Introduction to A/V Receivers

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Surround Sound

At the heart of the receiver is the surround-sound processor. The most basic surround processors use a Dolby Pro Logic II decoder. Pro Logic II is a "matrix" system that takes an encoded two-channel stereo signal and converts it to a five-channel full bandwidth (range) playback (Left/Center/Right/Left Surround/Right Surround),resulting in a surround experience. Most TV shows are encoded in Dolby Pro Logic II, as are the majority of VHS videos. You can also play DVDs through a Pro Logic II only-receiver because DVD players can synthesize a Pro Logic II signal that mimics a surround soundtrack. The newer Dolby Pro Logic IIx adds the ability of converting stereo or 5.1-channel surround sound for seamless 6.1 or 7.1 playback.

The reality though, is that every A/V receiver now has state-of-the-art, movie-theater-quality 5.1-channel Dolby Digital decoding. With five channels of discrete, full-frequency sound plus a low frequency effects (subwoofer) channel (the ".1"), Dolby Digital is the surround-sound standard that has revolutionized home-theater sound. A "Dolby Digital-ready" receiver accepts a decoded Dolby Digital signal, from a DVD player equipped with a built-in decoder, via analog inputs. Or, you can get a receiver with full Dolby-Digital-decoding built-in. Dolby Digital EX adds a center rear channel which helps clarify audio effects that pan from front to back.

In addition to Dolby Digital, many AV receivers now also include DTS (Digital Theater Sound) decoding, which is another 5.1 or 6.1 channel system. DTS uses a lower digital-compression rate than Dolby Digital and it tends to sound better. However, less compression does mean more than more disc space — in this case more three times. DTS Neo:6 provides up to six channels of matrix decoding from stereo matrix material. DTS Digital Surround is the DTS standard for providing 5.1 channels of discrete digital audio. DTS 96/24 is a 5.1-channel surround format that offers a higher standard of audio quality by encoding with 96 kHz sampling rate and 24-bit word length.

Top of the line A/V receivers include THX Surround EX and DTS ES.

These formats add a third surround channel that drives loudspeakers located directly behind the listener. Receivers with EX and ES decoding usually include seven amplifier channels rather than five. And they have the same goal: to allow film sound-mixers to position sounds and pans (movements of sounds) more precisely behind you.

DSP Processing

All modern A/V receivers use Digital Signal Processing (DSP) chips to perform Dolby Digital and DTS decoding. The same DSP chips also create the "soundfield" modes that process two-channel sources for surround-sound playback over multiple loudspeakers. These soundfield modes attempt to replicate the acoustics of various venues, from jazz clubs to stadiums, by adding simulated acoustic reflections to the signal and delaying the signals sent to the surround loudspeakers.

(Sometimes these emulations amount to little more than novelty effects, but depending on your program material, they can enhance your listening experience).

DSP chips also perform another important function in the receiver: bass management. Bass management filters bass frequencies from small loudspeakers and directs them to the subwoofer. While Dolby Digital, DTS, and DSP surround processing all occur in the digital domain, the final sound output must come from your analog speakers.

Receivers use a multi-channel digital-to-analog converter (DAC) to create the final analog signal passed on to your speakers. These DACs can vary greatly in sound quality so be careful.

Inputs

You should make sure your AV receiver has sufficient audio and video inputs and outputs to meet your current and future needs. Today a DVD player may be your only digital source, but tomorrow you may wish to add a DBS satellite dish and a CD recorder that requires their own digital inputs for optimum performance.

The heart of any home theater system, and the component you'll interact with most. A/V receivers handle a multitude of essential tasks that can make or break the quality of your home entertainment experience. It has a preamp allowing you to route audio and video from different sources DVD, cable TV, CD, turntable, etc. to your speakers and television. The receiver also does doubles as an AM/FM tuner, amplifier for you speakers, and typically controls everything from surround-sound decoding (such as Dolby Pro Logic, Dolby Digital, or DTS)to tone controls (bass and treble)and master volume. An A/V receiver is similar to the conventional "stereo" receiver you’ve had for years, with several important differences. Although both select sources, control the system’s playback volume, and amplify signals to drive your loudspeakers, the A/V receiver is unique in three respects: It includes some form of surround-sound decoding, it processes 5.1 or 7.1 channels (rather than the stereo receiver's two channels) and it has video inputs and outputs.

To get the most from DVD, make sure your TV has an S-Video, or even better, a component video input. Most receivers include S-Video switching and multiple S-Video inputs and outputs so you can conveniently switch between other S-Video sources like DBS satellite and a Hi8 or Digital8 camcorder. With its improved color purity, superior color detail, and reduced NTSC artifacts, Component video is superior to both composite and S-video. While it's ideal to have a receiver with S-Video or component video switching, there are some very good but affordable receivers that don’t have S-Video or component capability. If you plan to get an HDTV TV or projector with component-video inputs, look for a receiver offering component-video switching. Last, if you frequently use your camcorder with your home theater setup, be sure your receiver includes a set of convenient front-panel AV jacks.

Outputs

You'll want to make sure your receiver has enough outputs for all your speakers, monitors, and other devices. If you own a CD or DVD recorder, you'll probably want a receiver with a digital output jack, either coaxial or optical, depending upon your recorder's input. If you're going to be using a powered subwoofer in your home theater setup, you'll also want a subwoofer out. You might want to consider buying a receiver with "pre-out" line-level RCA jacks for each of its main channels, especially if your budget restricts you to a relatively low-powered model. That way you can easily add a more powerful outboard amplifier later, relegating your AV receiver to preamplifier status. If you're highly concerned about future upgradeability, you might want to consider a receiver that has six-or even eight-channel throughputs. This will allow your receiver to pass the signal from a future surround-sound format.

Because most home-theater speaker systems consist of five limited-frequency-range satellites and a subwoofer, you should buy a receiver with a flexible bass management system that permits you to direct low frequencies (anywhere from 180 Hz on down)from the five main channels to the subwoofer. This enables both the receiver and your speakers to perform more efficiently.

Amplifier Power

How much amplifier power do you really need for your home theater setup? For the best performance, a minimum of 100 watts per channel (20-20,000 Hz, all channels driven) is recommended .Power is the most misunderstood specification in home theater. For example, a manufacturer might give their receiver a two-channel-driven rating of 100 watts at 20-20,000 Hz. Their five-channel playback (all channels driven) rating might look similar but mean something very different: 100 watts at 1000 Hz, for example. Because it's only given for one frequency, not the entire range, the second spec probably means there is a significant power drop off at the frequency extremes when all five channels are in use. The bottom line is when looking at power ratings; try to get a rating that covers the full audio spectrum, not just one frequency.

Another factor influences how powerful a receiver you need: whether or not you have a subwoofer with a built-in power amplifier. With a subwoofer in the system, low bass frequencies are filtered from the signals driving the main loudspeakers, which reduce the amount of amplifier power needed to drive them. Low bass requires more amplifier power to reproduce than do midrange and treble frequencies. By directing the bass to the subwoofer and away from the main speakers, the receiver's power amplifiers don't have to work as hard. Adding a powered subwoofer reduces the receiver's power-output requirements by about 20 percent.

Plus, power output isn't always an indicator of how loudly your home-theater system will play. In fact, he speakers connected to the receiver have as much influence on the system's volume capability as does amplifier power. Specifically, loudspeakers vary in their sensitivity, a measure of how much sound they produce for a given amount of input power.

What does sensitivity difference mean in the real world? A great deal.

Every 3-dB decrease in loudspeaker sensitivity requires a doubling of amplifier power to achieve the same loudness. That means that speaker A (83-dB sensitivity) would need 16 times the amplifier power to produce a given sound-pressure level compared to speaker B (95-dB sensitivity).Yes –160 watts driving speaker A produces the same volume as 10 watts driving speaker B! That's why you should match the receiver's output power to your speakers’ sensitivity.

THX CERTIFICATION

Some receivers are billed as "THX Certified" ((Ultra, Ultra II or Select) which means they incorporate four Lucas film-developed technologies that better translates the soundtrack intended for theater reproduction into the home. THX-certified receivers also meet a set of technical criteria established by Lucas Film. These are:

A re-equalization circuit to reduce excessive brightness.

Surround de-correlation to increase the sense of envelopment by making the left and right surround channels slightly different from each other.

Timbre matching to assure that sounds from the front and surround channels have the same timbre.

A subwoofer crossover.

Choosing a THX-certified A-V receiver has many advantages. First, THX's signal processing (surround decorrelation, timbre matching, re-equalization, subwoofer crossover) is of unquestionable benefit in reproducing film soundtracks in the home. Second, you know that the receiver has achieved a certain level of technical performance.

Third, a receiver with THX certification will likely have all the output power you'll ever need. Finally, using a THX-certified receiver with THX-certified loudspeakers and subwoofer takes the guesswork out of matching the receiver to the loudspeakers.

However, that there are many excellent receivers that don't bear the THX logo —simply because their manufacturers decided not to participate in the THX licensing program –or pay Lucas film royalties.

 

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What about HDMI?  It is way past time for an update.