SSL Unveils Free X-ISM Metering Plug-In
Solid State Logic (SSL) has released X-ISM, free VST/AU plug-in for interpreting inter-sample peaks. The plug-in for Windows and Mac was developed to allow 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).
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? 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.
It must be understood that most DAW peak meters 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. In truth, it gives no indication that the resultant analogue waveform may contain >0dBfs peaks due to the 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.
The free SSL X-ISM plug-in is available in VST, AU and RTAS formats for PC and Mac. More information on SSL X-ISM.
