Subject:Noise reduction

Posted by: knightbeat

Date:10/23/2002 7:14:59 AM

What settings are need to get Dolby C noise reduction when recording a sound file?

Thank you.

Subject:RE: Noise reduction

Reply by: Chienworks

Date:10/23/2002 8:16:22 AM

Well, this concept doesn't really apply to digital recordings. Dolby noise reduction is intended to deal with noise inherent in recording on magnetic tape. In digital recording you're going directly to the hard drive, which has a noise floor of zero (infinite S/N ratio). Now, before various people point out that sound cards can be very noisy, let me say that i know that. However, after the signal is digitized by the sound card, there is no additional noise added. Any signal processing that can be done will happen after the signal is digitized, and has therefore already picked up the noise from the sound card. So, something analagous to Dolby for digital recordings really doesn't exist.

Dolby C does two things to the signal as it's being recorded. It amplifies the signal in the frequency range that contains the most hiss, and it also raises the quieter parts of the signal to help make them stronger than the tape noise. Playing back a Dolby encoded tape reverses these two processes, restoring the signal back to it's original state, and reducing the tape hiss at the same time. If you want to help reduce the noise and hiss added by the sound card in the computer, the best thing to do is make sure the signal coming into the card is as loud as possible without distorting. I suppose you could also use an external EQ before the sound card to boost high frequencies and an external compressor to boost quiet passages, then undo these effects in SoundForge after recording. The point is that this has to be done before the sound card gets the signal.

Subject:RE: Noise reduction

Reply by: knightbeat

Date:10/23/2002 3:33:58 PM

Thank you for the reply. My questions arouse from the fact the an audio file I has created from a tape has allot of hiss. It was a bad recording and just wanted to see if I could get rid of some of the noise using the filters after the fact the file had been made, only to get results where the sound level was reduced, or it sounded like it was in a tunnel. So I thought there might be a way to filter this stuff out like Dolby would do while the file was being created.

Thank you again.

Subject:RE: Noise reduction

Reply by: Chienworks

Date:10/23/2002 4:54:25 PM

Do you have SonicFoundry's NoiseReduction plugin? If so, you can use it to sample some of the hiss in the file and analyze it. It can then do a rather good job of removing that hiss pattern from the rest of the file. Other than that, i'd suggest playing with EQ. Hiss is quite often in the 4KHz and above range. Removing this range won't diminish the recording much, but it may make it sound a little dull and lifeless. You may find a setting that is a good trade-off.

Subject:RE: Noise reduction

Reply by: knightbeat

Date:10/23/2002 9:59:53 PM

Thanks again. When you say external EQ, are you meaning graphic equalizer?

Regards.

Subject:RE: Noise reduction

Reply by: Chienworks

Date:10/24/2002 2:39:06 AM

Yes. Get a 31 band or even a parametric EQ if you can get your hands on one. The typical home stereo 10 or 15 band units won't be of much help, and might be rather noisy themselves.