Signal Levels

                        Audio signal levels.

Audio signal or sound levels are measured using a decibel (dB) system. The dB system is used to compare two levels or a change in signal voltage or power. Sound signal level in dB can be considered either as a power or as a voltage. The level in decibels is 10 times the logarithm of the ratio of two power levels. Where P is the measured power in watts and P Ref. is a reference power in watts. Sound signal level in dB can be considered as a voltage ratio. V is the measured voltage and V Ref. is a reference voltage.

dBm: decibels or dB referenced to 1 milliwatt (.001 watt)

dBu or dBv: decibels or dB referenced to 0.775 volt (dBu is more commonly     used)

dBV: decibels or dB referenced to 1 volt 

These dB measurements all provide a dB value that indicates the level of the audio signal. The "m" indicates a dB reference based

upon 1 milliwatt. The dBu or dBv indicates the same reference which is 0.775 volts. You will see dBu used much more often. The "V" indicates a dB reference based upon 1 volt. 

Analog & Digital Audio signal levels.

Digital audio levels are measured differently than analog audio levels. Of cause another and different dB system is used. The dB system in digital audio starts at the top and defines the loudest sound level that is to be digitized. This top or full scale view of the audio levels results in a full scale or "FS" system of dB measurement. A 0 dBFS measurement unit is to be the highest audio level. Assuming this is to be at the highest audio level before clipping occurs, this corresponds to an analog level of 24 dBu. Therefore, 4 dBu (dBu =dBv) is the same as - 20 dBFS or 0 VU. In the same manner as an analog level meter indicates audio levels, a digital audio meter indicates the level of both the left and right channels.

Field Descriptions:

1. VU Reference Level: Scales the digital signal level dB FS to which the meter's OVU point is referenced. The level can    be adjusted from 0 to 24. This scales the 0 VU from 0 dBFS to -24 dBFS. (Standard setting would be "20 Ref" resulting in a -20dB FS = 0 VU)
2. VU Graphic Bar Level Meter: Indicates VU level of left and right digital audio.
3. VU Numeric Level Meter: Indicates VU level of left and right digital audio.
4. Peak Program Meter: A single bar the bounces above the main bar indicating peak program levels.
5. Maximum Peak Level Meter: Captures and updates the maximum or peak level of the audio program material.
6. Max Reset: Resets Maximum Peak Level Meter. Highlight field and click to reset.  

Impedance - Impedance is the ability of a cable to impede electrical signal flow. Impedance changes over frequency, because it is determined by the Inductance and capacitance of the cable, which, in a coax cable is determined by the ratio of diameter of the center conductor, the diameter of the shield and the dielectric constant of the insulator in between them. The common spec you will see advertised for coax cables is the "characteristic impedance". For audio signals which are relatively low in frequency, the characteristic impedance is meaningless, since the wavelengths of the highest frequency signals are usually thousands of times longer than the cables carrying them. Because of this, reflections are not a factor. As frequency increases and the wavelength's actual length traveling through copper wire approaches the cable length, characteristic impedance becomes very important. This is very generally in the MHz range, and why characteristic impedance is so important for video and digital audio signals. The Characteristic Impedance is the impedance at witch, with matching input and output impedances there will be no reflections on the cable. The output device and input device should match the characteristic impedance of the cable to minimize reflections and signal loss. Of course, in the real world, no cable is perfectly uniform in size and shape, so small imperfections in the manufacturing process can have a real impact on cable performance, especially as frequencies increase. .