Bit Rate Difficulty - Phone System Voice Recording

Special Interest Groups Audio Processing
1

I recorded voice prompts for a telephone system in the following specs: PCM 16-bit, mono, 8000 HZ and 8 Khz. However when I sent the files to .wav format, they seem to have converted to a bit rate of 128 kbs (that’s what each file reads when you hover the mouse over it). Additional however, when I click “properties” of the files it lists the recorded bit rate of 16.

I need this to work for someone’s telephone system–and want to send the files correctly, so that it can be used. What have I done wrong? Can I fix this without rerecording?

Thanks in advance, T

2

You’ve made a few typos, but I think I get what you’re trying to say.

You want 8-bit mono wav files at an 8KHz sample rate.

When you hover over those files you will see that it’s listed as 64 kbps. This is what it should say since 8000 (samples/sec) * 8 (bits/sample)= 64000 (bits/sec).

You are getting 16-bit files. This is because you have Audacity’s settings wrong.

In 1.2.6 the setting is in the Edit → Preferences → File Formats menu. You need to change the uncompressed export format to 8bit PCM.

In 1.3.3 the setting can be changed after you’ve clicked Export. Click Options in this dialog box and change the output format there.

Be aware that if you are converting the sample rate during output there is a bug in Audacity that will add extraneous silence to the exported file. This needs to be fixed but hasn’t been.

Also, Bit Rate and Bit Depth don’t mean the same thing. Bit Rate is the number of bits per second. Bit Rate is a function of both Sample Rate and Bit Depth. Bit Depth is the number of bits per sample. This was confusing me the first time I read your post.

3

Thanks so much. Sorry about the typos. Actually the specs SHOULD have read
“PCM, 16-bit, mono 8000 Hz (8 KHz),” which is what I set the recording to.

So, if I understand you correctly, it seems as if the 128 kps reading is OK, and should work in the system. Is that right?

Thank you for the explanation. (The 128 was confusing me, as that number was nowhere in the specs.) You have saved me from what could have been a lot of wasted time. T.