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Topic: Sampling rate for reduced bitrate MP3s? (Read 4596 times) previous topic - next topic
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Sampling rate for reduced bitrate MP3s?

These quotes are taken from a thread on the usenet:

Quote
What sort of bitrate is advised to encode FM speech radio broadcats into MP3 format?
Quote
Paul wrote:
>
> The sample rate (e.g. 44.1KHz) affects quality at low bit
> rates.  If you are lowering the bit rate (say to 56K), then
> you probably should lower the sample rate to 22KHz.
>
> If you are conserving space and you don't need stereo, try
> mono 22KHz @ 96K bit rate.  You can actually lower the bit
> rate more and still have very good quality.  Lower bit rates
> sound lousy when you don't lower the sample rate.


I too want to make some small MP3s of FM radio speech broadcasts and I don't think I need anything like 128 quality.  I will use a lower  bitrate.

Seems that I should lower the sampling rate if I lower the bitrate.

Are there any guideliness to help decide on what range of sampling rates are usually acceptable for given bitrate?

Or vice versa.

Sampling rate for reduced bitrate MP3s?

Reply #1 –
Lame will change the default sampling rates automatically for you. I think they switch to 32Khz at 96kb/s, and to 22khz around 64Kb/s but I could be off on this.

Load a few files into razorlame and see what happens, but the defaults havn't really been questioned too much.

You might want to consider vorbis for such apps though. Much better performance at low bitrates.

If it has to be mp3, FhG will be the best bet secondary to its intensity stereo mode which Lame has never used.

Sampling rate for reduced bitrate MP3s?

Reply #2 –
Vorbis q -1 on 44 khz mono material averages 20 kbps and sounds great. That's lower than MP3 can even go, much less sound good. For some stuff from the 1930s that was pretty lo-fi anyway, I downsampled to 8 khz and got it down to 10.25 kbps average. Managed to fit an hour long radio program into 4.5 MB. Booyah.

Sampling rate for reduced bitrate MP3s?

Reply #3 –
Quote
That's lower than MP3 can even go, much less sound good.

"Sound good" is of course subjective, but MP3 is capable of going down to 8kbps.

Sampling rate for reduced bitrate MP3s?

Reply #4 –
Vorbis does arguably perform better than MP3 at low bitrates (see various listening tests, and perform your own tests), but MP3 still has massively wider acceptance and platform support.  I'm assuming you want MP3 encodes (you stated MP3).

There are plenty of threads dealing with low bitrate MP3s (and their various sample rates already) for voice:

1st link
2nd link
3rd link

My preferences for low bitrate MP3 are: LAME 3.90.3 VBR 8-96kbps mono resampled @ 22050 Hz and lowpassed at 11kHz.  This typically yields bitrates between 40kbps and 64kbps.  I even transcode (wait for the flack) from LAME APS and get fine results.  I know I can get lower bitrates using different parameters and different codecs, but I've found this set up works well in terms of platform portability and when music is injected in to the mix.  I comfortably get 24 hours of audio on a 700MB CD.

If you're intersted in much lower bitrates and not fussed about the codec, then try Speex.

edit: added more info.

Sampling rate for reduced bitrate MP3s?

Reply #5 –
The only guideline there is, is to start with the presets. They will adjust sampling frequency and more importantly the lowpass frequency to what is appropriate for the target bitrate. Since we are dealing with speech you should also use mono.

lame --preset <bitrate> -a

where you replace <bitrate> with the bitrate you want of course. And it does not have to be the standard 64, 80, 96 etc.

Sampling rate for reduced bitrate MP3s?

Reply #6 –
Quote
I too want to make some small MP3s of FM radio speech broadcasts and I don't think I need anything like 128 quality.  I will use a lower  bitrate.

Seems that I should lower the sampling rate if I lower the bitrate.

Are there any guideliness to help decide on what range of sampling rates are usually acceptable for given bitrate?

Or vice versa.

In 3.96 I would choose

-V 6

which has a lowpass of 16000 an has a target of around 115 kbits

or

-V 7

which has a lowpass of 14900 an has a target of around 100 kbits and is resampled to 32kHz

or

-V 8

which has a lowpass of 12500 and has a target of around 85 kbits and is resampled to 32kHz