right idea of what it is. Or
should be.
(I call a single channel an amplifier, for the sake of simplicity)
Its function is to use the electrical energy from the mains to
transform an input signal of X Volts in an output signal of X * G Volts, G
being the gain.
Professional amplifiers have a normalized value for G, either 26 dB
or 32 dB (G=20 and G=40, respectively, as 20 Log 20 = 26 and 20 Log 40 = 32)
This is an ideal world situation.
Please note that X*G is a voltage, not a power.
In the real world, the output signal is: (X Volts * G = X.G + e1
+ e2
+ e3
+….. ex)
where e1, e2, e3…are harmonic
distortion, non-harmonic distortion, noise level, etc.
Here comes the maximum output power…
The X*G product is limited to a definite value which is inherent to
the amplifier. When X increases so that the output is becoming near to X.G
max, G is reduced and e1 is
seriously increased, especially in odd harmonics (the most unpleasant to the
ear).
So it seems straightforward to select an amplifier: Maximize X. G
max, minimize e1 + e2
+ e3.
The larger X. G max, the less you are likely to get near
to this limit, which is called clipping level.
In some literature it is stated that some valve amplifiers can sound
“louder” than what can be expected from their maximum output. This is supposed
to be because their clipping is more “musical” than the one of other
amplifiers, so you can drive the amplifier harder. Personally, I am very
suspicious about the “musicality” of clipping.
This does not mean that amplifiers all sound about the same when
they are below clipping level. My point is: I strongly believe that an
amplifier that does not clip sounds better than one clipping.
Of course it suggests selecting large power (voltage) amplifiers, which is not the cheapest solution.
Of course it suggests selecting large power (voltage) amplifiers, which is not the cheapest solution.
Now, let’s look at the power figures, which depend on the load
impedance. The impedance of an actual loudspeaker is not a constant, like a
resistance, but a function that varies with frequency.
The power figure is given by the Ohm formula: P = U2/R
Therefore, the power P is inversely proportional to the impedance (here
expressed as the resistance R). So, if you divide R by 2, you double the power.
The problem here is that X*G max is, in theory, constant
for a given amplifier. In reality, it is only constant within a certain
impedance range.
Having a look at the figures in the literature, we will see for
instance and amplifier capable of delivering 200 W in an 8 Ohms load can only
provide, say, 320 W in a 4 Ohms load.
Applying the Ohm law, we will find that the X*G max value is 40V
into 8 Ohms, and only 36 V into 4 Ohms. This shows that 4 Ohms is outside of
the linear range of X*G max.
Bad news!
Looking at various amplifiers figures, we will see that for, say,
200 W into 8 Ohms, some amplifiers will deliver 400 W into 4 Ohms, whilst
others will only deliver 280 W, for instance.
There are even amplifiers delivering less power into 4 Ohms than
into 8 Ohms. This sometimes explains why the power is expressed into a 6 Ohms
load: It is the load into which the power is maximum, this being used as a
(poor) selling argument.
In some instances, this can be due to the circuit design, but in
most cases it is a limitation from the power supply.
The technology of a good power supply is straightforward and well
known, so it is not a design issue.
The real issue is cost: In most power amplifiers, the power supply
is the main part of the cost (and weight in traditional analogue designs).
So, the trade-off in limiting the cost is limiting the ability of
the amplifier to drive low impedance loads.
And the more linear is the power vs.impedance relation, the better
the amplifier.
Well, this does not explain it all. There is more...
Nearly everyone knows, amplifiers have different sound qualities.
We are in the real world, actually very far from the ideal “black box with
gain”.
We can look in detail at e1 + e2
+ e3,
etc; beyond a certain level of “decent quality” it will not give us
any significant information. If distortion is below 0.01% (-80 dB below signal
level) it will not be perceived as it is masked by the sound of the signal
itself. Now the noise floor (which does not depend on the signal) will not be
audible if the s/n ratio is higher than 110 dB (even with high-efficiency horns). By the way, the s/n ratio is a very interesting indicator of
the built quality of the amplifier.
Stranger even is the significant variation of tonal balance between
one amplifier and another, which is never suggested by any frequency response
measurement (it is always ruler-flat).
About this, I would like to mention a strange experience I have
made.
I was working at the R&D of professional sound reinforcement loudspeakers.
Some products in our range still had passive crossovers (cost, again!), and we
were setting up a QC measurement of the crossovers.
We used to do it a rather high levels (circa 50 Volts), as they
were heavy-duty loudspeakers.
We measured the frequency response of the voltage across the
crossover load. Also this is not meaningful, what we were looking for is the
identical reproduction from one crossover to the other. It was only QC.
Once we found results that significantly differed from the
standard. Strangely enough, all crossovers in the batch had an identical
response, but significantly differing from the previous batch. The
implementation was correct, and the components were not different. The only
different element was the power amplifier that had been replaced by another
one.
So we checked the frequency response of the power amplifier itself,
driving the complex load presented by the crossovers under test.
We had the surprise to discover that it was very far from
ruler-flat, so we thought it was faulty.
Then we re-installed the former amplifier. It had a quite different
response, but still far from flat!
Then, we reduced the voltage by steps, and discovered that the
curves became nearer and nearer to the ideal straight line when the voltage was
reduced. About 10 dB below clipping, the response of both amplifiers was the
same straight line.
The most concerning part was the serious unbalance of the response
at high levels, with deviations of up to 6 dB for the “weakest” of the 2 amps.
I found reassuring that the “weakest” amp was the one which was not
so good sounding…
It is unfortunate that I did not have the time or later the
opportunity to further investigate this experiment, but I did think about it
and found a possible explanation, although it is only a not-verified
hypothesis.
Its is
therefore to be taken with great caution
Looking back at the available power from, for instance, amplifier
A:
- 200 W into 8 Ohms
- 300 W into 4Ohms
Whereas amplifier B delivers:
- 200 W into 8 Ohms
- 400 W into 4 Ohms
It is easy to understand that at frequencies where the loudspeaker
impedance is at its minimum, 4 Ohms for
instance, amplifier A will have its
output voltage limited at a lower value than at frequencies where the impedance
is higher. This of course only occurs at high levels when the voltage is near
to its maximum.
Still, the result is affecting the frequency response of the
amplifier, as the voltage will be higher at some frequencies than at some
others.
In the same conditions, amplifier B will not meet this issue, as it
is capable to deliver the same voltage into 4 Ohms
and into 8 Ohms. Its frequency response will remain unchanged.
The serious variation in amplifier frequency response I described
above suggests that the limitation phenomenon starts occurring at levels that
are far below the clipping level: There would be a progressive modification
from the straight ideal response to a load-limited clipping response as the
level increases.
It looks like a “soft-knee”
clipping, which would occur at different levels according to the load impedance
and the power supply ability to handle it.
I insist that this is only a hypothesis, I did not investigate this
phenomenon deeply enough to derive a definitive conclusion. However, this could
explain why the tonal balance seems to significantly differ from one power
amplifier to another one.
And there is no risk to deduct the following preferences when
choosing an amplifier:
1) When
the budget allows it, it is better to use one channel per loudspeaker driver
(multi-amplification) rather than passive wide-band loudspeaker systems: The impedance
of individual drive units is typically easier to drive for an amplifier than
the wide band load of a passive loudspeaker with a reactive crossover.
2) When
the budget allows it, it is better to use very powerful amplifiers. They are
more likely to work in their linear range anyway. Further, their (unpleasant)
clipping threshold is rejected at higher levels.
3) Choose
amplifiers provided with a properly dimensioned power supply and a significant headroom, so that the maximum
voltage does not vary too much with respect to the load impedance.
4) Select
preferably amplifiers with a high s/n ratio (>105 dB)
Well, this has been a bit long, and I just hope you did not fell
asleep.
To be followed soon…