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Topic: LAME: encode digital silence at the lowest possible bitrate (Read 7967 times) previous topic - next topic
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LAME: encode digital silence at the lowest possible bitrate

Using LAME 3.99.5 32bits.
Commandline:
Code: [Select]
lame -V 2 original.wav bad.mp3

Original file: [original.flac]. Resulting mp3: [bad.mp3]

This is an excerpt of a file with spoken parts and long periods of digital silence in-between. The silence was generated using Audacity's Generate -> Silence option. Yet, when encoding the resulting audio with LAME it uses mostly 64kbps frames for the silent parts instead of 32kbps.

Why is that? How can I force it to use 32kbps for the silent parts?

LAME: encode digital silence at the lowest possible bitrate

Reply #1 –
Did you make sure that Audacity did not add dither to the file?

LAME: encode digital silence at the lowest possible bitrate

Reply #2 –
There's noise shaped dither. The "silence" in the middle of the file has an RMS level of -80dB FS. The "silence" before "ten" and after "time" has an RMS level of -70dB FS.

You need digital silence (all zeros) to get the lowest bitrate. Disable Audacity's dither. Use a simple noise gate to process this file if you don't want to have to re-do the whole thing.

Cheers,
David.

LAME: encode digital silence at the lowest possible bitrate

Reply #3 –
-80 dB of dither sounds excessive to me. Is that normal?

LAME: encode digital silence at the lowest possible bitrate

Reply #4 –
Quote from: pdq on
-80 dB of dither sounds excessive to me. Is that normal?

It it is a noise shape then having response of +20dB?30dB is normal.

LAME: encode digital silence at the lowest possible bitrate

Reply #7 –
Thanks guys. I've found Audacity specific instructions for correcting this here.


Hah! After disabling dither and re-exporting the original audio, the resulting FLAC file is much smaller too. In fact, the FLAC copy is now smaller than the LAME -V2 conversion. Whereas the flac file has an average bitrate of 37kbps, the mp3 is stuck at 40kbps (because the mp3 specs don't allow a lower bitrate than 32kbps at the audio's sampling rate).

LAME: encode digital silence at the lowest possible bitrate

Reply #8 –
Lossless compression loves strings of zeros!

If you zip (or even better, rar) the mp3s, you'll probably find the resulting archive is really small if it's mostly digital silence. For a minute of "CD quality" digital silence, I get a 234kB -V2 mp3 which compresses to a 1.3kB zip or a 0.5kB rar (default/normal compression in WinRAR).

Cheers,
David.

LAME: encode digital silence at the lowest possible bitrate

Reply #9 –
Quote from: 2Bdecided on
a minute of "CD quality" digital silence


Aw! Man that just made my day, it really made me laugh. If there ever was any audiophile standup, then this would be it.