Limiting & Clipping section The final clippers and limiters.
Stereo Tool has 2 clippers built in:
Simple Clipper This clipper is indeed very simple. It does not handle distortion too well, so you should only use it to remove some occasional spikes.
Advanced Clipper The Advanced Clipper uses a lot of techniques to minimize or even completely remove digital clipping and intermodulation distortion. It can produce an extremely loud, yet still dynamic and distortion-free sound. The Advanced Clipper requires registration.
These clippers may still leave some very small spikes through. To remove those, Hard Limit output can be used. Final peak remover panel Removes small peaks above 0 dB in the audio.
Hard Limit output Removes any small remaining peaks after the rest of the processing.
This is the final step of the processing chain. If there are any peaks left above 0 dB, this removes them by quickly lowering the volume around the peak. This filter was built to avoid creating harmonics, not to sound good - so it should not be used to remove big spikes.
If the Post Amp slider is set close to 1.00x (0 dB), HARD LIMIT should always be enabled to protect against clipping. For FM output it should also always be enabled, to protect against overshoots.
Prepare for lossy compression (MP3/AAC/OGG/...) panel Protects the output against overshoots when using lossy compression (MP3 etc.) on it.
If you compress the tightly clipped output of Stereo Tool with a lossy codec such as MP3, AAC, Ogg, you will notice that you get spikes again. This is caused by the fact that almost any change in the tightly clipped signal will cause it to be less tightly clipped. To avoid getting distortion, you need to lower the output level.
But because differences in the signal occur much more in higher frequency ranges, it helps to boost the highs a bit before clipping, and lowering them again afterwards.
This graph shows the effect of different settings on a 128 kbit/s MP3 file.
Quickly lowers the volume if the input level exceeds this level. Use with care, may cause distortion by itself if the cause of the high input level is loud bass.
Pre-limiter volume The level at which the Pre Limiter starts to work.
Advanced Clipper section Clips the output, while removing distortion caused by clipping.
Boosts the output level without increasing peak level. The Advanced Clipper can make the sound up to 12 dB louder, without causing higher peaks (hence clipping) in the output.
The clipper in Stereo Tool detects and actively removes (almost) any distortion that is audible by humans - but nothing else! This results in a clean, open, dynamic and very bright sound.
This filter is intended for FM, AM or internet radio stations that want to sound loud. It can also be used to create a 'denser' sound.
Loudness leaves some small spikes in the signal. Therefore, if you use this filter, and the Post amplifier slider is set at (or just below) 0 dB, make sure that Hard Limit output is enabled to avoid distortion caused by clipping. If you use Stereo Tool to prepare an FM signal, always turn Hard Limit output on when using the Advanced Clipper, regardless of the Post Amp slider setting. Volume boost panel Volume settings that control the clipper behavior.
Advanced Clipper Enables the Advanced Clipper.
When the Advanced Clipper is enabled, the Simple Clipper is automatically disabled, but the advanced clipper level is still adjusted for the simple clipper's Amplification.
Clipper drive (Loudness) Sets the amplification before going into the clipper.
Clipping always occurs at 0 dB, this slider adjusts the input level to match.
Relative non-FM loudness
Oversample limiters and clippers panel Forces the clipper to use in 4 times oversampled mode.
This removes any spikes over 0 dB that are created in the sound card during playback. These spikes can cause distortion based on the sound card design, but they don't have to.
This means that the clipping is stronger, hence may cause slightly more artifacts for high frequencies. For FM processing, this is automatically enabled regardless of the setting.
Oversampled limiting and clipping Turns oversampled limiting and clipping on.
Composite clipping (FM) panel Composite clipping greatly improves the quality of loud FM transmissions.
The composite clipper can create upto 160% left and right channel modulation while keeping the total modulation at 100%. It also allows the usage of Single Sideband stereo coding and several reception improvements.
If the audio is clipped normally, and stereo encoding and RDS encoding are added afterwards, the output that is generated will only rarely peak to the maximum modulation. Multiple dB's of signal headroom are wasted by clipping the audio without looking at the stereo pilot, RDS signal and how the L+R and L-R modulated at 38 kHz contribute to the peak level.
To give a simple example: If there's a high frequency peak in the audio, and both the stereo pilot and RDS signals are peaking in the opposite direction, then the peak in the audio could be clipped at a much higher level. It is possible to 'hide' audio peaks inside the holes created by the pilot and RDS signals. The effect of the 38 kHz modulation of the L-R signal is even bigger but also much harder to explain.
When using normal audio clipping and then adding the stereo and RDS signals, the left and right channel audio level after demodulation on an FM receiver will always be a bit below 100%. FM composite limiter overdrive can get it closer to 100%, but it will never exceed 100%. When using composite clipping, values over 160% can be reached. The audio level coming out of the receiver still won't exceed 100% though, because only high frequencies can reach levels above 100%, and they are reduced with de-emphasis in the receiver.
The composite clipper can not be used if you are using an external stereo coder, and works slightly less good with an external RDS encoder.
Pre-limiter section Pre-Limiter and phase-optimizer panel
Reference clipping level (0 dB to match clipper level)
Never limit bands below (relative)
Smothering (0 for normal, 1 for top half only)
Tones only (0 for any sound)
Tones threshold (100 for any sound)
Pre-Limiter effect panel
Bass section Settings that affect the sound of bass. Bass shape panel Deforms the bass to achieve louder levels and sound better on some speakers.
Deforms low bass sounds if they are being clipped by making the tops 'flatter', to make them sound louder at the same peak level. This adds harmonics, so if too much is used it can sound bad, especially with electronic music which contains very clean bass sounds. Another effect is that bass is more audible on speakers that are bad at reproducing bass.
Bass shape: Strength Controls how much deep bass sounds are made louder.
Bass shape: Max harmonic frequency Controls upto which frequency the bass may be deformed to sound louder.
Bass sounds upto this frequency can be deformed to make them sound louder without getting higher peak levels. Setting this value higher gives more bass, at the cost of the sound quality of the bass. Values upto about 160 Hz should be fine.
Bass shape: Badness (may cause knocking sound)
DC offset panel Settings that control DC offsets in the clipper output.
Forcibly remove DC caused by Advanced Clipper (reduces quality) Don't allow Loudness to add a DC offset.
In some cases, clipping only occurs in one direction (for example because there are spikes in one direction, but not in the opposite). Normally, Loudness will move the DC offset level to reduce the needed amount of clipping. This may however cause a DC offset in the output for a longer period of time, which is not always accurately reproduced by a sound card. So if you are feeding an FM transmitter with your sound card, and it's not phase linear, it's usually a good idea to enable this setting to avoid getting too big spikes in your FM modulation.
Bass protection (Deprecated) panel Was needed in the past to protect against intermodulation distortion. Also boosts deep bass.
Highs section Settings that protect the clipper against excessive highs.
Excessive highs can punch holes in the rest of the audio, which is generally perceived as very annoying in music (it's ok with speech). This is usually only a problem for FM output, where the highs are boosted by Pre-emphasis and a lot of clipping is used to reach high output levels. Highs de-esser panel Reduces excessive highs caused by Pre-emphasis.
It tries to do this with as little impact on the sound as possible. This filter only responds if the total level would be too high, so if there are no other sounds present it will not reduce the highs a lot, regardless of how loud they are. This means that it normally has no impact on speech, only on music. For FM output, the de-esser is far less needed when using Composite Clipping than when using traditional clipping.
De-esser limit Sets the value above which the de-esser needs to work.
De-esser slope Ignore de-esser for small drops.
If the highs level is only slightly too high, this slider reduces the effect of the de-esser, leaving a brighter sound. If the highs get really loud, the de-essing behavior is not changed.
Small filter (latency 1024) panel
Extra filtering at latency 1024
Highs hole punch protection panel Protect against volume drops caused by loud highs.
Protects the sound against volume drops caused by extremely loud (often pre-emphasized) highs. The only reason why this is configurable is that it takes a lot of CPU power.
High quality mode uses twice as much CPU power as normal quality mode. For pre-emphasized audio it's wise to turn it on, because the high frequencies can be extremely loud.
Smoothe highs early (potentially more distortion)
CPU section Settings that control the CPU usage (vs. quality) of the Advanced Clipper. CPU panel
Strictness (CPU) Controls how strictly the clipper clips.
Lower values leave more small spikes in the output signal, which (if you don't want spikes) need to be removed by HARD LIMIT. And HARD LIMIT removes the spikes by lowering the output level. Strictness is configurable because higher Strictness levels cost much more CPU processing power.
Take some shortcuts (CPU, reduces quality) Some calculations are done less precisely. May slightly reduce audio quality.
Take oversampling shortcuts Some processing steps are done at a lower than optimal sample rate.
Stronger clipping Forces the clipper to more aggressively remove spikes.
This allows a lower Strictness (CPU) setting to reach the same level of remaining (small) spikes which can be cleaned up with Hard Limit output. The audio quality may get slightly reduced though.
Medium stop (CPU)
Distortion protection section Settings that protect the Advanced Clipper against intermodulation distortion.
Intermodulation distortion is the type of distorting that is caused by loud bass sounds, which create 'holes' in other frequency ranges. (Intermodulation distortion is actually any kind of distortion created by modulation effects between different frequencies, but the type caused by low bass frequencies is harder to clean up). Protect panel Intermodulation distortion protection settings.
Delay bass clipping at the start of a new bass sound for this amount of time. The onset of the bass is not reduced, leading to a very punchy bass sound. But also potentially to some distortion - although this type of very brief distortion is almost not noticeable.
Clipping level during delay The clipping level used during the delay.
Relative sensitivity Reduces the bass sensitivity effect during delayed clipping.
Despite what was said in Delay clipping, in some cases delayed clipping can cause noticeable distortion. Because of that, the clipping level will still be reduced a bit if Reduce bass for mids or Reduce bass for highs indicate that it should be reduced. This slider controls how strongly it responds during the onset of a bass.
Response speed Determines how quickly a new delayed clip can occur.
A punchy sound means that the volume goes up suddenly. To determine whether the volume goes up, we need to keep track of the average level - this value tells the detection algorithm what the lowest frequency is that it can expect in the input, which helps to determine how fast it should respond to lower sample values.
Reduce asymmetric bass Clips bass harder if it's asymmetrical.
Asymmetrical bass (just like asymmetrical mids, which are handled by Phase rotation) are as bad for the sound as symmetrical bass sounds at half the frequency - meaning that they can easily cause very noticeable intermodulation distortion. If this setting is checked, the detection of asymmetrical bass causes the bass clipping level to be lowered, which protects the other sounds.
Bass clipping strictness Determines how strictly the bass is clipped.
Higher is better, but also causes a higher CPU load.
Very low bass - Clipping from 0 Hz upto Bass clipping works fully on bass frequencies below this one.
Very low bass - Drop to no clipping at Bass clipping completely ignores frequencies above this one.
Always clip deep bass below Whatever the input is, the bass may never exceed this level.
So even if there's only bass present, if this level is set low, it will not reach full modulation.
Dynamically reduce deep bass to Reduce bass further if it causes problems.
If Reduce bass for mids or Reduce bass for highs indicates that the bass is damaging other sounds, by potentially causing intermodulation distortion, the bass clipping level is reduced further, at most this level.
Reduce bass for mids Reduces the bass clipping level if loud mid frequencies are present.
Bass combined with loud mid frequencies can easily cause intermodulation distortion. Setting this sensitivity higher causes the bass clipping level to be dropped further. This also means that the bass level goes down.
Constant tone distortion protection: Smoothe mid frequencies The number of bins around a spike that are protected against distortion.
Constant tones such as voices are especially susceptable to intermodulation distortion from bass sounds. Or actually, there is not more intermodulation but it is far more noticeable.
This slider controls, when a frequency that sticks out is detected, how big the area around it is that needs to be protected against bass intermodulation distortion.
Setting this higher gives more protection, but also slightly lowers the output level.
Constant tone distortion protection: Peak detection steepness Controls the detection of single frequency peaks for Link error '3398'.
Reduce bass for highs Reduces the bass clipping level if loud high frequencies are present.
Bass combined with loud high frequencies can easily cause intermodulation distortion. Setting this sensitivity higher causes the bass clipping level to be dropped further. This also means that the bass level goes down.
Allowed highs distortion Determines how strongly the high frequencies are protected against bass.
Without protection, a combination of loud bass and loud highs leads to quite horrible intermodulation distortion from the bass in the highs, as can be heard on my FM radio stations. Stereo Tool contains a unique filter that detects when this is noticeable, and fixes it.
This slider controls how strongly the protection works. Opposed to what you may expect, protecting it too much can by itself cause something that sounds like intermodulation distortion, and the protection slightly lowers the level of the highs frequencies.
Allowed highs distortion clipping strictness How strictly highs intermodulation distortion is repaired.
Affects both the quality and the CPU load.
Highs priority Selects between clean highs and constant volume levels.
High frequencies can get extremely loud due to FM Pre-emphasis. With this slider, you can choose if you want to keep the volume of all other sounds constant (low values), or if you want loud highs to be able to push other frequencies down. That allows the highs to sound brighter, but pumping caused by loud highs is very annoying in music.
If you are using Composite Clipping, higher values are less problematic, because highs are less problematic.
Upsampled highs clipping (useless)
Sound section Settings that control the sound. Open sound panel By allowing a bit of extra distortion in some situations, the highs sound far more open.
The tight noise filtering can cause the highs to sound somewhat 'squeezed', while in many cases allowing a bit of extra noise in the high frequencies can make the highs sound both louder and less 'restrained'/squeezed.
Airy Highs short Allows extra highs distortion for loud short-term noise sounds.
Airy Highs long Allows extra distortion for longer-term (~100 ms) loud high noise sounds.
Time spread Medium Slightly changes the sound when Airy Highs long is used.
Artistic effects panel Add some distortion, always or to give more headroom to loud sounds.
Originally, this section was intended to add distortion as an 'artistic effect', but for that purpose Controlled Distortion was added later which gives a much more precise control.
It can however also be used to allow a bit more distortion when it's most likely not noticeable, and the sound is really loud. This makes the sound less 'squashed'.
Dirty Bass Allows more distortion in bass frequencies, caused by bass frequencies.
Dirty Mids Allows more distortion in the mid frequencies.
Dirty Highs Allows more distortion in the high frequencies.
Protect tones Reduces dirty mids and highs when tones are present.
Only for loud highs Turns all the Dirty settings off if there are no loud highs present.
Sparkling Highs panel
Punch (distorts tones)
Controlled Distortion panel Adds a controlled amount of distortion, to replicate the sound of certain processors.
Most compressors don't have a clipper that delivers a signal that's as clean and free of distortion as Stereo Tool. Some people (not many...) seem to like this distortion. With Controlled Distortion you can replicate the distortion from those other processors, while still avoiding some really nasty types of distortion.
Enable Enables Controlled Distortion.
Base level Extra distortion allowed for all frequencies.
It is likely (but still needs to be checked) that with a very small amount of allowed distortion, the clipper might sounds more open. And the amount of extra distortion is so low that it's hardly noticeable.
IMD sensitivity Sensitivity for the introduction of intermodulation distortion.
With the added distortion, also certain really nasty types of distortion, such as intermodulation distortion in female voices combined with bass sounds can occur. This slider controls the detection of this type of distortion.
Stokkemask FM section ITU-R SM.1268 compliance settings.
The Stokkemask (ITU-R SM.1268) is a mask on the RF spectrum (see RF spectrum analyzer). Enabling this mask makes the RF spectrum less wide, which improves multipath distortion and reception strength of your station, and reduces disturbances caused to weaker stations at nearby frequencies.
Using Stokkemask is mandatory in the Netherlands, but it improves reception everywhere. For stations that don't need to be compliant but do want the improved reception, Multipath clipper can be used - it has less impact on the audio and the CPU load but it should have the same advantages.
The Stokkemask filter can be used without using Composite Clipping, but the sound quality will suffer a lot, and the effects of the RDS and stereo pilot cannot be taken into account, which means that compliance cannot be completely guarranteed. Stokkemask (same as under FM Transmitter) panel Enables Stokkemask clipping mode.
Sloppy smoothing (lower audio quality) Skip some steps in the Stokkemask filtering.
When spikes outside the Stokkemask are detected in the RF spectrum, the signal around those spikes needs to be clipped more strongly. Around means in both directions. If this checkbox is checked, this is only done in forward direction, because moving backwards through the data (at 16x oversampling) is very heavy, and doing this in one direction has almost the same effect as doing it in both directions.
You might want to keep this turned off if you really need to be 100% compliant.
Strictness (CPU) Decreases the effect of the Stokkemask clipper by using more CPU power.
If this is set higher, multiple RF measurements are done to check if the Stokkemask clipper manages to get the signal inside the Stokkemask - once it does, the amount of Stokkemask clipping can be reduced, which also reduces the effect of the Stokkemask clipper on the audio quality and stereo separation.