[MLA] Lossy encoder clipping

[Thomas Lund]

Companeros,

With lossy encoders, clipping can happen in the M/S domain, even if e.g. the S signal is nowhere near clipping, but the M component is. I’ve played several listening examples of that on panels with you, Bob and Bob. Sample rate converters is another place where you can’t take pre-attenuation let alone headroom for granted.

Better to stay below 0 dBTP, and to keep checking M/S.

cheers,
Thomas



> On 12 Oct 2018, at 18.34, Bob Katz <bobkatz at digido.com> wrote:
> 
> Thanks, Eelco, for that confidential information. You have a good contact at Spotify. Keep on pushing them to stop that egregious peak limiting!
> 
> Even though with a floating point codec, the clipping is NOT officially "baked in", I would always advise engineers to protect the path ahead of the encoder. It's the safest and probably the best place to deal with future clipping down the line. Apple performs the encodes and adds metadata. Engineers and producers do NOT send AAC files to Apple. And you won't find Apple changing data, even if it would prevent clipping down the line. Apple would NOT attenuate a master in front of the encode. This is a mastering engineer's prerogative. 
> And the chances of expecting a playback device to inspect ahead of time for overshoot during a streaming session are slim and none. Think about it, the real time decoder would have to have foreknowledge that the codec was going to overshoot, and know ahead of time to attenuate the whole song before it had started. It's just not practical. And I don't believe in adding peak limiting to already-mastered and approved material. At the least it changes the producer's intent, at worst it distorts and just goes one step closer to sucky broadcast-style processing.
> So I would not change my practices because of this discovery. Prevention is the best way to a cure.
> 
> 
> Bob
> 
> 
> 
> On 10/12/18 12:10 PM, Grimm | Eelco wrote:
>> Hi Kevin and Bob,
>> 
>>> Robert from Fraunhofer was just complaining because Ian claimed that the clipping was "baked in" to the encoder. Which is untrue to the best of my knowledge. The encoder/decoder are in a closed floating point relationship and so if you intercept the decode in the floating point domain, attenuate it and dither it to 24 bits and then send it on to the DAC or file or whatever, there will be no clipping. 
>>> 
>> 
>> I got a reply from my Spotify contact. Please treat this as confidential information!
>> 
>> "I've also found out the float/int situation, and unfortunately it was as I feared, we're using int:s throughout the pipeline. The decoder(s) is/are float, but we clamp them to int right after the decode step. This is also, for sure, on our roadmap to change, but no promises on timeline there either, I'm afraid."
>> 
>> So at the moment there’s not much they can do. But at least they are aware of the problem and the opportunities now.
>> 
>> All the best,
>> Eelco
>> 
>> 
>> 
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> 
> -- 
> 
> If you want good sound on your album, come to
> Bob Katz 407-831-0233 DIGITAL DOMAIN MASTERING STUDIO
> Author: Mastering Audio              
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> 
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