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Strange effect or bug in lowpass-filter? (3.53 Mp3 recoding)
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Author:  ZorroZ [ Thu Apr 16, 2009 1:06 am ]
Post subject:  Strange effect or bug in lowpass-filter? (3.53 Mp3 recoding)
Hi,
because it´s not easy to change my (good) old Car-Radio to a new one which can play
mp3´s from USB or SDcard, I get one of these MP3 Car cigarette Lighter FM Transmitter.
The transmitting power is sufficient, but the sound was dull/muffled because of the reasons we all know .... missing FM preemphasis. :roll:
I try and use the Commandline version 3.53 + Lame to process my mp3 songs (mostly not normalized but good coded >160kbit/s) and was very satisfied. :mrgreen: (I need only the preemphasis, a little loudness and compression, and smaller filesizes because of the lowpass)

For easy click/process and copy I´ve automated it with a little self-written Frontend-
Program that creates the needed .STS and .Bat files.
(haven´t found an easier way to process a great number of files and play around with the stereo-tool settings)

Thats my short .STS file, leaving all other parameters at default:
[Common]
Pre amplifier=1.0
Extra loudness=1.2
Hard limit output=1
Mode=Advanced
[Multiband Compressor]
Enabled=1
Difference=0
Very high quality enabled=1
Maximum volume - Band -1=9000 .....\
Maximum volume - Band 0=8000 ...... Don´t want to kill the bass ;)
Maximum volume - Band 1=7300 ..... /
[Output Filter]
Enabled=1
Lowpass filter=15000
[FM Transmitter]
Enabled=1
Pre-emphasize=1
Pre-emphasis time=50
Output is pre-emphasized=1
Stereo encoder enabled=0

Batchfile:
lame --decode -S %1 - | stereo_tool353 - - c:\fmwork\STS.sts | lame --preset standard -S - %2

*** Now we come to the interesting part (Bug?) ***

Only playing around with the Lowpass filter to recode a test-source (simple top10-pop 320kbit/s) I get the following filesizes:
Music_12khz.mp3 3306465 bytes
Music_13khz.mp3 5110075 bytes
Music_14khz.mp3 3550746 bytes
Music_15khz.mp3 3650746 bytes
Music_noFilt.mp3 4037351 bytes

Any idea from the expert(s) why the peak at 13khz ? :shock:

Regards
Author:  hvz [ Thu Apr 16, 2009 1:24 am ]
Post subject:  Re: Strange effect or bug in lowpass-filter? (3.53 Mp3 recoding)
Ok, that's odd...

The only thing I can think of is this: If you are using the lowpass filter when using FM options and HARD LIMIT, the lowpass filter first filters at the frequency that you set, then it hard limits the output which causes some (very soft) sounds at higher frequencies. It then filters the output again, but at a higher frequency - because otherwise new peaks would be caused.

My guess would be that for some reason, when the filtering is set at 13 kHz, the higher frequencies are treated by the MP3 encoder as "audible", while at other frequencies they are not. But I cannot explain why... Maybe a look at the spectrum will clarify things.
Author:  ZorroZ [ Fri Apr 17, 2009 1:36 am ]
Post subject:  Re: Strange effect or bug in lowpass-filter? (3.53 Mp3 recoding)
I will try to check if there are any this inaudible effects, that make it difficult for lame
to encode as expected.
I'll give you feedback in the next days .... thank you.
Author:  ZorroZ [ Mon Apr 20, 2009 12:57 am ]
Post subject:  Re: Strange effect or bug in lowpass-filter? (3.53 Mp3 recoding)
OK, I have played around some hours and find out the following .... hope it brings interesting input:

- I´ve found nothing in the waveforms or spectrograms etc.
- The 13khz-filesize-peak after encoding with lame is very "sharp" (13khz +-200Hz)
- Lacking high frequencies in source (before Stereotool) reduce/eliminate the effect, that lame produces to big mp3´s.
- Reducing the volume of the source has no significant effect
- Using the lame lowpass upto 15khz in addition eliminate the effect,
- Using the lame lowpass 16khz or higher have no effect. Seems to be the problem in the 15-16khz range. :?:

Current best workaround for my purposes: Using only the Lame lowpass, disabling that from ST,
because I always get smallest mp3´s at given quality.
And if I want to save 10-20% more bitrate I cut at 12khz (nearly inaudible over FM-Transmitter), but 12khz was also no problem using the ST-Lowpass.

I failed to put some short testfiles etc. (~6MB ZIP-package) in attachment, please leave a message if you want to analyze something with these ....

Regards.
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