Audio formats for video discs
There are many different audio formats used by DVD, Blu-ray and UHD Blu-ray discs. This article describes why there are so many and some of their differences.
Why it matters
Most of the time you should not have to worry about audio formats—the best available format is normally the default that gets used. But there are some rare situations where you need to deliberately choose the format, if you want to use the best quality audio that is available.
Discs usually have a setup menu where the audio track can be chosen, for switching to an audio track in a different language. Sometimes there will be multiple tracks in the same language, but in different formats. And sometimes the default track is not the one that uses the best available format—this is very rare, but does happen.
The table below lists audio formats roughly from lower to higher quality. Analog formats are included for comparison, but aren’t used on the discs covered by this article.
While discs don’t use analogue formats, they have been included in the table for comparison.
Note: some of the details in this article are not strictly correct, but have been simplified to make it easy to understand the main difference between the formats. In reality, these audio formats are much more complicated, with different limits when they are used with different parameters and constraints.
|Audio format||Type||Req||Obj||Channels||Connection||Bit rate|
|Dolby Surround Pro Logic||Analogue||-|
|Dolby Digital (AC-3)||Lossy||M||6||640 kbps|
|Dolby Digital Plus (E-AC-3)||Lossy||8||6.144 Mbps|
|DTS-HD High Resolution||Lossy||8||6.144 Mbps|
|Linear PCM (LPCM or PCM)||Lossless||M||8||HDMI||27.6 Mbps|
|DTS-HD Master Audio||Lossless||8||HDMI||24.5 Mbps|
|Dolby TrueHD||Lossless||16||HDMI||18.0 Mbps|
The formats that are mandatory for DVD and Blu-ray players to support are indicated with a “M” in the “Req” column.
Lossy vs lossless
Lossless formats are generally better than lossy formats, since they fully reproduce the original audio signal. Lossly formats discard some information, so can only produce an approximation of the original audio signal. But formats that use lossy encoding produce smaller audio tracks.
Formats that use lossless encoding produces larger audio tracks—which require higher bandwidth connections (i.e. HDMI) to transmit.
The LPCM format is lossless, since the data is stored on the disc without any compression. It requires the most data and bandwidth (depending on the sample rate, bit depth and the number of channels). Therefore, even though it is always available and is lossless, it is not practical to use in many situations.
The newer and better formats support more audio channels. Stereo and mono sountracks are commonly used by older content.
Multiple channels are required for surround sound. Six channels are needed for 5.1 surround sound (5 speakers plus a subwoofer). Eight channels are needed for 7.1 surround sound (7 speakers plus a subwoofer). While the object based formats support up to 32 channels, home theatre systems have fewer physical speakers. For example, 17 channels are used for 9.2.6 (9 ear-height speakers, 2 subwoofers, 6 overhead speakers).
Besides the very common 5.1 or 7.1 soundtracks, other arrangements are sometimes used. For example, the Australian release of Enemy Mine Blu-ray uses a 4.0 soundtrack.
More channels means more data, so some of the newer formats can no longer be transmitted over optical or coaxial S/PDIF connections. They must use HDMI connections. This article will not go into the complicated topic of the different HDMI versions.
Object based formats
Object based audio formats are indicated with a “Y” in the “Obj” column. The object based formats are more commonly found on UHD Blu-ray discs, but they can also be used in ordinary Blu-ray discs (e.g. Dolby Atmos is used on the Australian release of First Man).
Some formats are extensions to other formats. This allows them to be played on players that don’t support the newer formats: by only using the core format and ignoring the extensions.
- Dolby Digital Plus extends the Dolby Digital core audio format.
- DTS-HD High Resolution extends the DTS core audio format.
- Dolby TrueHD always includes a Dolby Digital (AC-3) fallback.
The maximum bit rate for the different formats are shown in the table. But the actual bit rate used depends on many factors, including the sample rate and bit depth (which are related to the quality of the sound).
The bit rate determines the amount of data required. The disc produce has to ensure the total size is small enough to fit onto the disc. They also have to ensure the data can be read off the disc fast enough to be processed.
Finally, the standards for DVD and Blu-ray discs have mandatory formats (that all players must support) and optional formats (that they don’t). For example, with non-UHD Blu-ray, “DTS” is mandatory but “DTS-HD Master Audio” (abbreviated “DTS-HD MA”) is optional.
So a disc producer has to decide which formats to use and which one to make the default track. Use an optional format as the default, and the disc might not play on some players (not without needing the viewer to first change the track). Use a mandatory format as the default, and the disc plays on all players but it might not be the best sounding format.
Disc players might not support the optional formats for a number of practical reasons. The optional format is in a newer version of the standard, so wasn’t available when the player was designed. The better formats require more processing power, increasing the cost of the player. The optional format requires additional licensing fees, also increasing the cost of the player. Therefore, older and cheaper players might not support the optional formats.
Most new players will support all the optional formats (unless new ones are added in the future). Since DVD players is a very mature technology most players support the optional formats. Most DVD discs these days use “DTS-HD Master Audio”, even though it is an optional format. But there can be some discs that stick to the mandatory formats.
There are many factors that need to be considered, and compromises made, with audio on DVD, Blu-ray and UHD Blu-ray discs. That is why there are so many different audio formats.
For completeness, upmixer formats should also be mentioned.
In the early days of surround sound, standards were created to simulate surround sound. These synthesised more channels out of fewer channels. For example, simulating a 5.1 soundtrack from a 2 channel stereo soundtrack.
|Dolby Digital EX|
|Pro logic II||2 to 5|
|Prologic IIx||2 to 7.1 or 5.1 to 7.1|
|Dolby Surround Upmixer (DSU)||Adds height channel|
|DTS Neural:X||Adds height channel|
|Auromatic||Adds height channel|
Upmixing techniques have largely been superceded by the newer multi-channel formats.