Blu-ray bitstream output vs LPCM output?

Question

My setup: Onkyo TXSR-805 receiver. Panasonic TH50PZ77U. Soon, a Blu-ray or HD-DVD.

My question:

My understanding of the benefits of bitstream output compared to LPCM are clouded. I understand, according to Onkyo, that no current HD/Blu can output bitstream audio to the receiver through HDMI 1.3 for receiver-onboard decoding (this, according to their site, which might be dated...). Given that I've seen a Pioneer BD, Toshiba HD-A35, as well as a firmware-upgraded Samsung BDP1400 that say they support bitstream output, I'm curious as to what benefits either format provides?

Internal decoding of Dolby TrueHD through LPCM is currently possible and only requires HDMI 1.1 (>35.5Mbps), which confuses me. Why does external decoding in my receiver require HDMI 1.3 and only 18Mbps (source: http://en.wikipedia.org/wiki/Dolby_true_hd)?

I guess it comes down to this: which format is better? LPCM or bitstream output to external decoder? If the question doesn't make sense, I'll adjust later.

Answer 1

Here's ALL you need to know to get the sound you want:

You need to know which current Blu-ray Disc players are compatible with your Onkyo, the difference between the THREE "lossless" (uncompressed) audio formats and how to set the "HDMI Audio" output for whatever player you choose.

A continually updated list of capable Blu-ray Disc players is near the bottom of this link (right after the capable receivers): http://www.avtruths.com/uncompressed.htm...

Here's a link to a complete list of Dolby and DTS's (the "Coke" and "Pepsi" of digital surround) audio formats.  You'll find the 3 competing "lossless" audio formats near the bottom of the page under the heading "New High Definiton Surround Formats": http://www.avtruths.com/dolbydts.html

The last part is easy.  If you pick one of those 4 players and hook it up using an HDMI cable, you'll need to make sure the "HDMI Audio" output in your Blu-ray Disc player is set to "Bitstream" and not "LPCM".  It has nothing to do with "sound quality", but if you want your Onkyo to properly decode the audio signal from the player, it needs to be sent in "Bitstream" form.

One more thing.  Most Blu-ray titles default to the LOWEST QUALTIY audio track.  Before you press "Play" you'll need to go into the "Audio Menu" (every time) and select one of the three "lossless" formats:

"Uncompressed 5.1"

"DolbyTrueHD"

"DTSHD Master Audio"

Unfortunately, not all Blu-ray titles include a "lossless" audio track.  Some studios simply transfer the original DVD audio to the Blu-ray Disc version of the same film.

(and while "Dolby Digital Plus" is less compressed than standard "Dolby Digital", it is NOT a "lossless" audio format)

Blu-ray Disc won the format war.  Check out this link which monitors the continuing demise of the HD-DVD format: http://www.avtruths.com/blurayvshddvd.ht...

Answer 2

Bitstream output means that the signal is being outputted in its native format whereas with LPCM, the signal is being converted into LPCM.

For example, if you are outputting a Dolby Digital signal and the digital output is set to "bitstream," the signal will be Dolby Digital. If it's 5.1 Dolby Digital from the disc, it will be 5.1 Dolby Digital when the decoder gets it.

LPCM, or Linear Pulse Code Modulation, may take the signal and convert it into a linear PCM signal.  For example, if you are outputting in Dolby Digital and the digital output is set to "LPCM," the signal will be LPCM instead of Dolby Digital. If it's 5.1 Dolby Digital from the disc, it may be 2.0 LPCM when the decoder gets it.

The LPCM option is for compatibility; if your decoder is unable to decode Dolby Digital (such as using an outboard PCM A/D converter), you set the output to LPCM instead so the decoder can work with the signal you are feeding into it.

As for why LPCM requires a higher bitrate: LPCM is uncompressed, requiring a higher bitrate whereas Dolby TrueHD uses a lossless CODEC to decrease the size of the bitstream without sacrificing fidelity but without requiring as much bandwidth to transmit it.

Answer 3

To tackle the later questions first.. LPCM is in fact both digital and can support up to at least 8 channels (7.1) audio. Don't let the fact that it falls under the same play mode on your system as analog signals confuse you.  Also, LPCM ouput should generally never downgrade audio from surround sound to stereo, unless of course that's what it's supposed to be doing. I think the misconception may stem from PCM as a recorded format on standard DVD (DVD-Video), which I think only supports 2.0 or 2.1 audio do to bitrate limitations. DVD-Audio discs however can do more channels, because there's no video output consuming most of the available bitrate ;)

Essentially as a format LPCM is higher quality then any compressed format . In the computer world, PCM is a Wavefile (.wav) and the various other compressed formats (Dolby, DTS, etc) are like Mp3, WMA, etc.  Essentially all of the audio standards like Dolby Digital, decode and decompress into the something like LPCM for further processing.

When Onkyo says no current HD/Blu-Ray player supports bitstream audio out, I believe they specifically meant for DolbyTrueHD.  Other formats, including any supported by regular DVD, should bitstream through HDMI.. as this was supported by newer regular DVD players through HDMI or S/PDIF.

I understand the confusion about HDMI 1.3 needed for bitstreamed TrueHD, because naturally you realize it's a lower bitrate then LPCM. Essentually it doesn't have anything to do with the supported bitrates or any such limitations, but simply what formats are written into the HDMI specification as a standard.

To finally answer your question, however...  the answer to anyone asking that question, in general, would likely depend what has a better audio decoder (and other relevant features), the player device or the reciever. Essentially it's a choice of which you are going to have decode the audio, not a question of how good LPCM is. In general, bitstream would be the choice for most, as the reciever will at least decode the audio as well, plus know what it's dealing with thus how to treat the audio better. This point is magified by the fact you have a fairly fancy reciever.  In matter of fact, now that I think of it.. any audio reciever that would decode audio worse then your player is pretty much c**p.

So yeah, bitstream's your safe bet. But LPCM as a recorded format is better.  Sending a lesser format via a higher one of course isn't going to magically make it any better though. I would certainly think it's possible for their to often be not much of a difference however, depending on the source audio the recorder format, and player. Obviously is someone has a low end audio setup, the difference could not be rendered undisernable.