Plugins to improve poor quality pre-recorded audio?


I am running an Internet radio station, Rural Catholic Radio, using a lot of pre-recorded audio.

One of the problems that I continually face is the poor quality of the source material. In many cases, the audio was not recorded professionally and suffers from ‘metallic-tunnel’ sound (that’s the best way of describing it), transient interference (high-pitched whistling, or hiss), or low-pitched rumble, or poor bass.

Now that I have our New Season up-and-running, I would like to redo some of the worst offenders. Unfortunately, I have little experience with plug-ins and would appreciate advice of which ones might be useful for correcting the worst offenders. Any suggestions?

I realize, that for the most part, little can be done to improve poor-quality vocals, but I’m hoping …

Thanks & hoping,

What can be done to fix the various problems depends very much on the ‘context’ it appears in - e.g. you can use a good quality EQ to correct for poor bass or remove rumble (and even to reduce hiss or whistling if you can tune in to the pitch it is happening at) but you will always affect some of the audio in the process. This may matter less for example on a voice only recording (if the focus is mainly on intelligibility) than it would for music - where you might affect the balance of the mix undesirably.
As for the ‘metallic tunnel’ sound - that seems like it could be bad room accoustics where the original recording was made (e.g. some kind of harsh echo / reverb from the walls) and there is no such thing as a plugin that can remove or produce ‘anti-reverb’ to compensate. (Which is why so much attention is paid to creating a good ‘neutral’ recording - you can add as much reverb / accoustic effects as you like later, but you cannot remove them if they are already in the recording)

Thanks linuxdsp,

The ‘metallic tunnel’ is the one I really wanted to adjust, but as you mention it was probably recorded with a tape recorder omni-dir mic, and judging from the other noises, in what must have been a large hall (lunchroom) with lousy acoustics.

I’ve already tried to adjust the sound in that series of recordings with little effect. I thought that if I increased the bass, and cut or limited the med-to-high freqs I might achieve something. Alas, to not much avail. Too bad - the content is excellent but is very hard to listen to for any length of time.

Btw, what is a ‘compander’ plugin? Is there a list that describes what each plugin does - the names are so cryptic to me (not being a musician) :frowning:


You’re unlikely to reduce the effects of room reverb by using EQ - although you may be able to make the recording less fatiguing to listen to.

Btw, what is a 'compander' plugin?

A compander is a term derived from ‘compressor / expander’ - essentially two processes which alter the dynamic range of a signal.

In a compressor, the dynamic range is reduced by a ratio, above a given threshold so for example if the threshold is -20dB and the compression ratio is 10:1 it means that for input signals louder than -20dB, a level increase of 10dB at the input will result in only a 1dB change at the output.

An expander provides the opposite effect, thereby expanding the dynamic range in a similar way.

Dedicated compander circuits used to be used as a form of noise reduction in analogue circuitry (mainly tape recorders). In that situation, the audio is compressed by a low ratio (e.g. 2:1) before it is recorded to tape. Then when it is played back it is expanded by e.g. 1:2 - this (mostly) restores the dynamic range of the audio but has the useful effect that noise created as a result of the recording / playback process - such as tape hiss - (and which therefore has not been subject to the compression) is also ‘expanded’ by the same amount which improves the signal to noise ratio by effectively reducing the level of the noise relative to the original signal.

You’ll probably need to make a mixture of EQ boosts and cuts. I’d start off by making sure you’ve got a full range of EQ tools: high pass, low pass, multi-band parametric and multi-band compression (check out some of linuxdsp’s own plugins).

Parametric EQ can be very useful to tune into a problem area then either cut or boost as needed.

Likewise multi-band compression. For example, you can try to take out just the “boom” in a boomy bass end without simply cutting all the bass frequencies in the signal.

I have a hunch that the said “metallic tunnel sound” is actually the result of a low quality mp3, or something of the like. I don’t know if the suggestions made would still follow in that case (others here probably know). In my experience, EQ tricks are best applied to steady background noise - not “mp3 noise”.

@booniesboy: What media is the audio currently on? (CD, tape, hard drive, etc), and do you have access to the original media?

Thanks for replying!

@mcgruff: I’ll give these ideas a whirl, now that I know the names of the plugins

@kelleydv: The source is low-quality CDs - all wavs (at 14H 44.1k – I have no idea what ‘14H’ means??). They sound like they were recorded in a very large hall with horrendous acoustics with a cheap mic (probably a built-in mic on a cassette recorder).

All sources are now on a separate hard drive in wavs.

I think I already did some basic EQ on the series in question, but not knowing what to use, I tried a minimalist approach. It improved them somewhat - I believe I used bass boost, and chopped some the mid/high freq. Too bad I hadn’t kept any notes on what I had used . :frowning:

I was in a rush to at least get them on-air, but now that I have more time, I’ll go back and try to improve the sound (somewhat).

The other series were on tape (required over 100 hours of real-time digitizing). I did a fair amount of 1khz damping and boosted low-end bass - they do not have the metallic-tunnel sound.

And the last bunch - supposedly professional CDs that I paid a pretty price for - most of them I had to throw out - they were beyond ‘repair’ - poor voice quality, poor diction, etc.

Thanks for the ideas!

I know this is quite old, but I thought I would throw in my two cents. One tool I have found helpful in dealing with really problematic audio is a multi-band compressor like in Jamin. This lets the filter kick in only when really needed and not affecting all the audio like a straight EQ would do. If the “metallic tunnel sound” is from MP3 compression or possibly noise removal processing, there are usually underlying frequencies that come through and may be dealt with this way.