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Inter-sample Peaks

Inter-sample Peaks

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A lot of Home Studio enthusiasts might be damaging their audio, without even knowing it.

A lot of Home Studio owners nowadays do everything, from the Production to the Recording, Mixing and Mastering. Almost 100% of the time the Mixing and Mastering takes place ITB (In the box – DAW). To fully understand what inter-sample Peaks are, we first have to look at audio in the digital domain.

D/A Conversion

Just as audio has to be converted from Analog to Digital in order for us to work with it in our DAW, it also has to be converted from Digital to Analog, in order for us to hear it. This is performed by a DAC (Digital to Analog converter). DAC's use an interpolation algorithm, also known as a FIR filter, to reconstruct the digital representation of audio into a smooth waveform.

Mixing/Mastering

With the Loudness War still in our midst, it is not unlikely for Mixing/Mastering Engineers to mix audio near or up to 0dBFS. Past 0dBFS is digital distortion!

ptmeters

Inter-sample Peaks

In some cases, it is possible for the DAC to produce levels that are greater than 0dBFS.

Take a look at the picture below:

Inter-Sample Peaks

(Photo courtesy of Solid State Logic)

In the picture above, four discrete samples are used to represent the audio. The smooth lines that connects the dots/samples are a product of the interpolation algorithm. Take note that none of the samples are above the 1 value (0dBFS in this case), but the smooth line that joins the samples has a value greater than 1 (greater than 0dBFS). It is possible when Mixing/Mastering music near 0dBFS, audible distortion may not be heard or clipping might not be seen, but a signal can be produced that is greater than 0dBFS on different audio players (iPods, MP3 Players, etc.)

Solution

Most DAW peak meters only look for samples that are at their digital maximum, and will only indicate a peak when 3 or more of these samples have reach their digital maximum in a row.

This might give the engineer an indication that he has run out of digital headroom, but it gives no indication if the resultant waveform might be over 0dBFS. The first solution would be to leave a reasonable amount of Headroom (-3dBFS – 0.3dBFS). The second solution would be to employ an inter-sample peak detector, like the X-ISM Plug-In from SSL. As mentioned earlier, DAC's employs a FIR filter, briefly explained, the FIR filter up-samples the audio to a multiple of the original sample rate. The FIR filter is then used to create the smooth curve in between the original sample points. X-ISM 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.

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1 comment

  • Jazlyn posted by Jazlyn Wednesday, 27 April 2016 08:01

    It's always a pleasure to hear from someone with exetrpise.

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