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seanadams
2003-11-21, 10:07
"PCM" stands for "pulse code modulation" - it is a generic term for how
digital audio is usually stored and transmitted. "Raw PCM" means that
the data being sent is simply a stream of audio samples.

WAV and AIFF are essentially the same thing, they just have different
header formats - the header contains informations which tells the
playback software how to interpret the stream (how many channels, how
many samples per second, and how big each sample is).

Squeezebox uses two data connections to communicate with the server,
one for control information and one for the audio stream. When playing
raw PCM, the server tells the client "go get this URL, and play it back
in 16-bit, 44KHz, 2 channels". Then the client connects to the server
and starts playing the stream. The server takes care of extracting the
raw PCM stream from whatever format you're playing, be it WAV, AIFF,
lossless compressed (Flac) or lossy compressed (AAC, Vorbis).

MP3 is a special case in that it can be decoded client-side. I suppose
we could also decode it server-side but there's not much point except
perhaps to do more advanced processing like cross-fading or
equalization.



On Thursday, November 20, 2003, at 06:21 PM, Adam Arthur Bier wrote:

> When you say it's sent as PCM, do you mean the stream is in AIFF or
> WAV format or another format? I ask because I'm contemplating encoding
> a bunch of AIFF/WAV files on the server as FLAC by hand, and then
> picking up the stream with iTunes as a client (iTunes doesn't have
> support for FLAC, but it does for AIFF & WAV streams--if your PCM
> stream is formatted as something altogether different, I guess I'd
> want to change the server mapping settings so as to transcode, or
> whatever is done to FLAC files to convert them to PCM, the FLAC files
> to AIFF or WAV).
>
> On Nov 19, 2003, at 11:52 PM, discuss-request (AT) lists (DOT) slimdevices.com
> wrote:
>
>> From: Sean Adams <sadams (AT) slimdevices (DOT) com>
>> Date: November 19, 2003 8:19:29 PM EST
>> To: SlimDevices Discussion <discuss (AT) lists (DOT) slimdevices.com>
>> Subject: [slim] flac
>> Reply-To: SlimDevices Discussion <discuss (AT) lists (DOT) slimdevices.com>
>>
>>
>>
>> That's correct, squeezebox will play decoded flac without transcoding
>> - it's sent as PCM, and the whole path is lossless - what comes out
>> the S/PDIF is what was on the CD.
>>
>> We are investigating the possibility of running FLAC client side, to
>> save network bandwidth. I've estimated the CPU requirement for our
>> processor and I believe it'll work, but the proof is in the putting,
>> as they say.
>>
> --
> Adam Arthur Bier
> New York University School of Law
>
>