Dolby TrueHD, DTS-HD Master Audio, DSD, PCM, FLAC, 5.1, 7.2, … A short explanation of the various audio codecs and their uses.
Terms explained
The Difference between lossless and lossy
There are two kinds of audio codecs:
- lossy codecs like the widely known MP3
- lossless codecs like FLAC
The main reason behind any audio compression is the reduction of file size. An uncompressed audio file found on a regular CD is about 10MB per minute in size; that’s more than 600MB for an hour. With 6 tracks (for 5.1 surround) instead of 2 that size triples. If the resolution is higher than 44.1kHz it takes up even more space on the disc. Before Blu-ray with it’s big capacity of up to 50 Gigabytes per disc, there was no way of fitting surround sound music on a disc without compression.
Lossy compression algorithms actually change the audio file, leaving out things apparently not important for the music or sound. This means, once it is compressed, there is no way of converting it back to it’s original state and there is always a loss of quality. Depending on the bitrate (bandwidth) and the codec used that quality loss can be negligible. Unless you strive for the highest quality possible, of course 🙂
Lossless compression is comparable to zipping a file on a computer. It repacks the data to reduce the file size, but the corresponding decoder can unpack it back to the original audio stream. Nothing is lost during the process, hence the name lossless.
5.1, 7.1, 4.0 – Say what?
To describe the number of speakers in a surround setup, first the regular speakers are listed followed by a dot and the numbers of subwoofers. So 4.0 menas 4 speakers and no subwoofer, while 7.2 means 7 speakers and 2 subwoofers.
The most common setup is 5.1 consisting of 3 front speakers (left, center and right), 2 surround speakers (sometimes called satellites) and 1 subwoofer.
Many home theatres have 7.1 or 7.2 setups for movies, but for music 7.1 is still rare and often just an upmix of a 5.1 recording.
Sampling Rate / Bit Rate
Digital Audio is defined with 2 parameters: the sampling rate and the bit depth. Basically the bits represent the grid used to capture the amplitude (loudness) of a soundwave and the sampling rate defines the number of times a picture of the soundwave is taken. As a technical sidenote: the waveform, once it is converted back to analog, does NOT look like the digital picture that is taken during the analog digital conversion. The digital data gets modulated and filtered to get as close to the original as possible.
In the graph above the red line represents the soundwave (a sine wave in this case), the vertical grid shows the number of bits and the horizontal grid stands for the sampling rate. The digital to analog converter (ideally) converts the digital data back to the red waveform.
CDs are encoded at 44.1kHz with 16 bits and for many there is no reason to increase this, as they believe there is no real benefit in it. In theory more bits and higher sampling rates are better (Blu-ray supports sampling rates up to 192kHz at 24 bits), but there is not many a topic as heavily discussed in online fora as this. I don’t want to go there now, but I’d like to quote sound engineer and founder of the label 2L Morten Lindberg and then leave it at that 🙂
All Audio formats on The Nordic Sound are sample rate converted from the same DXD master. Comparing them in our studio we find only subtle differences from DXD down to 192kHz and 96kHz. The obvious degeneration is from 96kHz down to 48kHz. We find DSD, as used in the SACD format, somewhat different in colour from PCM; in some mysterious way DSD is softer and more beautiful but slightly less detailed. In DXD we find the shimmering brilliance from the original analogue source as directly from the microphones. Linear PCM is offered in addition to DTS-HD Master Audio on this Blu-ray with the purpose of convincing audiophiles of the true lossless qualities of commercial encoding. (…) I personally prefer extremely high resolution PCM over DSD and I would claim that DSD is not as transparent. But it all comes down to what the sound from your speakers can do to your body and mind. I find that the placement of microphones has an infinite more important role in the final experience of music, than the difference between HiRes PCM and DSD. Sometimes a lie can be more beautiful than the truth!
If you want to compare the different sampling rates and codecs yourself, I can highly recommend the Pure Audio Blu-ray The Nordic Sound, from which the above quote was taken. You can buy the disc here
Codecs
Dolby Surround (not used for music)
Dolby Surround, developed by Dolby Laboratories, is mainly used for television broadcast. It’s a stereo file with information about the surround speakers ‘hidden’ in the audio (by means of phase shifting). These so called matrix encoded audio files can be played back on any stereo player, as the ‘hidden’ surround information is simply ignored. The LFE is handled by the playback device (anything below a certain frequency is routed to the subwoofer). A discrete channel distinction all around is not possible with this codec.
On the hardware side (DVD, Media or Blu-ray player) you often see the term Dolby Surround Pro Logic. This is a newer version used to decode the matrix encoded streams. In addition to a superior channel distinction Pro Logic also extracts a center channel. With newer Pro Logic II decoders, even an upsampling from stereo to surround or 5.1 to 7.1 is possible.
Dolby Digital (on DVD, DVD Audio and Blu-ray)
Dolby Digital or AC-3 is a lossy codec, allowing up to 6 discrete audio channels at a sampling rate of 48kHz to be encoded. There are different version of Dolby Digital, like Dolby Digital EX, which hides an additional back center channel in the signal by means of phase shift (same principle as Dolby Surround) for 6.1 or 7.1 output and Dolby Digital Plus with 8 discrete channels for true 7.1 and a higher bitrate (which results in better audio quality).
Dolby TrueHD (on Blu-ray)
Dolby TrueHD is a lossless codec found on many Blu-ray discs. It can carry up to 8 discrete channels (7.1) at 96kHz/24 bit and up to 6 channels (5.1) at 192kHz/24 bit.
DTS (on DVD, DVD Audio, DTS Disc and Blu-ray)
DTS is an alternative codec to Dolby Digital developed by DTS, Inc. formerly known as Digital Theater System. Just like Dolby Digital, DTS is a lossy codec capable of discrete 5.1 surround sound encoding, albeit at a higher bitrate, which is why film lovers preferred DVDs with DTS sound for their better audio quality. There are also different variants, like DTS-ES Matrix 6.1 (same principle as Dolby Digital EX) and DTS-96/24 for a higher sampling rate.
DTS-HD Master Audio (on Blu-ray)
DTS-HD MA is the lossless counterpart to Dolby TrueHD. It can also carry up to 8 discrete channels (7.1) at 96kHz/24 bit and up to 6 channels (5.1) at 192kHz/24 bit.
DSD MCH (on SACD)
Direct Stream Digital (short DSD) was developed by Sony and Philips for use with Super Audio CDs (short SACD). The DSD stream is stored as a sequence of 1 bit values at a sampling rate of 2.8224 Mhz, that’s 64 times the sampling rate of a CD. DSD has a dynamic range of 120dB and a theoretical frequency response up to 100kHz. However, the limit of human hearing and most recording equipment is at around 20kHz.
PCM (on Blu-ray)
Pulsce Code Modulation (short PCM) is uncompressed digital audio, found in stereo on every commercial CD, many concert DVDs and since the introduciotn of Blu-ray, also in surround.
FLAC (download)
Free Lossless Audio Codec (short FLAC) is an open source audio codec, that compresses the audio stream losslessly. A FLAC compatible player converts the file back to the original audio wave form, so no audio quality is lost and it is about half the size of the source file.
And surround? FLAC can handle up to 8 channels at very high sample rates (655350Hz/32 bit) and with it’s popularity growing in the audiophile sector (LINN, 2L among others) the future for FLAC does indeed look bright. On the forefront of innovation is (once again) the Norwegian label 2L. Many 2L surround sound recordings are available as multi channel FLACs, SACD and Pure Audio Blu-ray.
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