Remove pre-ringing caused by steep linear phase antialiasing filters
- Windows 7 – 10 / 32 & 64 Bit
- Mac OSX Lion 10.7.3 – Sierra 10.12.x
Six different levels of Apodization (Shallow, Normal and Steep as short and long apodization filters)
64Bit High Precision Audio Engine
Input Audio Formats: WAV, AIFF, FLAC, ALAC and MP3
Output Audio Formats: WAV, AIFF and FLAC
Metadata transfer (album, title, artist, cover, etc.) between all file formats that include metadata
Multi-Threading to allow the conversion of several audio files in parallel
Batch processing with freely configurable output file names
The Test Version is limited to one minute output file length
The activation key is valid for three parallel installations (Windows & MacOS X)
XiPodizer can be fully tested before purchase.
The activation allows 3 parallel installations. In fact, it is possible to use the Windows and MacOS X Version in parallel by just using one activation key.
WHAT IS PRE-RINGING AND WHY SHOULD I CARE?
One of the advantages of high resolution audio (sample rates higher or equal 96kHz) is the possibility to apply any low pass (antialiasing) filtering outside of the audio spectrum and therefore without any impact on the temporal resolution of the audio signal.
Standard resolution CD quality records with a sample rate of 44.1kHz ask for steep low-pass filters at around 22kHz to avoid any aliasing during the analog to digital conversion process. The same principle applies during playback, where the digital to analog converter must use a low-pass filter to reconstruct the analog signal.
Filters used to suppress aliasing and to reconstruct the analog signal are usually “linear phase”, which implies that they create pre-ringing and post-ringing (pls. see Figure 1) by limiting the bandwidth of the input signal.
The ringing amplitude is significant because the spectral energy of music above 20kHz is still quite high.
There is quite a debate in the Hi-Fi community that the unnatural and none-causal impulse response of a linear-phase filter, plagued by pre-ringing, causes artifacts that sound inferior.
In contrast to High Resolution Audio we do not have the possibility to apply the filtering outside of the audio spectrum but we can use algorithms like “Apodization” that are quite efficiently used within different fields of optics like microscopy and astronomy.
APODIZATION IN MORE DETAIL
Within the audio domain apodization tries to reduce the pre-ringing by applying shallow “minimum phase” filters that only exhibit post ringing. That kind of ringing is causal and therefore part of all natural effects like the filtering caused by your listening room.
A second design goal is to keep the filters as short as possible to avoid introducing other artifacts that change the tonal balance of the records.
Unfortunately, there is no free lunch in digital signal processing.
Applying apodization forces us to design filters that show an earlier frequency droop, limiting the bandwidth of the audio signal and therefore its temporal resolution.
It seems to be common sense in the group of Hi-Fi enthusiasts that the limitation of the bandwidth is a good compromise for getting rid of the pre-ringing issues.
Figure 2 displays a setup where a Dirac impulse with an unlimited bandwidth is filtered by applying a linear phase antialiasing filter, causing pre-ringing (pls. see figure 1).
After applying Apodization, the transient is cleaned and exhibits only post-ringing (pls. see figure 3), which is stronger than the post-ringing of the “linear phase” filter but causal as well as shorter and therefore less disturbing.
WHAT ABOUT THE DIGITAL TO ANALOG CONVERTER PRE-RINGING?
Digital signal processing in the case of linear-invariant systems allows us to change the order of the processing steps without changing the result.
So, applying apodization at any point in the chain leads to the same results.
The setup of figure 4 shows the same output signal (pls. see figure 3) as the processing chain displayed in figure 2.
APODIZATION IN THE WHOLE RECORDING AND PLAYBACK CHAIN
Figure 5 represents the full recording and playback chain, including the analog to digital and digital to analog processing but no apodization.
The resulting output signal (pls. see figure 6) exhibits strong pre- and post-ringing.
In figure 7 the recorded signal is apodized by applying the XiPodizer to reduce the effect of pre-ringing.
The XiPodizer is very effective in suppressing the pre-ringing (pls. see figure 8) by compromising a bit of the available audio bandwidth.
Figure 5: Normal recording and playback chain covering the ADC and DAC processing without apodization.
Figure 7: Apodized recording and playback chain covering the ADC and DAC processing