There are a number of Dolby and DTS audio formats supported by your AV receiver.
Dolby Pro Logic II, DTS 5.1, Dolby TrueHD - if you've just bought yourself a shiny new receiver and are flicking through the manual, it won't be long before you come across these terms.
Unless you have taken some time to study home theater surround audio formats, you can be forgiven for being somewhat confused by all these audio types. Don't be too downhearted, it's not too bad once you give it some thought.
In this article we will look at the various AV audio formats and listening modes you get on your equipment and try to understand the difference between them all.
The most important issue to be clear about when we talk about home theater audio types, is the difference between audio encoding and decoding.
Once we get this straight the subject becomes much clearer.
Audio encoding is the method that is used to store the audio on a DVD or Blu-ray disc. For Blu-ray discs there are seven codecs which are supported:
We won't find all of these formats on every Blu-ray disc, but if we look on the back of the box we will see which soundtracks are available for that particular disc. For more detailed information on all these codecs, go to the article on understanding Blu-ray audio codecs.
The important thing to understand is that these are the encoded formats. However, to hear this audio through our speakers these soundtracks need to be decoded.
Therefore, audio decoding is the process of reading the data from the disc and then sending the audio to the correct channels on our amplifier so that we can hear everything. If your hardware doesn't support the decoding of a particular audio format, then you won't be able to play that version of the soundtrack.
However, the main soundtrack will always be a mandatory audio type that all hardware will support - so you will never be in the situation where you cannot hear the sound from a Blu-ray disc.
The decoding process is usually done by the AV receiver, which will have audio decoders built-in.
The specifications of the AV receiver should include details of the AV audio decoders it has, and this will allow you to know which Blu-ray soundtracks it will directly support.
Therefore, if it has a DTS 5.1 decoder, then it will accept a DTS 5.1 soundtrack over an HDMI or coaxial/optical connection and play it in 5.1 surround sound. The front panel display should show 'DTS 5.1' (or similar) when it detects the incoming signal.
Similarly, if a receiver has on-board decoding for Dolby TrueHD audio, then it will be able to receive this signal type directly from a Blu-ray player, decode it, and then send the audio to the speakers.
In some circumstances the decoding may also be done by the Blu-ray player.
This is an important point. If your AV receiver doesn't support the decoding of a particular encoded format, it may be that your Blu-ray player does.
This is particularly important when dealing with the newer HD audio codecs Dolby TrueHD and DTS-HD Master Audio, although any format not supported by your AV receiver could be decoded by the player and sent as LPCM.
For example, if your AV receiver can decode DTS-HD Master Audio (like the Yamaha RX-V679 AV Receiver pictured here), then you can choose to bitstream the audio over HDMI from the Blu-ray player (you will need at least HDMI 1.3 to do this). This means the Blu-ray player sends the digital information directly to the AV receiver without decoding it. The AV receiver then decodes the data and you hear DTS-HD Master Audio over your speakers.
However if your AV receiver is not able to decode DTS-HD Master Audio, but your Blu-ray player does, then the Blu-ray player can decode the audio first and then send it as an LPCM signal over HDMI or multichannel analog outputs. Therefore, you can still hear DTS-HD Master Audio on your sound system even though your AV receiver cannot decode it.
Before you buy an AV receiver or Blu-ray player, you can check which audio decoding it supports and then you can be sure of the Blu-ray soundtracks you will be able to play.
In addition to onboard decoders for the various surround audio formats on Blu-ray and DVD discs, AV receivers will also come with other decoders and processors.
These extra audio processing features are often often called listening modes, although the actual terminology can vary between manufacturers.
There can appear to be a fine line between an AV receiver decoding a signal and processing a signal - and this is because AV receivers are designed to make the process transparent, with as little intervention from the user as possible.
So when we play a soundtrack that is directly decoded by the AV receiver, the receiver will automatically detect and switch to that listening mode. So an incoming Dolby Digital 5.1 signal will be detected automatically, the front panel display will show as 'Dolby Digital', and the audio will be sent around the speakers as 5.1 DD audio.
However, on your AV receiver there will also be other forms of audio processing (listening modes) where we can tell the receiver how to play back the audio. Therefore, the audio isn't decoded from the disc itself, it is processed after decoding. In the example of Dolby Digital above, we can change the 'Dolby Digital' listening mode to a different one - to Dolby Pro Logic IIx to create 7.1 surround sound for instance.
The Onkyo TX-SR343 5.1-Channel AV receiver pictured above has a grand total of 26 decoders and listening modes listed in the manual, ranging from the common all channel stereo mode, various Dolby Digital and DTS modes and some modes that are meant to improve the sound experience if you are using your system for gaming.
Most AV receivers allow us to set a default listening mode for a particular input, and we will usually set this to how we would normally like the audio from this input to be played. For example, we can set the receiver to always play stereo audio signals from our digital TV box as DTS Neo:6 Cinema, which will create a 5.1/6.1 surround sound effect from our 2-channel input signal.
However, we can change these listening modes at any time to suit the type of audio. If we are listening to music, we may switch to a stereo listening mode to make sure we hear the music through the traditional 2 front left/right speakers - or we may feel adventurous and select DTS Neo:6 Music to hear this stereo music track as 5.1 surround sound.
Some of the listening modes will only be available for certain types of audio, and if you consult the manual for your hardware (go on, it won't bite!), you will see a list of the types of processing available for stereo sources, multichannel sources etc.
There are various Dolby and DTS audio listening modes available on an AV receiver.
Some of these are the result of decoding the audio directly from the disc, and some are there to process the audio to suit our speaker setup. If we take a little time to understand the various options we have, then we will be able to get the best out of our receiver and speaker system.
Now that we know the difference between audio encoding on the disc and the audio decoding and processing that is done by the AV receiver or Blu-ray player, we can look at the different listening modes that we will come across.
On the next page we list the common types of AV audio decoding and processing you may find on your hardware and describe a little about what they do. Go to the guide to surround sound formats for more details.