Ringing in Conversion from Wav to MP3

[palmeroo]

We are recording voice educational lectures in wav with an H2 recorder and would like to compress them to place on a website. We notice a bad ringing at the 16000 Hz MP3 joint stereo setting compression in Audacity and there is still some ringing even at 40 kHz. The 40 kHz files are too large for email or slow downloads.

We got no noticeable ringing when using iTunes to do a mono conversion of the wav file at 16 kpbs. We would prefer to stay in Audacity because iTunes does not allow other editing and also requires us to export a wav file from Audacity for conversion.

I analyzed the spectra of the various recordings and attached a picture. The wav file and iTunes show a fairly linear drop in signal with increasing frequency and then it stops. Both the Audacity compressions show a precipitous drop to -140 dB. The fall off gets increased with the 40kbps but is still there.

1. Could this be the source of the ringing?

2. How can I make a clean sounding mp3 file in Audacity at 16 kbps?

(We had previously used a Sony ICD-P620 for the task and got great results and small file sizes, <20Mbyte/2hour lecture, but the AV techs wanted the H2. I tried a low pass filter to get rid of -140dB signal, but that didn't work.)

Frequency analysis from Audacity plotted in Excel for original wav file, Audacity 16000 Hz and 40 kHz MP3 conversion, and iTunes conversion of wav at 16kbps.

spectra comparison.png (44.16 KiB) Viewed 2041 times

[kozikowski]

<<<2. How can I make a clean sounding mp3 file in Audacity at 16 kbps?>>>

You probably don't. The minimum bit rate for a mono show is 32. Stereo is 64. Lower than that, the show rapidly gets bubbly, honky, and muffled.

You are warned that compressors only work when they're presented with perfect, clear, crisp original work. MP3 compressing a show that started out MP3 or otherwise damaged is a shortcut to gargly disaster. The transfer from the H2 to Audacity should be as high a quality WAV as the system supports. Do Not use MP3 in that step.

Segment the show in blocks of time that fit. Few people can email an hour show that sounds perfect.

You might also try AAC instead of MP3. Anybody with an iPhone, an iPod or a Mac can play those. You can get iTunes to create those or you might be able to get FFMpeg to do it in Audacity 1.3.11.

Koz

[Trebor]

1. You don't really need stereo for a lecture, for the same kbps rate mono will give better quality sound than stereo: less burbly compression artifacts.

2. For voice-only (not music) recording, 10Khz bandwidth is plenty (you could even get away with 5KHz but it'll sound like a telephone).
Resample* a copy of your original wav recording at 22050Hz (instead of the 44100Hz default) this will reduce its size with negligible loss in quality for a voice-only recording. Then make a mono version and save as mp3. Attached are examples of what it'll sound like at different kbps (mono not stereo).

original - 64Kbps - 32Kbps -16Kbps.mp3

(939.75 KiB) Downloaded 164 times

If I had to listen to an hour I wouldn’t use less than 64Kbps for a 22050Hz sample rate voice-only recording (mono).

tried a low pass filter to get rid of -140dB signal, but that didn't work.

-140db is silent, you're trying to remove something that can't be heard: the threshold of audibility is about -100db.

[* "Resample" is in the "Tracks" menu in Audacity 1.3 }

[whomper]

We are recording voice educational lectures in wav with an H2 recorder and would like to compress them to place on a website.
...

2. How can I make a clean sounding mp3 file in Audacity at 16 kbps?

(We had previously used a Sony ICD-P620 for the task and got great results and small file sizes, <20Mbyte/2hour lecture, but the AV techs wanted the H2. ....

nothign wrong with using H2
dont like H2 then buy another sony

set the H2 to record at the lowest mp3 rate you can tolerate
and set it for mono
then leave the file *alone*
you cant compress it any more and make it better

not sure what benefit audacity is bringing to this scenario

all you can do is make the file worse if you overprocess the minimum (and already compressed some) size mp3 file

[kozikowski]

If you let the H2 compress the work, then you can't bring it into Audacity for further processing and editing. Audacity will not edit MP3 tracks. It imports them, converts them to high quality format (with no increase in show quality) and then edits them. If you want an MP3 later, Audacity has to export one -- in effect doubled-compressing the work which always sounds terrible.

If you go that route, you don't need Audacity at all. You should be using one of the fine MP3 editors out there.

<<<If I had to listen to an hour I wouldn’t use less than 64Kbps for a 22050Hz sample rate voice-only recording (mono). >>>

Yes, 32 is seriously on the edge. 64 in mono is much better -- if you can stand the file sizes.

However.

What happens if you leave the original work at 44100 instead of down-sampling to 22050? Did you ever actually try that? I think downsampling is a serious wife's tale.

Destructive compressors all process much better with the best, highest quality work you can supply. Their job is to preserve as much quality as possible. Pre-damaging the work usually creates sound (or video) problems with no great benefit anywhere else.

Koz

[Trebor]

Having re-read the original post I think Palmeroo is downsampling to 16000Hz (i.e. 8000Hz bandwidth) ...

We are recording voice educational lectures in wav with an H2 recorder and would like to compress them to place on a website. We notice a bad ringing at the 16000 Hz MP3 joint stereo setting compression in Audacity and there is still some ringing even at 40 kHz. The 40 kHz files are too large for email or slow downloads.

Whatever the sample rate, making the stereo file mono will reduce the file size, and for any given kbps increase sound quality.

set the H2 to record at the lowest mp3 rate you can tolerate

If they do that they won't have the oppertunity to make a high quality presentation, say on CD, or via the internet when bandwidth improves.

[kozikowski]

<<<If they do that they won't have the oppertunity to make a high quality presentation, say on CD, or via the internet when bandwidth improves.>>>

Yes, that does make my teeth hurt. I come from a background of "camera master negatives" and "studio master sound tracks." You can always go downhill from those to any other product you wish. If you already have a degraded product and you haven't even finished speaking, that's just begging for trouble.

Koz

[Trebor]

If Palmeroo isn't aware how to convert a stereo track into a mono one on Audacity
"Stereo track to Mono" is on the "Tracks" menu ...

stereo track to mono1.png (25.66 KiB) Viewed 2037 times

Sacrificing stereo will reduce the file size and compression artifacts for a given kbps.

[whomper]

If you let the H2 compress the work, then you can't bring it into Audacity for further processing and editing. Audacity will not edit MP3 tracks. It imports them, converts them to high quality format (with no increase in show quality) and then edits them. If you want an MP3 later, Audacity has to export one -- in effect doubled-compressing the work which always sounds terrible.

If you go that route, you don't need Audacity at all. You should be using one of the fine MP3 editors out there.
Koz

that is the point --
if you let the H2 compress while recording and make mp3 you don't need to process with audacity. h2 will even normalise it.
note the h2 does not compress the mp3 after recording it.

if you are going to use audacity then start with a wav file at least 16x44100.

[whomper]

set the H2 to record at the lowest mp3 rate you can tolerate

If they do that they won't have the oppertunity to make a high quality presentation, say on CD, or via the internet when bandwidth improves.

true
but that was not what they want to do with it as i read their posting

else they should record at 32bitx192ksamples/sec without compression
and never have a sony or H2 at all.

the world today thinks mp3 is high fi
and anything "better" sounds strange to them