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How A/V Receivers Decode Surround-Sound Formats

The A/V receiver in a home theater decodes surround-sound formats in audio and video programming so that sound can be distributed to all speakers. Two kinds of chips in the A/V receiver handle all this digital magic: the DSP and the DAC.

Digital signal processors

The DSP (digital signal processor) is the brains of the decoding process. This chip handles all the steering of surround sound, sending musical signals to the correct channels. The DSP also can provide its own sound field enhancements — essentially electronic changes in tone and timing (echoes). One feature that DSPs enable is the ability to dynamically compress music and, more importantly, movie soundtracks. Dynamic compression makes the louds less loud and also makes the quiet parts not quite so quiet. This feature is often called nighttime mode.

Many receiver manufacturers make a big deal about which DSP they use. The key is that the DSP can support the decoding of the surround-sound formats you want and need in your home theater. At a minimum, you should pick a receiver that can decode Dolby Digital and DTS (the plain-Jane 5.1 versions of these standards) and Dolby Pro Logic II (for VHS and other nondigital sources). If you’re getting a fancy six- or seven-channel system, you want a receiver that can support DTS-ES or Dolby Digital EX (or THX Surround EX).

The Blu-ray disc player has brought Dolby’s TrueHD and Digital Plus as well as DTS’s DTS-HD Master Audio formats to the home theater. For the most part you’ll find the decoders for these formats inside the Blu-ray disc player and not in the receiver, but a number of fancier receivers are beginning to include decoders inside the receiver itself.

You can get these high-quality surround-sound formats into your receiver in three ways:

  • If you have HDMI and a decoder for these formats in your receiver, and your Blu-ray player supports it, you can send the TrueHD or Master Audio bitstream over HDMI to your receiver, and let the receiver’s internal decoder handle the surround-sound decoding.

  • If you have HDMI in your receiver and a decoder in your Blu-ray disc player (not in your receiver), you can send the surround sound as PCM audio over your HDMI cable. This is just about as good, soundwise, as decoding inside the receiver and is much more common.

  • If your receiver has no HDMI connection and your Blu-ray disc player has a built-in decoder, you can use analog connections for your audio. This is less convenient (eight cables for audio plus a video cable to your TV), but the quality is also quite good. This is also common.

Digital analog converters

The DAC (digital analog converter) takes the digitally encoded musical signals (from a Dolby Digital or DTS DVD soundtrack or the PCM from a digitally connected compact disc) and converts these digital signals into analog signals that the receiver’s amplifier and the speakers can understand.

DACs are rated by the frequency of digital signal and the number of bits they can decode (for example, 96 kHz and 24 bits). Some receivers have multiple DACs (up to 16 in a single receiver) rated at 192 kHz and 24 bits. You should choose a receiver with at least 96 kHz/24-bit DACs. If digital connections for systems such as SACD or DVD-Audio ever become common, those higher-capacity DACs will become a minimum recommendation.

Sometimes, you don’t want to mess with all that digital stuff. Analog signals coming into the receiver are typically converted to digital signals. The digital signals run through the DSP and are then converted back to analog signals with the DACs. Many audio purists feel that all this conversion can create minor distortions in the audio. If your receiver has analog bypass, you can stay in straight, old-fashioned, pure analog all the way to the amplifier section. Some don’t bypass it at all; so if you want this feature, check to see if your receiver will give it to you.

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