Xtreamer Support System

Introduction into Hd Audio


Bitstream audio refers to a succession of 1's and 0's transmitted from a player typically to a receiver or A/V preamp. The receiver then decodes this information and amplifies the resulting audio signal to drive your speakers. This information can be transmitted either through SPDIF (optical or digital coax) or HDMI connections depending on the compression method.

Bitstream audio as it comes on disc is usually compressed in 2 ways:

Lossy compression such as:
Dolby Digital
Dolby Digital Plus
DTS HD High Resolution

Lossless compression such as:
Dolby True HD
DTS HD Master Audio


PCM or Pulse Code Modulation "is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a digital (usually binary) code." (From the PCM WIkipedia article.) There are compressed forms of PCM audio but the PCM soundtracks that you see in BluRay discs are uncompressed.

Linear PCM or LPCM can be included on the disc itself or the player can derive LPCM from other digital tracks and transport this through the player's digital connections.

Is there an audible difference?

Given the right equipment and the right conditions a compressed lossless Dolby True HD soundtrack should sound the same as an uncompressed LPCM soundtrack.

See this comparison between the two on "The Departed." The LPCM soundtrack is on the BluRay disc and the Dolby True HD trak is on the HD DVD disc:



"Now, how do the PCM and TrueHD tracks compare? Given this historic opportunity, I decided to conduct a little experiment. I invited a friend over, who is a big movie and music buff, but not particularly technical. He knows good audio when he hears it, yet doesn't know a PCM from an RPM from R.E.M. In other words, he's Joe Six-Pack with a great ear. Anyway, together we conducted a "blind" audio test -- we select ten short sequences from the film, and listened to a compare of each. We took turns firing up each scene, and selecting which one sounded better, with no knowledge of which sample was the Blu-ray and which the HD DVD.

After writing down our answers on little scraps of paper (note that we didn't throw them into a hat -- we aren't that dorky), the results were interesting. Out of the twenty comparisons (ten for him, ten for me), we could only detect differences on four scenes total. But of those four, we both always preferred the PCM track, if only a smidgeon. For example, there is a scene in involving an attempted trade bust between the Costello character and a Chinese gang. There is a sound of a gun firing that we went back over a few times, and as silly as it sounds, the force and impact of the sounds was a shade more realistic in PCM. Also a beneficiary of the uncompressed mix is the music, as this is a film brimming with rock songs. The first scene we picked featured the Rolling Stone's "Gimme Shelter," and again the PCM track boasted a slightly more spacious feel to the music in all channels -- as if the very highest end of the frequency range was more palpable.

Granted, these are very slight differences and subjective preferences. Had we not blindfolded each other (figuratively speaking, of course) and been flipping back and forth between discs like one of those old Coke-Pepsi commercials, such deviations likely would have been imperceptible. It is also certain that the average listener wouldn't be able to tell the difference without possessing the ears of a dog. Still, in this case I give a slight edge to the PCM track, though a comparison between a single title hardly qualifies as the final word. If nothing else, it made me realize that if all the studios dumped this dueling audio format business and went all-PCM, I can't say I would be likely to complain... "

As long as the Dolby True HD soundtrack is decoded properly, the resulting LPCM signal should be exactly the same as the uncompressed LPCM track or the studio master.

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Last Updated
14th of April, 2011

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