Volume Display, 0dB, Reference Level - how is it all related?

[MDS]

Volume Display (absolute vs relative), 0 dB, Reference Level - what does it all mean and how is it all related?

These topics come up frequently and usually in the context of the relative volume display (negative numbers). I usally chime in and attempt to explain the concepts but it occurred to me that the reason some people have trouble wrapping their head around these concepts is that they mix and match the different concepts when they can mean slightly different things in different contexts (especially '0 dB').

This will be my attempt at defining the terms and hopefully make it easier to understand.

0 dB

0 dB means different things in different contexts:

- 0 dB Sound Pressure Level (SPL): Silence

- 0 dBFS (0 dB Full Scale): The maximum or 'peak' value of a digital audio sample, in other words all ones. A digital audio sample can never exceed 0 dB; when sampling analog audio to create digital samples, if the value necessary to represent any particular time slice of the analog audio waveform would exceed the maximum value the sample can hold (eg. +32767 or -32768 for 16-bit audio), it gets 'clipped' to 0 dB. Too many successsive 0 dB samples in a row (usually four) will light the clip indicator in any audio editor.

- 0 dB or 'unity gain': the point at which the receiver is not attenuating (reducing) the input level from the source component at all. All receivers have such a point, regardless of whether it uses an absolute or relative volume display. Modern receivers are decibel accurate and regardless of whether you are counting up from zero (absolute display) or counting down from zero (relative display) each increment changes the output SPL by 1 dB.

- Average level: The average level across the entire set of samples, often called the RMS level. The ear responds to average level and is how we perceive 'loudness'. Any particular sample value (or the average) is represented by negative numbers; ie, the 'distance', if you will, below 0 dB. The less negative number, the louder the sound is perceived to be and conversely the more negative the number, the softer the sound is perceived to be.

A modern CD with an average level of around -12 dB is MUCH louder, at the same volume setting, than an older CD with an average level of -18 dB. Movie soundtracks are typically mastered to an average level of around -30 dB (which not coincidentally is the average level of the test tones your receiver provides for 'calibration'). These concepts will be important when it comes to defining 'reference level' and what do the negative numbers on the relative volume display mean.

Reference Level and Calibration

A reference level is the PEAK level (in terms of output SPL in decibels) when the digital audio sample is also at its PEAK (ie '0 dB'). You can use any level of your choosing but generally when people talk about 'reference level' they mean Dolby Reference Level which has a well-defined meaning (from Dolby) and that is 105 dB (115 dB for LFE but the receiver automatically adds the +10 dB boost for the LFE channel).

So how do you get reference level? By the process of calibration and that is where the receiver's test tones (or an audio calibration disc's test tones) come in and where it is important to understand the prior discussion of digital audio levels.

The receiver's test tones are broadband pink noise and normally are at a level of -30 dB, which is a THX specification (and as mentioned above is the average level of a typical movie soundtrack). Some setup discs use a level of -20 dB, which is a Dolby specification. The level of the tones you use is only relevant when it comes to which number you want to see on an SPL meter when performing the calibration.

So...to get Dolby Reference Level, you run the test tones and adjust the channel trims on the receiver up or down as necessary until each channel reads roughly 75 dB on the SPL meter. Because the average level of the test tones is -30 dB, doing so yields 30 + 75 = 105 dB PEAKs when the digital audio sample peaks.

In other words, the majority of the time the SPL you hear will be 30 dB below the peak of 105 dB but when any sample is at a peak (again '0 dB'), you'll get blown out of your chair as the SPL instantly rises to 105 dB.

Also keep in mind that when you did this calibration routine, the average level of the test tones was -30 dB. This means that if you were to play a source whose average level is much higher than -30 dB, the output SPL will be much greater. A receiver calibrated using -30 dB tones will be much louder when playing a CD with an average level of -12 dB at the same volume setting.

But what number do you need to set the volume display on the receiver to in order to be listening at reference level? That's where the absolute vs relative volume display comes in.

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.

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...

[fightinkraut]

Bookmarked, I'll be passing this one on whenever I get these questions. Thanks!

[jostenmeat]

This = Sticky.

[1hagop]

this = sticky.

+1........