bit stream 和 pcm的分別

Bitstream

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
DTS HD High Resolution

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

PCM

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:

http://hddvd.highdefdigest.com/departed.html 

Thus,

Theoretically no difference. It is just the way the data is used.

Bitstream = Player sends audio as it appears on disc and receiver changes the data to something it can output and you can hear.
(player doesn't change it, just outputs it)

PCM = Player changes data and sends a playable source to the receiver(receiver doesn't change it, just outputs it)

Granted not all players and receivers are created equal. In regards to lossless audio(DD TRUEHD or DTS-HD-MA)Some players can just do PCM, but even those can be limited.

Prime example is the Playstation 3 - PCM only for lossless audio but it only supports Dolby TrueHD, no DTS-MA yet. It cannot output either via bitstream.