[MLA] Loudness example with distortion on Sound Check

[Ian Shepherd]

So it looks as if I was wrong about iTunes preventing positive gain changes that would cause clipping, sadly.

The example file Bob L. sent is indeed boosted by 8.8 dB when Sound Check is enabled.

What’s strange though is that the peak level output from iTunes is no higher than 0 dBFS. I’ve checked now with several files, and asked some contacts to confirm. 

iTunes seems to be hard clipping the output, as far as I can tell - which is strange, because I’d always understood it was 32-float throughout, these days. It’s a shame if that’s true, since I’m pretty sure Core Audio uses floating-point, in which case the clipping could be avoided for most users who don’t run the system at full volume all the time.

Can anyone shed any more light on this ?

Ian





> On 13 Oct 2018, at 21:22, Ian Shepherd <ian at mastering-media.co.uk> wrote:
> 
> 
> Hi Bob,
> 
> Interesting example, I’ll take a listen, thanks.
> 
> FWIW I think iTunes is intended to prevent positive gain that would cause clipping, although there are some bugs. 
> 
> Another example of why we need lower normalisation reference levels sooner rather than later, though...!
> 
> Ian
> 
> 
> 
>> On 13 Oct 2018, at 17:35, Thomas Lund <thomas at lund.one> wrote:
>> 
>> Hi Bob,
>> 
>> That’s still an excellent example, and points to the danger of allowing positive gain, especially when having a bad limiter in the system.
>> 
>> cheers,
>> Thomas
>> 
>> 
>> 
>> 
>>> On 13 Oct 2018, at 18.21, Robert Ludwig <gatewaybob at mac.com> wrote:
>>> 
>>> Hi Ian,
>>> 
>>> As you like to write about Loudness I wanted to give you something I came across before you joined the group.
>>> 
>>> We were once on the lookout for real world examples of tracks that needed boosting on iTunes Sound Check and that boosting would cause distortion I came across this example in my iTunes library.
>>> 
>>> Attached is a 5:26 Victoria Mullova & Kristian Beuldenhout Beethoven Violin Sonata movement on iTunes that peaks at -2.8 dBFS on the left channel at 4:26 on the 3 loud chords.  
>>> 
>>> Sound Check says it needs to be raised to +8.8dB!  It technically has 6dB of distortion at that peak if Sound Check was being used.
>>> 
>>> BS-1770 says it has a Program Loudness of -27.7, so if iTunes had used ITU as a measurement it would have raised it even more!
>>> 
>>> Best,
>>> 
>>> Bob Ludwig
>>> 
>>> 
>>> <02 Victoria Mullova & Kristian Bezuidenhout.m4a>_______________________________________________
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