What are the iTunes digital audio levels in and out

Question

I am having a lot of trouble with finding out and setting the audio levels in and out of iTunes.
Digital audio has 16bits at '1' with an analog signal level of +12dB, 24bit has all'1's at +18dB. Making the overhead capability for CD 16bit 12dB and that for HD 24bit 18dB.
I would like to record (using SoundStudio) a set of standard audio tones from 20-20kHz with digital levels of "all 1's" 16bit, or +12 dB analog level, "all 1's" 24bit ditto, and the same for levels of analog 0dB (-12dB digital) for 16bit and 0dB(-18dB digital) for 24bit.
What I am finding is that if I get Sound Studio to inert a tone at what it calls 0dB, save it as AIFF, then the output it from iTunes I get a much higher level, around +8dB or so!
Any ideas?

Answer 1

As an addition to my first question. I asked the makers of Sound Studio about levels. They tell me that the Mac standard is to have all '1's in the digital stream defined as 0dB.
Which helps as I can now record a tone at the CD level of -12dB and the HD audio level of -18dB. And if I make recordings I know to keep the input levels well into the green, not yellow and definitely not up to the red.
As you say the output of iTunes is boosted, and from my measurements this boost is in the analog circuitry and is about +8dB.
I woul dnot recommend turing down the iTunes volume control as this brings in the digital volume reduction, which depends on the algorithm they have implemented. This is not so bad for iTunes as the Audio Core software works at 32bit and can handle 16bit volume changes fairly well. But on the iPad it is different as iOS is only 16bit audio so the volume algorithm is poor. So always leave the volume at maximum and turn down your HiFi volume control!
Or use a digital output to an external DAC as the iTunes volume control is then disabled. Set Audio MIDI for 24bit output and you are ready to go with HD Audio.