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Bit Rate 128Kbps question

Posted: March 5th, 2009, 5:59 pm
by icequeen
I am new to Librivox and need to ask a question about the bit rate. Sorry if it has already been asked. The book that I would like to read wants the recording to have 128Kbps for the bit rate. I downloaded Audacity 1.2 and there is no setting for 128Kbps. It just has 16b, 32b and 32b float. Where do I find 128Kbps? Or should I download Audacity 1.3? Thanks for putting up with a newbie!


Posted: March 5th, 2009, 6:37 pm
by Steampunk
Go to Edit --> Preferences and select 128 from the pull-down menu next to Bit Rate in the File Format tab.



Posted: March 5th, 2009, 8:21 pm
by harvey

You are (understandably) confusing two different things, which both
have the word "bit" in their names.

When you record yourself, the mic picks up your voice as an analog
signal (i.e., it is continuously varying). Some place in your
recording set-up is an electronic device which samples that analog
signal to make it digital, the so-called analog-to-digital converter.
An individual sample is a snapshot of that signal, which represents
what your voice is doing at that instant.

There are two parameters involved in this digital conversion: the
sample rate, which is the number of samples (or snapshots) per second.
And the bit depth, or number of bits, per sample. The audio on audio
or music CDs always has a sample rate of 44,100 Hz (ie, that many
samples per second) and a bit depth (or "sample format", as Audacity
calls it) of 16 bits per sample. Those 16 bits are used to indicate
the volume or amplitude of your voice in that sample, at one of
65,000+ possible levels (2 raised to the 16th power = 65,536).

Audacity records sounds -- your voice in this case -- in Wave audio
format, which is an uncompressed format. Because the Wave format
takes up so much space, we have the MP3 format, which stores audio
more compactly. In fact, we can choose the degree of compactness,
called "compression", when we convert the Wave format to MP3. A
compressed MP3 file is said to have been "encoded" from the Wave
source. Compression is indicated by the bit rate per second. That's
the number of bits used to encode each second of sound. The standard
chosen for LibriVox is 128Kbps. That's 128,000 bits every second.

So when you record for LibriVox, you want the sample rate to be 44,100 Hz
and the sample format (or bit depth) to be 16 bits. You can set these as
the default in Audacity: Edit [menu] > Preferences > Quality [tab]

When the recording is finally finished and edited, and you are ready
to export it as an MP3 file, then, as Jim says, you want the bit rate
of that file to be 128Kbps.

Is that a bit clearer, now?

Posted: March 6th, 2009, 10:59 am
by icequeen
Thank you Jim and Harvey! I think I got it now!

Posted: March 6th, 2009, 3:53 pm
by icequeen
OK, now I know that I am a complete and total dork! I checked all the settings and they are already defaulted to the correct settings that I need. Thank you Jim and Harvey for your clear explanations and your patience. I really, really appreciate it! :D

Posted: March 7th, 2009, 4:01 am
by Cori
You're not at all a dork ... I'd call it "thorough", and it's also benefiting those who might have a similar problem but not be brave enough to post about it. :D Thanks for doing that, icequeen!

Posted: March 7th, 2009, 11:52 am
by icequeen
Cori wrote:You're not at all a dork ... I'd call it "thorough", and it's also benefiting those who might have a similar problem but not be brave enough to post about it. :D Thanks for doing that, icequeen!

Thanks for the pep talk. I have never done this before and I want to do it right. I just posted a sample at the book that I am recording and kb told me that I am basically ready to go. :D Thanks!