High Definition Audio

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Acknowledgement

Initial information for this wiki page were taken from an article on avforums.com, published by 'dante01'. See the full thread here:- http://www.avforums.com/forums/av-amplifiers-receivers/984906-attempt-explain-high-definition-audio.html

High Definition Audio

Blu-Ray brings with it new high definition sound formats to match its high quality pictures and video.

Sound can be stored on a disc in a number of ways:

  • Lossless Uncompressed (no data lost),
  • Lossless Compressed (smaller storage space needed, but no data is lost),
  • Compressed lossy (some data is lost).

Compression formats that lose data (“lossy” compression) use careful algorithms to throw away data that you are supposed to be unable to hear. DVD’s use these types of compression formats as space restrictions and don’t usually utilise lossless audio as the video information takes up the majority of the available free space. DVDs do allow for stereo PCM, and this is especially apparent on classical music DVDs, albeit nowhere else. Stereo LPCM is one of the mandatory audio formats that all DVD players and processors must support (unlike dts). The problem is that DVD doesnt support lossless surround sound.

With Blu-Ray, discs space isn’t as much an issue as it is with DVD. With 25GB per layer, multiple layer support, and dual-layer discs being the norm, the Blu-Ray disc has plenty of space to store lossless formats.

DVDs can deliver data at up to 10.08 Mbps (megabits per second) and only approximately 1.5Mbps of this is used for audio (that means all audio streams; 5.1 soundtrack, 2 channel soundtrack, director’s commentary, etc.). According to the Blu-ray Disc specification, Blu-ray 1x speed is defined as 36Mbps (data) and 1.5x 54.0Mbps (video/audio). Blu-ray has the potential for much higher speeds, the only limiting factor for Blu-ray is the capacity of the hardware. The Blu-ray Disc Association (BDA) plans to raise the speed to 8x (288Mbps) or more in the future, but the current maximum video is limited to 40Mbps and audio is limited to 27.648Mbps.

Audio streams can be sent to an AV receiver/amplifier as bitstream (encoded digital data) or LPCM (essentially raw digital data.) Bitstreamed audio from a DVD or Blu-Ray disc needs to be decoded. This can sometimes be done by the player itself and output as LPCM to the amplifier/receiver. If a player lacks the inbuilt ability to decode the data, bitstreamed data can be sent for a suitably equipped AV receiver/amplifier/processor to do the decoding. Regardless of which method is used, there is no difference in quality between LPCM and lossless bitstreamed formats like Dolby TrueHD and DTS-HD Master Audio.

Along with the lossless Dolby TrueHD and DTS-HD Master Audio formats, Blu-Ray offers Dolby Digital Plus and DTS-HD High Resolution audio. While being a “lossy” formats, these two new standards offer benefits that Dolby Digital and DTS from DVD discs can’t such as higher sample rates. Like Dolby TrueHD and DTS-HD Master Audio, they also offer support for 7.1 channels of audio, where DVD’s can only support up to 6.1 channels.

Dolby DTS-HD Master Audio has a “core” DTS soundtrack that older receivers can lock into (fall back on) as they do with Dolby Digital and DTS soundtracks from DVDs. Like all “lossy” compression systems, the less compression, the better. With the extra space on Blu-Ray and HD DVD discs, higher compression rates aren’t as necessary for the DTS “core” soundtrack, meaning better quality audio. For example, Dolby Digital 5.1 on a DVD will use a sample rate of either 384kbs or 448kbs. Blu-Ray discs will carry the same Dolby Digital 5.1 soundtrack at 640kbs. The “core” DTS soundtrack on a Blu-Ray disc is often delivered at an even higher bit rate. The Dolby TrueHD stream is actually two streams: one MLP (Meridian Lossless Packing), also known as Packed PCM and one standard Dolby Digital. TrueHD track are required to contain a standard Dolby Digital AC-3 track for compatibility with players that don't support TrueHD.


What audio codecs will a Blu-ray player support?

Dolby Digital (DD):

  • MANDATORY support on BD players
  • Lossy, non HD format commonly used on DVDs
  • Supports up to 6 channels of discrete sound
  • Sample rates: 32, 44.1, 48 kHz
  • Bit Depth: 16, 20 bit
  • Constant bit rate of up to 640 kbps

The audio format familiar from DVD, Dolby Digital (sometimes known as AC-3) is one of the base standards of Blu-ray. It works basically the same way that it worked on DVD in configurations from 1.0 to 5.1, though it does offer a higher maximum bit rate of 640 kb/s (which is considered audibly indistinguishable from Dolby Digital Plus at the same rate). 



Digital Theater System(s) (DTS):

  • MANDATORY support on BD players
  • Lossy non HD format commonly used on DVDs
  • Supports up to 6 channels of discrete sound (DTS Digital Discrete up to 7 channels)
  • Sample rates: 48 and 96 kHz
  • Bit Depth: 16, 20, 24 bit
  • Constant bit rate of up to 1.5 Mbit/s

Digital Theater System(s), also previously known as 'DTS Encore', is a holdover from standard definition DVD. DTS is an alternative and competing format to Dolby Digital. The basic difference between the two formats is the method of compression. The use of DTS is optional on DVDs. Blu-ray more ably supports the codec at a higher bit rate that used for DVD. With the extra space and bandwidth afforded by Blu-ray, DTS audio tracks can be encoded at data rates greater than 1.5 Mbit/s. DTS soundtracks found on Blu-ray discs usually have a higher bit rate than those normally found on a DVD.



Dolby Digital Plus (DD+):

  • OPTIONAL support on BD players
  • Compressed extension of Dolby Digital
  • Supports up to 8 channels of discrete sound
  • Sample rates: 48 or 96 kHz
  • Bit Depth: 16, 20, 24 bit
  • Constant bit rate of up to 6 Mbit/s

DD+ is a lossy format that uses a more efficient compression technique at data rates from 96Kbps to 6 Mbps, resulting in better sound quality. It can also support movie soundtracks with up to 7.1 discrete channels of sound. On Blu-ray, DD+ is encoded as an extension to a "core" Dolby Digital AC-3 track. The DD+ extension bitstream is used on players that support it by replacing the rear channels in the 5.1 setup with higher fidelity versions, along with providing a possible channel extension to 6.1 or 7.1. The complete audio track is allowed a combined bitrate of 1.7 Mbit/s: 640 kbit/s for the AC-3 5.1 core, and 1 Mbit/s for the DD+ extension. During playback, both the core and extension bitstreams contribute to the final audio-output, according to rules embedded in the bitstream metadata.



DTS-HD High Resolution:

  • OPTIONAL support on BD players
• Compressed extension of DTS
  • Supports up to 8 channels of discrete sound.
  • Sample rates: 48 or 96 kHz
  • Bit Depth: 16, 20, 24 bit
  • Constant bit rate of up to 6 Mbit/s

Similar to Dolby Digital Plus, DTS-HD HR is a lossy format that offers enhancements over standard DTS with higher bit rates and better compression. It delivers up to 7.1 channels of sound at frequencies from 48kHZ up to 96 kHz and 24 bit depth resolution. DTS-HD HR is selected as an optional surround sound format for Blu-ray with constant bit rates up to 6.0 Mbit/s. It is an alternative for DTS-HD Master Audio where disc space may not allow it. DTS-HD HR is also encoded as an extension to a "core" DTS track. The DTS “Core + Extension” structure contains data for a 5.1-channel system, operating at 44.1 or 48kHz, with a bit rate of 1.5Mbps (mega-bits per second). . If you have an older DTS-capable receiver, it will “ignore” any extensions and just decode the "core". The same “Core + Extension” structure is utilised within DTS-HD MA.


Linear PCM (LPCM):

  • MANDATORY support on BD players
  • Lossless encoding
  • Supports up to 8 channels of discrete audio
  • Sample rates: 48, 96, 192 kHz
  • Bit Depth: 16, 20, 24 bit
  • Constant bit rate of up to 27.7 Mbit/s

A PCM track is an exact replication of the studio master, encoded on disc without compression. The benefit to this is that it maintains the purity of the source without any loss of fidelity that may come from compression. The downside is that an uncompressed audio track takes up a tremendous amount of disc space, which may (especially on single-layer BD25 discs) negatively affect the video quality of the movie. While the Blu-ray format is capable of utilising PCM audio up to 24-bit resolution, studios may choose to encode at 16-bit resolution instead, depending on the bit depth of the original source or concerns about conserving bandwidth (downsampling a 24-bit master to 16 bits is technically not the same thing as compression). 



Dolby TrueHD:

  • OPTIONAL support on BD players
  • Compressed Lossless encoding
  • Supports up to 8 channels of discrete audio
  • Sample rates: 48, 96, 192 kHz
  • Bit Depth: 16, 20, 24 bit
  • Constant bit rate of up to 18.4 Mbit/s


Dolby TrueHD is a "lossless" compression codec. It is compressed to take up less disc space than a PCM track, but once decoded it is bit-for-bit identical to the studio master at either 16-bit or 24-bit resolution (at the discretion of the studio) and a bit rate up to a maximum of 18.4 Mbps.



DTS-HD Master Audio:

  • OPTIONAL support on BD players
  • Lossless encoding
  • Supports up to 8 channels of discrete audio
  • Sample rates: 48, 96,192 kHz
  • Bit Depth: 16, 20, 24 bit
  • Constant bit rate of up to 24.5 Mbit/s

DTS-HD MA (previously known as DTS++) is a lossless audio codec similar to Dolby TrueHD. Like Dolby TrueHD, a disc encoded with DTS-HD MA delivers ALL of the information from the original master recording — bit-for-bit. The difference between the two is that DTS-HD MA uses a core+extension configuration (just like DTS-HD HR). A DTS-HD MA track takes up more disc space than a TrueHD track, but does not require a secondary standard track for backwards compatibility. Since both DTS-HD MA and TrueHD are lossless, they are both 100% identical in quality to the studio master, and hence identical in quality to each other (lossless 24-bit/192 kHz).


The three lossless audio formats associated with Blu-ray players' high-resolution audio soundtracks are Dolby TrueHD, DTS-HD MA and PCM. These formats will all convey 8 discrete channels (7.1) of "lossless" audio that exactly duplicate the original studio masters.


Blu-ray players and recorders will have to support playback of the mandatory codecs, it will still be up to the movie studios to decide which audio codec(s) they use for their releases, but Blu-Ray titles must use one of the mandatory schemes for the primary soundtrack. A secondary audiotrack, if present, may use any of the mandatory or optional codecs along with the mandatory primary soundtrack.


Bits, Rates and Frequencies

Sampling Rate or Frequency Rate is defined as the number of times samples are taken per second to convert an analog signal to digital. A higher sampling rate allows for higher frequencies to be represented. The sampling frequency also impacts fidelity. The sampling frequency is essentially the number of times the sound event is quantized within a given time period (e.g., 96kHz equates to 96,000 samples per second). Sampling frequencies are specified in KiloHertz (KHz). CD-quality sound requires 16-bit words sampled at 44.1 KHz. Essentially this means 44,100 16-bit words (705,600 bits) are used to digitally describe each second of sound on a compact disc.

Bit Depth (Word Length) is the number of bits (8, 16, 24 etc,) used to represent the analog audio signal each time it is sampled in the analog-to-digital conversion (ADC) process. A higher bit number allows a more accurate representation of the amplitude of an audio signal, resulting in better dynamic range. Generally a larger Bit Depth equals higher fidelity. 24-bit is generally considered the current practical limit as this Bit Depth allows a signal-to-noise ratio exceeding that of most analog circuitry, which by necessity must be used at least twice in the recording/playback chain (ADC and DAC).

Bit Rate or Data Rate refers to the number of bits used per unit of playback time to represent a continuous medium such as audio or video. It is the number of bits-per-second that can be processed, calculated by multiplying (sampling rate) x (sample size) x (number of channels).


Digital Audio and LPCM

Digital audio begins when an analogue audio signal is first sampled, and then converted into binary signals. An analog signal is converted to a digital signal at a given sampling rate and bit resolution; it may contain multiple channels (2 channels for stereo or more for surround sound). Generally speaking: the higher the sampling rate and bit resolution the more fidelity, as well as increase the amount of digital data.

A digital audio signal starts its life with an analog-to-digital converter (ADC) that converts an analog signal to a digital signal. The ADC runs at a sampling rate and converts at a known bit resolution.

After being sampled with the ADC, the digital signal may then be altered in a process which is called digital signal processing (DSP) where it may be filtered or have effects applied. The digital audio signal may then be stored or transmitted to another digital device.

Digital audio needs to be converted back to an analog signal with a digital-to-analog converter (DAC). Like ADCs, DACs run at a specific sampling rate and bit resolution but through the processes of oversampling, upsampling, and downsampling, this sampling rate may differ to the initial sampling rate. All devices capable of accepting incoming digital audio and outputting analogue audio possess a DAC.

For audio, BD-ROM (Blu-Ray) players are required to support Dolby Digital, DTS, and linear PCM. Players may optionally support Dolby Digital Plus and DTS-HD High Resolution Audio, as well as lossless formats Dolby TrueHD, DTS-HD Master Audio (MA) and Linear PCM (Linear Pulse-Code Modulation). Blu-Ray titles must use one of the mandatory schemes for the primary soundtrack. A secondary audiotrack, if present, may use any of the mandatory or optional codecs.

Bitstream is usually the output of an audio or video encoder. The elementary stream contains only one kind of data, e.g. audio, video or closed caption. An elementary stream is often referred to as "elementary", "data", "audio", or "video" bitstreams or streams. The format of the elementary stream depends upon the codec or the type of data carried in the stream itself. The Source device does not decode the signal and the device receiving the bitstream must be capable of decoding it. Think of bitstreaming as passing data through untouched. For example, if your Blu Ray player cannot decode Dolby TrueHD / DTS-HD Master Audio but your AV receiver can, you need to set the digital output of your player to "Bitstream". In other words, the encoded audio will be streamed to the AV receiver / amp in order for it to do the decoding.

Linear pulse code modulation (LPCM) is a method of encoding audio information digitally. The term also refers collectively to formats using this method of encoding. LPCM can be included on the disc itself or the player can derive LPCM from other digital tracks if it posses the ability to decode them. The player then transports the raw LPCM or decoded as LPCM audio through the player's digital connections to other devices such as an AV receiver. For example, if your Blu Ray player can decode Dolby TrueHD / DTS-HD Master Audio but your AV receiver can't, you need to set the digital output of your player to "LPCM" or "PCM". In other words, the encoded audio will be decoded by the player in order for it to be streamed to the AV receiver / amp as LPCM.

LPCM is used for the lossless encoding of audio data in the compact disc Red Book standard and has been defined as a part of the DVD and Blu-ray standards as well as the digital audio standard frequently called AES/EBU. Stereo LPCM is one of the mandatory audio formats that all DVD players and processors must support.

The term PCM is often used to describe data encoded as LPCM. PCM is a digital representation of an analog signal where the magnitude of the signal is sampled at regular intervals and then mapped (quantized) into binary code. The quantization is necessary because CPUs are used to implement DSP (Digital Signal Processing). Computers can only process finite quantities at any one time. Signals need to be mapped to fit a finite resolution, so that they can be stored in a CPUs registers and memory while the DSP takes place. PCM ís basically a stream of "1s" and "0s" (bits, "ons" and "offs") that precisely describe the frequencies and relative loudness (amplitude) of a music signal. No data is discarded to conserve bandwidth or reduce file size. The "1s" and "0s" are used by the DAC to reconstruct the original audio in order that the original audio may then be amplified and output as analogue by your AV amp or receiver.

Quantization is the fundamental element that distinguishes lossy data compression from lossless data compression, and the use of quantization is nearly always motivated by the need to reduce the amount of data needed to represent a signal.

Aimost all new BD players can decode the Dolby TrueHD / DTS-HD MA lossless audio and send it to your receiver as a multichannel LPCM stream. Just about any modern AV receiver can decode LPCM even if unable to decode Dolby TrueHD / DTS-HD Master Audio. The LPCM audio is identical to the Dolby TrueHD / DTS-HD MA original format, so no quality is lost.


So What Do You Need?

Firstly you’ll need a Blu-Ray player and some discs encoded with the new formats.

You’ll also need an AV receiver/amplifier/processor. It needs to be compatible with high definition audio – either with onboard HD audio decoding abilities or the ability to accept and then decode multichannel LPCM. The format compatibility labels on the front of some units might only list Dolby TrueHD and DTS-HD Master Audio but Dolby Digital Plus and DTS-HD High Resolution audio will also be covered by these units. Some early 7.1 receivers and amplifiers may show no external signs of being compatible with the new formats and do not posses the inbuilt ability to decode these formats for themselves. However it is possible to send the required audio streams to them as LPCM from players with their own inbuilt decoders.

When you decode Dolby TrueHD / DTS-HD MA tracks, you get an identical copy of the original audio – this is why it's called "lossless." On Blu-ray discs, the Dolby TrueHD and DTS-HD MA codecs can include up to eight channels of lossless audio information, each channel with up to 24-bits resolution at a 96kHz sampling rate, commonly referred to as "24/96."

The vast majority of HD audio compatible receivers will be 8 channel, usually referred to as 7.1 (there are a few exotic HD processors (amps, receivers) that only support 5.1 LPCM over HDMI) and an increasing number of entry level 5.1 AV amps and receivers that also come with HD audio decoding abilities. The majority of Blu-Ray titles currently available only offer 5.1 DolbyTrueHD or DTS-HD Master Audio. It is acceptable to purchase 5 speakers and upgrade to 7 speakers at a later date when 7.1 channel audio on Blu-Ray discs becomes common place. Seven speakers plus a subwoofer isn't a strict requirement and you can happily use 5 speakers attached to the 7.1 AV receiver/amplifier.


Connections

In AV amps that use HDMI "Switches" (not "Repeaters"), the HDMI input is not connected to anything other than the HDMI output . This means the digital audio can only pass straight out the AV amp and into the display device (passthrough). The sound can NOT be heard via the speakers connected to the AV amp. The sound is still digital when it reaches the display device and as display devices generally only deal with analogue audio (2 channel stereo, not multichannel digital), they can not perform digital to analogue conversion on the digital audio and extract the full multichannel digital sound track that may be in the audio stream. You must connect the player to the AV amp with an additional digital audio cable, such as an optical (TOSLINK) cable (Not suitable for streaming HD audio), use multichannel analog input / output jacks (suitable for streaming HD audio, but not always a viable option), or utilise an AV amps that utilises an HDMI "Repeater".

In AV amps that use HDMI "Repeaters" (not "Switches"), the HDMI Repeater is connected to the internal electronics of the AV amp. This means the AV amp can perform the necessary DAC and decoding to the digital signal so that it can be amplified and output to the speakers connected to the AV amp. This means no further connections are necessary from your HDMI source device to the HDMI AV amp as all data can be sent and handled by the HDMI repeater.

S/PDIF transmission aren't compatable with HD audio streaming. Due to bandwidth limitations, neither coaxial nor optical digital connections support multichannel HD audio streams. If you connect your Blu-Ray player to an AV receiver with optical or coax, the audio will "fall back" to Dolby Digital, DTS or two-channel PCM (lossless, but only two channels).

Note that you don't specifically need a receiver equiped with HDMI v1.3. First-generation HDMI provides full-bandwidth playback of Dolby DigitalPlus/Dolby TrueHD and Dts High Res Audio/dts Master Audio signals decoded to LPCM inside the player while HDMI 1.3 (version number refers to the interface on the receiver/amp and not the cable used) can handle/transport Dolby Digital, Dolby Digital Plus, Dts High Res Audio/dts Master Audio and Dolby TrueHD bitstreams (from players equipped with bitstream output capability) directly to AV receivers equipped with Dolby Digital Plus/Dolby TrueHD and Dts High Res Audio/dts Master Audio decoding.

If you lack HDMI connectivity between the player and the AV receiver or the type of HDMI interface on your AV amp is only an HDMI "Switch", you'll need a player and AV receiver with a set of multichannel analog input / output jacks. You'd need the player to perform the decoding in order for it to send the LPCM data to the AV receiver via its multi-channel analog outputs to the corresponding inputs on the AV receiver. Multichannel analog outputs give full-bandwidth playback of Dolby Digital Plus/Dolby TrueHD and Dts High Res Audio/dts Master Audio soundtracks decoded to LPCM inside the player and connected to an AV receiver equipped with multichannel analog inputs. However, most receivers do not apply DSP postprocessing to analog input signals. In these instances, a Blu-ray player that provides bass management (particularly important if you have a sub/satellite speaker system) is preferable.

It should be noted that when sending LPCM rather than the Dolby TrueHD / DTS-HD MA bitstream, the AV receiver's Dolby TrueHD / DTS-HD MA indicator will not light up. This is because the AV receiver is not decoding Dolby TrueHD / DTS-HD MA – the BD player already did the decoding. Instead, the receiver will (correctly) indicate that it is receiving a LPCM audio stream.


The Sony Playstation 3 (PS3): HD LPCM, but no bitstream

If your Blu-ray player can decode Dolby TrueHD or DTS-HD Master Audio, it's said to have onboard decoding for that format. That means the player itself converts the soundtrack to Linear PCM, which it can then be sent to a compatible receiver.

The PS3lacks multichannel analog outputs and lacks the ability to bitstream HD audio to an external decoder, but can decode Dolby TrueHD or DTS-HD Master Audio onboard. This allows the PS3 to send the resulting multichannel Linear PCM (LPCM) stream to a suitably equipped AV amp/receiver via its HDMI interface.

You'll need to output to a device that can read multichannel Linear PCM (LPCM) at 7.1 channels via its HDMI input. If the receiver/amp cannot handle multichannel LPCM, the audio will be output at 5.1 channels instead. If you use LPCM, but output via the PS3's digital optical connection, you are limited to 2 and 5.1 channel sound. The multi-A/V-out (either component or composite video) only supports 2 channel audio.

In order to take advantage of the the lossless codecs with the PS3, you must stream multichannel Linear PCM (LPCM) via the PS3's HDMI output. To do so, go to the PS3's XMB interface:

Settings -> Video Settings

Once there, scroll down to the BD/DVD Audio Output Format (HDMI) and set the PS3 to stream Linear PCM. This will force the PS3 to decode the Dolby TrueHD / DTS-HD MA tracks onboard and send them out as lossless LPCM via the HDMI connection.

You can ensure everything is is set-up correcly by going to:

Settings -> Sound Settings

Make sure the PS3 audio is set to go out over the HDMI connection and that the necessary LPCM formats (24-bits at 96kHz) are enabled.

For more information regarding the PS3's "Audio Output Settings": http://manuals.playstation.net/docum...diooutput.html


New Caveat: The PS3 Goes Slim

The slim Ps3 supports Dolby TrueHD and DTS-HD Master Audio bitstream output to your receiver. The HDMI chip on previous generations of the PS3 didn't support bitstream output of the new(ish) high def codecs like Dolby TrueHD and DTS-HD MA. As such, the PS3 had to decode it internally before sending it over to your receiver via LPCM. PS3 Slim owners can bitstream the HD audio formats and have the indicators for TrueHD or DTS-HD MA light-up on their receivers.

New to the PS3 via Firmware update 3.3: Bitstream (Direct) and Bitstream (Mix) have been added as options under Video Settings > BD Audio Output Format (Optical Digital).


Note: It should be said, as previously noted, that an AV amp/receiver with onboard HD audio decoding abilities will not indicate the resulting input as being anything other than Linear PCM i.e the receiver's Dolby TrueHD / DTS-HD MA indicator will not light up if the HD audio is being streamed as LPCM.