This term is known primarily for describing the quality of lossy compressed audio (eg MP3). Unfortunately, this makes it even more difficult than with channel-separated compression, the bit rate is split between the two channels: stereo MP3 (not to be confused with dual channel) with a 320 kBit / sec bit rate. uses only 160 kBit / sec per channel.
Or different bit rates per channel: set stereo (mono signal calculated with additional stereo information), on the other hand, it works with volume differences between the two channels and therefore can use the bit rate much more efficiently .
DAC, bit rate
What exactly is the obviously so essential bit rate? For a CD based on the Red Book standard, the bit rate is calculated as follows: 2 channels * 44,100 Hz sample rate * 16 bit depth per sample results in 1,411,200 bits / sec., That is, 1.4112 Mbit / sec. Bit rate. Obviously, this is considerably more than what compressed formats (should) provide. You can see how the compression processes work: last but not least, they more or less cleverly reduce the bit depth per sample at a given sample rate of 44.1 kHz, for example. The amount of data decreases, and that is exactly the goal of every data compression: halving the bit rate means exactly halving the amount of data.
As an example, I have compared some common file formats for digital audio:
Format
Codec (s)
Multichannel
Sampling rate
Bit depth or resulting bit depth from bit rate
Compression
/
subject to acoustic losses
Wav
PCM et al.
yes
any
any
Optionally, depending on the codec, also lossless
AIFF
PCM et al.
Not
any
any
Not
FLAC
FLAC (Free Lossless Audio Codec)
yes
0.001 kHz-655.350 kHz
4, 8, 16, 20, 24, 32
without losses
Apple loses
less
MP4
ALAC (Apple Lossless Audio Codec)
yes
0.001 kHz-384,000 kHz
16, 20, 24, 32
without losses
MP3
MPEG I Layer 3 in various incarnations
as B. LAME
Not
8-48 kHz
8-320 kBit / sec. CBR / VBR or 640 CBR for free MP3 format
with mandatory loss
Most readers will know that lossy compression is often based on psychoacoustic models or natural limitations of human hearing: what humans cannot (should) hear is not stored in the musical signal and is irretrievably lost, apparently not. you need it. The most popular example of this is the old MP3 format already mentioned. For some audiophiles, lossy compression is by definition useless for serious music listening, regardless of whether or not they would notice the loss of compression. For others, the sound is 320 kBit / sec. MP3 encoded for pop music are identical to CDs, they are satisfied.
DAC bit rate
Lossless compression, on the other hand, has become increasingly popular since Internet bandwidths and storage capacities have steadily increased. An example is the FLAC format, which fortunately is also “open source”, which means that it can be used freely and even changed in terms of the program. Meanwhile, most of the time it is directly compatible with proprietary audio hardware, so FLAC files can be played without the help of a computer, and in some cases even created (ripped CDs). As the table above shows, FLAC supports very high sample rates and bit depths, as well as multi-channel sound.
With FLAC, the audio signal is encoded based on computable fixed-point algorithms that conserve computational power, in which blocks are formed step by step and stereo separation is converted to mid-side separation and performed the remodeling of the signal with differential storage. No information is lost, it is stored more efficiently than, for example, on a CD; Depending on the complexity of the audio signal, compression rates of up to 30% can be achieved.
FLAC
It should be clear once again that FLAC or MP3 are file formats and therefore cannot be directly compared to the PCM of the Red Book encoding of a CD or DSD (see next section). Let’s leave it at that on the subject of data compression.
