[MLA] Lossy encoder clipping

[Grimm | Eelco]

Hi Kevin,

Interesting point. The question then becomes how many fixed point based subband decoders are still around (in percentage of the total). Another question is whether the path from the decoder to the volume block (that likely will perform the normalization) is floating point. 

All the best,
Eelco

> I'm pretty sure integer decoders are still in use. Not on PCs or even smartphones but in smaller audio appliances (e.g. Bluetooth speakers). In any case, you've got to get back to integer at some point to feed the DAC. So you either give the encoder the extra headroom as Spotify is suggesting (see Ian's blog) or you attenuate after the decoder as you've suggested. Maybe you end up doing both conservatively in which case you've lost perhaps 6 dB compared to 0 dBFS PCM which is obviously not a small discrepancy.
> 
> Kevin Gross - AVA Networks
> 
> 
> On Mon, Oct 8, 2018 at 7:45 AM Bob Katz <bobkatz at digido.com <mailto:bobkatz at digido.com>> wrote:
> Ian, it's been a while since I performed this test. I would have to retest to be sure, but to the best of my recollection, the clipping is ONLY on the repro side and only when converted to fixed point. Remember that all the codecs (both encode and decode side) are 32 bit floating point. 
> If you run an attenuator in the floating point domain after the decoder, you can fix any over issues which would manifest as clipping when converted to fixed. Test it yourself. Run Apple's AU lab with the round trip codec and Bitter placed in critical places. 
> Then put a 24 bit dithering plugin and run a distortion test on a sine wave to make sure it's not an illusion. If you guys don't perform this test before the weekend, I'll try to do it myself again. 
> 
> Best wishes,
> 
> Bob
> 
> 
> On 10/7/18 7:39 PM, Ian Shepherd wrote:
>> Hi All !
>> 
>> My recent blog post about lossy encode clipping has had quiet a bit of attention:
>> 
>> http://productionadvice.co.uk/spotify-upload-true-peak/ <http://productionadvice.co.uk/spotify-upload-true-peak/>
>> 
>> Including an email from Robert Bleidt @ Fraunhofer. He’s asking if I have specific examples of encodes that have “baked in” the clipping, because his position is that it shouldn’t happen, these days.
>> 
>> I’d like to give him a helpful reply and I’ve experienced problems myself in the past with encoding from Logic and Audacity, for example, but the truth is I don’t have a collection of examples on hand to offer him.
>> 
>> Do any of you have recent examples that I can point him at, or know for sure of current encoders that consistently have issues with extremely loud content ?
>> 
>> Thanks for any suggestions,
>> 
>> Ian
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