Audacity appears to be increasing the bit rate?

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Re: Audacity appears to be increasing the bit rate?

Permanent link to this post Posted by Submariner » Fri Jan 13, 2012 1:36 am

Yet again thanks Steve,

Just a thought: If this Project Phonobox V is 16 bit.
A] Is it still a good thing to bring it into Audacity at 32-Bit do all the processing Click & Pop reduction and then do a one time final conversion to 16-bit with triangular dither on?
Or
B] should I keep it in 16-Bit from start to finish. Sounds like as Audacity does all it's processing in 32-bit, it would still be advantageous to do the former?
Any views
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Re: Audacity appears to be increasing the bit rate?

Permanent link to this post Posted by steve » Fri Jan 13, 2012 2:09 am

If you are only doing cut/paste/trim type editing, then it is (marginally) better to keep everything in 16 bit and (temporarily) turn dither off.
If you are doing any "processing" (which includes such things as "fade in/out", "Amplify", "Normalize", "Noise Removal" ...) then it is better to work in 32-bit (float) and leave dither enabled to help with the final conversion to 16-bit.
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Re: Audacity appears to be increasing the bit rate?

Permanent link to this post Posted by Submariner » Fri Jan 13, 2012 12:57 pm

Hi Steve
Thanks again for all the knowledge - saved e loads of time trialing different approaches , plus if your not sure what your doing - you never if the result is not as good as another or you did something wrong.
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Re: Audacity appears to be increasing the bit rate?

Permanent link to this post Posted by Makaveli84 » Sat Apr 06, 2013 3:25 pm

steve wrote:"Dither" is a technique used to improve the conversion quality when down-sampling from a high bit format (bit depth) to a lower one.
http://en.wikipedia.org/wiki/Dither

In the case that waxcylinder is describing, the audio is being converted from 32 bit float to 16 bit.
16 bit audio is an "integer format" - that is, it uses whole number values between 0 and 65,535 (well actually +32,767 and -32,768), whereas 32-bit float can also represent fractional values which provides for much higher precision.

Clearly,when dropping down from 32-bit float to 16-bit integer there needs to be some method for "rounding" the sample values.
The most simple way to do this is to either round up, round down, or round to the nearest whole number.
As you can probably imagine, compared to the 32-bit waveform, the 16-bit waveform will be less smooth and there will be small "steps" appear where there were previously smooth curves. This is referred to as "quantize error" and it is a form of distortion that adds a little harshness to the sound.

It has been found that when rounding the 32-bit values down to 16 bits, this type of distortion can be eliminated by adding some randomness to the rounding. For example, when rounding 76.5, rather than always rounding to 77, it will sometimes round to 76 and sometimes round to 77.

The added randomness is called "dither" , and what it is doing is adding a tiny bit of "random" noise which helps to "smooth out" the steps between 16 bit values.

Even though it is helping to smooth out the steps and reduce the audible artefacts of quantize errors, it is obviously desirable to make the dither noise as unobtrusive as possible. To do this, the noise can be "shaped" so that frequencies in the noise are distributed into ranges where they are least noticeable. One of the most effective ways of doing this is with "triangle dither". Another good one is simply called "shaped dither". Both of these forms of dither produce good results.

Note that to minimise the dither noise, it should only be applied once, otherwise there can be a noticeable build-up of what should be insignificant noise.
If all of the processing is done at 32-bit and the audio is converted to 16-bit right at the end, then it will only be during that final conversion step that dither will be applied. The disadvantage of using 16 bit audio tracks in Audacity is processing is done at 32-bit, so dither will be applied with every processing step.


I'm sorry to resurrect this old thread, but regarding this issue, I have come across a 16-bit audio track that is being compressed with a medium quality variable bitrate compression to 198 kbps with shaped dither on and 220 kbps with dither off. How can this be explained in terms of Audacity's dither and compression encoder??
And what is advisable in this case, dither off, dither on, or avoid mentioned codec?
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Re: Audacity appears to be increasing the bit rate?

Permanent link to this post Posted by steve » Sat Apr 06, 2013 4:29 pm

Makaveli84 wrote:I'm sorry to resurrect this old thread, but regarding this issue

No problem - you could have started a new topic, but your question is related to this topic and I don't think that it has been covered already.

Makaveli84 wrote:I have come across a 16-bit audio track that is being compressed with a medium quality variable bitrate compression to 198 kbps with shaped dither on and 220 kbps with dither off. How can this be explained in terms of Audacity's dither and compression encoder??

Variable Bit Rate (VBR) encoding attempts to retain as much of the audible information as possible while keeping within a range of bit-rates. When the audio is easy to compress the bit rate is reduced (which reduces the file size and overall bit rate) and when the audio is more complex the bit rate is increased so as to retain more information. VBR is not concerned about the precise overall bit rate as long as it is within the defined range.

If we take a simple case of "silence". Absolute silence can be compressed to a very low bit rate with perfect sound quality - this is one place that VBR encoding can drop the bit rate down to minimum and save a lot of "bits" without compromising sound quality.

Now consider the case of "dithered" silence. The LAME encoder is pretty smart, but not smart enough to know that dithered silence is supposed to be inaudible so the bit rate is raised a little so that the dither is preserved in the MP3. This may seem like a waste of "bits", and in the case of absolute silence it probably is, but silence is usually only a tiny proportion of most recordings.

If we take a more typical case of a music recording, Audio that has been exported with dither will have low level, high frequency dither randomisation throughout the recording, and LAME will use some bits to encode these high frequencies, thus an MP3 file that has been exported with dither may be a few percent larger than if exported without. In the example you gave, the difference was 200 kbps rather than 198 kbps, which is a 1% increase (probably not enough to worry about).

Dither probably does not have as much benefit for MP3 files as for WAV files, because the MP3 encoding is reducing the sound quality anyway, but it does still reduce the harmonic distortion and increase the dynamic range, so if you can afford the extra 1% file size then I'd say that it is worth using dither.
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Re: Audacity appears to be increasing the bit rate?

Permanent link to this post Posted by Makaveli84 » Sat Apr 06, 2013 5:15 pm

Steve,
Thx for the reply. However, there have been a misunderstanding it seems.
In my case, the bitrate difference was around 20 kbps (i.e 10% in this case, and not 1%). And the other misunderstanding, and the most disturbing part, is:

- NO DITHER ~= 220 kbps
- WITH DITHER ~= 198 kbps

One detail however is that those bitrates are for 5.1 audio, which means less bitrate difference per channel, but still, the idea of lower bitrate on dithered audio remains!
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Re: Audacity appears to be increasing the bit rate?

Permanent link to this post Posted by steve » Sat Apr 06, 2013 5:25 pm

It also remains that MP3 encoding is not exact. VBR encoding aims for a "quality" within a set "range" of bit rates, and works out the exact bit rate on the fly.
I've found that dithered audio is usually a couple of percent larger in VBR MP3 than not-dithered audio, but that is not necessarily the case - it all depends on how LAME can achieve the best encoding within the selected encoding settings.

So that we can look at the specific details of what you are doing, perhaps this discussion would be better in your other topic: viewtopic.php?f=46&t=71864
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