# Query about decibels and the voltage standard

I’m reading the voltage standard, specifically:

Signals should typically be 10Vpp​ (peak-to-peak). This means that audio outputs should typically be ±5V (before bandlimiting is applied), and CV modulation sources should typically be 0 to 10V (unipolar CV) or ±5V (bipolar CV).

Absolute decibel measurements (e.g. for VU meters) should be relative to 10V amplitude. For example, a ±10V signal is 0 dB, and a ±5V signal is approximately -6 dB. You may alternatively use dBV for measurements relative to 1V amplitude.

I would expect the ±10V signal to have an amplitude of 20V, and the ±5V signal an amplitude of 10V, corresponding to +6dB and 0 dB respectively (rather than 0 / -6 dB as stated). Am I missing something?

Unfortunately, the word amplitude on its own is not specific enough. The second paragraph you quoted would be consistent with ‘peak amplitude’, where your expectation is ‘peak-to-peak amplitude’ .

I think your expectations are reasonable, but I guess the specific examples in the voltage standard are there to try to clarify things.

Also the reference to VU meters in the text is pretty misplaced, since a VU meter reading is never in reference to either peak or peak-to-peak amplitude, it’s in reference to volume units, hence the name.

Decibel readouts should be based on distance from zero, divided by 10V, in order to be consistent with other Rack modules. In other words, instantaneous dB is 20 \log_{10}(x / 10V) so 10V \rightarrow 0 \,\text{dB} and 5V \rightarrow -6.02 \,\text{dB}. Typically you’d want to display decibels over time, so I personally always use a “leaky peak” filter that gradually falls exponentially but rises immediately.

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The drop in level between ±10V and ±5V is -6dBV. The 0dB reference point is not established so it is hard to say if it’s above or below 0dB. A good practice is to remember that dB is meaningless on its own, it always relates to another unit.

I think the documentation is just trying to provide context for a potential application (eg. making a VU meter). As David points out, it really depends on what is being used as a reference level in the context of a VU meter (10dBV=0dB as answered by Andrew)…

However, in the context of a VU meter in VCV Rack, that doesn’t state what this means in the digital domain (dBFS). In the context of a VU meter, I would assume people use -18dBFS as the ref point for 0dBVU… although, as in hardware, this may vary (±6dBFS).

Some corrections to be picky:

A drop is a ratio, which is unitless, so it’s dB, not dBV.

Ratios are meaningful. (But I get what you’re saying.)

Actually the 10V \,\text{amplitude} = 0\, dB standard can be said to be 0 dbFS. Although there’s no maximum/minimum voltage in Rack, Rack’s audio interfaces map [-10V, 10V] \rightarrow [-1, 1] so \pm 10V will be the “full scale” of the voltage range and therefore be called 0 dBFS.

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haha, picky corrections are good… avoids confusion in a topic which is confusing enough as it is!

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Btw, I learned the hard way that doubling voltage isn’t exactly 6db. It’s very close, but not exactly 6. I wasted half a day on that recently.