I am using the compressor block and am wondering how the values are obtained in dB.
I'm assuming the following formula is used:
V_dB = 20*log10(V/Vreference)
What is Vreference?
Voltage cannot be expressed in dB, because decibel is a relative unit, one can only measure gain (or loss) using decibels. Voltage can be expressed in dBV for example. Since dBV means "volts relative to 1 volt" your Vreference will be 1 volt. According to the formula 1 V will be equal to 0 dBV; 2 V will be equal to +6 dBV and 0.1 V will be equal to -20 dBV.
Yes. So the question is, what exactly does Sigma Studio use as the reference voltage.
The diagram below shows the nominal full-scale signal levels at the input, inside, and output of a ADAU1701:
The setup below demonstrates how the compressors work. A test signal that switches between 0.5 peak (-6dB) and 2.5 peak (+8dB) is applied to a compressor. The peak value of the compressor's output is captured by the Envelope follower and appears on the Real-Time Display.
I set up the compressor as a limiter at 0 dB. When the test signal jumps up to +8 dB, the compressor's output follows it momentarily before catching up, regulating down to 0 dB. The -6 dB input level is below the limiter threshold and remains at -6 dB.
In the good old days, 0db was always 0.775Vrms.
I wonder what made the industry change that?
I also wonder if 0.9V is a standard in digital audio or just that only AD is following this scale.
The 0.775V standard (1mW into 600 ohms) = 0 dBm remains the standard for interconnections between professional audio boxes. Inside the boxes and at the circuit board level, impedances are rarely 600 ohms and engineers design each stage for maximum performance regardless of any standard level. Thus at each point within the equipment, "0 dB" may be defined as the maximum achievable voltage before clipping -- which depends on DC supply voltage and such. Thus chip manufacturers, including AD, specify their inputs and outputs by the maximum voltage they can handle. Any dB specifications will be relative to this level. For example, if the output can manage 0.9V max, and quiescent noise is 0.1 mV, the S/N would be rated as 20 log (0.9 / 0.0001) = 99 dB.
Inside the DSP there are no volts at all, only numbers. SigmaDSPs are fixed-point, with a number of bits from 5 to 8 before the binary point, and 23 or 24 bits afterward. Where a numbers' magnitude spills over from after the binary point to before it, is a logical place to spot the 0 dB level. This also corresponds to the clipping point for the chip's ADCs and DACs if included.
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