A Quick Guide to HD Audio
Chris has put together a short guide to the wonders of high-definition audio...
By and large, DVD used two main formats: Dolby Digital and DTS. These are what is known as ‘lossy’ audio formats, in that they ‘discard’ the parts of the soundtrack that the human ear theoretically can’t hear, which greatly reduces the amount of space that the soundtracks occupy on the disc. This was more important for DVD than Blu-ray, because of the relatively restrictive size of the discs (8.5GB for a dual layered disc, as opposed to 50GB for Blu-ray).
Because of the massive storage afforded by Blu-ray, not only can it deliver superior video, but also high-definition audio. As with DVD the two dominant formats come from Dolby and DTS, but they differ slightly in their implementation and there are also more variants. Let’s take a look at the available Blu-ray audio formats, some of which are mandatory and must be supported by Blu-ray players, and some of which are entirely optional:
- Linear PCM (Mandatory)
- Dolby Digital (Mandatory)
- Dolby Digital Plus (Optional)
- Dolby TrueHD (Optional)
- DTS (Mandatory)
- DTS-HD High Resolution Audio (Optional)
- DTS-HD Master Audio (Optional)
Which format is used is down to the content makers, but most Blu-ray Discs contain at least one lossless soundtrack. However, because these soundtracks are not mandatory, not all Blu-ray players and amplifiers can decode them (more on that later). Let’s take a look at the formats in a bit more detail.
Dolby Digital is the most commonly used format for DVD soundtracks and it also appears on a large number of Blu-ray Discs. As previously stated, Dolby Digital is a lossy format that discards information in order to compress a soundtrack down to a more manageable size. Common bitrates for Dolby Digital on DVD were 384Kbps and 448Kbps, but Blu-ray ups this to a maximum of 640Kbps. Although capable of delivering excellent audio, it’s not high-definition and supports a maximum of 5.1 channels.
Dolby’s newer format, Dolby Digital Plus, is similar to standard Dolby Digital in that it is a lossy format. However, it uses more efficient compression at higher bitrates (up to 1.7Mbps) and offers support for 7.1 channels. This produces superior results to standard Dolby Digital, but it is rarely employed on Blu-ray Disc because of its optional status. On HD DVD, where it was a mandatory format, it had a maximum bitrate of 3.0Mbps and was used on many titles.
Dolby’s most advanced format is TrueHD, which is lossless. What this means in simple terms is that although the format is compressed, no audio information is lost (in this respect it is analogous to a zip file). It allows for up to 7.1 channels of audio at bitrates of up to 18Mbps. TrueHD is widely used as the primary soundtrack on many Blu-ray Discs, but because it is an optional format it is always accompanied by a standard Dolby Digital track at 640Kbps for legacy compatibility. This track is usually hidden, but some studios (such as Warner Brothers) offer it as a selectable item in their menus.
Digital Theater Systems
DTS was a popular lossy audio format on DVD due to its alleged superiority over Dolby Digital (and no I don’t want to get into that debate here). DTS was commonly found on ‘premium’ editions of certain titles and was often used as a marketing tool to attract consumers who demanded the best audio-visual quality (Sony’s Superbit rage springs to mind). Standard DTS supported up to 5.1 channels of audio and offered bitrates of 768Kbps and 1.5Mbps, although the majority of titles were encoded at the lower bitrate.
DTS’s answer to Dolby Digital Plus is DTS-HD High Resolution Audio. This is a lossy format that offers up to 7.1 channels at a maximum bitrate of 6.0Mbps and its use is more widespread than Dolby Digital Plus on Blu-ray (at least in my experience). It is usually employed on titles where space is an issue, but it occasionally crops up on some big-name titles (the German release of Fight Club springs to mind).
Now this is where DTS’s naming conventions get a bit confusing, as its answer to Dolby TrueHD is called DTS-HD Master Audio, but more often than not it is referred to simply as Master Audio. It supports up to 7.1 channels of audio at a maximum bitrate of 24.5Mbps. Whereas Dolby TrueHD includes a separate Dolby Digital track, DTS-HD (both variants) works on a slightly different principal. There is a single stream containing both the standard lossy DTS Core and the lossless extension, which combine to create the full DTS-HD track (this is referred to as Core+Extensions). This lossless extension contains the 'difference' between the standard Core audio and the full DTS-HD soundtrack. If a player does not support DTS-HD then the lossless extension is discarded and you are left with a 1.5Mbps DTS Core track.
PCM is the other main format in use on Blu-ray and is seen by some as the ‘holy grail’ of audio. Because it is uncompressed and identical to the studio master it occupies far more disc space than the compressed Dolby and DTS formats, and as such it is used less frequently. For example, many early releases from Sony featured PCM tracks, but they have now largely switched to TrueHD (and to a lesser extent Master Audio).
As you can see, high-definition audio is a bit of a minefield, what with all of the different formats and their variants. However, for the best possible quality you’re going to want to seek out Blu-ray Discs with Dolby TrueHD, DTS-HD Master Audio or PCM soundtracks. Although compressed, TrueHD and Master Audio are bit-for-bit identical to the studio mater, so there is theoretically no difference between these three formats other than space requirements and occasional volume differences (Dolby tracks tend to be quieter).
Now that you know about the available formats you’re probably wondering what equipment you need. Well, that’s just as confusing as the formats themselves. Much of the following depends on the capabilities of your Blu-ray player and your amplifier, so there’s no ‘one size fits all’ answer. Blu-ray players and amplifiers are usually equipped with two or more audio connectors, as outlined below:
- S/PDIF (Sony Philips Digital InterFace): Optical or Coaxial digital audio connection
- HDMI (High-Definition Multimedia Interface): Digital connection for both audio and video
- Multi-Channel Analogue: Analogue audio connection
Most amplifiers will support one or more of these interfaces, although only newer models will offer more advanced options. Ideally you will be looking to connect your Blu-ray player to your amplifier via HDMI 1.3 as it is the most elegant solution, but this is not always possible. Below is a summary of what you can expect to hear with each format over each interconnect.
S/PDIF utilises either coaxial RCA or optical TOSLINK (TOShibaLINK) connectors and is capable of transmitting both Dolby Digital and DTS bitstreams from your player to your amplifier, where they are decoded. However, it lacks the bandwidth necessary to transmit Dolby Digital Plus, TrueHD and DTS-HD (both variants). In such cases most players will use the standard lossy Dolby or DTS track instead, but a few are capable of re-encoding soundtracks into 1.5Mbps DTS for arguably superior results. It's also worth noting that PCM soundtracks are downmixed to two channels due to bandwidth limitations.
This is the newest technology and the easiest way of obtaining HD audio from your Blu-ray player. The latest HDMI version (1.3) can support bitstreaming of all of the audio formats we’ve been talking about, so if you are in the market for a new amp it makes sense to go for one that supports this standard. Most recent modestly-priced amplifiers will decode all of the HD audio formats (check your manual if you are unsure).
However, not all amplifiers can decode HD formats or support HDMI 1.3. So, what if you have an older amp? Well, in this situation there could still be a way for you to enjoy HD audio without replacing an expensive piece of equipment. Many Blu-ray players are capable of internally decoding the HD audio formats and outputting them as PCM over HDMI. All HDMI versions can handle up to eight channels of PCM audio, so if you have an amp with HDMI 1.1 or 1.2 you could still hear the audio in all of its high-definition goodness providing it supports PCM over HDMI (if in doubt, consult your manual). For all intents and purposes there is little difference between the player and the amp handling the decoding, other than the thrill of seeing the Dolby and DTS lights appear on the amp when it decodes!
Another benefit of allowing the player to perform the decoding is that it can mix primary and secondary audio, which is not possible when bitstreaming. Why is this beneficial? Well, the main benefit is that it allows you to hear the audio on picture-in-picture tracks, but it also allows you to hear the various clicks that often accompany menu navigation. However, it is worth noting that some players cannot do this for DTS-HD tracks (notably the Sony BDP-S550).
Of course some Blu-ray players cannot decode all HD audio formats. Historically a number of players have lacked the ability to decode Master Audio, and in these cases the player would fall back on the 1.5Mbps DTS Core (which is still no slouch). The same is true of Dolby TrueHD etc., so you will need to consult your manual for details of which formats your player can decode. The PS3 was a favourite among early adopters because it could internally decode all formats to PCM except Master Audio, but support for that format arrived some time ago as part of an update. In addition to decoding, the newer PS3 Slim is also capable of bitstreaming the audio should you chose to do so.
What if you have a really old amp that doesn’t have HDMI? As we know, S/PDIF isn’t an option for HD audio, but if both your amp and Blu-ray player have multi-channel analogue interconnects there is a light at the end of the tunnel. Providing your player can decode all of the available formats, connecting it to your amp via six (or eight) RCA cables will allow you to enjoy HD audio with one or two caveats. Firstly, because the analogue decoding is done inside the player you must set speaker delay, distance and size manually using the player’s audio settings, rather than those of your amplifier. Secondly, the quality of the final audio is dependant on the quality of the DACs in the player, rather than those of the amp. I used to use multi-channel analogue for my HD audio, and while it is an effective way of wringing a bit of extra life from an old amp, it's really no substitute for an all-digital set-up.
I hope this guide is useful to those of you dipping your toes in the high-definition waters. It's not designed to be an in-depth technical analysis of the formats, and as such some concepts have been 'dumbed down'. However, if you think that the article could benefit from more information or believe any of the above information to be erroneous, why not add your thoughts in the comments section?
Editorial by Chris Gould
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