Dolby AC-3, LPCM and DTS 5.1 - Blu-ray Audio Codecs Explained

Dolby AC-3, DTS-HD Master Audio and LPCM are just a few of the audio codecs that can be present on a Blu-ray disc.

Some are mandatory, some are optional, some may play on your Blu-ray player and some may not.

In discussions about home theater technology we may come across terms like Dolby TrueHD, DTSNeo:6, Dolby Pro-Logic II and DTS 5.1 Surround - but do you understand the differences between them all and what they do exactly?

In this article we will look at the audio codecs we may find on a Blu-ray disc and try to understand the different types.

Audio Encoding and Decoding

The first thing we need to understand is the difference between audio encoding and audio decoding.

Much of the confusion people have with the audio formats relating to home theater is that they don't understand the difference between the two......or is that just me?

Audio encoding is the method used to store the audio onto the Blu-ray disc itself. There are a number of formats which are used to encode (and sometimes compress) the audio onto the disc, and these are the audio formats that we are talking about in this article.

These different types of audio encoding have to be supported by the Blu-ray player so that they can be read from the disc, and these are the names we will see printed on the back of the Blu-ray box.

On the other hand, audio decoding, or processing, is where the audio that is on the Blu-ray disc is read so that it can be played through our amplifier and speakers. This process is often done by the amplifier or receiver, but can also be done by the player. The decoding/processing stage involves splitting the audio into multiple channels and sending it to the speakers.

It is easy to believe that these two processes are essentially the same thing, but there are more decoding/processing types than there are encoding formats - so there must be a difference somewhere.

Therefore, it is useful to try and understand the difference between audio encoding and decoding/processing because some of the terms we come across relate to encoding, and some to the decoding process - and sometimes both.

We will discuss the different types of audio decoding/processing in another article.

Blu-ray Audio Formats

So what are the audio codecs that we may find encoded on a Blu-ray disc?

Well, there are actually seven audio formats which are supported by Blu-ray players. Of these, three are mandatory and have to be supported by a Blu-ray player, and four are optional.

This doesn't mean that all seven formats will be on the Blu-ray disc that you buy. The studios have a choice as to which audio formats they put on a disc. The only rule they have to follow is that the primary audio track has to be one of the mandatory formats (which all players will support), and then any further audio tracks can then be either mandatory or optional (so your player may support them or may not).

Therefore, when you are buying a Blu-ray player, you will have to pay attention to the audio formats that your player supports if you have particular soundtracks that you need it to play.

These are the seven Blu-ray audio codecs with some details on each.

1. Linear PCM (mandatory)

Also called LPCM or sometimes just PCM.

PCM audio is the standard for CD and DVD technology and has also been made a standard for Blu-ray. It is an uncompressed audio format so there is no loss in quality on the disc, but the big disadvantage is it takes up a lot of space.

It supports up to eight channels of audio, so can provide encoding for 7.1 soundtracks, however it is more common to have stereo (2.0) or 5.1 surround sound in this format. Although it can support 24-bit sample rates, often LPCM audio will be provided in 16-bit to save disc space.

Just be clear, LPCM audio has no loss in quality as it is not compressed. Therefore, an LPCM soundtrack will sound as good as a Dolby TrueHD and DTS HD-Master Audio version providing they all derive from the same master soundtrack. However, the lossless HD versions are more likely to come from higher resolution masters as they can be compressed to use less space on the disc.

LPCM 5.1 surround audio can be passed through any version of HDMI (i.e. version 1.0 to 1.3), but will have to be downmixed to stereo for transfer over an optical/coaxial digital audio connection.

2. Dolby Digital (mandatory)

Also commonly known as Dolby AC-3 or DD 5.1.

Dolby Digital supports up to six channels of audio and so is limited to 5.1 surround sound and below. The maximum sample rate supported is 48 kHz.

It is a lossy format, or in other words it compresses the audio to save space on the disc, but some of the original audio information is lost in the process. It is the most common form of 5.1 surround sound and is also found on DVD.

Like LPCM, Dolby AC-3 can be sent through HDMI or digital optical/coaxial connections.

Although commonly used to refer to 5.1 surround sound, the term 'Dolby Digital' is actually a generic name for the encoding system as a whole, and on its own shouldn't really be used to describe the number of channels used. For example, we can have stereo Dolby Digital - or DD 2.0.

We may also see some Blu-ray discs with Dolby Digital 5.1 EX. This version of Dolby surround sound has an extra rear channel encoded into the two surround left and right channels to create a 6.1 mix. In a 6.1 surround sound setup there is one extra speaker behind the viewing position between the left and right surrounds.

3. DTS (mandatory)

Also known as DTS 5.1 or DTS Surround. This is effectively the DTS version of Dolby Digital.

Like Dolby Digital, DTS encoding is limited to a maximum of six audio channels and it compresses the audio to make the footprint on the disc smaller.

One difference is it supports a higher bitrate than Dolby Digital (1500 kbits/s compared to 640 kbits/s), although in reality the actual bitrate on disc is often lower than the maximum.

As with Dolby Digital 5.1 EX, there is also a version of DTS which is aimed at 6.1 surround sound systems - DTS-ES. The extra rear channel is encoded as part of the DTS 5.1 surround data, and will only be available on a system with an extra rear surround speaker.

There is much debate as to the difference between the audio quality of Dolby Digital vs DTS. If they have a choice, some people will always use one rather than the other. However, you will also find other people who say they can't tell the difference between the two.

In my view, there is a noticeable difference between DTS and Dolby Digital soundtracks, but which one you prefer just comes down to personal taste. We will look into this issue in more detail in another article.

4. Dolby Digital Plus (optional)

Also known as DD+ or E-AC-3. This audio encoding format is an extension of standard Dolby Digital.

Dolby Digital Plus supports 7.1 surround sound on Blu-ray discs, although the majority of Blu-ray movies only come with a 5.1 soundtrack. The maximum bitrate of this format is also greatly increased to about 6 Mbits/s, therefore allowing for better audio quality, although the maximum bitrate will be nearer 1.7 Mbits/s on Blu-ray. Like Dolby Digital, the maximum sample rate is 48 kHz and it is still a lossy compression format that loses some of the original audio fidelity.

Dolby Digital Plus cannot be sent over coaxial or optical digital audio connections, and if they are used, then the player will automatically use the standard Dolby Digital track instead.

Therefore, an HDMI connection is required to make use of this format (unless your player has multichannel analog audio outputs). Also, unless the Blu-ray player decodes the DD+ signal on-board first, you will require an HDMI 1.3 connection with a decoder on the AV receiver.

5. DTS-HD High Resolution Audio (optional)

This codec is an extension of DTS 5.1 and increases surround sound support to 7.1 channels.

It has an increased bitrate on Blu-ray of 6 Mbits/s, which is much greater than DTS 5.1, however it is still a lossy format which compresses the original audio to disc and loses some of the resolution in the process.

As far as the connections you can use for DTS-HD High Resolution Audio, the same rules apply as for Dolby Digital. Optical and coaxial connections aren't supported (and the player will switch to DTS 5.1 for this type of connection) - or an HDMI 1.3 connection is required unless the player decodes on-board and sends the decoded signal as LPCM.

6. Dolby TrueHD (optional)

Dolby TrueHD is a lossless format which compresses the audio onto disc, but creates an exact copy of the original studio master tapes.

Therefore, it provides the best possible audio reproduction of a movies soundtrack but also enables the data to be compressed on to disc. Compared to DD 5.1 or DD+, the soundtrack will have a greater dynamic range, better surround imaging and you should hear more high and low frequencies - if your sound system is capable of doing justice to these improvements of course!

TrueHD is capable of supporting up to 14 channels of audio, which means it can easily cope with the 8 channels required for 7.1 surround sound playback on a Blu-ray disc. 7.1 channel audio is supported at 24-bit/96 kHz resolution - and 5.1 channels are available at 24-bit/192 kHz.

In this format of Dolby surround sound the maximum bitrate is increased to 18 Mbits/s which means that an HDMI connection is required to transport this type of audio. As with DD+, if you want to bitstream this signal through HDMI to be decoded by your AV receiver, then you will need an HDMI 1.3 connection and an AV receiver with Dolby TrueHD decoding. However, if your Blu-ray player is capable of decoding this format on-board, then it can send the decoded signal as LPCM over any version of HDMI.

7. DTS-HD Master Audio (optional)

DTS-HD Master Audio is the DTS version of lossless audio.

If you have this on a Blu-ray disc then you have an exact bit-to-bit copy of the master tapes from the movie studio.

A Blu-ray disc with a lossless soundtrack may have this DTS version, the Dolby TrueHD version, or both. The annoying thing is some players may not support both types and so you may not always be able to use these higher resolution versions.

Master Audio has the highest bitrate of all at 24.5 Mbits/s on a Blu-ray disc, with a resolution for 8 channels the same as Dolby of 24-bit/96 kHz.

Dolby AC-3 Summary

So now we understand the difference between Dolby AC-3 and DTS 5.1, Dolby TrueHD and DTS-HD Master Audio, we can begin to understand what support to look for when we are looking to buy a Blu-ray player.

We need to think about what we want our Blu-ray player to be able to play as the support for these audio formats isn't the same for all players.

Also, now that we understand the audio that is encoded on the disc, it will make it easier when we think about the different types of audio processing that we find on our AV receivers. This is discussed in this article on Dolby and DTS audio formats and listening modes.

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