The various surround sound formats available on modern AV receivers can appear confusing.
Dolby Digital, DTS and THX listening modes are all available to process your audio, but what is the difference and what do they all do?
On this page we list some of the audio decoding and processing formats you will find on your hardware, and give a few details as to when you might use them.
You may come across other audio formats in addition to those listed here, but these are the most common and well-used. You will find that some of the processing modes on your AV receiver are only available for certain types of input - i.e. for analog/PCM stereo inputs or for Dolby Digital/DTS multichannel inputs.
This will be selected automatically by an AV receiver which decodes this format - and most receivers will support this. The most common type of Dolby Digital is 5.1 surround sound, but there are some Dolby Digital 2.0 soundtracks for systems with stereo speakers rather than surround sound setups.
Dolby Digital can be sent over HDMI or optical/coaxial connections, and the AV receiver will then decode the signal and play it as 5.1 surround sound (or stereo for DD 2.0). 'Dolby Digital' will be displayed on the front panel display confirming it is receiving and decoding the data stream.
This is the most common 5.1 surround sound format. It is found on most DVDs and Blu-ray discs, and will be supported by almost all AV receivers. It is a lossy format, which means the original audio from the film studio is compressed in order to fit the data on a disc. This compression will lose some of the original sound quality, although it still sounds pretty good on a decent sound system.
Some Dolby Digital soundtracks are encoded as 5.1 with an extra channel added as part of the surround channels. This allows for an extra surround channel at the rear - thus creating a 6.1 system. A Dolby Digital EX enabled receiver can decode this type of codec and accommodate an extra surround speaker for a 6.1 setup. If there is no extra surround speaker attached to the receiver, then it will automatically play the 5.1 version.
This codec is optional for Blu-ray and extends Dolby Digital to 7.1 surround sound. Therefore, this option may be present on a receiver that supports 8 surround channels. However, with the lack of 7.1 soundtracks, most studios will provide a DD 5.1 soundtrack on a disc rather than Dolby Digital+.
This type of audio will be detected automatically by an AV receiver if it has an onboard decoder. If decoded by the AV receiver, this codec can be sent over HDMI 1.3 as a bitstream from the Blu-ray player, and the front panel will display 'Dolby TrueHD'. However, the ability of AV receivers to decode this format isn't universal.
If the receiver cannot decode Dolby TrueHD, it can still play this audio type if the Blu-ray player decodes Dolby TrueHD first, and then sends it as LPCM. In this instance, the front panel will display 'Multichannel Audio' (or words to that effect) - but you will still be hearing the proper HD audio.
The advantage of Dolby TrueHD over standard Dolby Digital is it uses lossless compression to transfer it to a disc. So, although it is still compressed, there is no loss of audio quality from the original studio master tapes.
On a good sound system, this will result is crisper high frequencies, deeper bass and a more-defined surround effect.
TrueHD soundtracks are only available on Blu-ray discs - they are too large for DVD discs.
Unlike the previous four codecs which are encoded on to the disc, this is a processing format that takes a stereo audio signal and creates a virtual 5.1 surround sound effect. Therefore it allows those with a 5.1 speaker setup to enjoy surround sound from sources that only provide a stereo signal - such as most TV channels. It also works on a Dolby Surround 4-channel signal. It's not as good as a true 5.1 soundtrack, but it can be quite effective.
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There will often be three different versions of Dolby Pro Logic II - Movie, Music and Game. These different types have all been optimised to suit differences in the source audio. The 'Movie' version is designed for stereo and Dolby Surround movies and TV shows. The 'Music' version allows the user to adjust the sound stage width for music sources, and the 'Game' version is optimised for video games. Choose the relevant type depending on what you are listening to.
This is an enhancement of Dolby Pro Logic II. It converts a stereo or 5.1 soundtrack into 6.1 or 7.1 - depending on the speaker setup you have in your room. This will only be available on receivers that support speaker setups greater than 5.1 and it also comes with Movie, Music and Game versions.
An improvement on Dolby Pro Logic IIx. This version adds a new dimension to surround sound - height. The idea of Dolby Pro Logic IIz is to add two speakers above the usual front left and right speakers to create an even more realistic feel to certain sound effects - especially things like wind and rain. These height speakers can be added to a 5.1 system to create 7.1 (with front height speakers instead of back left and right) - or to a 7.1 system to create a 9.1 system. Obviously the AV receiver will need to support these speaker configurations for this to be available.
This format will be detected and decoded automatically by an AV receiver with a DTS decoder. Therefore, if you play a DTS soundtrack on a Blu-ray or DVD, the receiver will accept the signal over HDMI or coaxial/optical connections, display 'DTS' or 'DTS 5.1' on the display, and send the audio to the 5.1 speaker system.
Like Dolby Digital, this is a lossy format, so the compression onto disc will lose some of the original sound quality from the studio master. However, DTS is transferred at a higher bit rate than DD, and because of this many people say the sound of DTS is slightly better than the Dolby version. However, the average home user will probably not notice the difference.
This format takes a stereo sound source and creates virtual surround sound - either 5.1 or 6.1 depending on your hardware setup. In this respect it does a similar thing to Dolby Pro Logic II and IIx. However, there are differences in the resulting sound between the two systems and so it is advisable to try both and see if you prefer one over the other.
DTS Neo:6 comes in two versions - Cinema and Music. These are optimised for different types of audio - movies/TV shows and music. Switch the mode depending on the type of audio you are listening to.
It can also create a 6.1 surround mix (with the additional center surround speaker) from a 5.1 soundtrack.
This will be available on some AV receivers for decoding DTS-ES soundtracks. These versions are an extension of DTS 5.1 with an extra surround channel encoded in the standard left and right surround channels. The extra channel will only play on an AV receiver that has the extra speaker installed to create a 6.1 system.
An AV receiver that can decode this type of audio on-board will automatically switch to this setting when it detects the incoming signal over HDMI. It will then play the multichannel audio according to the encoded data on the disc and display 'DTS-HS Master Audio' on the front panel display.
As already mentioned with Dolby TrueHD, there are many AV receivers that cannot decode this type of HD audio. In this case, we will need a Blu-ray player that can decode DTS-HD Master Audio on-board. We can then set the Blu-ray player to send the decoded audio to the AV receiver as LPCM over HDMI or multichannel analog outputs. In this case the front panel display will read 'Multichannel Audio' as it is receiving LPCM rather than the original Master Audio data stream.
Like Dolby TrueHD, DTS-HD Master Audio uses lossless compression. Therefore, it will sound as sharp and clear as the original studio soundtrack.
As they are both lossless audio, there shouldn't be any difference in audio quality between DTS-HD Master Audio and Dolby True-HD. Also, they are only available on Blu-ray discs as the data is too large to fit on a DVD.
Some models of AV receiver (often the more expensive ones!), are THX certified and may offer extra THX listening modes.
THX certification means that a product meets certain standards defined by THX, who aim to set the highest standards for home theater video and audio equipment. This certification isn't limited to AV receivers, and we can find THX certified products across all types of audio-visual hardware. Whilst we should never assume that a product without THX certification must be of poor quality, where we find this mark of approval we should be confident of a high-quality product.
A THX certified receiver will have THX Surround Sound Modes which use a technology called THX Advanced Speaker Array (THX ASA). The three listening modes - Cinema, Games and Music - are similar to Pro Logic in that they can play back any type of source material and match it to the speaker setup you have in your room.
The difference with the THX listening modes is they also add equalisation to the audio in order to tailor the original soundtrack mix to suit a home environment. The theory is that movie soundtracks are mixed to sound good at high volumes in movie theaters, but will sound different at lower volumes in normal rooms. Therefore, these modes are designed to alter the frequency response of the sound you hear and allow for the differences between a real movie theater and your room.
If you have these listening modes on your equipment, you may find that you can improve the sound in your room by activating these settings. Give it a try and see what you think. They can be used on their own or alongside the various Dolby and DTS modes listed above.
The types of decoding/processing listed above are the most common versions that we will find on most AV receivers.
However, each manufacturer will often provide further DSP (Digital Signal Processing) listening modes for you to choose from.
These extra versions will provide other processing options to alter the sound that you hear through your system. Such types can include alternative 'Cinema' or 'Music' modes, Classical/Rock/Pop settings designed to enhance certain styles of music, or allow you to do things like playing a stereo source around all your speakers whilst keeping the original stereo image intact.
Feel free to play with these and see if you like the resulting sound.
As you can see, the surround sound formats and listening modes that AV receivers have can seem quite complex.
However, the idea is to give you the flexibility to tweak the audio you hear to suit your room and hardware setup. There is no way a movie can be made to sound the same in all types of rooms and with different speaker configurations, and so at least we have the option of adjusting the audio slightly to suit our hardware and taste.
Fortunately, many of these options will be selected automatically by your hardware depending on the type of audio you are playing, but it can be useful to understand the options you have for changing the settings because it can allow you to get an even better sound in your room.
To understand how these surround sound formats fit into the bigger picture you may want to check out our guide to Blu-ray audio formats; or find out the difference between decoding and processing in our guide to Dolby and DTS audio listening modes.