IPB

Welcome Guest ( Log In | Register )

 
Reply to this topicStart new topic
LAME: encode digital silence at the lowest possible bitrate
hydrOH
post Oct 8 2013, 09:07
Post #1





Group: Members
Posts: 2
Joined: 8-October 13
Member No.: 110746



Using LAME 3.99.5 32bits.
Commandline:
CODE
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?

This post has been edited by hydrOH: Oct 8 2013, 09:08
Go to the top of the page
+Quote Post
pdq
post Oct 8 2013, 12:41
Post #2





Group: Members
Posts: 3305
Joined: 1-September 05
From: SE Pennsylvania
Member No.: 24233



Did you make sure that Audacity did not add dither to the file?
Go to the top of the page
+Quote Post
2Bdecided
post Oct 8 2013, 12:50
Post #3


ReplayGain developer


Group: Developer
Posts: 4945
Joined: 5-November 01
From: Yorkshire, UK
Member No.: 409



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.
Go to the top of the page
+Quote Post
pdq
post Oct 8 2013, 13:00
Post #4





Group: Members
Posts: 3305
Joined: 1-September 05
From: SE Pennsylvania
Member No.: 24233



-80 dB of dither sounds excessive to me. Is that normal?
Go to the top of the page
+Quote Post
nu774
post Oct 8 2013, 15:28
Post #5





Group: Developer
Posts: 476
Joined: 22-November 10
From: Japan
Member No.: 85902



QUOTE (pdq @ Oct 8 2013, 21:00) *
-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.
Go to the top of the page
+Quote Post
pdq
post Oct 8 2013, 17:02
Post #6





Group: Members
Posts: 3305
Joined: 1-September 05
From: SE Pennsylvania
Member No.: 24233



QUOTE (nu774 @ Oct 8 2013, 10:28) *
QUOTE (pdq @ Oct 8 2013, 21:00) *
-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.

I assume in this case that would be relative to -96 dB?
Go to the top of the page
+Quote Post
nu774
post Oct 8 2013, 18:47
Post #7





Group: Developer
Posts: 476
Joined: 22-November 10
From: Japan
Member No.: 85902



QUOTE (pdq @ Oct 9 2013, 01:02) *
QUOTE (nu774 @ Oct 8 2013, 10:28) *
QUOTE (pdq @ Oct 8 2013, 21:00) *
-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.

I assume in this case that would be relative to -96 dB?

Exactly.
Go to the top of the page
+Quote Post
hydrOH
post Oct 10 2013, 17:37
Post #8





Group: Members
Posts: 2
Joined: 8-October 13
Member No.: 110746



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).

This post has been edited by hydrOH: Oct 10 2013, 17:50
Go to the top of the page
+Quote Post
2Bdecided
post Oct 11 2013, 10:07
Post #9


ReplayGain developer


Group: Developer
Posts: 4945
Joined: 5-November 01
From: Yorkshire, UK
Member No.: 409



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.
Go to the top of the page
+Quote Post
Rescator
post Oct 12 2013, 02:05
Post #10





Group: Members
Posts: 73
Joined: 13-January 09
From: Trondheim
Member No.: 65515



QUOTE (2Bdecided @ Oct 11 2013, 11:07) *
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. smile.gif


--------------------
"Normality exist in the minds of others, not mine!" - Rescator
Go to the top of the page
+Quote Post

Reply to this topicStart new topic
1 User(s) are reading this topic (1 Guests and 0 Anonymous Users)
0 Members:

 



RSS Lo-Fi Version Time is now: 19th April 2014 - 11:29