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Comparisson of Audio Compression
I've
compiled a table containing the same audio sample compressed into
different bit rate using several common compression techniques. Namely
MP3 (constant bit rate and VBR,) windows media audio, ogg vorbis, AAC
and flac. My intention is
that
you can use this to work out the lowest bit rate at which the audio
sounds clear to you. this will depend on the equipment you're using,
and your hearing. I firmly believe that if you can't hear the difference
then there is no point wasting space.
I
discuss re-encoding from one format to another below
Notation:
k
means kilo which is 1024. b means bit. B means Byte
Bandwidth
As
you can imagine this page is quite bandwidth intensive, for me as well
as you. I'd ask you to be sparing. That is to say if you've decided
that
MP3 at 192kbps sounds the same to you as the original file then there
is
probably no need to download all the higher bitrates too. Finally i
would ask that you try and only download the larger files once. Right
click 'Save As...' and save them to your desktop. If you would like all the
files you can download them in one 57 meg zip file from rapidshare here.
Details
of the sample and the various encoding techniques used can be found
below the table together with some
thoughts....
Below the table is a Blind Test,
it's been
pointed out to me that some of the differences you can hear maybe
because you already know, so it seemed worth while to create a test
where you listen blind.
|
MP3 |
WMA |
AAC |
Ogg
Vorbis |
FLAC |
WAV |
| 32kbps |
150kB
CBR VBR |
158kB |
151kB |
|
|
|
| 40kbps |
186kB
C V |
|
187kB |
|
|
|
| 48kbps |
222kB
C V |
235kB |
226kB |
220kB |
|
|
| 56kbps |
258kB
C V |
|
260kB |
|
|
|
| 64kbps |
295kB
C V |
305kB |
298kB |
297kB |
|
|
| 80kbps |
368kB
C V |
379kB |
375kB |
370kB |
|
|
| 96kbps |
440kB
C V |
453kB |
441kB |
437kB |
|
|
| 112kbps |
513kB
C V |
|
510kB |
509kB |
|
|
| 128kbps |
585kB
C V |
601kB |
589kB |
587kB |
|
|
| 160kbps |
731kB
C V |
749kB |
734kB |
740kB |
|
|
| 192kbps |
876kB
C V |
896kB |
888kB |
882kB |
|
|
| 224kbps |
1,021kB
C V |
|
1,038kB |
1,013kB |
|
|
| 256kbps |
1,166kB
C V |
|
1,185kB |
1,149kB |
|
|
| 320kbps |
1,457kB
CBR |
|
1,485kB |
1,146kB |
|
|
| 467kbps |
|
|
|
2,110kB |
|
|
| 968kbps |
|
|
|
|
4,384kB |
|
| 1.4Mbps |
|
|
|
|
|
6,389kB |
A
Blind Test: I've also added a blind test, the first 12
seconds of the
sample has been taken from the 64, 128, 192 and 320 CBR MP3
files and
converted back to wav so the files appear identical, the first 12
seconds of the uncompressed file is also included, each file is 2meg.
Can you tell which
one is which??? The full 37 second versions can be downloaded from
rapidshare HERE
(30meg)
The Sample (also WAV
file):
The Sample contains three songs: Dido – My
Lover’s Gone, Daft Punk – Short
Circuit and R.E.M. – Everybody
Hurts the Tracks were ripped from cd using CDex 1.70 beta 2.
And joined using Audacity 1.2.4.
The sample is 37.086176 seconds long.
MP3
The CBR MP3s were encoded using the Lame MP3 Encoder Version 1.32
engine
3.97 Beta 2 (MMX), constant bit rate (CBR), J-Stereo, quality factor 2
(high) this is my encoder of choice. The variable bitrate MP3s were
encoded with lame.exe 3.97 with the default options left inplace apart
from the --abr option. the target abr was adjusted to match the file
sizes of the cbr.
Variable rate encoding adjusts the bitrate many times a second
depending on the complexity of the sound currently being encoded. There
are two methods of variable bitrate encoding available, one uses target
quality and the other uses a target bitrate. The target quailty method
will produce a consistent quality, the file size changing depending on
the complexity of the sample, the target bitrate method will produce a
consistent size, the quality varying with the nature of the sample. it
is the target bitrate method used here, to provide a
comparison to the CBR encoding.
The target quality method uses a notation -V0 (best) to -V9 (worst), i
also encoded the sample using this method, the resulting file is
directly comparable to a target bitrate file of the same size. The
bitrates are given here:
-V0= 232kbps, -V1=
200kbps, -V2=
182kbps, -V3=
152kbps, -V4=
138kbps, -V5=
126kbps, -V6=
114kbps, -V7=
98kbps, -V8=
89kbps and -V9=
66kbps
It's worth stating explicitly, if it is not obvious, that these quality
level / bitrate comparisons are unique to this file, though the may
server a a rule of thumb guide for other files.
WMA
Microsoft's Windows Media Audio encoder v9.2
AAC
This is not a codec i know much about but many of you will be familiar
with it because it is iTunes' native format. the wav file was converted
using 7.6.0.29, AAC, CBR. I haven't included apple's lossless
format, simply because i do believe them if they say it's lossless. but
the conversion resulted in a file of 4,081kB corresponding to around
882kbps. notably better than FLAC.
Ogg Vorbis
Ogg Vorbis 20051117
The audio was encoded using the quality factor technique rather than
artificially specifying minimum maximum and nominal bit rates, as this
is recommended
for ogg vorbis. The quality factor (shown) was then adjusted so the
file size most closely matched the mp3. it is then compared to the
mp3’s bitrate. The last Ogg file is a quality factor of 10,
this is the maximum quality for an ogg file.
FLAC
The FLAC codec allows a compression level from 0 to 8. I encoded the
sample with all of these levels which resulted in files with bit rates
from 901kbps (level 8) to 968kbps (level 0). Since this represents a
variation of less than 10% i have only included the one file of
compression level 0. I should mention that the only difference is the
encoding time.
Re-Encoding (Transcoding)
It is generally accepted that re-encoding from one lossy form of
compression to another, (transcoding) is undesirable as each algorithm
will have it's own set of rules when it comes to eliminating different
aspects of the source audio, meaning you get the 'worst of both worlds'
you have sounds eliminated by both codecs.
Worse than this, the second codec will waste effort trying to mirror
the aspects of the first codec that may be undesirable. For instance if
you have that familiar mp3 squelch in the first file the second codec
must try to reproduce this in the same way that if you copy a video
tape to dvd the dvd must use up valuable bandwidth in
reproducing
the VHS noise.
Whilst I've always taken this as fact i thought it would be interesting
to demonstrate it so i converted the above 96kbps mp3 file to wma, then
to ogg, then back to mp3. Compare the first and last files.
MP3
> WMA > OGG
> MP3
Conclusion
When not really a conclusion so much as some thoughts.
It's immediately clear that at very low bitrates the MP3 codec does not
produce the clarity of WMA, AAC or Ogg Vorbis. This may lead you to
conclude
that the MP3 codec is inferior. However, the codec was not designed to
work at this level, and you would never use it so. to form a valid
comparison you need to simulate real world conditions and listen in
your usual way....
One more thing it is also immediately apparent that the MP3 codec is
infinitely better when used in variable bit rate mode. It is designed
to work this way, and you do have to wonder why so many people encode
in cbr.
Nigel Coldwell 2011
LINKS
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