The HD Advisor Number 23

Posted Fri Jul 31, 2009 at 12:00 PM PDT by

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Answers by Joshua Zyber

Bitstream vs. PCM

Q: I've read how in theory Dolby TrueHD, DTS-HD Master Audio and PCM should all be the same quality and audibly indistinguishable. I have an A/V receiver that can handle all the current HD Audio formats, so I have my Blu-ray player set to Bitstream. I was wondering since I have the Blu-ray player set to Bitstream, is it in fact handling discs with PCM 5.1 soundtracks differently than when it would decode Dolby TrueHD and pass it to the receiver as PCM? If the player is set to Bitstream, does the AVR then receive the signal in a way that would bypass the “PCM filters” and be treated like a codec or does the player recognize that it is a PCM track and flag it as such to the AVR and then the AVR may apply its filters?

A: Uncompressed PCM is the native (digital) format of any movie soundtrack. The studio archive masters for any soundtrack will always be in PCM form. Some Blu-rays (mostly older titles) are authored with uncompressed PCM audio tracks. For these, a direct copy of the studio master is simply pressed onto the disc. Other soundtracks are compressed with the various flavors of Dolby and DTS.

Dolby TrueHD and DTS-HD Master Audio are just compression codecs, lossless in both cases. They function like a ZIP file. You take the space-hogging source data (the PCM), ZIP it, and press the resulting file onto the disc with considerable storage savings. Upon playback, the file must be decoded, which is akin to unZIPing it. Because the codecs are lossless, the extracted results will be 100% identical to the original PCM.

The decoding process can be done either in the Blu-ray player (if it offers that function) or in the A/V receiver. In either case, the basic steps are the same. You start with TrueHD or Master Audio. That's decoded to PCM. The PCM is converted to analog, and then the analog signal is amplified out to your speakers. The only difference is where each of these steps takes place.

If you do the decoding in the player, you'll be left with a PCM file that must then be converted to analog. Again, this can be done either in the Blu-ray player or in the receiver. For the purposes of this answer, I'll assume that you're using an HDMI connection, and the PCM would be passed to your receiver for the D-to-A step.

I'll also assume that you have a Blu-ray player with an HDMI 1.3 connection that can transmit the high-def codecs in native form (not all can, unfortunately). In that case, if you choose the "Bitstream" option in your Blu-ray player, the TrueHD or Master Audio signal will simply be transmitted as is, for the receiver to do the decoding. Regardless, at some point, you will always wind up with that same PCM file in your receiver.

So, what happens when you play a Blu-ray with a PCM 5.1 soundtrack that hasn't been compressed? Essentially, you just skip over the decoding step. Everything else is the same. Even though you've set your Blu-ray player for "Bitstream," it will know enough to pass the raw PCM data without altering it. It will wind up at the same place in your A/V receiver as a Dolby or DTS signal would after decoding, and it will be treated the same from that step forward.

Moving Supplements to BD-Live

Q: Should studios put more bonus content on BD-Live and less on the disc? Even if the video quality of the features might be degraded on BD-Live, wouldn't that be a way to make more room for higher video and audio bit rates on the main movie?

A: Personally, I don't think that's such a good idea. Although most current model Blu-ray players support BD-Live, a great many households still own legacy units that can't access BD-Live content. Even if a certain player supports BD-Live, there's no guarantee that every owner will have it connected online. It would simply be unfair for all those viewers if every bonus feature was shifted to BD-Live rather than pressed on the disc where any player could access it.

You must also take into consideration the logistical challenges of online content. How fast or reliable is the user's internet connection? How long will it take to download each piece of content? For how many months or years will the home video studio continue to host that content on their servers before discontinuing the feed? We've already run into a number of situations where certain online features have stopped being supported by the studio.

I'm of the feeling that BD-Live should only be used for features that require an internet component, such as live chats with the filmmakers, or various interactive games and social networking functions. Newly-created content not available at the time of disc authoring, or content that requires regular updating are also good uses for BD-Live. When it comes to the standard featurettes, deleted scenes and whatnot, those should remain on the disc. If space is truly an issue with any particular movie's encode, the studio should put the bonus features on a second disc in the case, and leave the movie to its own primary disc.

Connecting a Blu-ray Player to a Standard-Def TV

Q: I am about to purchase another DVD player to replace a model I have had for ten years now. The one that has drawn my attention is a Panasonic Blu-ray model. However, I do not currently own an HDTV or an updated A/V receiver. I just figure that it may be worth the upgrade to get a future-proof unit that can handle Blu-ray and DVD. But I do have two questions: My TV is a 36-inch RCA (7 years-old) with an S-Video input but no Component input. What type of picture detail can I expect to see hooked up with the S-video cable? The receiver I am using is a Yamaha RX-V795. It has 5.1 analog inputs on the back side to accommodate, hopefully, Dolby TrueHD and DTS-HD Master Audio. I am a novice with most of this. Is this something that would be worth my while until I can upgrade my TV and receiver or am I wasting my time?

A: As far as video quality goes, you will see little to no difference between a DVD and a Blu-ray on your current TV. Blu-rays will usually have less compression artifacts, so you might find some improvement there, but it will be small. The resolution and picture detail of the high definition Blu-ray will be downconverted to the same quality as a DVD over the S-video connection.

On the audio front, things are a little better. So long as you pick a Blu-ray player that can decode the high-res Dolby TrueHD and DTS-HD Master Audio audio formats internally, and that has 5.1 analog outputs, you will be able to hear movie soundtracks in full lossless quality.

With a Blu-ray player, you'll also be able to access Bonus View (and potentially BD-Live) supplements not found on DVD. Whether any of this is worth the investment without any improvement in picture quality is up to you.

If you have to replace your DVD player anyway, and you're not afraid to spend the extra money, a Blu-ray player might be a good future-proofing investment. A number of newer models have really come down in price to affordable levels. It might even spur you on to finally replace that TV as well. Trust me, you'll be glad when you do.

A Blu-ray Player Region-Free for DVD?

Q: Is there a good Blu-ray player on the market that has region-free DVD playback? I currently own a DVD home theater system but want to get most out of my Blu-rays. My DVD collection is made up of many regions. If the Blu-ray player is region-locked for DVD playback, then I'm going to have problems.

A: A few weeks back, I addressed the question of region-free Blu-ray players, but that was specific to the region coding of Blu-ray discs, not DVDs. However, the two things go hand-in-hand. I'm not aware of any Blu-ray players that are region-free for DVD but not Blu-ray. So my answer to that question should apply to yours as well.

Homework Assignment: You Be the Advisor

Some questions that the HD Advisor receives are best answered by our readers. If you can help to answer the following question, please post your response in our forum thread linked at the end of this article. Your advice and opinions matter too!

Downmixing Lossless Multi-Channel Audio to Stereo PCM

Q: When you configure a Blu-ray player to output stereo LPCM from the S/PDIF output, with what soundtrack does the player begin, assuming the disc does not have a stereo PCM track? Does it downmix and downsample the 640 kbps Dolby Digital track or 1.5 Mbs DTS core track? Or rather does it start with one of the lossless tracks to create the stereo LPCM output? The technically superior solution would be to begin with a lossless track.

JZ: That's a good question. I'd assume that the player starts with the lossless track and downmixes from there, but I don't know the technical workings of the process for certain. Perhaps one of our readers with a more in-depth audio background can help answer this?

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.

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