-=> Digital Man wrote to tracker1 <=-
DM> By the normal definition of "lossy" versus "lossless" audio encoding,
DM> the pulse-code-modulation (PCM) used on CD-Audio discs (and .WAV files)
DM> is considered "lossless". There is no compression of the data
DM> whatsoever (e.g. 60 minutes of silence consumes exactly the same number
DM> of bits as 60 minutes of music on a CD-A disc).
DM> FLAC is a compressed lossless format, so 60 minutes of silence produces
DM> a much smaller FLAC file than 60 minutes of minutes, but still, it's a
DM> lossless compression.
The test of lossy versus lossless is whether the reconstructed waveform (or in
the case of codecs, the sequence of data bits) is identical to what was encoded
in the first place. If the result of compression and decompression is the same
as the input, then the compression is lossless. If the result is different,
the compression is lossy. CDs are pure PCM, just a digital representation of
the audio waveform, and by definition lossless (within the sampling bandwidth
of 20 kHz).
The trick with lossy compression is (the codec) knowing what to throw away,
while leaving the resulting data still useable for its intended purpose. For
example, you can generally throw away a lot more to transmit intelligible
speech than you can to transmit reasonable sounding music. Some of the
extremely high compression "vocoders" used on speech circuits can generate a
lot of artifacts (and sound a bit like a robot - try listening to 700 bit/sec
Codec2 encoded speech! ;) ), but they still produce reasonable speech quality
for communication purposes.
DM> MP3, AAC, and many other formats are "lossy" because they throw away
DM> audio data (it gets "lost") in the encoding process. How much of the
DM> data is "lost" and how important that data is to the fidelity of the
DM> audio depends on the encoding scheme/processor, the target bitrate,
It's a tradeoff between audio quality and data rate. Having a choice of
bitrates allows you to find the best balance.
... An Elephant; A Mouse built to government specifications.
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■ Synchronet ■ Freeway BBS in Bendigo, Australia.
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