Bit Rate vs. Bit Depth

Can someone explain the difference between "bit rate" and "bit depth"?

And why, when exporting to mp3, do we always talk about bit rate (e.g. 128kbps), but when recording we only care about bit depth (e.g. 24bit)?

The reason why I'm suddenly curious after all these years is that I'm having some old analog tapes converted to digital. I need to specify what format, so I just told them "24 bit, 96 kHz". Should I also specify a bit rate?

P.S. Out of curiosity, I checked the properties of the files in my Audio folders, and they all seem to be 2304kbps (at 24/96). Is there a setting in Sonar that controls this?

SD,

This is such a GREAT question and one that has plagued me for years too. I THINK (please notice i said think, cuz I'm hoping someone more knowledgable will come along and say it correctly) that bit rate is speed-based, whereas bit depth is size-related.

Bit depth directly corresponds to the resolution of each sample in a set of digital audio. Bit rate refers to the amount of data, specifically bits, transmitted or received per second. Does that help? (got it from Wiki, so I'm going on the fact that its correct- also below is from Wiki)

Technically speaking, bit depth is only meaningful when applied to pure PCM devices. Non-PCM formats, such as DSD or lossy compression systems like MP3, have bit depths that are not defined in the same sense as PCM. This is particularly true for lossy audio compression, where bits are allocated to other types of information, and the bits actually allocated to individual samples are allowed to fluctuate within the constraints imposed by the allocation algorithm. Recently, many lossy formats such as DTS and WMA Pro have been promoted as 24-bit. However, this is not correct. A lossy file will not actually contain 24 bits worth of information per sample, but is actually a file that was originally mastered at 24 bits and then compressed.

One of the most common bit rates given is that for compressed audio files. For example, an MP3 file might be described as having a bit rate of 160 kbit/s or 160000 bits/second. This indicates the amount of compressed data needed to store one second of music.

The standard audio CD is said to have a data rate of 44.1 kHz/16, implying the audio data was sampled 44,100 times per second, with a bit depth of 16. CD tracks are usually stereo, using a left and right track, so the amount of audio data per second is double that of mono, where only a single track is used. The bit rate is then 44100 samples/second * 16 bits/sample * 2 = 1,411,200 bit/s or 1.4 Mbit/s.

To fully define a sound file's digital audio bit rates, the sampling rate, word size, number of channels, (e.g. mono, stereo, four-track), and format of the data also need to be known. Someone else said: "Bit depth determines the range of decibels that the digitization process can capture. The more bits, the wider the range." That seems to make sense.

I sure hope this is helpful, as I initially had a hard time wrapping the ole brain around this as well......
Bestest,
CakeMaker

try this
bit rate = sample rate * bit depth * channels
44100 * 24 * 2 = 2,116,800 (~2116kbps)
48000 * 24 * 2 = 2,304,000 (~2304kbps)
48000 * 32 * 2 = 3,072,000 (~3072kbps)
 

BURN IT!

This is no doubt an over-simplification, but when I always see it when compared to tape as:
 
Bit Depth = width of the tape
Bit Rate   = speed of the tape
 
A higher Bit Depth gives a greater resolution with respect to the amplitude of the waves, whereas a higher Bit Rate gives a greater resolution with respect to frequencies.
 
M.
 

The higher the bit depth, the greater the signal to noise ratio (S/N). For a given bit depth, increasing bit rate increases the highest frequency that can be reproduced.

John T
You're thinking of sampling frequency (for "speed of the tape"), which does not directly relate to bit rate.

Yes, you're right!  my bad.