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Thursday, April 06, 2006

Levels Redux

News and Notes
Direct link to the mp3: http://www.archive.org/download/Levels_Redux/FDA-11.mp3
If you want to include a short promo send an mp3 to digiaudio.podcast@gmail.com

Sony Announces new Minidisc recorders

Other podcasts with a focus on Digital Audio

Look for a review soon of the Boss 1200CD: http://www.bosscorp.co.jp/products/en/BR-1200CD/

Audio Hijack Pro: check it out - very useful. Inexpensive $32 or $16 -10 minute limit in Demo. Classic podcast use is to record Skype interviews: http://www.rogueamoeba.com/audiohijackpro/

Levels Redux
Peak Levels vs. RMS levels - what's the difference? We need to keep track of both
I became infatuated with the idea of getting the RMS levels spot on (-17.5 db) right from the recording. The problem with this is it requires recording at very high or hot levels. This means I would at times shoot past a peak of 0db which causes terrible distortion called clipping. I thought I could use a software peak limiter to control this, but I have learned that once it is clipped it stays clipped (or distorted) in the digital domain. So you need to keep track of both

What to do about this?
Invest in a hardware limiter that limits the signal before it is convered from analog to digital. This probably mans investing in a compressor/limiter, xlr mic and mixer with inserts.
Or, record at lower levels and boost in post - best way is to use the RMS Normalization feature of apps like Peak Pro, Audition or SoundForge, but those are expensive

Step by step in Garage Band
  • Set input levels
  • Check levels with RMS buddy on the Master track. I get about -25db average or RMS level and a peak of about -4 to -6db.
  • Garageband lets you boost a track by a max. of 6db. So i've boosted by 6db and added a peak limiter on the vocal track. In this case the peak limiter is set to -5db. I'm finding that the limiter can't react fast enough to limit at a level of -1db - I still get clipping so I had to lower it to -5db.
  • After boosting I get RMS levels of about -21 db or so with peaking still under 0db. We are shooting for about -17.5 so we are a little short. Still this gets us at an RMS level about the same as public radio uses - all in all not bad.
If you are really bent on getting to -17.5 db you can export the vocal track out of Garage band, import it back in and boost it by 4 to 5 db, and apply a peak limiter again. May not be worth the trouble.
It's a lot easier to use a tool that has RMS normalization like Peak Pro or Audition, Sound Forge, Wave Editor, etc...

Wrap up
digiaudio.blogspot.com for show notes and links
digiaudio.podcast@gmail.com for comments, feedback or audio promos
Next time Boss BR 1200CD
Still working on into to DSP as promised

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