Revision of Processing Cassette Tapes from Wed, 02/27/2013 - 10:00

Audio File Processing

Feb 27, 2013

General Notes on Audio Library Creation and Noise Correction

How ANRL’s Local-Web-Page was built

  • ANRL’s Historic Audio Cassette Tapes were played one by one with a cassette tape player. I connected the Aux Out to an XITEL INPORT (http://www.xitel.com/manuals/INportDeluxe_manual_vista.pdf) and fed that into Audacity on my Mac.
    Audacity is a well-known program for processing audio files.
  • One of my Mac computers was capturing audio files, a second Mac was running Audacity to do the noise reduction and editing.
  • I saved both the original and edited .mp3 files to my Drobo storage
  • I saved each updated .mp3 file to my Drobo storage.
  • I used DropSync to automatically copy files from my Drobo to an external ThunderBolt 1TB storage device.
  • I wrote a custom perl program to build ANRL’s Local-Web-Page
  • I use a text file as the master database file. All of the track information and notes are in that file.
  • I re-run my perl program to rebuild ANRL’s Local-Web-Page whenever the master data file changes.
  • The updated web page is then copied for other members of ANRL to review.

Audio Capture

I used the Audacity program to capture audio from the cassette tapes and also to apply noise correction. Audacity is free ‘Open Source’ software and you can download it for both PC and Mac from http://audacity.sourceforge.net

A note about me…

  • I’m Bob Proctor and I live at Cypress Cove. You can contact me though the ANRL web site or by the Webmaster link on ANRL’s Local-Web-Page.
  • I’m a beginner at this sort of conversion. I know some computer tricks and a little about .mp3 files and noise correction but I’m no expert.
  • I’m open for suggestions for better noise correction techniques, and even better ways of getting good audio from bad cassettes.
  • Please contact me with your suggestions and tips!

Basic Noise Correction Strategy

We have over 200 audio cassettes which are mostly over 50 years old. I don’t have a clue what the original recording equipment was. The media (cassette tapes) is not the same throughout the collection and the quality of the media is fairly low quality when compared to modern tapes, for example: TDK IECI/Type–1 Dynamic Performance D90.

I made two libraries of .mp3 files: the original copy and an edited copy in which I applied noise reduction filters.

The original .mp3 files had simple edits: cutting off dead time between Side 1 and Side 2 and the dead time at the beginning and ending of the tape. The original .mp3 files could be used by anyone wanting to attempt different noise cancellaton techniques than the ones I used.

Tapes all have a background humm noise which is a combination of 60cycle and 120 cycle noise. Some tapes have airplane noise and dogs barking. There seems to have been a loose wire on the microphone used because when the microphone was moved between speakers, there was huge static and audio output would settle down to different levels on the two channels. There was no felt pad under the microphone, therefore sliding the mic on a table was another source of huge noise. Attempting to filter out all the bad audio also removes part of the good audio. I learned that it’s better to not be very aggressive with the filters.

When I started this project, I was quite aggressive and was able to reduce audio down to around –40db in the pauses between spoken words for some speakers. I then was able to use the ‘Truncate Silence’ filter and reduce length of some tapes by more than 5 minutes. Problem was that the audio was awful and even harder to understand. I may want to go back and re-process those recordings.

Sterio vs Mono

All the tapes were recorded in Sterio but the levels of R & L channels vary considerably on some tapes. I started doing the conversion in Mono (after ~ 25 were done in Sterio) because of the low quality of audio. Seemed to me that we could get no advantage of using Sterio.

Recording Level

I tried to keep the max level just under –1db. Some tapes started out with low audio but then audio increased and lots of clipping and distortion took place. We may need to re-record some tapes.

Audio Filters

  • Envelope Adjust: This was used in some cases where a segment of audio was lots highewr than other segments. I lowered the big audio to match the lower audio
  • Click Removal: Hard to know optimal control settings. I used 150/35 for most.
  • Noise Removal: Get a noise profile from a part of the tape with just noise then apply Noise Removal to entire tape. It’s hard to know what the optimal settings are for this process.
  • Noise Reduction: The basic idea is to Select a quiet area in the sound track that has just noise. You select this and then: Effect -> Noise Removal… -> “Get Noise Profile”. Then, select the entire track and do this: Effect -> Noise Removal… “OK” after adjusting parameters.
  • Normalize: I used –1db or 0db and mostly always applied this as the last filter. Here again, you select the entire track and do: Effect -> Normalize… -> “OK” after adjusting parameters.

Basic Workflow

  1. Cut the dead space, if any, from the ending, middle splice between cassette sides and beginning.
  2. Apply the Effect -> Click Removal filter
  3. Apply the Effect -> Noise Removal… filter. Find a couple seconds of quiet audio which has noise you want to remove, and get the Noise Profile from that after adjusting parameters. 9db seems to be good number experimentation is in order. Sometimes I tried 12db. Sometimes I applied Noise Removal twice while using different noise profiles.
  4. Optional use of the Envelope Tool.
  5. Apply the Effect -> Normaliz… filter. I mostly used either –1db or 0db.
  6. Workflow changed as I got into this project and learned more stuff.

Tips and Tricks

  • It’s handy to have a cassette tape repair kit avialable
  • Tapes break in many different ways
    • Broken take-up reel: I replaced take-up reels by harvesing old reels from old tapes I had around the house. I spliced in the leader from old reels.
    • Broken tape:
    • Mangled tape: I removed a few feet here and there that was so mangled it just wouldn’t work. That seemed to fix the problem.
    • Seriously mangled tape: I had two tapes (014 and 004) which became bound up on the take-up reel to the extent that the tape stopped turning. There is no easy way to take the cassette apart other than to destroy it. The tape is nearly impossible to fix when this mangling starts. Tape becomes twisted, folded, and doesn’t feed on the take-up reel. The thickness of tape becomes bigger than the cassette can hold and the tape stops feeding. What to do with these tapes?
    • Tape spinning ahead at 5–10X Speed: No clue why this happens, but rewinding and trying to play the tape again and again may fix it.
  • It’s handy to glance at Audacity’s History file to make sure all steps were taken. Some steps take several minutes and while doing three things at once it’s easy to get confused and lose track.
  • A super fast computer really helps speed the process and keep a nice flow while working on each tape.
  • We don’t have a good way of doing OCR on the handwritten notes. Many of them are written into the Notes section of tracks. If you display Master Index sorted by Tape Number or Master Index sorted by Date, oldest first, you will have an opportunity of using your Browser’s Search engine to find text that otherwise might not be found. That may get you to a specific track and help find stuff.

A Challenge for you:

  • Listen to the original version of a track, and then listen to the enhanced version. If you know better noise reduction techniques then please contact me.
  • Listen to the the audio tracks and take notes, then send them to me. I’m wanting to get names, dates, places, and other information into my database so I can better cross-reference this material. I’m also wanting to correct any errors I may have made.