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format  Audio
Signal
Processing A receiver may perform a number of audio signal
processing functions, depending on the input source and depending on user
preferences. These include digital audio (PCM)
decoding, simulated soundfields, bass
management, and THX post-processing. Digital
Audio (PCM) Decoding For digital sources
like audio CD, you can connect the CD player using a pair of analog stereo
connectors. In this case the audio signal is already converted from
digital to analog form by the CD player and no further decoding is
required. Alternatively, you can use the CD player as a CD transport and hook it
up to the receiver using the digital audio output.
If you use the digital audio connection, then the receiver's audio digital-to-analog converter (DAC) is used to convert the raw
Pulse Code Modulation (PCM)
digital audio signal to analog. The receiver's audio DAC is usually
24-bit word length and samples at 96 kHz. Better receivers will have audio DACs with higher
sampling rates (up to 192 kHz), or quality audio DACs made by Burr-Brown or
Analog Devices. Unless you have a high performance
audio CD player or any digital source with high quality audio DACs, we recommend
that you connect the CD player to the receiver via one of the digital audio
connections. This will allow you to use the receiver's
audio DAC and simultaneously minimize
degradation that usually plagues analog connections. Simulated
Soundfields Many receiver manufacturers include a number of
simulated soundfields (sometimes referred to as DSP modes) effects to imitate the
acoustic environment of various real-world venues (e.g., jazz club, concert hall, cathedral,
stadium) by adding synthesized early reflections and reverberations. While listening to these soundfields can be fun, audio purists will find
little value with these simulated soundfields as they do not serve the
artists' or performers' intentions. And while it's
nice for receivers to include this feature, it should not become a primary
consideration when choosing among different receivers. Bass
Management The receiver's bass management
function re-directs the bass frequencies from any channel to those
loudspeakers that
can handle the low frequency bass information (e.g., full-range floorstanding loudspeakers, subwoofer).
This ensures that the
low frequency audio signals are not lost when you're using a system of bookshelf or
satellite speakers. If you have five full-range loudspeakers and a
subwoofer (few of us do), bass
management is not a concern. But if you don't, it is essential that
you look for a receiver with complete bass management functionality. Bass management is usually performed in
the digital domain by DSP circuitry. With bass management,
you set up your system by telling the receiver whether you have large
(full range) or small (satellite or bookshelf) loudspeakers for the center
channel, the two front left/right mains, and the surround sound
channels. If you have a subwoofer, you should note the crossover
frequency. Frequencies above the crossover frequency are allowed to
pass, while frequencies below the crossover frequency are
re-directed to the subwoofer (if present) or to a full-range speaker (if
any are present). Many receivers will have a fixed crossover
frequency of 80 Hz (the standard set by the THX specification), while
better receivers will allow you to fine tune the crossover frequency to suit your
system's performance. Editor's Note: If you plan to
adopt the new high resolution, multi-channel DVD-Audio and
Super Audio CD formats, you will also need bass management if you have
small bookshelf, satellite, or surround speakers. When buying a DVD-Audio player or SACD
player, look for one that has full bass management functionality, because
few receivers offer an appropriate bass management function for these high
resolution audio formats. If your DVD-Audio player or SACD player
lacks bass management, and you want a receiver that performs bass
management, look for one that performs bass management in the analog domain. This will avoid additional
analog-to-digital and digital-to-analog conversions that would severely
degrade the pristine high resolution
analog signal coming from the DVD-Audio or SACD player. Digital
domain bass management requires A/D conversion, to get the audio
signal into the digital domain, where the digital signal processing
techniques are used, then another D/A conversion to get the signal back in
analog form for amplification. Digital domain bass management may be
acceptable if the DACs are at least 24-bit/192 kHz. THX
Post-Processing The
more expensive receivers will include THX post-processing, as part of
the Home THX certification program.
THX post-processing consists of the following:
-
THX Re-Equalization™: THX-certified movie
soundtracks are originally mixed and optimized for the acoustic space of commercial movie
theaters. The THX Re-Equalization post-processing function is
simply an equalizer preset to Home THX specifications to remove just the
right amount of brightness (excessive energy in high frequencies)
that sound designers use to compensate for high frequency dissipation
in a
large movie theater. In our opinion, this is the most
value-added of the four THX post-processing functions listed here.
-
THX Surround Decorrelation™: For
Dolby Surround soundtracks where the surround channel is mono, THX
includes a decorrelation algorithm to make the left and right surround
channels slightly different from each other in order to achieve a more
natural and enveloping surround sound experience. This function
is not used when the surround channels are discrete (independently
encoded) and are truly different as in many Dolby
Digital 5.1 and DTS
5.1 soundtracks.
-
THX Timbre Matching™: THX
recognizes that the same sound coming from different directions will
sound differently because of the nature of human hearing, so they
attempt to match the timbre (the
quality of the sound) of the sound coming from various
loudspeaker locations in a typical home theater configuration.
Note, this does not attempt to match the sound produced by different
brand or type of loudspeakers. In fact, this function assumes that you
already have a home theater loudspeaker system that already matches in
tonal characteristics. We think this is the second most
value-added THX post-processing.
-
THX Subwoofer Crossover™: This
THX post-processing is similar to the bass
management function we discussed above. Here, THX re-directs
all bass frequencies below 80 Hz from the front channels (left,
center, and right) to the subwoofer output. For Dolby Digital
and DTS 5.1 sources with a LFE channel, the re-directed bass
frequencies are added to those in the LFE channel. This function
works when you have a subwoofer as part of your loudspeaker array.
The Home THX standards for amplification performance
will be discussed in a later section, as
well as the difference between "THX Ultra" and "THX
Select". THX
certification is a nice-to-have feature for mid-priced receivers
($700-$1500),
but is a serious consideration for high-priced receivers (over $1500). Amplification You
may have heard "more power is better" when it
comes to receiver amplification. Well, we're here to tell you that
it's pretty much true (to a certain extent)! But here's the kicker: there are a few other facts you must
know and consider in choosing the amount (and quality) of amplification that you will need.
Amplification is the process where a receiver takes a low voltage analog audio signal
(usually a few Volts) and amplifies it to a powerful analog signal with
enough current to control and drive the loudspeaker's array of drivers to
produce the sound that we hear. First, we'll talk about the
number of channels, then how receiver
amplification power is specified and how much
power you will need. Number
of Channels As we mentioned above, the current
de facto standard are the 5.1-channel
surround sound formats such as Dolby
Digital and DTS.
This means 5 channels of full
frequency, plus a single ".1" low
frequency effects (LFE) channel for an optional subwoofer.
5.1-channel receivers have 5 channels of amplification for the full frequency
channels, while the ".1" subwoofer output is not amplified
for use with an active subwoofer (one with a built-in power amplifier) or a
passive subwoofer in conjunction with a separate amplifier. The
new and emerging surround sound formats are the 6.1-channel
extended
surround sound formats, with the addition of a full-frequency back surround
channel. Though there is a single audio signal for the back surround,
two loudspeakers driven off of that signal is usually recommended for a more
continuous soundfield. So if you are considering the 6.1-channel
formats, many mid-priced receivers will have 6 channels of built-in
amplification, some with an option to use a separate power amplifier for the second
back surround loudspeaker. The more expensive receivers will have 7
channels of built-in amplification to allow the use of two back surround
loudspeakers. In any case, the ".1" subwoofer output is not
amplified. Amplifier
Power Ratings There are a number of
aspects to a receiver's power rating, all of which should be considered
carefully: Continuous (RMS)
Power. Amplification power is usually given in units of Watts (just as the wattage of a light bulb
is a general indicator of how much light the bulb can emit). Since music
and movie soundtracks can have long loud passages, we are interested
in how much power a receiver can continuously put out to drive the
loudspeakers during these extended demanding aural passages. For this reason, we should not be interested in the peak power of a receiver,
the amount of power a receiver can deliver for a very brief duration before
running out of "steam". Rather, we are interested in a
receiver's continuous power output, which ensures that the receiver will have
enough "steam" to drive the loudspeakers during long passages of
demanding audio material. Continuous power is usually expressed in terms
of "Watts
RMS" (root mean square) or "Watts continuous". Do not
give credence to or compare receivers based on power specifications that say "Watts peak". Frequency
Range. Ideally, our ears can hear sounds as
low as 20 Hz (bass frequencies) to as high as 20,000 Hz
(treble frequencies). This full
range of audio frequencies should be reproduced by a home theater
system. In doing their part, receivers should be adept at amplifying audio signals in the full
range of 20 Hz to 20 kHz in order to maintain a realistic reproduction of
music and movie soundtracks. The frequency range portion of a receiver's
power rating is usually given as "20 Hz to 20 kHz", or equivalently
"full bandwidth". Anything
less than this range of frequencies is unacceptable. Channels Driven
Simultaneously. Movie soundtracks often have loud passages where
all channels (center, left, right, left surround, and right surround) erupt to
loud sound levels. These loud passages are very demanding as all
5-channels are driven hard at the same time. Better home theater receivers
will have power specifications that say "all channels driven
simultaneously" for the rated power output. Those receiver with
specifications that don't say "all channels driven simultaneously"
are likely to provide the rated power output for only one or two channels
driven simultaneously. Like the continuous
power specification, those that lack this capability may run out of "steam"
during loud passages where all channels are driven hard. But this
specification is less important between the two. Only high performance
receivers will have "all channels driven simultaneously" in its
power rating, so unless you're in the market for a receiver of that caliber,
this is something you can forego. Speaker Impedance.
Most loudspeakers have a nominal impedance (or resistance) of 8 ohms.
Some will have a value of 4 ohms, which means that they will draw twice as
much current from receivers than those with 8 ohms (assuming all else are
equal). Most receivers have no problems driving loudspeakers with 8 ohms
nominal impedance, but only some have enough "juice" to
effectively drive the more demanding loudspeakers with 4 ohms nominal
impedance. Most receivers are specified with power output "into 8
ohms". If you have a loudspeaker with 6 ohms or 4 ohms nominal
impedance, look for receivers that specify power output "into 4
ohms". Those that only specify "into 8 ohms" will still
drive 4-ohm or 6-ohm loudspeakers, but its performance will be much less than if
they were driving 8-ohm loudspeakers. Additionally its power output into
the 4-ohm loudspeaker will not be known. If you have 4-ohm loudspeakers, it
would be ideal if a receiver's specification give two numbers, one for 8-ohm
loudspeaker loads and another for 4-ohm loads. Editor's
Note: A perfect power amplifier can deliver double the power when the
loudspeaker impedance is halved. For example, a high-end power amplifier
can deliver "120 Watts into 8 ohms" or "240 Watts into 4
ohms". But few, if any, receivers can achieve this level of performance. Total
Harmonic Distortion (THD). This is a measure of how
"clean" a receiver can amplify audio signals. If you
enjoy music at louder volume levels, you will want to pay extra attention to
this part of a receiver's power specification. Since harmonic
distortion is a bad thing, lower numbers for THD are
better. Typical values are 0.05% to 0.08% THD for a
"clean" receiver, but any value below 0.1% THD is
acceptable. Don't compare THD values for receivers of different brands
too closely since there are some slight variations as to how this number is
measured across different manufacturers. Use it to compare receivers
with the same brand, but across brands only a difference of 0.03% THD or
greater should be
considered noteworthy. If you want a
good home theater experience and your budget is flexible enough,
we strongly
recommend choosing a receiver with a power specification that
reads something like "100 Watts RMS into 8 ohms per channel,
20 Hz to 20 kHz, all channels driven
simultaneously,
with no more than 0.08% THD". This kind of
specification will ensure that your loudspeakers are getting the power
they need to reproduce demanding movie soundtracks and music with the highest
possible fidelity. (We'll talk about how much power you really need in the next
section.) Compared to the two-channel stereo receivers of
a decade ago, today's home theater receivers have at least five channels of
amplification. While the digital components of a receiver have
significantly increased in computational power at the same time as getting much cheaper, the power amplification
components have not benefited from an equal drop in price. Consequently,
to make home theater receivers affordable and price competitive with the
two-channel stereo receivers of yesteryear, manufacturers have started to use innovative
technologies as well as "cut corners" to meet specific price points. Specsmanship.
To meet certain consumer-friendly price points, equipment manufacturers are
practicing what is called specsmanship. This is a practice where
manufacturers dress up or cover certain performance specifications in a manner
where a casual, ill-informed consumer is likely to miss out on the
not-so-subtle differences. The three parts of the power amplification subject to specsmanship are: continuous
(RMS) power, frequency range, and channels
driven simultaneously. Instead of specifying continuous or RMS
power, manufacturers use peak power in the specification for their entry-level
receivers. Instead of specifying power over the full range of audible
frequencies, manufacturers specify the power for only one frequency (usually
1,000 Hz or 1 kHz). And instead of specifying power as "all channels
driven simultaneously", manufacturers simply say nothing. So a power
specification with specsmanship reads something like: "100 Watts peak
@ 1,000 Hz, 0.10% THD". In our opinion, the two most onerous
practices in specsmanship are in the continuous (RMS)
power and frequency range part of the power
specification. It is to our disappointment that the FTC (Federal Trade
Commission) has allowed such specsmanship practices for peak power and
power specification for a single frequency, without making a concerted effort
to educate the consumer. There's nothing wrong with
buying an entry-level receiver from a manufacturer who uses this type of
specsmanship, so long as you (the consumer) are aware of the not-so-subtle differences. Many
receivers under $600 are subject to specsmanship, but those that cost more
are not necessarily exempt. And
now that you're informed, you can intelligently compare receivers
apples-to-apples and know what you're getting. How
Much Amplifier Power Do I Need? Now that
you know how amplification power is rated, the next step is determining how
much power you will need. Surely, more power is better, but most of us
live with fixed budgets, so practically speaking, we need to know how much is
enough. The amount of power you need will depend on a number of
factors. The most important factors are your loudspeaker's sensitivity,
the
size of your room, and how much headroom
you want in your system. Loudspeaker
Sensitivity. A loudspeaker's sensitivity is a measure of how
much sound the loudspeaker puts out for a given amount of power input from a
receiver. Usually, it is expressed in terms of decibels (dB) sound
pressure level (SPL) per 1 Watt of amplifier power measured at 1 meter from
the speaker. Different loudspeaker designs will have different
sensitivities. Most loudspeakers have sensitivities in the range of 85 to
91 dB SPL/1W/1m. Frequently, it is understood that sensitivity is
measure with 1 Watt of power input at 1 meter, so sensitivity is usually expressed
as just "dB". The higher the number, the more sensitive or the louder the
sound output from a loudspeaker given the same power input. Since
sensitivity is measured in decibels, a 3 dB difference in sensitivity means a
doubling or halving of amplification power to achieve the same loudness (SPL).
In other words, a loudspeaker with a sensitivity of 87 dB requires twice
the amplification power of a loudspeaker with a sensitivity of 90 dB. A
power amplifier delivering 100 Watts of power into the 87 dB loudspeaker
produces the same loudness as a power amplifier delivering 50 Watts of power
into the 90 dB loudspeaker. This is why loudspeaker sensitivity is an
important consideration when determining the power required from a receiver. Editor's
Note: For an 8-ohm loudspeaker, power input of 1 Watt corresponds to 2.83
Volts. But some loudspeaker manufacturers specify their 4-ohm
loudspeaker's sensitivity with a 2.83 Volt signal, which corresponds to 2
Watts of amplification power. This gives a false impression that their
4-ohm loudspeaker is more sensitive than it really is. Size of Your
Room. Larger rooms will require more amplification. As a
rule of thumb, use the following table to determine the power amplification of
your receiver. This table assumes that you want your home theater system
to be able to play at movie theater reference volume levels. First, look up your speaker's sensitivity in the first
column, then find the size of your home theater room as measured in cubic feet
in columns 2-4. Compute the room size by multiplying the width and
length and height of your room in feet. Then look up the value of the minimum
suggested amplification power (expressed in Watts per channel).
(Example: A loudspeaker with a sensitivity of 88 dB in a room that is 2,500
cu. ft. should be driven by a receiver with at least 64 Watts per channel.) If you find that your power amplification
requirements exceed 170 Watts per channel, you may want to consider a system
of separates, a pre-amplifier and a power amplifier (that can output
more than 170 Watts per channel) instead of an integrated one-unit
receiver. The most powerful receiver (Denon AVR-5803) produces 170 Watts
per channel into 8 ohms.
Guideline
for Minimum Receiver Power Amplification
(minimum recommended power, in Watts per channel)
(v1.0)
Loudspeaker
Sensitivity
[dB] |
Room
Size |
Small
(less than 2,000 cu. ft.) |
Medium
(between 2,000 and 3,000 cu. ft.) |
Large
(larger than 3,000 cu. ft.) |
| 85 |
96 |
128 |
192 |
| 86 |
76 |
101 |
152 |
| 87 |
60 |
81 |
121 |
| 88 |
48 |
64 |
96 |
| 89 |
38 |
51 |
76 |
| 90 |
30 |
40 |
61 |
| 91 |
24 |
32 |
48 |
| 92 |
19 |
25 |
38 |
Note 1: This table is based on the Home THX Program's recommendation of
achieving a peak sound pressure level (SPL) of 105 dB, which can be
achieved with 64 Watts into a loudspeaker sensitivity of 88 dB. Typical
listening levels are usually in the range of 80 to 90 dB SPL.
Note 2: If your array of loudspeakers vary in terms of
sensitivity, use the lowest sensitivity number from the front, center, or
right loudspeakers. The sensitivity of the surround sound loudspeakers
are secondary when it comes to figuring the minimum power amplification.
Headroom. Keep in mind that
the table above indicates the minimum amplification power you'll need. Usually, you will want more power in case you move your home
theater to a larger room or simply to have what is called headroom. Headroom
is the power reserve that is above and beyond what you normally need in
steady-state that allows your system to respond to very brief loud
passages. This is particularly important if you're a big fan of action
movies with lots of explosions. So yes, buy the most powerful receiver you can
afford, but keep in mind how amplifier power ratings are specified. THX Certification:
Comes in Two Sizes For home theater receivers, the Home THX
program
covers certain post-processing functions (discussed
in a previous section) and power amplification
requirements. The post-processing functions make sure that the
soundtrack mixed for a commercial movie theater is compensated for proper
reproduction in a home theater environment. The power amplification
specifications make sure that when a THX-certified receiver is used with THX-certified
loudspeakers, the system produces sound quality that meets THX
standards. And that means an awesome home theater experience. For receiver power amplification (and loudspeakers
too for that
matter), there are two basic levels of THX certification. THX Select certification is for components that are
certified to perform in a mid-sized home theater environment of up
to 2,000 cubic feet. (The volume of a room, expressed in cubic
feet, is computed by multiplying the length by the width by the
height of the room. For example, a room that is 18 feet wide
by 14 feet wide by 9 feet high, has a volume of 18 x 14 x 9 = 2,268
cubic feet.) The more stringent THX Ultra
certification (which is equivalent to the original, plain
"THX" certification) is given to components that meet the THX performance
standards for larger home theater environments of up to 3,000 cubic
feet. Theoretically, if you have a home theater
environment that is 2,000 cubic feet or less, THX Select certified receivers and loudspeakers should deliver about the same presentation standards as
the higher-end THX Ultra certified components. Both THX Ultra
and THX Select levels of certification call for the same post-processing
functions. 
THX Select
THX Ultra
THX Ultra2, more of a good thing? In
September 2001, THX updated the Home THX program to the THX Ultra2
specifications. In short, THX refined the THX post-processing functions,
refined the receiver performance specifications, and added the THX Ultra2
music surround sound mode, where a 7.1-channel surround sound experience can
be derived from music and movie program materials originally recorded in
stereo or encoded as Dolby Surround Pro-Logic in 2-channel. Right now,
THX Ultra2 certification is only available for high-end receivers.
Do
you need to buy a home theater receiver with THX certification? Certainly not. While a THX-certified receiver paired with a THX-certified
loudspeaker system can guarantee excellent results, you can achieve
the same (or better) performance with components that are not THX-certified. Certain receiver and loudspeaker system manufacturers produce
excellent products that perform very well, but they have chosen not to seek
THX certification and pay the associated Home THX program licensing
fees. Obviously, these costs would be passed onto the consumer as higher
prices. If you want to put together a home theater system in a fool-proof
manner that guarantees excellent performance and you don't mind paying a little
extra for THX-certified components and loudspeaker systems, by all means look
for the THX Select, THX Ultra, or THX Ultra2 logos.
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