DSP Loudness Control
Reply #12 – 2012-03-30 12:12:36
First, the Fletcher-Munson data is not accurate. See the work of S. S. Stevens, much better. Ever notice how a Fletcher-Munson loudness compensation control never sounds right? That's because they got it wrong to begin with. That's true, but you're putting up a straw man. I haven't said which "loudness curve" I based my reasoning on, either here or in the thread I referenced. In fact, I used ISO 223:2003, which was referenced by someone else early in the thread. Is Stevens still about? I haven't seen any work from him for many years. Second, Woodinville is right, it IS a hard problem, it's not a fixed curve, and it is highly dependent on the specific acoustic play level, so it has to be dynamic. Ah, yes, but that doesn't make it hard to solve, at least approximately. Say you increase the level at 1 KHz by 6 dB. The change required to produce a similar perceived level increase at, say, 20 Hz is about 3 dB. This ratio holds true over a wide range of phons. So all you need is a coupled level and bass equalisation control that, for every 6 dB of level reduction, adds equalisation resulting in 3 dB of boost at 20 Hz relative to 1 KHz. (Every time the midrange level drops by 6 dB, the bass level drops by 3 dB). Next...why number them...BIG assumption that everybody mixes to a standard level in a standardized monitoring environment. Not in the music industry! Film, yes, but not music. And that -20dbFS would be nice, but doesn't happen after mastering, especially pop stuff. Not even close. Pretty much have to ignore dbFS in this case, it's not relevant. System acoustic play level is though. But in the context of correcting for differing hearing response at differing levels. You're in no way matching the mix environment, there's just no way to know what it was, and it's not important anyway. Standardised, or at least similar, monitoring levels are more common than you might think once you move up the ladder a bit. Ask Bob Katz, he could "bore for Africa" on the subject. And the -20 dBFS is relevant for monitoring when mastering, it has no relevance to the final released media level. As for matching the mix environment, try it yourself, assuming you have a competent reproduction chain. For most genres other than the highly artificial (electronica etc), there is a definite SPL at which they sound "right". So even though it may not match the mix environment levels, it sounds balanced to you on your system. No, you can't do it based on a volume control setting. Been tried by many people for many years, but it doesn't work. The reason is simple: the correction required is dependent on SPL, which a volume control may influence but doesn't predict and is not the only thing that affects it. Hotter signal into it, and you turn it down, but that would change the compensation inappropriately. There were even attempts to calibrate the compensation by adding another control, but it doesn't work because program dynamics are not fixed. No, the correction must be tied to specific SPL, not a control setting. That's actually where many people trying this messed up. You need two controls. One to set the initial volume level so that it sounds "right", then the coupled control to change the volume to the setting you want to listen at. In theory you would need to do this for each track, or at least each album, but in practice most sources of a given genre and age have similar levels. If you play old vinyl, you should be familiar with the way that the majority of LPs end up being played within a relatively small arc of the volume control. Ditto but different setting for old CDs, and again for current "loudness war" CDs. Apple's Soundcheck and MP3 Replaygain standardise the levels even more. ... And finally, it's been done, and done quite well. It's called Audyssey Dynamic Volume and Dynamic EQ. Rather than base their idea on existing loudness research, their algorithm is based on what was essentially reverse-engineering human loudness perception. They took LOTs of data on lots of subjects, with lots of different program material and the result is pretty darn good. The big advantage is, once an Audyssey system has been calibrated it knows the exact SPL at every moment regardless of volume control setting or variations in program material, so it can apply the right correction dynamically. Pretty darn smart, those guys. ... and missing the point when it comes to music dynamics. Chris acknowledges that the Audyssey dynamically changes the EQ in response to changing program levels. But this is exactly what you do not want when listening to music. As I said elsewhere: ".... Take Ravel's "Bolero". The double bass initially comes in while the levels are still moderate. The loudness of the bass is chosen to be audible but not overpowering. As the piece progresses and the overall levels rise, the bass level also rises but still in proportion to the rest of the players - if the overall level rises by 10 dB, the bass level rises by somewhat less. The point is that "loudness compensation" is built into music by the composers / musicians / mix engineers, and if you make a static adjustment to the volume, you only need to make a static adjustment to the loudness compensation. The rest is already taken care of in the music. " And why I think loudness compensation is needed: "... In my opinion, music is best listened to at the SPL at the listener position that it was created for. (Creation may mean the original performance, or the engineer's creation of a mix of separate components recorded at different times in different acoustics - or no acoustic at all in many cases.) If we normally listened at this SPL, there would be no need for any loudness compensation. But we do like to listen at different levels for several good reasons, and when we do so we no longer hear the intended tonal balance. Many of us like to adjust the tonal balance at our chosen listening level so that it is similar to the perceived tonal balance at the "correct" level. Done properly, we find this adjustment effective and pleasing. It is an effective mitigation of the degradation forced by having to listen at a different level to that which the work was intended for. ..." And on the topic of tonal balance change with level: "... In the specific case of loudness compensation, we aren't correcting for human hearing deficiencies. We're compensating for deficiencies in the reproduction environment. In a "live" situation, if we move away from the source we experience an overall level decrease. In addition, the treble decreases somewhat faster than the midrange, and the bass somewhat less. We perceive this as a natural tonal balance change, which needs no correction. In a reproduction scenario, if we reduce the volume by a similar amount, the tonal balance does not change. Compared to a distance increase, we have too much treble and not enough bass. We perceive this as unnatural. This is why I believe in leaving the HF compensation alone and just boosting the bass. The natural change in HF sensitivity of the ears, as illustrated by the "loudness curves", takes care of the required additional HF attenuation, so only the bass requires compensation. ..." I suggest you read the referenced thread in DIYAudio, if you haven't done so already. All of the points you raised were also raised there.