Thursday, March 4, 2004 at 1:54 AM Actually, this will work for any codec that has a low-pass feature (HINT: like Ogg). This will reduce file size, and give you as good or better audio quality, while still using the same bit rate. Yoinked from: http://www.jthz.com/mp3/ LAME, FM-radio quality and Sample-rate
Many people have asked us how we use LAME together with FM-radio. Let's explain:
Before FM-stereo audio goes into the transmitter it's being filtered at 15 kHz (this - among other things - has to do with bandwidth of the transmitter-signal and stereo-keying at 19 kHz). All high frequency content above 15.* kilohertz will be cut off entirely. Some tuners will give you 15.7 kHz as their highest 'clean' audio frequency output, but most transmitters will not even air anything above 15 kHz.
Conclusively;
Encoding MP3 audio intended for analog FM-radio, or
encoding MP3 from audio that's recorded from analog FM-radio (tuner) outputs,
can safely be done using a commandline like:
lame in.wav out.mp3 -V5 --vbr-new -q0 -b96 --lowpass 15.4 --cwlimit 10 or
lame in.wav out.mp3 -V6 --vbr-new -q0 -b112 --lowpass 15.4 --cwlimit 10 or
lame in.wav out.mp3 -V7 --vbr-new -q0 -b112 --lowpass 15.4 --cwlimit 10
with these you'll achieve around 10 times compression, while not losing anything from the FM original. Cutting off from 15 Kc will actually give you higher quality files, since there's more room left for other audio in the spectrum.Logically, people then asked at what sample-rate
they should record their FM-transmitted radio-audio.
If the anti-aliasing filter on a 32kHz digitizer is steeper than the original 15kHz filter used when the broadcasts were first made, it's possible that 32kHz digitization might lose a tiny bit of HF energy. The solution to this is to use no anti-aliasing filter before digitization (the material is already band-limited at 15kHz, so it doesn't need further filtering). An absolutely safe choice of sample frequency would be 38kHz, but this is almost never supported. If you are uncertain about the anti-aliasing of your soundcard (the SB Live! for example is safe to use with 32 kHz for FM-stereo mastering) use 44.1kHz, but both 44.1 and 48kHz are certainly overkill for FM-radio... Although, I wouldn't use VBR. If you read the lame docs, they don't seem to have very much faith in the feature. Instead, I would use ABR (Average Bit Rate). The most important part is using the low-pass filter to cut off at 15 kHz.
Along the same lines, (I don't mean to be a back seat encoder :) ) but those of you who were recording from MojoRadio, and now 91x, would be better off leaving the files as WMA's, as converting from one lossy format to another causes a significant decrease in fidelity, especially at low bit rates. Most people have something that will play WMA's, and if they really want to convert it to mp3, they can do it on their own.
—ZT |
