PCM is one of the most used audio codecs; most computers (as well as many devices) use it as
their default for audio output/input. All audio heard on this device, whether from a file, or
a microphone input, is in PCM, and that is used to regenerate an approximation of the original
analog sounds.
When you make a digital audio recording using a mic on a system that is using PCM for audio
output/input, the audio stream is PCM. When you losslessly save that recording as WAV, FLAC,
WMA Lossless, ALAC, AIFF, and etc, there is no encoding process, the audio stream is just put
into the new container file, it's still the PCM stream your system interpreted from the analog
real-world sounds.
Lossless formats can be divided into 2 types: compressed, and uncompressed. Uncompressed formats
like WAV and AIFF just store the PCM audio stream. Compressed formats like FLAC, ALAC, and WMA
Lossless run the stream data through a compression algorithm to save space, like a zip archive.
The data isn't changed, it's just stored more efficiently; it's still encoded as PCM.
If you then take the lossless export, and convert it to a 128kbps MP3, there is re-encoding involved.
Encoding means organizing the audio stream data in a new way; this is a lossless process. However,
there are no codecs to my knowledge that do this, because it would be pointless. The file would
sound the same, playback would require the audio to be decoded which means more system resources are
needed, and it would be the same size as the original file. Therefore, codecs like MP3, WMA Lossy,
AAC, Vorbis (OGG), and etc also perform additional operations. A common operation being to discard
data deemed to be of lesser importance. Discarding data results in a smaller file size.
The below isn't a perfect analogy, but it gets the point across I think.
Let's say that a "I hate you Sarah!" written on a piece of paper represents the PCM audio stream in
the WAV file you losslessly exported after making a recording.
Encoding that data in a different encoding format would be equivalent to jumbling the letters up
to say "h Iyae oSr! haatu". The decoding software knows how this codec arranges data so it can unjumble the
message.
However, as discussed, formats like MP3 also discard data during the encoding process. So the MP3
message would be more like "h I Sr! h tu", and when decoded (unjumbled), it would be like
this "I h t u S r h!". If you read this back, you still get the message, but excluding letters
does alter the sound a bit. The more you exclude, the worse it gets, until you reach a point where
you can no longer understand the original message.
The MP3 audio stream represents the original PCM audio stream. When you play the MP3, it's being
decoded back to PCM, but the discarded data doesn't return obviously. It sounds plausible in this
example to add back in the missing letters, but remember, computers aren't as smart as us, and
this is a very very very simple example.
If you were to convert the MP3 to a WAV file, technically you are decoding to PCM, and then saving
the PCM stream, which is why the WAV is bigger than the MP3. There is no data loss because the MP3
stream was always just a representation for a PCM stream. The codec specification tells the decoder
how to decode the MP3 back to PCM format. If you were to convert the MP3 to a 128kbps AAC, what is
actually happening is the MP3 is decoded to PCM, and then the PCM stream is encoded as AAC. The
encoding process from PCM to AAC results in further data loss, because "I h t u S r h!" is treated
as the original message. Notice the letters aren't squished together. When the AAC file is made, it
won't know spaces from excluded letters aren't part of the message when trying to figure out what is
safest to discard, which is why re-encoding data at the same bitrate results in quality degradation.
6Generally speaking it is only decoded, then stored as the uncompressed. All the losses occured when stuff was tossed out to make it a MP3. A better way to say it is "there is no more loss". If you have a Jpeg, and decompress it to the full bitmap (which occurs in most paint&processing programs) you dont have losses added, because it is just decoded. If you then go to save it out as something that does toss stuff to make it smaller , then you are re-encoding it, and will have loss again. If you were cross-coding it to another lossy format, it would be de-coded and en-coded again with loss. – Psycogeek – 2014-12-03T07:46:40.083
5Conversion between PCM containers in only lossless if and only if the sampling frequency and the resolution/bit depth is exactly the same. Even resampling to a higher frequency or resolution results in a lossy result. – agtoever – 2014-12-03T11:33:01.597