Absolute vs Relative - what's in a scale?
Some receivers only use the absolute scale which counts up from zero to some other positive number - such as 0 to 100. Some receivers only use the relative scale, which counts up from a negative number through 0 to a positive number slightly higher than zero, such as -80 to + 20. Some receivers, namely higher end Onkyo models, allow you to choose either display and also switch between the two whenever you want.
The use of any particular volume display, especially on receivers that allow you to choose which to use, is equivalent. But there are reasons, mostly for convenience, that the relative display is preferred - and that is mainly related to the reference level and calibration discussion.
So what's in a scale and why choose any particular one?
Absolute (say 0 to 100): It is simple and matches up perfectly with the notion that 'volume' is a numeric quantity. 0 is no sound and 100 is a whole lot of sound.
B]However, it is not convenient for determining the output SPL. It is still decibel accurate and moving the volume control from 50 to 60 still increases the output SPL by +10 dB. But when performing calibration you need to pick some number on the display to be your 'reference volume', such that when the volume display is showing that number you get the 'reference level' discussed above.
Typically you'll need to choose a number that is near 80% of the range because lower numbers might not let you get to the required 75 dB when playing a -30 dB test tone, simply because the channel trims don't have enough range.
Say you pick the number 80 when the scale is 0 - 100. If you turn the volume control so that the display reads 60, then the output SPL will be 20 dB below reference level but it is not convenient to note that (if you even care). You have to do the mental gymnastics and think, my reference volume is 80, but I'm currently listening at 60, so 80 - 60 = 20 dB. I'm actually listening at -20 dB from reference levels.
Relative (say -80 to +20):
Our -80 to +20 scale is absolutely identical to the 0 to 100 absolute scale. Remember Onkyo receivers let you switch at any time and the manuals tell you explicitly that 82 on the absolute scale is the equivalent of 0 on the relative scale.
But the relative display is far more convenient. If you use '0' as the reference volume setting, then the display indicates directly how far above or below reference level is the output SPL. If the volume number is -10, it is 10 dB below reference level (95 dB peak SPL); if it is +10, it is 10 dB above reference level (115 dB peak SPL).
Many people refer to the '0' on the relative scale (where 0 is not the max) as '0 dB'. It is 0 dB in this sense: it is the volume number you have to turn to so that when a digital audio sample peaks, the output SPL will also peak to the level you set during calibration.
Relative, with '0' being the MAX
This is fundamentally no different than the absolute scale either but is not found on most surround AVRs for the same reason that the absolute scale is not convenient for calibration. If '0' is the max range of the volume control, you have to pick a more negative number as your reference volume point and then you're back to doing calculations in your head to determine the output SPL, but this time it's even worse because now you're doing those calculations with negative numbers!
If you have a receiver that offers either the absolute scale or the relative scale where 0 is the top of the range, the question of which to choose comes down to this: do you see the glass as half-full or half-empty? Do you want the display to show how much of the range you've already used or how much of the range is left to be used?
This concludes our tour of the arcane terminology of digital audio and home theater..
