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Blu-ray Audio FAQ

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Blu-ray Audio FAQ

Welcome to the Blu-ray audio FAQ, hopefully you can find the answer to your audio setup questions here.

Click on one of the links below to go to that section:

Audio Formats

Audio Output Options

To discuss the contents, point out mistakes or suggest inclusions, please use the thread below:
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Linear PCM (Uncompressed HD Audio)

PCM is uncompressed high definition or lossless audio. It is in most cases the raw format used to create audio, and on Blu-ray is finally available in more than stereo.

One of the most useful things to know about PCM is what bitrate it uses for various configurations.

48KHz, 16-bit, 5.1 audio = 4.6mbps
48KHz, 24-bit, 5.1 audio = 6.9mpbs
48KHz, 24 bit, 7.1 audio = 9.2mbps
96KHz, 24 bit, 5.1 audio = 13.8mbps
96KHz, 24-bit, 7.1 audio = 18.4mbps
192KHz, 24-bit, 5.1 audio = 27.6mbps

Why are these figures important? Simply because they represent the maximum bitrate that you could ever see for a particular audio configuration. The first three cover about 99.9% of all Blu-ray films, as film soundtracks are generally mixed at only 48KHz during actual production. (The main reason for this is that on a big film, they may have over 100 tracks of audio to build up all the sound effects for the film. Using 96KHz would double the data rate of an already demanding system.) The higher sampling rates are usually found on music orientated releases with 192KHz, 24-bit 5.1 audio being the highest resolution that Blu-ray supports, and that's way beyond DVD-Audio capabilities.

PCM is generally what studios use to create the audio in the first place, and is what TrueHD and DTS-HD decompress back into for playback.

So, even though you will see claims from TrueHD and DTS-HD that they support up to ~18mbps and ~27mbps of audio respectively, don't be disappointed when your players bitrate meter shows significantly less, especially if it's a Hollywood film. Films compress at an average rate of 4:1, compared to music at 2:1, so a DTS-HD MA track of as little as 2mbps may not indicate a problem, or that you have been conned.

Support for multi-channel PCM is mandatory in ALL Blu-ray players, however because it is uncompressed, it takes up more space and also more bandwidth when reading from disc. Now that TrueHD and DTS-HD decoding in players and amplifiers has become more widespread, it's use on Blu-ray discs is becoming much rarer. Only music titles seem to continue with the format for now.

Not all players or amplifiers will support 192KHz mult-channel audio, so if that's what you are after and discs DO exist now with this on, then you need to check carefully that the player will handle this and not downsample.
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Dolby Digital Plus

As the name implies, this new format is very similar to normal Dolby Digital, but supports higher bitrates and more channels. It can go up to 3.0mbps on Blu-ray, and support 7.1 channels. However, you will never probably see a DD+ Blu-ray release...


Simply because on Blu-ray, it only works for 7.1 tracks. If you want just 5.1 which is still what the vast majority of discs contain, you just get full bitrate standard Dolby Digital 5.1. DD+ adds to normal DD by giving two extra surround channels and boosting the front L+R.

The trouble is, if you are using 7.1 on a disc, then you may as well use TrueHD. As DD+ is lossy, it's a fixed bitrate, so over the course of a film, TrueHD probably doesn't take much more space as it's variable rate, and on Blu-ray, audio bandwidth is not as restricted as it was elsewhere.

So, yes it's a supported format, but don't expect to see a release using it anytime soon.


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Analogue multi-channel outputs

If you do not have a receiver, amplifier (they're not all receivers, check out Denon's high end units) or processor that accepts at least multi-channel PCM over HDMI, then analogue output from the player is the only way you can hear true high definition audio from Blu-ray.

There is an argument that says once you get into high end equipment territory, the player may even do a better job over analogue than it can letting something else create the sound elsewhere in the output chain.

There are however some signficant pitfalls to consider before embarking down this route. By far the biggest one comes next, bass management and how well it is implemented.

Bass Management

When is no bass management in use?

This will happen if you set ALL of the speakers to have a size of large. This tells the player to send the full signal for that channel to the speaker. This may contain deep bass. In this instance, any crossover frequancy is ignored, and the player sends all frequencies to each channel as recorded on disc. The LFE channel on disc is the only thing sent to the subwoofer.

So what do the in-player options mean?

Speaker sizes

Small - This means that any bass information under the crossover frequency will be removed from that channel and sent to the subwoofer instead.

Large - All bass information contained in that channel will be sent to the speaker. No bass will be redirected to the subwoofer.

Crossover Frequency - This is the frequency below which all information will be sent to the subwoofer IF a speaker is defined as Small in the speaker setup. This may be fixed or variable depending on the player.

Speaker distance - This will tell the player how far from the listening position each speaker is. This allows the player to delay audio as required to make sure that the sound reaches you from all speakers at the same time. Without this option, sound may reach you too early or too late, causing the image to blur a little and resulting in less precise sound effects. Of course if all your speakers are the same distance away from where you sit, then this setting doesn't matter... but how many of us have such a setup?

So how does Bass work on analogue outputs?

When a player outputs analogue audio, the subwoofer/LFE channel will be at least 10db quieter than it should be. This is because a normal analogue channel is designed to deliver roughly a 2V signal which should produce a maximum volmue sound (when heard at reference level) of 105db. The LFE channel for effects in a film is designed to produce 115db to make the earth move!

If nothing was done, to get 115db over analogue at the same levels as the other channels could generate a 6V signal at full volume. This is significantly more than the 2V reference standard for analogue inputs. Some equipment will actually handle 6V, but most won't. A maximum of around 4V is I understand more common.

So, by default, to keep the LFE signal within standards, the LFE channel is usually toned down by 10db by the player before output. This -10db reduction applies in all scenarios even when NO bass management is in use.

What happens when bass management is used in the player?

If you use small speaker settings and set a crossover frequency below which sound gets redirected to the subwoofer channel then things get more complex. In this case, the summing of bass together from mutliple channels can also boost the signal, so a further 5db reduction to the LFE channel is applied.

That means, IF you are using analogue outputs AND you have any speakers set to small, the LFE out from the player could be -15db quieter than it needs to be.

The common course of action at this point is to whack up the sub channel on the player to make up for the lost bass... However, if we go back to why it was reduced in the first place, we can now hopefully see why that may not be the best course of action.

On the Sony S550 for example, to adjust the indivdual channel volume on the player, you can only decrease channel level relative to other channels, you cannot simply boost the sub by 10db for example.

So, how do you fix the -15db issue?

The answer rests with your amplifier. What you are looking for is something called an LFE boost for the multi-channel analogue inputs. Recent Denon units have this option and you can choose between 0, 10 and 15db boosts. If your player speaker settings are all large, then you would choose a 10db boost. If you use satellite based speakers and they are all small you would choose the +15db option. These options ONLY apply to the analogue multi-channel input.

Why wouldn't I simply set the sub level in my amplifier to +10/15db?

If your amp lets you boost that far it is an option, however, the amp levels apply to ALL sources connected to it. So, if for example you have Sky+ which will send a normal Dolby Digital signal for some programmes, or even prefer to use an older DVD player for DVD's, then you will have increased their sub output by +15db too. Digital sources don't need the +15db correction, so they will sound extremely loud in the bass dept compared to your Blu-ray player.

Of course if the ONLY thing connected to your amp is your Blu-ray player, then you can boost in the amp or even on the sub itself by turning up the volume.

What if my amp has no 10db boost for multi-channel analogue inputs?

Then you may not be able to get analogue output working correctly. Not all players can provide a 15db swing between channels. The current (2008) Sony and Pioneer models for example will only do a maximum of 12db. Also, if you do this the recommended way, by reducing other channels relative to the LFE one, then obviously general sound will be much quieter, and you will need to turn the amp volume up much higher. This could cause 2 problems:

1) If you listen at reference level your amp may not allow you to turn the volume up enough to keep at reference levels with a -15db reduction

2) If you switch to a different digital input on your amp, a jump of +15db in general volume could be quite a loud shock to the system if you forget to reduce the volume first. Remembering this could become tedious and is extremely significant other unfriendly.

In this case, if you are contemplating analogue outputs from a player as a cheap way of getting HD audio from Blu-ray, you would be well advised to think again. You are likely to be very disappointed and find it very frustrating to setup.

What if my amp only has a +10db boost, but I use small speakers?

This scenario is not so bad. It's not ideal, but should be fixable. As quite a few amps will take a higher signal than 2V, you probably have a bit of room for manouvre. You could simply set the normal channels to -5db relative to the LFE. This is the 100% safe solution.

However, if that produces too quiet a signal, then you might want to experiment by reducing the main channels a little, say -2db and booting the sub by say +3db. This will minimise the general volume difference between inputs.

What you are looking to avoid is a clipped LFE signal going from the player to the amp. As part of an explosion this might be difficult to recognise. As part of a music track with distorted bass, it should be easier to spot.

Some players such as the Pioneer models, actually offer two output modes for analogue out. In fixed output mode you cannot adjust the channel levels, but it outputs a full reference signal. If you select variable output mode, it actually reduces by default the signal by 6.0db. It then allows you to adjust the level by +/- 6.0db. This protects you from overloading the analogue outputs. so for these players you could simply boost the LFE by +5db with no risk. The downside will be a 6.0db drop in volume.

This however is player dependent so, you are highly recommended to download an online PDF of the pl;ayer manual and see what options it offers BEFORE purchase. Of course you can ask here on the forums too, where in most cases someone will own the machine you are interested in and be able to answer questions.

Why don't digital inputs have all these issues?

Simply because it's all hidden from you and is part of the decoding process to sort it all out. For many this is reason enough to stay digital.
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Legacy S/PDIF over Coax or Toslink (optical)

These outputs are limited to stereo PCM, standard Dolby Digital along with the EX extension, and standard DTS along with it's ES and 24/96 extensions.

You cannot transmit any of the new HD audio codecs via this interface. This is mainly because none of the legacy kit out there supports it, and also because in the case of TrueHD and DTS-HD, the current hardware implementations don't have the bandwidth either.

Keeping these interfaces as legacy only, simplifies what is already complicated enough by not allowing us to connect new players to old equipment and send them stuff they won't be able to decode.

Both normal Dolby Digital and DTS can deliver better sound than DVD either supports or normally delivers.

In the case of DD, the DVD standard has a limit of 448kbps as the maximum bitrate for audio. In many cases such as "The Matrix" DVD, you actually only got 384kbps.

For DTS it was slightly different in that DVD could support it's maximum bitrate for audio of ~1.5mbps, and some DVD's (again mainly music releases) did use the full bandwidth. However, if you check most of your DTS DVD's using something like PowerDVD on a PC, you'll see the vast majority of DTS discs are 768kbps.

All very interesting you might say, but what relevance does this have to Blu-ray?

Well, any amplifier that carries the Dolby Digital logo MUST be able to decode it at it's maximum bitrate of 640kbps. Likewise DTS.

As Blu-ray has much more space than DVD, and in the case of standard DTS is part of a DTS-HD Master Audio track anyway, you will find in the vast majority of cases you get 640kbps Dolby Digital and 1.5mbps DTS being sent to your old amplifier.

If you recall that 384 wasn't uncommon on DVD for DD, and most DTS that you've previously heard is 768kbps, those maximum bitrates found on most Blu-ray's are almost double the audio bandwidth that you are used to. Result? Even if you don't use HDMI or analogue outs for full HD audio, you will still get a decent boost in sound quality, and full bitrate DTS can sound very, very good.

What is DTS re-encode?

Because full bitrate normal DTS can sound excellent, and all DTS capable amps will support it, some players have implmented a feature called DTS re-encode.

What this means is that the player will still decode TrueHD internally to PCM. Obviously if the disc has a PCM track there's no decoding to do. In the case of DTS-HD Master audio, because of the way it works, the player already has a 1.5mbps core DTS, and may opt to use this, or it may possible do a full decode.

When the player has mixed in any other audio, it then sends that high quality mixed PCM audio stream it has to a special chip that then enocdes that PCM on the fly to a normal full DTS bitstream to send out digitally.

The result, especially for PCM and TrueHD, is that instead of using a 640kbps DD track, you get a better quality DTS one instead.

Currently the Samsung BD-P1500 and upwards support this feature and so do LG from the BH200 upwards. The new Oppo BDP-83 sort of supports this, but only for TrueHD and DTS-HD tracks, not curently PCM. This may change with later firmware.

So, if you don't have an HDMI amp, are put off by the setup issues around analogue, then one of these DTS re-encode players may be the best compromise. We will try and add to this list of re-encode models as time progresses.

Why don't all manufacturers do this?

We don't know. It's an excellent compromise for older players. The only problem is for amplifiers that do not support DTS, however, such equipment is usually very old if it's a separate receiver, or an all-in-one DVD surround system.

Perhaps Dolby don't like people selecting TrueHD on the Blu-ray and seeing DTS light up on their amplifier? Then we get pushed back to the DD vs. DTS wars... All players should provide DTS re-encode IMO if they provide legacy S/PDIF outputs.

What if my amp doesn't support DTS?

Well, the obvious answer would be to change it as it must be old, or not up to HD audio anyway, however, all is not lost.

Some players allow you to configure how they handle DD and DTS independently when outputting of S/PDIF. This allows you to send Dolby Digital as a bitstream and get the player to decode DTS for you into a stereo PCM stream that all amps will be able to understand.

Additionally, some have a DTS downmix option of either straight stereo or something called LtRt. If you send straight stereo to a Dolby amp that has ProLogic, you can still get surround sound, but it will be guessed at.

However, if the DTS downmix includes LtRt information, this allows a ProLogic receiver to recreate quite a few of the proper surround effects using old Dolby Surround encoding during the downmix. This means you get a matrixed front, centre, left and mono rear channel. Better than nothing!!

So you may wish to look out for this if your amp does not do DTS, but you want a Blu-ray player. Currently the Sony BDP-S350 and upwards, and the Pioneer 2008 models and onwards support this backwards compatibility feature.
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