Optimize bitrate and size preserving high audio quality in tracks, podcasts, tunes with AQuA!

April 15, 2011 by
Filed under: Voice and Sound Quality Testing Software 

AQuA Music is the tool that will help you to save up to 40% on bitrate and up to 50% on audio size when encoding your audio files!

AQuA Music is based on Sevana’s patented technology (AQuA Wideband) for perceptual evaluation of audio signal quality.

When considering digital recording and digital playback, sound quality depends on the range of sound signal, the rate at which it is sampled, and various conversions that occur in sound reproduction system. In lossy codecs like MP3, sound quality is a factor that determines how much of the sound signal data the encoder is allowed to loose in order to reduce file size. In MP3-encoded signals for instance the quality is defined by its bitrate, in kilobits per second (kbps).

The frequency range of sound (in Hertz) which equipment is capable to sample and reproduce affects sound quality. Humans can hear frequencies ranging from about 20 Hz to approximately 20 kHz, so sampling that does not extend far enough will take effect on the sound quality.

Sound signal wave is continuous and has some real value at every instant. The digital quantization of the analogue sound wave means that much of the continuous sound wave is not recorded. The rate at which the sound is sampled refers to the amount of information the detection equipment records for each second of the sound. The higher the sampling frequency, the more accurate the final samples will be. However, there is a mathematical proof that in order to preserve all frequencies in the original sound it’s enough that the sampling frequency is two times higher than the highest frequency in the sound signal spectrum. Thus for example when dealing with speech one can easily use sampling frequency of 8kHz, because human speech spectrum is limited by 4kHz and telephone channel spectrum by 3.4kHz.

When recording a podcast, grabbing music from your CD into MP3, OGG or AAC, uploading an audio book to your iPhone or dumping your favorite tracks on a memory stick to play while driving, we always face the same problem – we want to make the file smaller but sound as good as ithe original. In many cases when one usess MP3 encoder the choice he or she makes is simple – to use 320kbps and that will surely preserve the quality. In many cases it may work, but what if you store thousands of tracks? What if millions? What if you are going to transmit your podcast over the Internet? What if your audio is transmitted over a mobile network? In such cases every megabyte saved turns into real money saving and the lower the bitrate preserving desired audio quality the better performance of your service.

The way it works is very simple. AQuA has a perceptual audio quality model that acts as a very well trained human ear that can hear even tiny changes in audio quality and tell how much quality was lost due to audio processing. AQuA returns two types of scores:

- percentage of quality similarity compared to the original audio
- MOS (Mean Opinion Score), which is a value between 1 and 5

MOS Quality Impairment
5 Excellent Imperceptible
4 Good Perceptible but not annoying
3 Fair Slightly annoying
2 Poor Annoying
1 Bad Slightly annoying

So, the process of choosing the best audio compression parameters (OGG, MP3, AAC compression optimization) when optimizing f.e. bitrate with Lame MP3 encoder is split into the following phases:

1. set minimal bitrate = X
2. set bitrate incremental step = S
3. compress original audio into MP3 with bitrate X
4. decompress MP3 into uncompressed WAV
5. test quality comparing original WAV and uncompressed MP3 using AQuA Wideband
6. if the quality is lower than MOS 5 increase bitrate so that new bitrate = X + S and go to step 3
7. if the quality is higher or equals MOS 5 then we have found optimal bitrate for this audio

Let’s use MP3_Torture_Test.wav taken from this location: http://recording.org/daw-pro-audio/28203-mp3-encoder-torture-test.html

We are going to use mp3opt.bat file to automate bitrate optimization and the result we get is:

“% = 97.59″
“MOS = 5.00″
“Bitrate = 104″
“Passes=13″

So, we preserve 97.59% of original file quality if encode MP3_Torture_Test.wav using Lame encoder with the following parameters:

lame.exe -b 104 MP3_Torture_Test.wav MP3_Torture_Test.mp3

In the post where we took the MP3_Torture_Test.wav from 128kbps gives a very good quality according to the poster, but we could save 24kbps on bandwidth and about 400K on the file size!

lame.exe -b 128 MP3_Torture_Test.wav MP3_Torture_Test-128kbps.wav.mp3

WAV 15882228 Bytes Bytes Ratio
128kbps 1441122 11
104kbps 1080842 15

Now it’s time to check how much quality was preserved in the audio compressed with 128kbps, so we use AQuA again:

aqua-wb tst.lic -mode files -src file MP3_Torture_Test.wav -tstf MP3_Torture_Test-128kbps.wav -acr auto -npnt auto -miter 1 -enorm on -grad on -mprio on -tmc on -ratem %m

Here is the output:

Sevana Audio Quality Analyzer – AQuA-Wideband v.5.3.11.720.
Copyright (c) 2009 by Sevana Oy, Finland. All rights reserved.
—————————————————————
test license
—————————————————————
File Quality is
Percent value 97.52
MOS value 5.00

Amazingly, but the quality is the same as at 104kbps! And if it sounds the same (you are welcome to download these files and listen to them yourself) why to spend extra space and bandwidth?

And now another application of AQuA Wideband – audio transcoding. Let’s compress MP3_Torture_Test.wav using Lame with 320kbps keeping in mind that this is a typical choice of those who want to preserve maximum of quality.

AQuA shows us that at 320kbps (MP3_Torture_Test-320kbps.wav.mp3) we preserved

aqua-wb tst.lic -mode files -src file MP3_Torture_Test.wav -tstf MP3_Torture_Test_320kbps_mp3.wav -acr auto -npnt auto -miter 1 -enorm on -grad on -mprio on -tmc on -ratem %m

Sevana Audio Quality Analyzer – AQuA-Wideband v.5.3.11.720.
Copyright (c) 2009 by Sevana Oy, Finland. All rights reserved.
—————————————————————
test license
—————————————————————
File Quality is
Percent value 99.50
MOS value 5.00

almost 100% of audio quality. Let’s now transcode this audio file and check what lowest bitrate we can use and how much quality we’ll preseve from 99.5%

“% = 97.53″
“MOS = 5.00″
“Bitrate = 104″
“Passes=13″

And we got a pretty much the same result from tanscoding. Of course when dealing with uncompressed original audio we have a better chance to avoid compression artifacts, but still it is possible to transcode with a lower bitrate and audibly the same quality. Of course compared to the original uncompressed audio we may get a different result:

Sevana Audio Quality Analyzer – AQuA-Wideband v.5.3.11.720.
Copyright (c) 2009 by Sevana Oy, Finland. All rights reserved.
—————————————————————

test license
—————————————————————
File Quality is
Percent value 97.41
MOS value 5.00

But it is still of a high quality level.

WAV 15882228 Bytes Bytes Ratio
320kbps 3602807 4.4
104kbps 1081469 15

After transcoding we saved 216kbps bandwidth and 70% of space!

What’s in it for me?

Private customers (AQuA Service):

• Optimized audio CD grabbing
• Increased storage on MP3 player, iPod and mobile devices
• More audiobooks to fit the same storage
• More tracks to fit the same memory stick
• Simplified quality based personal podcasts encoding – optimized bitrate and audio size

Corporate customer (AQuA Server):

• Tremendous space saving on large audio files
• Space and bandwidth saving on podcasts, tunes, audio books hosting and streaming
• Optimized audio for transmission over communication channels
• Bandwidth saving on mobile network audio streaming
• Automated audio encoding with pre-defined quality level

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Comments

4 Comments on Optimize bitrate and size preserving high audio quality in tracks, podcasts, tunes with AQuA!

  1. Robert Ojok on Wed, 20th Apr 2011 10:58
  2. Brilliantly informative!

  3. admin on Wed, 20th Apr 2011 11:20
  4. Thank you!

  5. Ove Larssen on Mon, 25th Apr 2011 20:28
  6. Can this process take only 2-3 passes to optimize audio?

  7. admin on Mon, 25th Apr 2011 21:01
  8. Ove,

    It is surely possible! One just has to play a little bit to figure out optimal step to increase the bitrate, or to do audio pre-analysis. Please contact us via web form to get details http://www.sevana.fi/contact_us.php

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