Monday, 12 September 2011

Mulichannel Audio

Below is a very interesting article from the Bang & Olufsen technical team re Dolby DTS surround sound

Multichannel Audio
People familiar with movies in the cinema or on DVD-Video will know that “surround sound” is comprised of what is known as 5.1 channels of audio. The 5 channels are sent to the Left, Centre, Right, Left Surround and Right Surround loudspeakers, and the .1 channel refers to the LFE (Low Frequency Effects) channel which is usually produced by a subwoofer. This channel is called a .1 channel since it has about one-tenth of the frequency content of the other 5 channels.
The current high-performance medium for distributing films is BluRay which is a disc capable of supporting up to 8 channels of audio. This is typically used as a 7.1-channel system which includes the 5.1 channels of DVD-Video plus an additional 2 channels, probably used for extra surround information. There are different options for the placement of the two extra loudspeakers, depending on the movie’s mix.
In order to get the 7.1 channels of audio from your BluRay player into your surround processor - a name for a very fancy volume knob with a switch to allow you to choose between your BluRay and other players,- you can either connect 8 separate wires that send analogue audio signals between the two devices, or one single cable called an HDMI cable which supports not only the 8 channels of audio, but also the video signal for your television. If the audio signal is sent on the HDMI cable, then it is transmitted as a digital signal, a stream of 1’s and 0’s, referred to as a PCM (Pulse Code Modulation) signal. PCM is probably the most common method of using a digital signal to represent an audio signal. It is the method used in compact discs, but in fact has been used for decades before the introduction of the CD. When the PCM digital audio signal on the HDMI cable contains more than one channel, it is known as a “multichannel PCM” signal.

Audio Compression
In order to send a large document via email, it is common to compress the file in advance, usually using a .zip format, to reduce its size for sending. You create the document, “zip” it to shrink it, and send it. The person receiving the file “un-zips” it to return it to its original size and content. The same is true for audio on a BluRay – in order for all 7.1 channels to fit into the storage available on a BluRay, the audio signals must be compressed to make them smaller. There are a number of compression formats that are available with different levels of quality for fitting the audio on the BluRay disc. Some of these compression systems are “lossy” which means that they throw away information to make the storage requirements smaller. Some of the systems are “lossless” which means that, although they reduce the amount of storage space required, they do so without any loss in information.
Two examples of these “lossless” compression systems are Dolby TrueHD and DTS-HD Master Audio

Dolby TrueHDTrueHD is Dolby’s system for lossless compression and is optional as a format on BluRay movies and players. This means that not all players are able to decode a movie that uses the TrueHD compression algorithm, and that not all BluRay movies have audio that uses the format. In order to ensure that, in either of these cases, you will still be able to hear audio from your film, the soundtrack is always available in Dolby Digital format, the same lossy compression format used on DVD-Video, as a “backup” solution.
Some quick facts about Dolby TrueHD:

- it is a successor to the Dolby Digital, also known as “AC-3”, lossy compression format
- it can support up to 14 channels of audio, although most films are mixed in either 5.1 or 7.1
- if higher-resolution audio is used, then the number of channels is reduced. 8 channels of 96 kHz, 24-bit audio or 6 channels of 192 kHz, 24-bit audio


DTS-HD Master Audio
The competitor for Dolby TrueHD is from DTS (Digital Theatre Systems) and is called DTS-HD Master Audio. It is an extension of the original DTS lossy compression format available on some DVD-Videos. This means that the original format forms the “core” of the new format. If your player cannot play the DTS-HD Master Audio format, it will play the portion of the audio signal that is in the DTS signal, however, more capable players can decode the entire stream and give you a lossless stream equivalent to the original signal.
Some quick facts about DTS-HD Master Audio:

- it is an extension of the DTS compression format
- like Dolby TrueHD, if higher-resolution audio is used, then the number of channels is reduced (8 channels of 96 kHz, 24-bit audio or 2 channels of 192 kHz, 24-bit audio

Summary
It is important to not confuse “multichannel PCM” with either Dolby TrueHD or DTS-HD Master Audio. The latter two are the compression systems used to fit the audio on the distribution medium, like BluRay, for example, whereas multichannel PCM is the format that is probably used to transport audio on your HDMI cable between devices in your home. For example, it is likely that your BluRay player reads either the Dolby TrueHD or the DTS-HD Master Audio file on the disc, decodes it, and then sends it to your surround processor on the HDMI cable using multichannel PCM digital audio.

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