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Digital Audio Anatomy

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Digital Audio Anatomy

Analog audio works by converting sound pressure waves into positive and negative voltages. In an analog to digital conversion, the audio is captured over time and stored as digital values.

Sample Rate

This is how frequently a digital sample is “taken” of your analog waveform. This will directly influence the frequency response of your audio capture. For simplicity’s sake, think of frequency as pitch. The sample rate must be double the frequency you’re capturing. Humans can’t hear beyond 20kHz so theoretically 40khz would be sufficient. (Humans generally hear around 20Hz–20kHz.) For dialog do not worry about recording above 48kHz.Common sampling rates include:44.1khz48 khz96 khz


Bit Depth

This controls the number of discrete levels your analog waveform is divided into. Sort of like dividing a $100 dollar bill into 2 $50 bills or 10 $10 bills. Similar to picture, the more levels the more detail can be produced. This directly affects the dynamic range (difference between softest and loudest sounds) you can capture. With sound, the effect of bit depth is particularly apparent at the noise floor because the increased quantization means greater separation between signal and noise.16 bit is a common audio bit depth.24 bits is worth the increase in file size because of aforementioned noise floor. If you have the option to record 24 bits then use it and record at a lower level, giving yourself headroom before clipping. Higher than 24 bit doesn’t hold much value.

In this image the bit depth is four so we have 16 possible values (4^2).


Input Levels

Digital audio is measured in dbBFS or “decibels below full scale”. A digital audio signal is peaking at 0–it only measures sound as values less than the maximum it can record which is 0. Digital levels are always negative. You’ll always want to keep your peaks below 3DBFS. About -12dBFS is a very general level to aim for recording dialog, but if you can, record at 24 bits and at a softer level (e.g. -18 dbFS) to maintain headroom.DB (decibels) is a relative scale and has no quantitative meaning. A sound isn’t simply 50 decibels, though it could be 50 decibels louder than another sound. A 1dB level change is generally noticeable and a 6dB level change is perceived as being twice the volume. dBu or dBv: references decibels to .775V. Frequently seen on VU meters, it’s an analog scale where 0 is a reference level, not a peak level. 0VU=+4dBu=-20dBFS. 0DBFS = 24dBu.dBV: referenced to 1 volt and usually seen on “semi-pro” gear. 0dBV = 1 VoltVU: Analog audio is measured in “Volume Units” where 0dBVU is 1.228 volts if you really want to know. This doesn’t affect us much for the purposes of digital audio though it helps us understand some plugin user interfaces and mastering tools.Gain StagingAn analog audio signal exists at different levels which must be managed at every point in the chain.

Mic Level

The level generated by most microphones.1.5mv–70mv (1 millivolt is a thousandth of a volt)

Line Level

Level used for most audio mixers.Pro line level is +4DBu (1.23 volts) (4 decibels above 0dbU)Consumer line level is -10dBV (again, the “V” here means relative to 1 volt so this signal is closer to .32 volts or -7.8 dBu)(Keyboards and guitars are generally between mic and line level).

Level Measure

RMS is the average sound level, verses “peak” levels which represent the loudest levels. A limiter lets you bring RMS average up without clipping/distorting the peaks.

Speaker Level

10 voltsThis is an output level designed for monitoring–the very end of the chain.


Preamp

The quality of the pre-amplifier (“preamp”) affects your sound quality. This is the amp that boosts the weak mic-level voltage from your microphone. The preamps included inside prosumer DSLRS are notoriously mediocre. You can plug a very nice microphone into a DSLR but you’re still limited by the preamp. One option is to use a ‘hot mic’ with a strong enough level that the preamp isn’t actually amplifying much. In this case you turn your camera’s preamp to its lowest setting. Another option which adds bulk but increases control is to use an external preamp. This usually comes with additional benefits: physical gain controls, phantom power, XLR inputs.


Phantom Power

Most professional mixes will have a +48 volt “phantom power” for powering condenser microphones.An audience is often enormously forgiving of picture quality, but not so with the sound, making bad audio a curse of amateur filmmaking. 

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