[Correction: Subsequent data has called most of the theoretical speculation below into question. It turns out that SACD's do not have consistently low ISOs, they can possibly have the highest ISOs of all, as I detected with the the 'Poulenc' SACD released by Linn. This is an orchestra with a large pipe organ, and if I had associated low ISOs with higher quality, let alone previously believing all SACD's to have low ISO, I would have believed this to require a minimal 2.0dB headroom for ISOs. But in fact it set my current record...it required a a mind boggling 7.5dB headroom. And it was somewhat inconsistent. It might work with as little as 6dB headroom....or it might not. It apparently produces a peak around the same size as the 96kHz sampling interval, and how high it registers depends on how much of the peak occurs in any one sampling interval. So there is a large random element to how much headroom is required. I've also noticed that on some DVD-Audio discs with 96kHz sampling rate.
It does still seem that Reference Recordings and 'Discipline' (the brand behind the 40th Anniversary King Crimson DVD-Audios) produce consistently low ISOs. I would venture they seem to know what they are doing. But the peak ISO level seems not to depend on DVD-Audio vs SACD. Nor does it even have a fixed relation to music genre (rock vs classical) or does it necessarily have anything to do with the recording "Dynamic Range" or how much compression was obviously used---though it's possible some compression was slipped into the Poulenc at key moments and that's what gave it such high ISOs.]
I have begun fairly systematically copying all my SACD's and DVD-Audios into 24/96 copies for my hard drive. And I'm noticing a few weird things.
Remember that I set my gain structure so that with an 880 Hz tone recorded at precisely 0dB (generated by Audacity) a +4 level would be the highest possible before clipping.
But for actual recordings, I have to set the level between -2.5dB and +2.5dB, giving them at least 1.5dB extra headroom and as much as 6.5dB appears to be needed in some cases. This headroom is required for inter sample overs (ISOs) where in between the samples the signal peaks higher than 0dB.
Benchmark claimed that ISOs could be as high as 3.01dB and that many digital decoders failed to allow sufficient headroom.
BUT I am finding 3dB headroom is way insufficient.
EXCEPT, in some cases it isn't. Some discs seem to cluster around 2dB headroom required, up to 2.5dB in some cases. Which discs are these?
1) Reference Recordings HRx (174kHz/24bit) seem to only require 2-2.5dB headroom. These are some of the best sounding recordings ever.
2) SACD's generally only seem to require 2dB headroom (I would have expected SACD to be the 'wildest,' but in actuality, it's the 'tamest.')
3) The DVD-Audios in the King Crimson 40th anniversary DVD-Audio boxes, which seem only to require 2.5dB headroom. These are truly spectacular in audio quality (or in the case of Court of the Crimson King, merely way better than ever before).
On the other side?
1) Many hot sounding recordings on DVD-Audio, including Elton John, and especially Steely Dan. (I've long noticed that. I figured Steely Dan cranked up the compression so high it's bleeding out the ISOs).
I wrote this off for awhile, but now I'm getting a pretty clear feeling that the recordings which require the least ISO headroom are the ones that sound the most natural, laid back, and 3D. The recordings that require the most headroom sound highly processed.
You may be astonished to see some of the pictures when I post them. Having the huge ISO's means the rest must be scaled back in a recording, losing dynamic range in the midrange.
I think the excessive ISO's occur when the the anti-alias filtering is insufficient, and full scale high frequency garbage gets into the digital encoding. THAT's what's causing so much overshoot.
DSD has such a high rate it captures and controls all the HF crap by design. So SACDs do not suffer from excessive ISO's.
Likewise, the PMI analog to digital encoders used by Reference Recordings, which must have superb filtering.
And whatever King Crimson was using in the 40th anniversary DVD-Audio set.
Now, it would be interesting to know what Steely Dan used in such things as the Everything Must Go DVD-Audio, that produces such high ISOs.
Perhaps it's not the digital process, but the amount of processing (including compression) used before digital encoding that is the culprit. But the biggest ISOs look too big for just that, IMO. Still, the sonic variation might just be coincidental, the least processed recording just happening to use the digital processes which produce the least ISOs.
This is also a function of the Oppo BDP-205, which is apparently not headroom constrained itself. But I've found the height of the ISOs not to change with different reconstruction filters. I think they are inherent in the data and and the degree of oversampling used, with higher amounts of oversampling 'revealing' the true ISO height (because you are filling in more of the points in between the points).
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Update:
Now it appears that that the ISOs in DVD-Audio discs are all over the map. They can be < 2.0dB, for example, in Queen's A Night At The Opera. They can be < 2.5dB such as in every King Crimson 40th Anniversary DVD-Audio box. Or, they can be as high as 6.5dB, as in Steely Dan's Everything Must Go, Elton John's Goodbye Yellow Brick Road, or Frank Zappa's QuAUDIOPHILIc.
SACD's still appear to be consistent at around 2.0dB, and everything from Reference Recordings is there too.
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