What is the bit rate?

Question

I am new to audio programming, But I am wondering formula of bitRate,

According to wiki https://en.wikipedia.org/wiki/Bit_rate#Audio,

bit rate = sample rate X bit depth X channels

and

sample rate is the number of samples (or snapshots taken) per second obtained by a digital audio device.
bit depth is the number of bits of information in each sample.

So why bit rate = sample rate X bit depth X channels?

From my perspective, if bitDepth = 2 bit, sample rate = 3 HZ then I can transfer 6 bit data in 1 second

For example:

Sample data = 00 //at 1/3 second.  
Sample data = 01 //at 2/3 second.  
Sample data = 10 //at 3/3 second.

So I transfer 000110 in 1 second, is that correct logic?

Would "yes" be a sufficient answer to this question? – Karsten Koop Oct 08 '18 at 09:31 — Karsten Koop, Oct 08 '18 at 09:31
What’s the question? Are you asking whether 2 x 3 is 6? – matt Oct 08 '18 at 10:18 — matt, Oct 08 '18 at 10:18

score 1 · Answer 1 · answered Oct 08 '18 at 10:40

Bit-rate is the expected amount of bits per interval (eg: per second).

Sound cycles are measured in hertz, where 1 hertz == 1 second. So to get full sound data that represents that 1 second of audio, you calculate how many bits are needed to be sent (or for media players, they check the bit-rate in a file-format's settings so they can read & playback correctly).

Why is channels involved (isn't sample rate X bit-depth enough)?

In digital audio the samples are sent for each "ear" (L/R channel). There will always be double the amount of samples in a stereo sound versus if it was mono sound. Usually there is a "flag" to specify if sound is stereo or mono.

Logic Example: (without bit-depth, and assuming 1-bit per sample)...

There is speech "Hello" recorded at 200 samples/sec at bitrate of 100/sec. What happens?

If stereo flag, each ear gets 100 samples per sec (correct total of 200 played)
If mono, audio speech will sound slow by half (since only 100 samples played at expected bit-rate of 100, but remember, a full second was recorded at 200 sample/sec. You get half of "hello" in one second and the other at next second to (== slowed speech).

Taking the above example, you will find these audio gives slow/double speed adventures in your "new to audio programming" experience. The fix will be either setting channels amount or setting bit-rate correctly. Good luck.

score 1 · Answer 2 · answered Oct 08 '18 at 14:30

The 'sample rate' is the rate at which each channel is sampled.

So 'sample rate X bit depth' will give you the bit rate for a single channel.

You then need to multiply that by the number of channels to get the total bit rate flowing through the system.

For example the CD standard has a sample rate of 44100 samples per second and a bit depth of 16 giving a bit rate of 705600 per channel and a total bit rate of 1411200 bits per seconds for stereo.

What is the bit rate?

2 Answers2