Bit Depth and Bit Rate

When you export the song to disk you have the following options in GarageBand 10.0:

Use the "Share" menu > "Export song to Disk":

Please be aware that Bit Depth and Bit Rate are two completely different things!

They belong to a group of buzz words that belong to Digital Audio and that is the field we are dealing with when using GarageBand or any other DAW. Some of those terms pop up even in iTunes.

Digital Audio

To better understand what they are and what they mean, here is a little background information.

Whenever dealing with Digital Audio, you have to be aware of two steps, that convert an analog audio signal into a digital audio signal. These magic black boxes are called ADC (Analog Digital Converter) and “on the way back”, DAC (Digital Analog Converter).

Step One: Sampling

The analog audio (in the form of an electric signal like from an electric guitar) is represented by a waveform. The electric signal (voltage) changes up and down in a specific form that represents the “sound” of the audio signal. While the audio signal is “playing”, the converter measure the voltage every now and then. These are like “snapshots” or samples, taken at a specific time. These specific time intervals are determined by a “Rate”, it tells you how often per seconds something happens. The unit is Hertz [Hz] defined as “how often per seconds” or “1/s”. A Sample Rate of 48kHz means that the converter takes 48,000 Samples per second.

Step Two: Quantize (or digitize)

All these Samples are still analog, for example, 1.6Volt, -0.3Volt, etc. But this analog value now has to be converted into a digital form of 1s and 0s.This is done similar to quantizing a note in GarageBand. The value (i.e. the note) cannot have any position, it has to be placed on a grid with specific values (i.e. 1/16 notes). The converter does a similar thing. It provides a grid of available numbers that the original measured Sample has to be rounded to (like when a note get shifted in GarageBand by the quantize command). This grid, the amount of available numbers, is called the Bit Depth. Other terms like Resolution or Sample Size are also used. A Bit Depth of 16bit allows for 65,535 possible values.

So the two parameters that describe the quality of an Digital Audio Signal are the Sample Rate (“how often”) and the Bit Depth (“how fine of a resolution”). The very simplified rule of thumb is, the higher the Sample Rate, the higher the possible frequency, and the higher the Bit Depth, the higher the possible dynamic.

Uncompressed Digital Audio vs. Compressed Digital Audio

So far I haven’t mentioned the “Bit Rate” yet. There is a simple formula that describes the Bit Rate as the product of Sampel Rate and Bit Depth: Sample Rate * Bit Depth = Bit Rate. However, Bit Depth and how it is used (and often misused and misunderstood) has to do with Compressed Digital Audio.

Compressed Digital Audio

First of all, this has nothing to do with a compressor plugin that you use in GarageBand. When talking about compressed digital audio, we talk about data compression. This is a special form how to encode data to make the size of the data set smaller. This is the fascinating field of “perceptual coding” that uses psychoacoustic models to achieve that data compression. Some smart scientists found out that you can throw away some data in a digital audio signal and you wouldn’t even notice it, the audio would still sound the same (or almost the same). This is similar to a movie set. If you shoot a scene on a street, then you only need the facade of the buildings and not necessary the whole building.

Although the Sample Rate is also a parameter of uncompressed digital audio, the Bit Depth is not. Instead, here is the Bit Rate used. The Bit Rate tells the encoder the maximum amount of bits it can produce per second. This determines how much data it has to throw away in order to stay inside that limit. An mp3 file (which is a compressed audio format) with a Bit Rate of 128kbit/s delivers a decent audio quality. Raising the Bit Rate to 256bit/s would increase the sound quality. AAC (which is technically an mp4 format) uses a better encoding algorithm. If this encoder is set to 128kbit/s, it produces a better audio quality because it is smarter to know which bits to throw away and which one to keep.

Conclusion

Whenever you are dealing with uncompressed audio (aiff, wav), the two quality parameters are Sample Rate [kHz] and Bit Depth [bit] (aka Resolution, aka Bit Size)

Whenever you are dealing with compressed audio (mp3, AAC), the two quality parameters are Sample Rate [kHz] and Bit Rate [kbit/s]

If you look at the Export Dialog Window in GarageBand, you can see that the Quality popup menu is different for mp3/AAC and AIFF. Hopefully you will now understand why.

Hope that helps

Edgar Rothermich

http://DingDingMusic.com/Manuals/

'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'