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Topic: Optimal sample rate for FM recording? (Read 9250 times) previous topic - next topic
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Optimal sample rate for FM recording?

Hello,

I frequently record FM shows with my computer and I have wondered what is the optimal sample rate to get the best sound quality?

Originally I used to record them at 44.1khz but then read somewhere that recording at 32khz and later resampling it to 44.1khz is better, because FM won't transmit signal over 15-16khz and there would be less noise as the signal above 15-16khz is not recorded. Is this true? I myself do not hear any difference in sound quality when recorded with 44.1khz or 32khz.

Optimal sample rate for FM recording?

Reply #1 –
Well, if you can't feel a difference between 44.1kHz and 32kHz, then I think you will be better off keeping your files at 32kHz.

Resampling to 44.1kHz later seems dangerous to me.

Optimal sample rate for FM recording?

Reply #2 –
Quote
Well, if you can't feel a difference between 44.1kHz and 32kHz, then I think you will be better off keeping your files at 32kHz.

Resampling to 44.1kHz later seems dangerous to me.

Thank you for your suggestion, but I burn these shows on CD, so keeping them  at 32kHz is out of question.

Is upsampling so dangerous I would be better off recording them straight at 44.1khz?

Optimal sample rate for FM recording?

Reply #3 –
I would record at 44.1kHz and later lowpass at 16kHz.

Optimal sample rate for FM recording?

Reply #4 –
If you intend to burn them to CD-R I see no reason to use anything else than 44,1 kHz, 16 bit, stereo when recording. Those are the properties of CD-DA.

Optimal sample rate for FM recording?

Reply #5 –
there's a couple of good reasons to sample at higher than the 15k bandwidth of FM radio (apart from convenience of burning CDs)...

if your soundcard has an inadequate brickwall lowpass in it, then any frequencies outside the recording bandwidth will manifest in lower frequencies, causing more noise.  i'd say anything not sold as "audiophile" (i hate that word) would be like this... my sound card is shocking with it.  FM radio pretty much always has high frequency tones in it at varying places (depending on where you're tuned).  i've had then at 18-19khz, and as low as 14k on my rather old tuner.  these tones would alias like mad if they weren't lowpassed properly by the sound card before sampling.

there's another matter of soundcard resampling, but as far as i'm aware it only affects playback.  this is where Creative cards will play back 44.1k files resampled (badly) to 48khz.  even the digital outputs do this.  if this happens in recording (i've no idea how to test this ) then i'd simply record at 48k, lowpass, then resample to 44.1k for CD burning.

these things probably wont be too important seeing as FM isn't exactly HIFI extreme...

Optimal sample rate for FM recording?

Reply #6 –
If you're only getting signal up to 15khz with FM radio, you should probably invest in a good FM antenna. 

I would simply record at 44.1/16.  I don't think Creative cards resample on recording.  They only resample output for mixing purposes.

Optimal sample rate for FM recording?

Reply #7 –
Quote
If you're only getting signal up to 15khz with FM radio, you should probably invest in a good FM antenna. 

?!?!

It probably depends which country of the world you're in. I've heard a couple of people say they can receive up to 18kHz, but I'm sure nothing above 15.5kHz is broadcast in the UK.

Surely the 48k/44.1kHz resampling problem does happen when recording? If I had an SBlive I'd check, but thankfully I don't.

Whatever, if the destination is a CD, then the chocie of sampling rate is a sound card issue: 44.1kHz with a good sound card, or whatever works best with a bad one.

Cheers,
David.

Optimal sample rate for FM recording?

Reply #8 –
Quote
If you're only getting signal up to 15khz with FM radio, you should probably invest in a good FM antenna. 

Erm,  what?

Audio signal is lowpassed at 15 kHz before the FM transmission. What you pick up above that is noise + 19 kHz pilot signal which helps tuner to extract stereo information. None of that is really what you need to record.

Optimal sample rate for FM recording?

Reply #9 –
Quote
Audio signal is lowpassed at 15 kHz before the FM transmission.

Are you shure that it is lowpassed at 15 kHz? The recordings i made show a lowpass at 16 kHz.

Optimal sample rate for FM recording?

Reply #10 –
Quote
Quote
Audio signal is lowpassed at 15 kHz before the FM transmission.

Are you shure that it is lowpassed at 15 kHz? The recordings i made show a lowpass at 16 kHz.

Yes. Filter starts at 15 kHz. How sharp it is highly depends on radio station hardware/software, I guess it can go up to 15.5 - 16 kHz but not farther (signal must vanish at "safe" distance from 19 kHz pilot, which means about 3-4 kHz).

Optimal sample rate for FM recording?

Reply #11 –
Many radio stations do use a brickwall filter at 15 kHz.  It all depends on what processing options that particular station uses.  Most broadcast processing chains allow the station to select options such as where to lowpass.

However... many recordings I've made myself and looked at with Cool Edit have signal up to 18 kHz.  Signal.. not static  Then a quick dropoff, then the 19 kHz pilot.  The pilot occupies a pretty narrow notch of frequencies.  Try recording some FM radio if you don't believe me.. it's a fun diversion. 

Optimal sample rate for FM recording?

Reply #12 –
It might be helpful if all contributors regarding the lowpass used with radio broadcasts indicated which country their knowledge covers. Surely this is not the same all around the world?

Optimal sample rate for FM recording?

Reply #13 –
AFAIK it is true for EBU as well as for American broadcasters... 

And Cyaneyes: I've been recording from radio a lot and I've seen spectral view of my recordings many times. All I can see above 16 kHz is some symmetric noise around pilot tone, and I suppose it would strongly interfere with L+R and L-R signals if they weren't lowpassed far enough from pilot.

Optimal sample rate for FM recording?

Reply #14 –
I don't know the standards,  but there should be some findable broadcast standards out there.  They may vary from area to area,  part of the problem is you need to maintain how much RF bandwidth your signal takes up.. and there are hard rules about that. 

Some countries may have different channel spacing,  if wider in EU lets say..  they could have a higher cutoff because there will be a bit more bandwith.  But I'd bet its more as  Messer is thinking,  19Khz pilot is standard, and you need a certain spacing around it.

Best thing though I would think, would be to use one of the programs that will give you a spectral display...  and check out the local stations in your area.  Then you'll KNOW..  and station's choices, your reciever's response, and area differences won't matter.

BTW Messer...  nice looking plot,  I dont' recognize it,  what program?

Jon

Optimal sample rate for FM recording?

Reply #15 –
After a quick check around with my FM tuner card and Spectrogram,  I'm finding all the local LA area stations in Southern California start rolling off about 15.0 - 15.3Khz, and finish around 15.6 - 15.9Khz...  Check your own out!

Here's one, marks are on 15 and 16Khz.

Oops..  hmm.. need to have a site to source a link for an image,  rather than just attaching one?  Oh well..  guess I wont upload that graph..

Optimal sample rate for FM recording?

Reply #16 –
Just to reinforce what has already been stated... yes, FM radio is brickwall lowpassed at 15kHz - as required by the FCC.  Are the spectrum graphs that you guys looking at from lossless or lossy copies of transmitted audio?

Edit:  sp

Optimal sample rate for FM recording?

Reply #17 –
What Messer shows on his plot is exactly what I see in the UK.

It's been suggested that the "side bands" around the 19kHz pilot tone may not actually be broadcast, but are an artifact of how radios handle either the stereo decoding, or the RDS (or SCA in USA) information. I haven't heard a clear enough explanation to settle this matter in my own mind.

Whatever - it's not useful audio information, and you should filter it out before lossy encoding in order to save bits. It might also be good to filter it out before burning to CD, because (whilst you're unlikely to hear it) there is no benefit to leaving it there.

Be careful with very hard brick wall filters though - they're trivial to run in Cool Edit Pro, but will add huge amounts of ringing at the cut-off frequency. If this cut off frequency is within your hearing range (which it might be when lowpassing FM) this can be audible.

Before anyone quotes TOS 8 (i.e. prove it can be audible David!) - ringing from a brick will filter is totally obvious if you do it at 5kHz. There's no chance of me ABXing it at 16kHz, but YMMV.

Cheers,
David.

Optimal sample rate for FM recording?

Reply #18 –
Quote
Quote
If you're only getting signal up to 15khz with FM radio, you should probably invest in a good FM antenna. 

?!?!

I lot of FM receivers gradually reduce the amount of stereo separation as the signal strength lessons, eventually to mono. It was called "the blend adjustment" on the stuff I worked on.

Maybe some also adjust a lowpass to reduce noise. I have never checked for that. If they did, a better antenna might help, if the signal was weak of course.
flac > schiit modi > schiit magni > hd650

Optimal sample rate for FM recording?

Reply #19 –
hi
I sometimes record FM broadcasts using a separate hifi CD recorder, when I rip these CDs to .wav there is still some noise above 15hkz using CoolEdit's spectral analyse.
Is there a way to lowpass a lot of files at the same time ("batch lowpass") without having to open, lowpass and save each file separately?

thanks
 

Optimal sample rate for FM recording?

Reply #21 –
To cleanly remove the pilot signal from an FM recording using SoX, try the following:
Code: [Select]
sox "input.wav" "output-lowpass.wav" filter 0-18800 1024

  SoX can also be used quite effectively for the removal of DC offset, and other minor corrections.

    - M.

Optimal sample rate for FM recording?

Reply #22 –
Can anyone recommend a recording program that uses lame.exe rather than just the dll for recording fm radio - preferrably that can be scheduled to record a certain amount of time (2hrs) - plus suggestions for lame settings

- I used to record fm to wav and then CD but now listen to mp3 versions mainly so I've started recording to mp3 256 using X-Ripper

- I'm not that technical but my one big tip is to plug an old analogue fm receiver (I use a big old TEAC tuner from the 70's) into your sound card for best quality - digital tuners have a lot of internal noise

- the station I record is also online http://www.pbsfm.org.au/ - they play an amazing range of music and a 2-hr program makes a nice 1 hr CD usually of music I wouldn't hear otherwise or even know to look

glad to see other people record fm - I thought I was a nutter