Editor's Note: Each Friday, High-Def Digest's own HD Advisor will answer a new round of questions from our readers. If you have home theater questions you need answered, send an email to HDanswers@gmail.com.
Answers by Joshua Zyber
Blu-ray Audio Decoding & PS3 Settings
Q: My AV Receiver (Sony STR-DH510) only does LPCM for Blu-ray discs. Is this inferior to a receiver that decodes DTS-HD Master Audio and Dolby TrueHD? Or is LPCM the same thing, just decoded – the only difference being that I don't see the Dolby or DTS logos light up on my receiver? For this specific receiver, does it matter at all what the PS3 setting – BD/DVD Audio Output Format (HDMI) is – Linear PCM or Bitstream?
A: Uncompressed PCM is the native format of the digital audio masters for movie soundtracks. Dolby and DTS are merely compression formats that reduce the size of those original files. Standard Dolby Digital and DTS are "lossy" compression formats, meaning that they throw away parts of the original data in order to shrink the file size. By design, they do this by removing portions of the signal that human ears are less sensitive to first. However, with careful listening, you can often tell that something is missing.
Dolby TrueHD and DTS-HD Master Audio are both "lossless" compression formats. Once decoded, they will restore the original files without any loss of data. Think of them like ZIP folders that store the original PCM soundtrack. When you play a Blu-ray with either Dolby TrueHD or DTS-HD Master Audio, those formats must first be decoded to PCM (like un-Zipping the folder). The resulting PCM file will be bit-for-bit identical to the original master. That PCM must then be converted to analog and amplified out to your speakers.
Decoding of lossless audio formats can occur in either the disc player (in your case, the PS3) or the A/V receiver. When you do it in the PS3, you can transmit the resulting PCM file to your receiver by HDMI. In theory, it should not make much difference whether you decode the TrueHD or DTS-HD Master Audio tracks in the player or the receiver. They all wind up as PCM anyway. Some audiophiles may argue that transmitting PCM from a disc player to a receiver can introduce jitter, but the severity of this problem (if it is really a problem) are highly debatable.
All versions of the PS3 have the ability to decode Dolby TrueHD and DTS-HD Master Audio internally. You should set all audio output options to Linear PCM for this, and connect the console to your receiver by HDMI.
Only the PS3 Slim can transmit Dolby TrueHD and DTS-HD Master Audio in native bitstream format, so that the receiver will do the decoding. For that, set the PS3 Slim's output options to Bitstream.
The original ("fat") PS3 does not have this ability. If you set that one to Bitstream, it will downcovert the TrueHD or DTS-HD Master Audio tracks to standard lossy Dolby Digital or DTS quality first. To avoid this, you must use the Linear PCM output setting.
7.1 Soundtracks in a 5.1 System
Q: My home theatre setup is 5.1, but with so many Blu-ray discs now sporting 7.1 audio tracks, is it best to switch over to the 5.1 track on the disc versus just going with the default 7.1? Would it make a difference? If not, what happens to those other two channels if I just leave it on the default 7.1?
A: Off the top of my head, the only Blu-ray disc I can think of that offers separate mixes of the same soundtrack, both in lossless quality (and both of the same original language without dubbing), is 'Top Gun'. That disc includes both an older 5.1 mix in Dolby TrueHD format and a newer 6.1 remix in DTS-HD Master Audio format. I admit that there may be others out there that I'm not aware of, however.
In most cases, when a Blu-ray offers a 6.1 or 7.1 soundtrack, that's the only main lossless option on the disc. You might get a dub in 5.1, or possibly a lossy Dolby Digital or DTS version of the 5.1 mix. But generally, you won't be offered separate lossless tracks in the same language just for the difference between 5.1 and 7.1.
The reason for this is that there would be little point to it. When a 6.1 or 7.1 soundtrack is played in a 5.1 system, any audio cues intended for the surround back channels are simply folded over to the main left and right surrounds. You will not lose any audio. The only thing you might lose is the added directionality that the extra speakers would provide.
So, have no fear. Select that 7.1 soundtrack from the disc menu and let your equipment take care of converting it to 5.1 for you.
Why Do Studios Favor DTS-HD Master Audio over Dolby TrueHD?
Q: I had a question regarding HD Audio. I know that lossless is lossless, but if that is the case, why have almost all the Hollywood Studios publishing Blu-ray discs chosen DTS over Dolby for lossless? Even those who have been loyal to Dolby (Sony, Warner, Paramount) have begun publishing all movies with DTS-HD Master Audio soundtracks recently. Is this because of the disc space savings DTS allows with their "Core+Extension" format over the separate Dolby Digital and TrueHD tracks? What is the future for Dolby TrueHD since most studios have seemed to abandon the format?
A: When a similar question was asked about twenty columns ago, I mentioned that DTS-HD Master Audio's "core + extension" format offers greater convenience for backwards compatibility purposes than Dolby TrueHD, which requires that a separate Dolby Digital 5.1 track be authored onto the disc as well – and speculated that this might be a major selling point that was winning over some studios.
However, I recently had the chance to sit down with some Dolby executives, and I asked them this exact question. What they told me (and keep in mind that this is of course filtered through the Dolby perspective) is that DTS's encoders had faster run-times than Dolby's. In Hollywood, time is money. Because of this difference in encoding times, studios were finding it less expensive to author their discs with DTS-HD Master Audio.
The Dolby executives claim that the company has recently made improvements to the efficiency of its encoders, and has sped up the processing times. They indicated that this should result in an upswing of titles authored with Dolby TrueHD in the future. They also pointed to 'How to Train Your Dragon' as a major title to recently use Dolby TrueHD.
This week's Homework question comes from the Advisor himself. If you have suggestions on this topic, please post your response in our forum thread linked at the end of this article.
Questions for Dolby
JZ: As mentioned in the last question, I was recently given the opportunity to talk to some Dolby executives. I'll be speaking with one of them again next week, and wondered what questions about the company's products or services would our readers like to see answered. Would you like some info or clarification about Dolby TrueHD, ProLogic IIz, Dolby Headphone, theatrical 7.1 sound, Dolby 3D, Dolby Volume, or Dialogue Normalization, etc.? Post your questions in the forum thread for this article, and I'll see if I can get you some answers.
Check back soon for another round of answers. Keep those questions coming.
Joshua Zyber's opinions are his own and do not necessarily reflect those of this site, its owners or employees.