Stereo Tool

I've just put a "preview" for version 3.31 online. It's a preview because it is not yet finished. But it contains some features that might be very useful for some users, which is why I'm publishing it here now.

New in version 3.31:
- Greatly improved Loudness filter, which causes far less artifacts than previous versions. (1)
- Hard limit and FM Overdrive are now much better in avoiding peaks in the FM signal. (2)

(1) Works best when "Quality" is set to High. But it takes more CPU power than previous versions. At quality Normal, the CPU usage is only slightly higher than in version 3.30, at quality Low it's a lot lower. The difference in output quality of the Low quality version is quite audible though, so if you can, run it at "Normal" or "High" quality.

(2) Stereo Tool now uses "oversampling": It looks at the output signal at a much higher sampling frequency (768 kHz instead of 192 kHz). This enables detection of peaks that would otherwise go unnoticed. Previously peaks of over 10% of the maximum volume were possible in the FM signal, now that has been reduced to less than 1%. (5% if Quality is set to Low). Unfortunately this also causes a higher CPU load.

If both options are used at quality High, the CPU load is about 50% higher than in version 3.30. At quality Normal it's about 30% higher, at quality Low it's only a few percent higher.

You can download the new versions at:
Winamp plugin: http://www.stereotool.com/download/dsp_ ... r_BETA.exe
Stand alone version: http://www.stereotool.com/download/ster ... r_BETA.exe
Command line version: http://www.stereotool.com/download/ster ... d_BETA.exe

I will update these versions regularly until the new version is finished.

Note: At the bottom of the FM window you'll find a lot of new sliders. They don't do anything yet, so please ignore them for now. When version 3.31 is ready, they can be used to calibrate your transmitter to achieve better stereo separation and a more constant MPX signal - especially useful if you have a low quality sound card, a low quality transmitter or long cables to your transmitter.

FM calibration filter is now working.

Using my (very cheap) transmitter, my stereo separation increased from 20 dB to 35 dB using this filter.

IF you have a very good quality transmitter, a very good sound card and extremely short high quality cables, you probably won't need to do this (and if you do, you'll end up with all the sliders at the same value). Of course it never hurts trying - the signal MIGHT improve.

How to use it:
Don't forget to turn the filter on first! It's on the FM configuration screen. Then, click on the "Calibrate" button.

1. Make sure you have a method of measuring the MPX volume on your FM receiver. (You'll probably have to find a place on the print of your radio where you can fetch the MPX signal from, before the stereo decoder.) You can also use a device to measure the FM signal such as http://pira.cz/fm_broadcast_analyzer/ .

Note: If you don't manage to measure your MPX signal: I'll create a different method for calibrating the transmitter later. It will probably be less precise though.

2. For each of the sliders on the top of the calibration page, set the slider such that the volume you measure on your FM transmitter is identical. Note: If you use the MPX signal, chances are that you'll loose some signal at the high frequencies in the receiver or between the receiver and the measuring device. There are methods of fixing that later on, but using a separate device will probably work better.

3. Move the 19 kHz phase slider until you hear almost silence on a normal FM receiver.

4. Move the 20 - 10240 Hz phase sliders until you hear almost silence on the right channel on a normal FM receiver. If necessary, move the matching volume sliders also to approach silence even better. If that's necessary, it means that your measurements at step 1 were not precise enough. (You'll probably need to increase the volumes of the 20-10240 Hz bands a bit.) Correcting it in step 4 will improve your result, but you won't reach a perfect result this way. I will add a different way of calibrating it later on, where you don't need the MPX signal anymore.

Better audio quality with different settings!
The old Loudness filter needed a signal without peaks above a certain value. This is no longer the case for the new Loudness filter, which means that you can make some changes to greatly improve the audio quality.

1. You don't need the final (pre) limiter anymore! But you'll have to turn down the Loudness slider a bit.
How:
- Turn off the Final Pre Limiter
- Change the Final Limiter volume such that you get very little compression (black bars coming from the right). You'll probably end up somewhere around 0.60 - 0.80. Let's assume 0.70.
- Then move the Loudness slider to the value you were using, multiplied by the final limiter volume. For example if Loudness was set to 3.0, move it to 3.0 * 0.70 = 2.10. Turn the Final Limiter off.

Because we're not using the final (pre) limiter anymore, peaks in the signal are allowed. So, now it's possible to increase the clipping values in the multiband filter. That gives a much more powerful, less flat sound. And the great thing is: You'll still be able to reach the same output volume - and lower the CPU usage.

Note: When you turn the Loudness slider too high, the highs sound less clear. I've noticed that turning on the Final Pre Limiter (without the Final Limiter) seems to help a bit here. I'll look into this further later. If you don't set the Loudness slider higher than the value calculated above the highs should be fine.

I will update the presets for this later.

Another day, another update.

I have added settings (and measurement methods) to correct phase differences in the 28-48 kHz area.

And I have added 3 test methods where normal music is broadcast on the left channel, silence on the right channel - using 3 different methods: One with normal stereo, and 2 methods where only half of the stereo signal is used to broadcast silence. This helps creating a good stereo separation and a more constant maximum volume in the MPX signal.

Performance slightly improved.

Normal quality: Loudness filter quality slightly reduced for a performance boost.

Small performance improvement (5% more efficient - measured for FM Loud preset, quality Normal). Also the delay caused by the sound processing has been slightly reduced.

Another 4% (Normal, FM Loud) performance improvement, and a very small reduction of the delay. At quality Low the performance gain is 5%.

And another 1% (Low and Normal quality).

I've noticed a small bug in the stand alone version: The bars below the Final (Pre) limiter are not displayed correctly. Will look into that later.

Important bug fix: The "++" filter didn't filter out the distortion that was caused by the extra loudness boost. It does now. Also, the sound has become a bit clearer.

[Edit: New version sounds really crappy when there are a lot of bass sounds. Will look into this later. For now, you probably shouldn't use ++]

"++" now gives a gain of 3.5 dB instead of 2 dB in previous versions. Loud bass sounds cause some distortion, this will be fixed later. (probably by adding a filter that limits the bass sounds before the ++ filtering)

Feedback about the sound quality is very welcome.

Hint: When using "++", don't put the Loudness slider too high, or you'll get a lot of distortion. (I've tested it so far at values around 2.20).

some feedback ...

I noticed the same thing about the loudness ++

Overall, looking better and better - keep up the good work!