Survival Guide - Digital Audio/Video

Digital Audio or Audio/Visual is one of those areas that crosses the border between pure technical knowledge about computer'ish subjects - file formats, bit rates, sampling and all that good solid stuff. But it also moves into other territory to do with sound frequencies and if the material is musical - tempo and pitch and harmonics and...and .... And the maths - oof! - Fast Fourier Transforms (we had assumed Fast Fourier was a foreign relative of Fast Eddy) and RMS and Logarithms - stuff we thought we had long, and mercifully, left behind. We were non-plussed (gotta love that word) on our first excursion into the depths of this dark and frequently secretive world. And the shock when we discovered that you had to pay for some specifications. And Patents. We have only recently recovered.

Contents

Our history, needs and plans with digital audio (we love to write this motherhood stuff)
Sound Primer (Frequencies, Notes, Harmonics, loudness etc)
Digital Sound Primer (Quantization, Time Domain, Frequency Domain)
Musical Notes by Frequency
Frequency Ranges of Things
Equalization Notes
Audio/Video Formats
Acoustic Calculator
Audacity Notes
MoreAmp Notes
Glossary

Our History, Needs and Plans for Digital Audio

History: The project is partly professional, partly hobby. We started trying to restore a number of digital audio tracks that had mostly been captured from very old 78 RPM records, occasionally vinyl, but mostly good old 78's in the majority of cases from the 1920's, 30's and 40's. In some cases we had access to the source material and could re-capture but mostly not. We had what we had and were expected to perform miracles. Which as we all know can take a bit of time.

We started to use Audacity and that took us a long way. The noise removal and audio repair functions are very impressive. Then we moved into equalization in order to improve sound quality. We made some bad mistakes here. But since we always kept the original files nothing catastrophic occurred - we could always start again. But there are limits to Audacity when using some services - that does not detract in any way from the excellent quality and functionality for many, many other purposes and we still regularly use it. In particular we found that the batch mode of spectrum analysis extremely limiting. Of necessity it uses an averaging method to compute the spectrum graph and initially this was sufficient.

Then we got greedy. We wanted to be able to see the dynamic frequency. So we experimnted with a number of real time frequency analysers. Then we got very unhappy - some used 9 bands, 7 bands, 12 bands. Some use green and red leds to show intensity (what's intensity and does that relate to loudness). Almost none provided any useful documentation of their selected frequency bands or the reasons for the colors. Some came with equalizers which almost never were tied to their frequncy displays. A 10 band frequency analyzer with a three position equalizer (bass, middle and treble) is not too exciting especially when it does not even define the cut-offs for bass. Seemed this was more of an art form than a serious endeavour - not that artists are not serious people. Some of them are extremely serious.

If we were prepared to pay $gazillions we could get all we wanted. But we were not. So we finally found MoreAmp which has a great 31 position (1/3 octave) equalizer. So using our ears as a frequency analyzer (not too reliable) we played with that and it took us further down the road.

Finally we got past greedy - we got gluttonous. What we really wanted was:

1. A real-time frequency analyzer with at least 1/3 octave (31 bands)
2. The ability to drill-down even further within one or more 1/3 octaves - 1/12 octave or even lower
3. VU-Meter/PPM with peak detection showing on a dB (SPL) scale not these meaningless undimensioned scales of the original meters. These was an excuse 70 years ago when this stuff was originally developed but not today.
4. An equalizer that would reflect directly the frequency analyzer (1/3 or lower)
5. A hard limiter on the equalizer, so we could set the gain level but be able to cap it.
6. An equalizer that would be optionally harmonic sensitive - we could set a profile so that if we were picking up (amplifying) say a violin we could define a harmonic profile that would automatically apply proportional gain to the first, second etc harmonics. eventually we could build a series of these and call up one or more - boost the violins and reduce the violas. In real-time.
7. We wanted to be able to change equalizer settings at various times within a track. Thus the ability to have multiple equalizer profiles that we could set against the track time. Use equalizer profile 1 until 35 seconds, then switch to profile 7 until 1 minute 2 seconds etc., etc..
8. Since in our old recordings we felt that we had some rogue frequencies we wanted an equalizer that could move through a selected range (scan) and knock out (or boost) one frequency range at a time (with or without harmonics) to see the effect.
9. We also wanted an enhancer that would allow us to add or replace harmonic frequencies from a given starting point - based on the instrument's characteristics.
10. Clearly we wanted to able to both playback the effect and/or capture to a file in at least WAV and FLAC format (we could subsequently convert to any other format required) once we were happy.

None of this seems too, too unreasonable and some of it is pure speculation on our part that it will yield the reults we want. At this time we have decided that we need to develop our own tool-set and have chosen to base them on MoreAmp. We like Open Source but will release our code under the more permissive BSD/MIT license rather than GNU GPL.

Don't hold your breath waiting for this stuff to appear.

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Problems, comments, suggestions, corrections (including broken links) or something to add? Please take the time from a busy life to 'mail us' (at top of screen), the webmaster (below) or info-support at zytrax. You will have a warm inner glow for the rest of the day.