SSL releases free X-ISM plugin

X-ISM is a state of the art VST/AU plug-in which can interpret inter-sample peaks, allowing engineers to make informed judgements about the resultant sound quality of the mix.

There is currently a trend in the in the audio industry known euphemistically as ‘The loudness war’ in which the engineering process is driven to produce a product which is as loud as is technically possible within the constraints of the medium. Debate rages as to whether this produces problems that degrade and distort the audio. One reason for this distortion is often blamed on the presence of ‘inter-sample peaks’, where signals that are usually missed by the Fs sampling of DAW meters, may actually exceed 0dBfs in reconstruction from digital to analogue domain in some Digital to Analogue Converters (DACs).

– Why is this important?
Reliable metering and monitoring is a key to consistent results. Recording studios of all sizes choose the best equipment in order to produce the best results possible, and this invariably includes audio interfaces. So it is very likely that the circuitry used in professional audio interfaces will be more sophisticated than those used in consumer devices. It is therefore entirely possible for an engineer to produce a mix which would sound great in the studio but may sound different on some domestic systems.

There have been several tests performed on domestic CD players with signals known to produce inter-sample peaks, with interesting results. The tests indicate that some players are quite capable of handling the peaks without audible results, but some models produce audible clipping.

So if the recording engineer has no idea whether the mixes he/she produces will sound distorted on some domestic playback systems how can the problem be fixed? Probably the best solution would be for the studio to have a variety of types of players to play mixes on, along with an engineer with very good hearing!

A practical alternative is to use a meter which simulates the oversampling DAC filtering processes used most commonly, and can therefore indicate the presence of >0dBfs inter-sample peaks, even if the peaks can’t be heard in the control room. Preferably, this meter would take the form of a VST plug-in that can be inserted at the end of the DAW mix. In this way the pure digital signal can be monitored and results predicted.

– How does X-ISM work?
Firstly it must be understood that most DAW peak meters (including plug-ins) simply look for samples which are at digital maximum. They will indicate a peak condition when there have been enough of these 0dBfs samples in a row (usually 3 together). Although this can give an indication that the engineer has run out of digital headroom (and that remedial action is necessary), it really gives no indication that the resultant analogue waveform may contain >0dBfs peaks due to the inter-sample peaks.

It is possible that a mix could have many values at digital maximum but no inter-sample peaks, similarly a mix could have no values at digital maximum but lots of inter-sample peaks!

Commonly, current mass market DAC designs do the vast majority of their reconstruction process by employing a digital Finite Impulse Response (FIR) filter design tailored to provide a brick wall low-pass response. This approach overcomes the less perfect slope response issues of pure-analogue filter designs. These FIR filters work as follows; firstly, the audio is up-sampled to a multiple of the original sample rate, then the FIR is used to create the smooth curve in-between the original sample points. Of course different DACs have various ways of doing this and indeed, the finer details of the process are important design elements of the DAC design.

X-ISM uses significant processing to provide a combination of up-sampling and filtering that mimics the operation of an oversampling DAC’s reconstruction process. The result is a meter that shows inter-sample errors and provides a useful tool that most DAW metering misses.

– Compatibility
The plug-in is available in VST, AU and RTAS formats making it compatible with virtually all audio software currently available on both PC and Mac.


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