Secrets Q & A

Home Theater Audio Formats?

Question:
Can you explain the popular audio formats for home theater? I have a media-center PC with a sound card connected to a receiver. Depending on the content, my receiver indicates DTS, PCM, Dolby Digital, etc.

Also, when my receiver indicates it's receiving DTS or Dolby Digital, does that mean it's 5.1? I ask because after further research, some reviewers of the sound card say that it doesn't support 5.1, just 2-channel. But my receiver shows DTS or DD when playing. Can there be a "fake" signal but it's really only getting stereo?
- Alex Ehrhardt
Carterville, IL

Answer:
All the audio formats you're asking about—as well as the newer, more advanced formats called DTS-HD and Dolby TrueHD—are simply different ways of encoding digital-audio data. In most cases, the amount of data is also reduced, a process called data compression. These formats are often referred to as "codecs," which stands for "coder/decoder." Audio is coded into one of these formats and then decoded at the receiving end.

When your receiver indicates DTS or Dolby Digital, that does not necessarily mean it's 5.1. All codecs can carry any number of channels up to some maximum. Dolby Digital can encode up to six channels, most commonly in a 5.1 configuration, while Dolby Digital Plus can accommodate up to 14 channels, as can Dolby TrueHD, though Blu-ray limits it to eight channels at a resolution of 96kHz and 24 bits or six channels at 192kHz/24 bits.

Conventional DTS can carry up to six channels, while DTS-HD Master Audio can support a virtually unlimited number of channels, depending on available bandwidth. Blu-ray also limits DTS-HD Master Audio to eight channels at 96/24 or six channels at 192/24. There is no limit on the number of uncompressed PCM channels, other than the available bandwidth.

All of the codecs allow "downmixing," in which the center, surround, and LFE (low-frequency effects) channels in 5.1 or 7.1 content are mixed into the front left and right channels. This allows you to hear all channels when you play the content on a system with only two speakers.

If your sound card can deliver only two channels, I suspect it is downmixing multichannel content in this manner. Even so, the data is still encoded as it was before—say, DTS or Dolby Digital—which is why the receiver indicates that it is receiving one of these formats. Or you might be playing DTS or Dolby Digital files that include only two channels, and the receiver would still indicate the codec being used.

In addition to a codec indicator on the front panel, many receivers also include an indication of which channels are active. This tells you if the signal being received from the sound card is 2-channel or multichannel.

Another way to tell what your sound card is sending is to make sure that all "upmixing" functions in the receiver are turned off. These include Dolby Pro Logic II, IIx, and IIz as well as DTS Neo:6 and Neo:X and Audyssey DSX. Such functions can take 2-channel audio and spread it out to 5.1, 7.1, and beyond, making it more difficult to determine by ear what the sound card is sending.

BTW, I assume you're using an optical or coax digital-audio connection between the sound card and receiver. These connections can carry conventional DTS, Dolby Digital, and PCM, but not DTS-HD or Dolby TrueHD. The newer formats can be conveyed only via HDMI.