strange results using low-pass filter ( lame 3.98/3.99.5 )
Reply #50 – 2013-08-16 11:58:53
Using the foobar ABX component, the provided mp3 files sound very different. One is silent to my ears, the other is buzzing. See log below. However, using regular foobar playback, they are both silent. After some tinkering with/without DSP, WASAPI vs. DirectSound, RG on/off, the reason -- apparently -- is that I apply -3dB for non-replaygained files by default, which of course is ignored by the ABX component. When going closer to 0dB gain, the buzzing is added. I opened the files in Audacity and even after applying a gain of -12dB, one file is still buzzing and the other is silent (when played back via Audacity). I assume the clipping manifests itself when Audacity initially decodes the file to fixed-point (default sample format 16bit) and is therefore unaffected by the following gain. When I set the default sample format to float32 and re-open the file, the buzzing can be heard at FS but disappears with increasingly negative gain. This should not be news/surprising to any of the regulars but maybe helps the OP to understand what is going on: On this type of synthetic signal, LAME's processing leads to signals which exceed FS which causes audible clipping when decoded to fixed-point PCM. The clipping can be avoided by reducing the gain prior to decoding (e.g. using replaygain) or prior to encoding. As far as I understood [JAZ], the lowpass filter in LAME might actually be the one causing the larger-than-FS signal but can not be easily fixed due to the implementation. The question remains whether this is in any way relevant for real-world signals. Previous experience suggests it is not. But it seems like good engineering practice to add as little content above FS as possible during the processing.foo_abx 1.3.4 report foobar2000 v1.2.6 2013/08/16 11:34:25 File A: F:\incoming\music\lame lowpass\out_v0_lp20.1_99lame.mp3 File B: F:\incoming\music\lame lowpass\out_v0_lp20.2_99lame.mp3 11:34:25 : Test started. 11:34:33 : 01/01 50.0% 11:34:36 : 02/02 25.0% 11:34:38 : 03/03 12.5% 11:34:41 : 04/04 6.3% 11:34:43 : 05/05 3.1% 11:34:45 : 06/06 1.6% 11:34:47 : 07/07 0.8% 11:34:49 : 08/08 0.4% 11:34:52 : Test finished. ---------- Total: 8/8 (0.4%)