I am encoding some radio shows I recorded onto my computer from an FM tuner via my sound card's line in. I will be using lame --alt-preset standard to convert them to MP3. I was wondering if should I also force lame to use a lowpass at 15 kHz since any signal above that is probably noise?

I'd say to look at the original's frequency response to see where the radio station's cutoff is. AFAIK, there is no "default" cutoff, just that it has to be lower than 19khz (where the stereo pilot is). Adding a lowpass in the MP3 is probably a good idea, but I don't know about where it should be.

why not just use "--alt-preset standard -Y"
it will save some bits...

I assume that you cutoff to save bitrate!

I'm also curious... but I thought that --aps or --aps -Y is a bit overkill for a signal grabbed from a FM radio because I've heard that the bitrate of FM radio quality is somewhere around 64kbps MP3...
Where's the truth? Can anyone clear it up for us?

Edit: typo

Maybe true.
I recently recorded some songs from my favorite station and compared them directly to the lossless versions (once ripped from CD) that I have in my collection. I was very disappointed to realize that they all seemed to have heavy artifacts that could have been caused by heavy lossy data reduction-at least less than 128kbps.

APS isn't necessarily overkill. APS introduces less quantisation errors than presets with lower nominal bitrates. It doesn't matter that the FM input signal is not of high quality...you don't want to make it any worse and that's why someone might choose to use APS.

Encoding an FM signal at 64kbps would add insult to injury. The FM signal is already degraded so the high quantisation errors inherent in using a low bitrate would only make matters worse.

Almost all commercial broadcasters use some form of dynamics compression before transmitting their signal. This allows their signal to travel further.

If your broadcast signal reaches 50 miles in all directions, you cover 1963 square miles. Increase that signal's reach to 60 miles and your coverage increases to 2827 square miles, a dramatic increase for what is only a 20% increase in distance. More coverage means more advertisting dollars.

Gold is king so quality takes a backseat. That is often why for-profit broadcasters often have the worst sound (i.e. least faithful to the original recording) while non-profits like NPR and college stations transmit signals that sound more like real music.

What's more, broadcasters have been using lossy compressed music for at least the last 10 years. It wouldn't surprise me if their encodings were based off a 5-year old Fraunhofer encoder at 128kbps CBR.

I did some tests with a one minute sample from one of my recordings. Lame 3.90.3 with --aps was used for all tests:



I did an abx test with --aps vs. --aps --lowpass 15000 but was unable to tell them a part.

Here is some frequency analysis which may help explain the bitrate differences:

original wave file

Note the steep drop off at just after 15 kHz and the pilot tone at 19 kHz.

--alt-preset standard

No signal above 16.5 kHz even though the default lowpass filter for --aps appears to be 18.5 kHz (?) based upon the output from lame.

--lowpass 15000

Slightly less high frequencies then --aps.

-k

Some of the noise between 16.5 kHz and 18.5 kHz has gone but the pilot tone survives.


Conclusions:

Lame seem to remove most of the noise above 15 kHz without any extra prompting, presumably because it is already considered to be inaudible. The default lowpass filter for --aps is low enough to remove the pilot tone, which does appear increase bitrate if it is kept.

I think I will just stick with vanilla --aps.

compression means more signal in AM, but not FM. What it gets you is more loud more of the time, so somone just tuning around is less likely to pass by your channel without hearing anything. Also it will likely sound better to someone in a car with a lot of background noise.

@mithrandir:
Maybe you didn't notice, I was only referring to lossy data compression. Radio's music is also dynamically compressed, of course, and I noticed it during my comparison (and it didn't surprise me).

FM=frequency modulation carries all the sound info only in its frequency. The amplitude decreases with longer distance, but it has no effect on loudness. opposite: AM=amplitude modulation