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Audio Frequency Question

post #1 of 6
9/26/04 at 2:15pm
Thread Starter 
I wasn't sure which forum to post this in but here is my question. I'm taking some tapes and converting them into .wav files on my computer. The tapes have quite a bit of hiss on them. The program I am using allows me to basically cut off any sounds beyond a certain frequency. I randomly chose 10khz which seems to get rid of the hiss. Am I cutting off any of the "music" at this level? Should I set the cutoff point higher? Or can I go even lower? Thanks for any help.
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post #2 of 6
9/26/04 at 3:02pm
Anytime you are cutting off audible frequencies, you will likely cut off music, which will depend on what is recorded within the music (instruments, human voice, etc.). If you can get a hold of a std. equalizer (graphic, parametric would have more control and likely fewer bands) and cut down the level around 8k you will reduce hiss. You will also reduce the level of the music at that frequency though. If you can find a DNR or Phase Linear (e.g. model 1000 series one or series two - such as http://cgi.ebay.com/ws/eBayISAPI.dll...722298361&rd=1 or http://cgi.ebay.com/ws/eBayISAPI.dll...722527056&rd=1) single ended noise reduction unit you could try that in between the tape and the recording source. Basically this has a knob that let's one adjust that takes out stuff like hiss that would be appropriate to set at a level that does not impact the music. A long time ago, before stereo TV, I used one (series II) along with an outboard TV tuner to remove hiss on broadcasts of old movies and other sources.
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post #3 of 6
9/27/04 at 2:05pm
If you're doing a high-cut operation at 10KHz, you are cutting off some harmonics, with the result that you will be changing the audible characteristics of the sound; it may be "fuzzy" or grate on the ears. Of course, the noise is already doing that, so it's a tradeoff. If you absolutley can't get any kind of noise-reduction equipment [and are these "tapes" encoded with Dolby B, C, or XR noise reduction? if so you should be using it on playback, or the sound will be distorted as well as noisy], I would suggest a somewhat higher cutff. FM stereo, for instance, cuts off at 15KHz, which is usually acceptable, although the tape may also have a cutoff there which would leave all of your hiss below that frequency. Just in general, the more bandpass you have, the less "raggedness" there will be in the sound [it sounds to me like a saw, if that makes any sense :b ].
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post #4 of 6
9/27/04 at 8:51pm
I'd recommend using software that will let you take a "profile" of the noise (by looking at a section of the tape that should be quiet, and so only has noise, no music). Then it uses that profile to try to get rid of the noise in other places. That should do a much better job than just cutting off everything above 10kHz.

CoolEdit will do that, and I think the demo version is fully functional for a certain number of days, so that may be worth a shot. Alternatively, it looks like Audacity (the free, open-source audio editor) has a noise removal function. I have no idea how it works or if it's useful at all, but I'd be surprised if it didn't sound better than what you're doing now...
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post #5 of 6
9/28/04 at 7:00am
Thread Starter 
thanks for the input guys. I'm using goldwave, and i did finally figure out how to make a profile of the hiss before the music starts, and do noise reduction based on that rather than cutting it off at 10khz. It seems to work much better this way.
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post #6 of 6
9/28/04 at 10:05am
I'm glad you found a better way. Hopefully you can adjust the amount to your liking. Just cutting off above 10k does nothing for the remaining hiss. Background tape noise actually exists in full-spectrum so you would still have all the noise below 10k to deal with.

There are harmonics that can reach into the 20KHz range, but whether the tape used can go that high or if your hearing can detect that or not is another issue
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