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Digital Compression - was New SSM

Subject: Digital Compression - was New SSM
From: madl74
Date: Wed Jul 10, 2013 2:14 am ((PDT))
> Are there any hints to "minimum quality" that works best on SoundCloud?
> Which is the "best compromise"??

I thought I would have a go at explaining audio digital compression, not to 
be confused with audio volume compression. 

Lossless compression 

There is a lot of repetition in an audio data stream such as a WAV file. 
About half of the space can be saved by only storing the difference between 
one sound sample and the next. An example is FLAC (Free Lossless Audio 
Codec) and if you want the full works on that see Wiki. 

FLAC is a suitable format for original files on Soundcloud which will make 
better use of your free allocation of data than WAV. The problem with WAV 
files is that they are huge, and often more efficient compression algorithms 
are needed such as MP3. 

Lossy compression 

MP3 is the standard audio compression format but there are other similar 
formats like The "open source" Ogg Vorbis without copyright restrictions and 
as such is used by Wiki. These work by analysing the frequency content of a 
track and reconstructing it later. 

MP3 and OGG are primarily designed for music compression, and the good news 
is that they work well for birdsong. They work in a similar way to an 
audiogram, sampling chunks of data at intervals for frequencies, then 
decoding by reversing the process. There are small hiccups between samples, 
but the ear does not hear these at high sample rates. 

Where MP3/OGG fail is with random sounds like background noise and water. On 
compression they have to allocate frequency bands and they reproduce these 
later as tinkly notes. At 64Kb/s this can be very noticeable. 

I've accumulated gigabytes of audio recordings as I haven't taken the time 
to edit then all down, all recorded MP3 at 320Kb/s. First, I listened to 
various WAV recordings and compressed copies at different data rates and 
compared them on a sound editor by cross-switching with the original. 

WAV to 320Kb/s MP3 was inaudible on both tuneful and random sounds, so I 
master on that format. This also means that I can record long runs on my 
fixed mics and select interesting bits afterwards which I would have 
otherwise missed. 

At 192Kb/s, I heard a difference on background noise but my gurgling stream 
sounded fine. Without an A-B cross-switching check, it would be hard to 
define the loss of quality. 

At 96 Kb/s, I reckon most music is OK, but random sounds are getting tinkly. 
Compressed random noise starts to sound like a gurgling stream. 

BBC radio online is at 64 Kb/s and OK for general listening but is a bit 
rough. I don't know what the compression is with various pocket devices, but 
I've heard dictaphone recordings which are unpleasantly tinkly. Low 
compression rates can store a lot of pop and keep youngsters happy. Like 
vinyl enthusiasts, they would probably miss the distortions on a high 
quality feed. :-) 

David 

David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce 










"While a picture is worth a thousand words, a 
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.



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