Stereo Tool

One thing that I've been fighting with forever, it seems, is to have a good, quality AGC (automatic gain control). Stereo Tool has features for pre-limiting/compression (single band), multiband limiting/compression, and final limiting/compression. However, limiting/compression are relatively fast methodologies. AGCs are much slower.

With a compressor, of course, you have a fast to moderate attack time, and a fast to moderate release time. With AGCs, however, you have a moderate (250 ms) to slow (2 seconds) attack time, and a slow (2 seconds) to very slow (6+ seconds) release time. A typical AGC circuit might sample audio for a second, and then step up the audio by 0.5dB until it reaches the "target" average level. Conversely, it would behave the same to step down the audio.

This is a good way to effectively "Replay Gain" everything without having to actually do it.

A GOOD agc circuit would have the following settings:
-Target average level
-Average level sample time (the length of the sample to determine an average audio level)
-Rise rate (in dB, this would be the rate which the audio rises per sample)
-Fall rate (in dB, this would be the rate which the audio rises per sample)

I have worked to develop a solution for this, as I have switched to a Mac, and am developing in Audio Units. This is just a constructive thought for Stereo Tool as well. My stream uses my audio units, and respective configuration.

My Mac stream can be heard at the same place as my former Windows stream: http://superh.dyndns.org:8000/listen

I do check into Windows occasionally to investigate the latest Stereo Tool. I must say, it has gotten very impressive!

I very often miss Stereo Tool.

SAM Broadcaster has a HORRIBLE AGC. I'm sure yours could be much better, should you choose to make it!

Hi SuperH,

It's not as configurable as what you are describing, but have you tried the Pre Limiter combined with "Remove remaining loud peaks" in the Pre Limiter window?

The Pre Limiter should basically do what you are describing (it responds slowly to loud sounds, and afterwards the volume is increased slowly). In the latest versions of Stereo Tool the highs are protected against getting far too loud if there's a lot of low and little high content in the original signal.

The Final Limiter leaves some peaks when the sound suddenly gets very loud, "Remove remaining loud peaks" attempts to fix that.

Also, in the last few versions, Pre Limiter now looks at how loud something sounds, as opposed to the peak level that was used before version 4.00.


This means that the output volume of JUST the Pre Limiter, with everything else turned off, should already be more or less constant.

I will have to try this next time I am using Windows (and tweak that preset a bit).

If you listen to my stream, and know what to listen for - you'll hear a couple of things.
1. Quiet parts of the music get very slowly stepped up.
2. Loud parts of the music do not drop suddenly.
3. The output level is fairly constant.
4. There is near-source-original dynamic range.

How am I going about this?

I am AGC-ing the audio to maintain an average level of -14.5dB in the digital domain. Then, this is sent through a fast-acting multiband compressor with a high compression ratio and a lowered threshold. This compressor has a relatively fast-attack and a moderate-fast release. From here, the dynamic range is flattened out slightly, but the levels across the board are constant. The gain has been dropped considerably. So, the multiband compressor output is equalized to apply a "house curve", but fairly consistent, relative to original timbre.

From there, the output is passed into a multiband compressor with a moderate attack and a slow release, with a threshold at -40dB and headroom of -40dB. This ensures consistent output performance.

It is still a work in progress, however.

Reading this, I think your in-between multiband compressor corresponds a lot to my "remove remaining peaks" option in the pre limiter...